C S I R O L A N D a nd WAT E R

Balances of Water, Carbon, Nitrogen and

Phosphorus in Australian Landscapes:

(1) Project Description and Results

M.R. Raupach, J.M. Kirby, D.J. Barrett and P.R. Briggs

CSIRO Land and Water

Technical Report 40/01, December 2001

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Balances of Water, Carbon, Nitrogen and Phosphorus in Australian Landscapes:(1) Project Description and Results

M.R. Raupach, J.M. Kirby, D.J. Barrett and P.R. Briggs

CSIRO Land and Water Technical Report 40/01, December 2001

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COPYRIGHT

© 2001 CSIRO Land and Water.

To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water.

IMPORTANT DISCLAIMER

To the extent permitted by law, CSIRO Land and Water (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.

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Executive SummaryEquation Section 1

This is the first of two technical reports summarising the outcomes of Project 5.4A of the National Land and Water Resources Audit (NLWRA). This report provides an overview of the framework and results of the project, while the second provides supporting technical detail. The goal of the work is to assess the productive capacity of Australian landscapes, as determined by both water and nutrient availability, by • quantifying the linked balances of water, biomass (C) and key nutrients (N and P) on

Australian landscapes, expressed as spatial distributions of the major stores and fluxes in all these balances;

• quantifying the responses of stores and fluxes of C, N and P to changes in agricultural practice (nutrient inputs and offtakes, and water inputs through irrigation).

The major findings are as follows.

Net Primary Production: The most important driver of the coupled balances of water, C, N and P is Net Primary Productivity (NPP), equal to plant photosynthesis less plant respiration. This is the carbon or biomass yield of the landscape, available for use by animals and humans. On the Australian continent the spatial distribution of NPP broadly follows rainfall, but with additional influences from saturation deficit or air dryness (through its effect on water use efficiency), and also from light. The influence of light is significant only in Tasmania, because light is not a limiting resource elsewhere. The influence of saturation deficit implies that there is less NPP per unit rainfall in the north of the continent (where the air is dry on average) than in the south. This is a basic physiological constraint on plant growth and thence on agricultural productivity. NPP is also strongly increased in agricultural regions by nutrient inputs, and by water inputs through irrigation.

Carbon stores: The C stores in biomass, litter and soil are strongly controlled by NPP (hence rainfall and saturation deficit), so these C store distributions strongly resemble the NPP distribution. In addition, C stores are modulated by temperature: for a given NPP, there is less C storage in the tropics than in temperate regions because tropical C stores decay faster than temperate stores.

N and P stores: The stores of plant-available N and P, including both organic and mineral stores, are strongly coupled with NPP like the C stores. Only a small fraction (1 to 2%) of the plant-available N and P is in mineral form.

Dissolved N and P Concentrations in Soil Water: Dissolved N and P concentrations in soil water have a completely different spatial pattern to NPP, as the main climate driver on these concentrations is saturation deficit (air dryness) rather than rainfall. Hence, the N and P concentrations in soil water decrease as the climate-average air dryness increases from temperate to semi-arid tropical environments.

Effects of agriculture on NPP and the landscape stores of C, N and P: Agricultural nutrient inputs (including N and P from fertilisation and N from sown legumes) have led to regional-scale increases (relative to pre-agricultural conditions) of up to a factor of 2 for NPP and the stores of C, organic N and organic P, and up to a factor of 5 for mineral N, plant-available mineral P and the N and P concentrations in soil water. These increases are concentrated in the southern agricultural regions in WA, SA, Victoria and NSW. The influence of irrigation on NPP and the stores of N and P is locally large (especially in economic terms because of the

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prevalence of high-value commodities in irrigated areas) but its effect on continental aggregate stores and fluxes is relatively small because the irrigated area of the continent is small. Likewise, there is only a small influence at continental-aggregate scale on NPP and the landscape C, N and P stores from offtakes of agricultural products, though local effects may be significant.

N Balance: Before the advent of European-style agriculture, the N balance was dominated by input of N from natural fixation, with a small contribution from atmospheric N deposition. The balancing losses of N occurred through a mixture of volatilisation, leaching and disturbance (herbivory and fire). The spatial distributions of all these N fluxes were closely connected with the NPP distribution. With the advent of European-style agriculture, the N budget changed substantially: the largest term remains fixation, greatly enhanced in agricultural areas by sown legumes. Losses occur through disturbance (primarily herbivory by stock), leaching and volatilisation. The contribution of agricultural offtakes is negligible continentally but can be significant locally.

Continental-aggregate stores and fluxes of C, N and P: We estimate that the mean continental NPP is 0.96 GtC/year, and that nearly 60 Gt of carbon is stored on the continent in biomass and soil. Agricultural nutrient inputs have increased the continental NPP by 5%, the continental mineral N store by 13% and the continental mineral P store by 8%.

Implications: Nutrient inputs to the cultivated portions of agricultural landscapes often exceed those required to achieve optimum production levels and are approaching diminishing returns. Also, the costs from nutrient leakages into the environment (both through water and through air) increase sharply with increasing nutrient inputs. Therefore, there is an incentive to increase nutrient use efficiency so that agricultural production can be maintained or increased while total nutrient inputs, stores and leakages are reduced at landscape and regional scales. Mechanisms are also required for equitably distributing the environmental costs (largely off-site), production benefits (on-farm and within industries), and the costs of increased nutrient use efficiency (also on-farm).

Uncertainties: On regions of around 100 km by 100 km, the NPP has an uncertainty of around 30%, the organic C, N and P stores around 50%, and the mineral N and P stores around 100%. Estimates of change (current / pre-agricultural ratios) have uncertainties of around 50%. The predictions should never be interpreted at single-cell (5 km) scale, but rather should be used to analyse large patterns.

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Table of Contents

1 Introduction ......................................................................................................................42 Background .......................................................................................................................5

2.1 Mass Balances as NRM Accounts..............................................................................5 2.2 Landscape Function and Mass Balances ....................................................................6 2.3 Why Mass Balances Are Useful .................................................................................8

3 Methods .............................................................................................................................83.1 Model Formulation .....................................................................................................8 3.2 Spatial Scales, Temporal Scales and the Statistical Steady State.............................10 3.3 Forcing Variables and Input Data Sources ...............................................................11

4 Results..............................................................................................................................124.1 Water Balance...........................................................................................................12 4.2 Carbon Balance.........................................................................................................17 4.3 Nitrogen and Phosphorus Balances ..........................................................................21 4.4 Continental-Aggregate Stores and Fluxes ................................................................31 4.5 Uncertainties .............................................................................................................32

5 Discussion and Conclusions ...........................................................................................335.1 Implications ..............................................................................................................335.2 Strengths, Weaknesses and Next Steps ....................................................................34

Acknowledgments...................................................................................................................35References................................................................................................................................36

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1 Introduction

This is the first of two technical reports summarising the outcome of Project 5.4A of the National Land and Water Resources Audit (NLWRA), "Landscape Nutrient Flux and Water Balance". The original proposed title was "Living Within Our Means: Landscape Productive Capacity and its Dependence on Balances of Water, Carbon and Nutrients". The longer title remains an accurate statement of the aspirations of the project. Our goal is to assess the productive capacity of Australian landscapes, as determined by both water and nutrient availability. More specifically, we seek a spatially explicit determination of the linked balances of water, carbon (biomass) and nutrients on Australian landscapes, and the responses of these balances to changes in agricultural practice as quantified by nutrient inputs and offtakes, and water inputs through irrigation. These spatially explicit balances offer answers to the following questions at a broad (continental) scale: • What is the spatial distribution of the "natural" or "undisturbed" productivity of the

Australian environment, that is, the productivity prior to the introduction of European-style agriculture?

• How has this natural productivity been changed by the nutrient inputs and offtakes associated with European-style agriculture?

• What were the balances of nutrients (nitrogen and phosphorus) in Australian landscapes prior to European settlement? How have these balances been altered since then?

At the outset of the project, the goal was expressed in the following formal specification:

Broad Goal: To assess the productive capacity of Australian landscapes, and the roles of environmental nutrient fluxes in changing that capacity, by providing spatially explicit information on the balances of water, carbon and nutrients across basin-scale areas.

Enabling Objectives: Using national spatially resolved data on soil type, soil-landscape associations, climate, vegetation and land use, to: 1. Provide water balance terms (precipitation, transpiration, soil evaporation, surface and

subsurface runoff, deep drainage) for the soil root zone. 2. Provide estimates of the major terms in the vegetative carbon balance (net assimilation,

heterotrophic respiration, removal by grazing, harvesting and burning) and thus obtain estimates of amounts and changes in above-ground and below-ground biomass which are properly responsive to fluctuations in land use and climate.

3. Provide estimates of the following environmental fluxes of the nutrients N and P, in the same spatial and temporal framework as for the water balance: • gaseous N exchange; • leaching of N and P by deep drainage; • transport of N and P in solution by runoff and through sediment transport (in

conjunction with a related Audit project on sediment movement).

The purpose of the present report is to provide a non-mathematical account of the framework and results of the project. The companion report, Balances of Water, Carbon, Nitrogen and Phosphorus in Australian Landscapes: (2) Model Formulation and Testing (Raupach et al.2001, hereafter Report 2), describes in full detail the hypotheses, mathematical formulation and detailed testing of the model used to deliver the results specified above.

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This document (Report 1) is organised as follows. Following this introduction, Section 2 outlines the background and introduces informally the mass-balance approach which underpins this work. Section 3 provides a brief, informal description of the methods, referring for all details to Report 2. Results are described in Section 4, including the water balance, the carbon balance, the balances of N and P, and the changes to the C, N and P balances brought about by the introduction of European-style agriculture on the Australian continent. Finally, Section 5 summarises the implications of the project, assesses the strengths and weaknesses of the project by offering a critique of our own work (recognising that it has been completed to a time deadline rather than to scientific finality), and suggests next steps.

2 Background

2.1 Mass Balances as NRM Accounts

Land use for agriculture depends on harnessing key natural resources: light, water and nutrients. The availability of these resources determines the capacity of the land for natural or agricultural yield. The long-term availability of resources, and the consequent potential for generating yield, can be assessed by examining the mass balances of key resources, which in Australian conditions are water and nutrients. The nutrients studied here are nitrogen (N) and phosphorus (P). Like a financial balance sheet, a mass balance gives a picture of resource inflows or sources, resource outflows (including both use for commodity production and losses or leakages), the resource stock (the amount available for use), and the way that the stock changes with time in response to the various inflows and outflows.

The mass balances of water and nutrients on landscapes are linked by carbon (C) or biomass (roughly, plant biomass is about 50% carbon). Carbon is central to the balances of both water and nutrients for several reasons: first, the acquisition rate of plant biomass determines the resource available for human harvest and for maintaining animal populations, both natural and farmed. Second, the production of plant biomass is closely linked to plant transpiration or water use, which is usually a dominant outflow in the landscape water balance. Third, production of plant biomass is likewise also linked with the uptake of nutrients from soil into plants, after which the nutrients are locally recycled through litter or removed in plant or animal harvest. The balances and cycles of water, N and P are therefore intimately connected with the C balance and cycle, to the extent that all these cycles interact and constrain one another.

The most important carbon flow is the net rate at which plants build up carbon from the atmosphere by photosynthesis, the "net primary productivity" (NPP). This is the carbon gain per unit time by plants through photosynthetic assimilation, less the carbon loss per unit time by plant (autotrophic) respiration. NPP is the fundamental measure of "landscape yield" expressed in carbon units (for instance tonnes C per hectare per year). The NPP is significant not only as a measure of landscape yield, but also because plants acquire carbon from the air and nutrients from the soil in constrained ratios, so that the NPP provides information about the plant-soil cycle of nutrients through growth and decay.

In addition to the local nutrient cycles associated with carbon turnover, the nutrient balance depends on several nutrient inflows and outflows. For N, inflows include atmospheric deposition, fixation and fertilisation; outflows include gaseous loss, leaching from the root zone, export in surface runoff, and removal by harvest. For P (considering only the part of the

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total P in the soil which is chemically available to plants) the inflows include physical or biological weathering which releases P from inert soil stores, and fertilisation; outflows include leaching, export in runoff (in both dissolved and sediment-bound forms) and removal by harvest.

In summary, a biophysical balance sheet for Australian landscapes involves the linked mass balances of water, carbon and nutrients - here restricted to a consideration of N and P. This balance sheet has both spatial and temporal variability. The spatial variation is characterised by mapping the major stores and flows of water, C, N and P across the continent. A first insight into the temporal variation can be gained by comparing the stocks and flows in the mass balances under natural (prior to European-style agriculture) conditions with the corresponding stocks and flows under agricultural land management regimes in which nutrient inputs may be enhanced by fertilisation or introduction of legumes, and water inputs by irrigation. This gives an initial estimate of the changes in the biophysical balance sheet brought about by the introduction of European-style agricultural practices.

2.2 Landscape Function and Mass Balances

The flows of water, C, N and P in a landscape act to transfer matter through a set of stores or pools with different functions. Figure 1 shows the main flows and stores of water, C, N and P considered in this work.

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Linked terrestrial cyclesof water, C, N and P

Water flow

Respiration

Fertiliserinputs

N fixation,N deposition,gaseous N loss

C flowN flow

P flow

PLANTLeaves, Wood, Roots

ORGANIC MATTERLitter: Leafy, WoodySoil: Active (microbial)

Slow (humic)Passive (inert)

Photosynthesis

ATMOSPHERECO2 H2O N2, N2O

SOILSoil water

Mineral N, P

Leaching Sediment transport

N,P Cycles

C Cycle Transpiration

Runoff

Disturbance(herbivory, fire, offtake)

RainWaterCycle

Figure 1: Major pools and fluxes in the linked water, C, N and P cycles through the atmosphere, plants and soil. NPP is the sum of photosynthesis and plant (not litter and soil) respiration.

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The stores include leaves, wood, and roots and soil in various layers (two layers are considered, corresponding with the A and B soil horizons). The stores in the soil are further subdivided for modelling purposes to account for components of the organic C, N and P stores with different turnover times or chemical reaction properties, representing the active (metabolic), slow (humic) and passive (inert) components of soil organic matter. In addition, mineral N and P stores describe the reservoirs of N and P in the soil that are available for plant uptake and exchange with organic matter. In the case of N, this store includes all plant-available mineral forms (nitrate, nitrite and ammonium) while in the case of P it includes the soluble phosphate directly available for plant uptake, but not the (much larger) stores of P which are ionically bound to soil particles and therefore unavailable to plants on the time scales of plant growth and decay (for more details, see Report 2).

The key flows which exchange matter between the stores include: • conversion of light into plant biomass by photosynthesis; • local cycling of water through rainfall, soil evaporation and transpiration; • uptake of nutrients by plants and their return to the soil through litter decomposition; • grazing and nutrient cycling by animals; • harvest of product by humans.

These flows involve both internal, local cycling between the stores in the landscape, and also exchanges between the landscape and its external environment. Primary flows in this latter category are the inputs and losses of energy (through light, heat and evaporation), water (through rainfall, plant transpiration, soil evaporation and runoff) and carbon (through photosynthesis and respiration). Landscapes are also subject to small (and sometimes not-so-small) inputs and losses of nutrients through atmospheric deposition, fertilisation, fixation, leakage in runoff and leaching, gaseous loss to the atmosphere, and human harvest of product.

Box 1: The overall balances of water, carbon, nitrogen and phosphorus in the plant-soil system. In each case, the upper (light green) frame indicates the major contributions to the total landscape store, and the lower (light yellow) frame indicates the flows contributing to changes in the landscape store with time.

[ ] [ ][ ] [ ]

Net Primary Production (NPP) Heterotrophic Respiration

Transport (Water, Wind) Disturbance (Herbivory, Fire, Offtake)

TotdCdt

= −

− −

[ ] [ ] [ ][ ] [ ][ ] [ ]

N Fertilisation + N Fixation + Atmospheric N Deposition

Gaseous N Loss N Leaching

Transport (Water, Wind) Disturbance (Herbivory, Fire, Offtake)

TotdNdt

=

− −

− −

[ ] [ ] [ ][ ] [ ][ ] [ ]

P Fertilisation Atmospheric P Deposition Weathering of P

P Leaching Exchange with Soil-Bound P

Transport (Water, Wind) Disturbance (Herbivory, Fire, Offtake)

TotdPdt

= + +

− −

− −

[ ] [ ]Plant Water + Soil WaterTotW =

[ ] [ ] [ ]Plant C + Litter C + Soil CTotC =

[ ] [ ] [ ] [ ]Plant N + Litter N + Soil Organic N + Plant-Available Mineral NTotN =

[ ] [ ] [ ] [ ]Plant P + Litter P + Soil Organic P + Plant-Available Mineral PTotP =

[ ] [ ] [ ][ ] [ ] [ ]

Rain Canopy Transpiration Soil Evaporation

Interception Evaporation Runoff Drainage

TotdWdt

= − −

− − −

Water

Carbon

Nitrogen

Phosphorus

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For the complete plant-soil system, the mass balances for water, carbon, nitrogen and phosphorus are shown in Box 1. In each of these balances, the left hand side is the change in store per unit time, and the quantities in brackets on the right hand side are the fluxes contributing to the storage changes.

Over a sufficiently long time period it is often true that a store (say Ctotal) on average is neither increasing nor decreasing (though it fluctuates continually about an average value). The time-averaged value of storage change (dCtotal/dt) then approaches zero as the averaging time increases, and the time-averaged fluxes on the right hand side of the mass balance sum to zero. This is the "statistical steady state" condition.

2.3 Why Mass Balances Are Useful

A mass balance approach aids the fundamental task of determining the cycles of water, carbon and nutrients by provide basic physical constraints on material flows. These constraints can act in several ways:

1. For an individual entity (say water), measurements or models of all terms except one in the balance equation provide an estimate or constraint on the last, unmeasured term (subject to the accumulated errors in the other terms). If all terms in a single mass or energy balance are measured or modelled, the balance provides a check on measurement error or model consistency.

2. When several interacting entities are together subject to mass balances in the same control region, stronger constraints emerge because of the existence of relationships between the fluxes of different interacting entities. A key example is the link between carbon, nitrogen and phosphorus fluxes in the plant-litter-soil cycle, imposed by the fact that N:C and P:C ratios in plant, litter and soil organic stores are constrained by the biochemical compositions of the stores. These relationships transfer information between the balances of different entities, so that information about the fluxes in the C balance constrains both the N and the P balances.

3. When a single entity is subject to mass balances in adjacent stores, fluxes out of one store are related to (often, are identical with) fluxes into adjacent stores. This further constrains the behaviour of the system.

3 Methods

3.1 Model Formulation

A new model of the landscape balances of water, C, N and P has been developed for this work. Two versions of the model have been constructed: an evolving or time-dependent model (BiosEvolve) and an equilibrium or statistically steady-state model (BiosEquil). The structure of the governing equations in both versions is identical, apart from some simplifications in the treatment of the water balance in BiosEquil. All results presented here are from the statistically steady-state version, BiosEquil. The idea behind a statistically steady-state model is described in the next subsection.

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Full details of the governing equations are given in Report 2. Briefly, the model is based on two kinds of equation, the first being conservation or mass balance equations such as those in Box 1. These are of the generic form

[ ] [ ]Sum SumChange of store with time

Fluxes entering store Fluxes leaving store

=−

(1)

Second, phenomenological equations specify the fluxes appearing in the mass balance equations. These are generically of the form

( )Function ; ;Flux Stores Forcing Variables Parameters= (2)

where: • Stores include all water, C, N and P stores represented in the model; • Forcing Variables include both meteorological drivers (rainfall, solar radiation,

temperature, humidity) and land management drivers (nutrient inputs from fertilisers and legumes, product offtake in harvest, water inputs from irrigation); and

• Parameters are of three kinds: process parameters (such as light use efficiency or maximum photosynthetic capacity); properties of the soil or landform (such as soil depth or layer structure, and the hydraulic and thermal properties of soil layers); and properties of the vegetation and land surface (such as height, leaf area index or albedo).

The phenomenological equations are scale dependent, and are chosen here to describe landscape function at large scales (cell size about 5 km ×5 km or 25 km2). Referring for all details to Report 2, we briefly summarise here the principles used to determine the phenomenological equations at landscape scale, and also the tests of these equations.

Water fluxes (evaporation, transpiration and drainage): In the fully time-dependent model BiosEvolve, the treatment of the water balance involves explicit temporal resolution of all water fluxes in two soil layers. In the statistically steady-state model BiosEquil (the model supplying the results reported here), the following much simpler formulations are used for the time-averaged water fluxes: • Long-term average total evaporation is determined by water supply (rainfall) in dry

environments and by energy supply (radiation) in wet environments. • The energy-limited evaporation, the maximum attainable in wet environments, is

quantified as the Priestley-Taylor evaporation (1.26 times the available energy from net radiation), for reasons set out in Raupach (2000, 2001). Taking the maximum evaporation rate attainable in a wet environment as the definition of potential evaporation, we quantify potential evaporation by Priestley-Taylor evaporation.

• A single-parameter hyperbolic function interpolates between dry (rainfall-limited) and wet (energy-limited) total evaporation rates.

• The total evaporation is the sum of plant transpiration and soil evaporation. The partition between plant and soil is determined by the leaf area index.

• A single water store is considered, encompassing the entire soil root zone. Its time-averaged relative water content is approximated as the ratio of actual total evaporation (including plant and soil contributions) to potential (Priestley-Taylor) evaporation.

• A rule-based method is used to estimate deep drainage, involving assignment of a drainage rate constant on the basis of soil texture, with modification to account for

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cultivation. The actual time-averaged drainage is the product of this rate constant and the time-averaged total water content.

• Testing: The formulation for total evaporation has been tested against a set of data from small catchments (Report 2). Also, the water balance formulation has been tested (Report 2) by comparing predicted mean runoff with the observed mean runoff for 245 ANRA (Australian Natural Resources Atlas) Drainage Basins (NLWRA 2001).

Net Primary Productivity (NPP): We note first that GPP (Gross Primary Productivity) is the carbon flux into plants by photosynthetic assimilation, and NPP is the GPP less plant or autotrophic respiration. These quantities are modelled as follows: • Autotrophic respiration is modelled by expressing NPP as a fixed fraction (0.45) of GPP,

following Landsberg and Waring (1997). • An expression is found for the NPP limited only by light and water. This NPP is

distinguished by an asterisk, as FC*. It is determined jointly by transpiration, via a water use efficiency, and from incident radiation via a light use efficiency (an approach which recognises that the primary limit on NPP in Australia is water availability).

• The NPP limited by light and water (FC*) is then used to find the actual NPP (FC)

resulting from nutrient limitation in addition to light and water limitation, by applying a combination of biophysical and scaling arguments.

• Testing: The NPP formulation has been calibrated and tested (Report 2) against a set of 185 point data from the VAST database (Barrett 2001).

• A key property of the NPP formulation is that the NPP is predicted to have a strong dependence on saturation deficit (dryness of the air), through the effect of saturation deficit on water use efficiency: that is, an increase in saturation deficit implies a lower water use efficiency, which implies a lower NPP (all else being equal). Data confirm that this dependence is real (Report 2).

Carbon and nutrient fluxes between stores: The treatment is as follows: • Carbon fluxes out of stores are governed by rate constants dependent on temperature and

soil moisture. • Nutrient (N, P) fluxes follow carbon fluxes in stoichiometric ratios determined by

destination (not source) stores. • The stores and flows of C, N and P in litter and soil are modelled following the Century

model of litter and soil biogeochemistry (Parton et al. 1987, 1988, 1993). • Testing: The predictions for biomass, litter and soil carbon have been tested (Report 2)

against several hundred point data, also from the VAST database.

3.2 Spatial Scales, Temporal Scales and the Statistical Steady State

The spatial resolution is 0.05 (about 5 km) and the spatial domain encompasses the Australian continent.

The time step of BiosEvolve is 1 day. BiosEquil has no time step, as it determines long-term average values of the stores and fluxes in the water, C, N and P balances. Provided that the forcing variables (meteorological and land management) are steady, these long-term average values coincide with the steady-state solution arising when the storage changes in Equation (1) are much less than the fluxes, so that these equations reduce to Sum[Fluxes] = 0. The model then reduces to a set of algebraic equations, far easier to solve than the fully time-dependent model which involves differential equations in time. The statistically steady state solution arises when the forcing variables in Equation (2) are variable in time but are

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statistically steady (that is, have no temporal trends). In this case the stores and fluxes found by solving Equations (1) and (2) are also time-variable but statistically steady, and the steady-state solution describes the average state of the system after the temporal fluctuations are smoothed out. To find this average state algebraically by this route, the phenomenological equations (2) must apply to the time-averaged quantities. Raupach et al. (2001) show how the time-averaged phenomenological equations can be related to their short-term counterparts.

In BiosEquil, a mean annual cycle is retained in calculating evaporation, transpiration and NPP (that is, these quantities are seasonally varying). All other stores and fluxes are calculated as long-term averages, as above.

3.3 Forcing Variables and Input Data Sources

The model requires specification of a number of external forcing variables in several categories (see Table 1 for a summary).

Variable Source Spatial resolution

Temporal resolution

Temporal duration

Climate QDNRM 0.05° ≈ 5 km Daily 1980-1999

Land Use NLWRA (BRS) 0.01° ≈ 1 km Static 1996

Land cover Carnahan vegetation maps

Pathfinder AVHRR

0.05° ≈ 5 km

0.08° ≈ 8 km, resampled to 0.05°

Static

10 days

1750, 1988

1981-1994

Farm fluxes NLWRA (Reuter) SLA Annual 1987-1994

Soils Atlas of Australian Soils 0.05° ≈ 5 km Static present

Table 1: Major classes of input variable, showing space-time resolution, data source and period of available coverage.

1. The external climate variables are rainfall, solar radiation, temperature and humidity. These are available from Bureau of Meteorology records, interpolated and gridded by the Queensland Department of Natural Resources and Mines (QDNRM) (0.05° spatial resolution, continental spatial domain, daily time step) (Jeffrey et al. 2001). We used this invaluable and extensive climate data set for the period 1980-1999.

2. Land cover is specified by two leaf area indices for each spatial grid cell, one for woody and one for grassy vegetation. These were obtained from a companion project (Lu et al.2001) which undertook time series analysis of Normalised Difference Vegetation Index (NDVI) data from the Pathfinder AVHRR dataset for the period 1981 to 1994.

3. Land use was specified from the 0.01° (about 1 km) land use map developed by the Bureau of Resource Sciences (BRS) for 1996-97 (NLWRA 2000). We restricted the land use specification to four categories: crop, pasture, horticulture and non-agricultural land.

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4. Irrigation was determined from the BRS land use map as an areal extent, by assigning each grid cell an irrigated area fraction. Irrigation amount was estimated for the present primary purpose (the determination of continental C, N and P balances) by assuming that irrigation occurs to the extent necessary to raise the transpiration of the irrigated vegetation to the potential (Priestley-Taylor) evaporation rate.

5. Agricultural nutrient inputs and offtakes were determined from data supplied by Dr Doug Reuter through the project "Farm Nutrient Balances" (NLWRA Project 5.4D). Using data from fertiliser companies and the Australian Bureau of Statistics, that project provided estimates of farm inputs and offtakes of many nutrients. The ones relevant here are: • N input from sown legumes • N input from fertiliser • P input from fertiliser • Offtakes of N and P in farm produce, both plant and animal. All data were available as aggregates at Statistical Local Area (SLA) level, for those SLAs in the agricultural parts of the country. The land use map was used to disaggregate these data to estimate the above quantities for individual grid cells, preserving totals for SLAs (see Report 2 for details).

6. Soil data were obtained from the Atlas of Australian Soils. We used a gridded version of the Atlas in which the polygonal map of soil type (from a set of 725) was resampled at 0.05 , and cells assigned the dominant soil type. Revised interpretations (McKenzie et al.2000) were used to derive soil physical properties from the categorical specification of soil type. The physical properties derived in this way were (separately for an upper and a lower soil layer): layer depth, bulk density, silt and clay fractions, saturated volumetric water content, and saturated hydraulic conductivity.

4 Results

4.1 Water Balance

Rainfall: Rainfall largely drives the water, C, N and P balances, so it is appropriate to review its distribution prior to presenting the main results. Australia's overall continental water balance is unusual in global terms (Figure 2): in comparison with a global average annual rainfall of 777 mm on land surfaces, the Australian continent receives an average annual rainfall of 465 mm. Of this, almost all is evaporated, leaving only 52 mm of water annually for runoff, much less than the 310 mm of annual runoff averaged over the earth's land surfaces. It is also well known that the year-to-year variability of Australian rainfall is very high by global standards, and is linked with changing currents and water temperatures in the Pacific, Indian and Southern oceans. For example, there is a significant correlation (about 0.5) between annual continental rainfall and the ENSO (El Nino - Southern Oscillation) phenomenon in the Pacific Ocean (Figure 3).

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0

500

1000

1500

2000

2500

3000

All l

and

Aust

ralia

Ocean

ia

N Am

erica

S Am

erica

Ann

ual W

ater

Flu

x (m

m) Precipitation

RunoffEvaporation

P: 777R: 310E: 467

P: 476R: 52E: 424

Figure 2: Terms in the annual water balance (Precipitation = Evaporation + Runoff) averaged over all land surfaces on earth (left bars) in comparison with the same terms averaged over the continents of Australia, Oceania, North America and South America.

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

SOI

-30

-20

-10

0

10

20

Annu

al A

vera

ge R

ainf

all (

mm

)

300

400

500

600

700

800

900

Figure 3: The annual average rainfall over the Australian continent during the twentieth century (blue) and the annual average of the Southern Oscillation Index (SOI, the normalised difference between atmospheric pressures at Darwin and Tahiti).

The spatial distribution of rainfall across the continent is strongly nonuniform (Figure 4). About one third of the continent is classed as arid (receiving less than 250 mm average annual rainfall) and another third as semi-arid (250 mm to 500 mm). The rainfall distribution also changes dramatically through the annual cycle (Figure 4), so that the seasonal cycle of rainfall is quite different in different climatic regions. In the northern parts of the continent, rainfall is dominated by a humid, monsoonal wet season (October to March) followed by a hot, dry season (April-September). In the southwestern parts the pattern is Mediterranean with hot, dry summers and cool, wet winters, watered by frontal rain from the Southern Ocean. In

14

southeastern parts the rainfall is more uniformly distributed through the year, under the combined influences of winter rain from the Southern Ocean and summer rain from the Pacific Ocean, brought by weather systems from the northeast. The combined effect is that the seasonal pattern of Australian rainfall has a "flip-flop" character, being wet in the tropics and dry in the south during the (southern) summer, and the reverse in the southern winter.

Figure 4: Rainfall: mean annual (left panel) and mean monthly (right panels). Source: Bureau of Meteorology(BoM) data, interpolated by the Queensland Department of Natural Resources and Mines (QDNRM).

Evaporation and transpiration: Potential evaporation (defined here as energy-limited or Priestley-Taylor evaporation) is generally high over the Australian continent, significantly exceeding rainfall in all but the wettest areas (Figure 5). It is extreme (approaching 10 mm/day) in the northern inland in summer. It decreases with decreasing solar radiation, that is with increasing southern latitude and in winter. This measure of potential evaporation also decreases with proximity to coasts, because of increasing cloudiness.

Figure 5: Priestley-Taylor (potential) evaporation: mean annual (upper panel) and mean monthly (lower panel). Source: BoM-QDNRM gridded data on solar irradiance and near-surface temperature.

Actual evaporation (Figure 6) and transpiration by plants (Figure 7) are both far more spatially variable than potential evaporation, reflecting the effects of water limitation in most areas of the continent. Hence, both the annual mean and seasonal patterns in these quantities

15

broadly resemble the corresponding patterns for rainfall. The difference between total evaporation and canopy transpiration is soil evaporation, which is a large fraction of the total where there is little plant cover - broadly in arid environments. Hence the canopy transpiration maps have more "contrast" (relative variation) than the total evaporation or rainfall maps.

Figure 6: Total evaporation (canopy transpiration plus soil evaporation): mean annual (left panel) and mean monthly (right panel).

Figure 7: Canopy transpiration: mean annual (left panel) and mean monthly (right panel).

Runoff and drainage: Because potential evaporation significantly exceeds rainfall in all but the wettest areas, significant runoff (Figure 8) is confined to these wet areas and occurs in the southeast (including Tasmania), over the eastern ranges and in the north. Drainage (Figure 9) has a broadly similar pattern, though with added variability through the influence of soil texture.

16

Figure 8: Mean annual total runoff (surface plus subsurface).

Figure 9: Mean annual deep drainage.

The broad spatial pattern of the entire steady-state water balance can be seen by plotting its constituent terms as spatial averages across each of the 12 ANRA Drainage Divisions (AWRC 1987, NLWRA 2001). These are the largest hydrological units for the Australian continent (Figure 10). At this level of aggregation, the time-averaged water balance is

[ ] [ ] [ ]Rainfall Total Evaporation Total Runoff= + (3)

where total evaporation includes both canopy and soil contributions and total runoff includes both surface and subsurface routes. Figure 11 shows the rainfall, total evaporation and total runoff for the 12 ANRA Drainage Divisions, along with the canopy transpiration. The runoff is negligible in the drier Divisions and is more than half the rainfall only in the wettest Division, Tasmania. Canopy transpiration is less than half of the total evaporation in the drier Divisions, especially 7, 10, 11 and 12.

17

4.2 Carbon Balance

Landscape Yield (Net Primary Productivity): The mean annual NPP, with present agricultural inputs and offtakes of nutrients and present irrigation, is shown in Figure 12. This map is central to the whole project, because of the key role of NPP in determining fluxes and stores in the terrestrial C, N and P cycles (Figure 1). Dominant features of the NPP map are: • The NPP broadly follows rainfall, but with additional modulation by saturation deficit

through its effect on water use efficiency (see Report 2), and also by light. • The modulation by saturation deficit implies that there is less NPP per unit rainfall in the

north of the continent (where the saturation deficit is high on average, because of high air temperatures) than in the south (where the saturation deficit is lower on average).

• Modulation by light is significant only in Tasmania (see below). Elsewhere, light is not a limiting resource for NPP in Australia.

• The NPP is also strongly modulated in agricultural regions by nutrient inputs (and offtakes to a much lesser extent) and by water inputs through irrigation. Respectively, these inputs remove the nutrient and water constraints on plant growth.

Figure 11: Terms in the time-averaged water balance (rainfall = total evaporation + runoff) averaged spatially across each of the 12 ANRA Drainage Divisions.

North-East Coast 1South-East Coast 2Tasmania 3Murray-Darling 4South Australian Gulf 5South-West Coast 6Indian Ocean 7Timor Sea 8Gulf of Carpentaria 9Lake Eyre 10Bulloo-Bancannia 11Western Plateau 12

16

712

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Figure 10: the 12 ANRA Drainage Divisions.

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18

Figure 12: Mean annual Net Primary Productivity (FC) with current climate and current agricultural inputs.

Carbon Stores in Biomass and Soil: Figure 13a shows the C store in biomass (including leaf, wood and roots, that is, all above-ground and below-ground biomass). Figure 13b shows the summed C stores in all litter and soil stores. Key features are: • All these C stores are strongly controlled by NPP (hence rainfall and saturation deficit), so

the C store distributions strongly resemble the NPP distribution. • However, the C stores are also modulated by temperature because low temperatures slow

the decay of plant material and high temperatures promote rapid decay. More formally, temperature influences the rate constants or time constants (inverse rate constants) which parameterise decay. This in turn influences the steady state value of each C store, which is the product of a C input flux (proportional to NPP) and a time constant. All time constants decrease with temperature over the relevant range, so the steady state stores also decrease (for an otherwise constant NPP).

• Consequently, the ratio of tropical to temperate C storage is lower than the corresponding ratio for NPP, because the tropical C stores turn over faster than temperate stores.

Figure 13: a) mean biomass carbon, including leaf, wood and root pools. b) sum of all litter and soil carbon pools. Both with current climate and current agricultural inputs.

a b

19

Figure 14a shows the steady-state NPP per unit area, spatially averaged across the 12 ANRA Drainage Divisions defined in Figure 10. The ranking of the highest six drainage divisions for NPP per unit area is SE Coast (2), Tasmania (3), NE Coast (1), Murray-Darling Basin (4), SW Coast (6), SA Gulfs (5). This is quite different from the ranking for rainfall per unit area (Figure 12), which for the top six drainage divisions are Tasmania (3), Timor Sea (8), SE Coast (2), NE Coast (1), Gulf of Carpentaria (9), Murray-Darling Basin (4). These changes in ranking are brought about by all three of the modulating factors (in addition to rainfall) identified above as exerting significant controls on NPP: saturation deficit, light and agricultural inputs: • Light limitation causes Tasmanian NPP to be lower than regions in southern Australia,

despite these regions having a lower rainfall. • The strong influence of saturation deficit causes the northern regions (Timor Sea and Gulf

of Carpentaria) to have low NPP despite having high annual rainfall. This is a climatic influence which cannot be removed by either irrigation or nutrient inputs, and is a fundamental limitation on plant growth in northern Australia.

• The NPP in the primary southern agricultural drainage divisions (Murray-Darling Basin, SW Coast, SE Coast, SA Gulfs) is significantly enhanced by agricultural nutrient inputs and irrigation. Hence these divisions have a relatively high present NPP despite being far from the wettest regions on the continent.

Figure 14b shows the steady-state C stores in biomass and litter plus soil, as spatial averages across the 12 ANRA drainage divisions. In this case the top six divisions are Tasmania (3), SE Coast (2), Murray-Darling Basin (4), NE Coast (1), SW Coast (6), SA Gulfs (5). This differs from the ranking for NPP mainly because of the effect of low temperatures in slowing plant decay in the coolest parts of the country (Tasmania, SE Coast). These regions therefore have a relatively high C storage per unit NPP.

Figure 14: Net primary productivity, biomass carbon (CPla) and litter + soil carbon (COrg0), averaged spatially across each of the 12 ANRA Drainage Divisions (Figure 10).

Changes in the Carbon Balance Brought About by European-Style Agriculture: In Figures 12 to 14, all external climate and land management variables were those for current climate and current agricultural practices (irrigation and N and P inputs and offtakes). We also wish to assess the impact on the carbon cycle of changes in land management since 1788, associated with the introduction of European-style agriculture. To do this, we compare the steady-state

Net Primary Production of C: 12 Drainage Divisions

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20

NPP and carbon stores with two different (steady) external forcings: an "agricultural" case with present climate, vegetation and agricultural practices, as in Figures 12 to 14, and a "base" case with the same climate and vegetation but with no agricultural inputs and offtakes (that is, without water input by irrigation, N input from sown legumes or fertilisation, P input from fertilisation, and N and P agricultural offtakes). Thus, the comparison is between an Australian continent which is fully equilibrated to current agricultural practices, and a continent in equilibrium with external forcings in the absence of European-style agriculture.

Figure 15 shows the ratio of the steady-state agricultural NPP to base or non-agricultural NPP. Over the bulk of the continent (the grey region) the ratio is 1 because agriculture does not occur there, but in agricultural areas the NPP has increased locally (at the scale of 5 km cells) by up to a factor of 2 in response to N and P fertilisation and N input from sown legumes. The ratio (or factor by which NPP has increased) is even higher in irrigation areas. The largest regional-scale increases occur in the WA wheatbelt, with large increases also seen in the SA, Victorian and NSW wheatbelts. The very large increase in WA is associated with the low nutrient status of the sandy soils in that region prior to European-style agriculture.

This is consistent with the NPP formulation (Report 2), which predicts that the response of NPP to removal of nutrient limitations is an NPP increase by up to a factor of 2 from its natural value in the absence of artificial nutrient inputs, and that the response to removal of water limitations can be much larger.

In Section 4.4 we consider the separate contributions of nutrient inputs, irrigation and nutrient offtakes to the changes in Australian NPP which have occurred since 1788, by taking a whole-continent view.

Figure 15: Ratio of mean NPP with current agricultural inputs (irrigation, N and P inputs and offtakes) to mean NPP without agricultural inputs.

21

4.3 Nitrogen and Phosphorus Balances

Plant-available N stores: Figure 16a shows the steady-state store of total plant-available N, consisting of organic N in litter and soil plus the mineral plant-available N (including ammonium, nitrite and nitrate). The mineral component of this total store is shown separately in Figure 16b. Both maps are for current agricultural practices (irrigation, nutrient inputs and offtakes). Key features are: • Both of these N storage maps strongly resemble the maps of C storage and NPP, because

the N stores are coupled to C stores through well-defined (though not constant) N/C ratios in leaves, wood, roots, litter and soil organic matter.

• In particular, saturation deficit and temperature exert similar controls on N stores as they do on C stores, in the absence of agricultural inputs of N.

• Agricultural N inputs have a relatively higher impact on mineral N stores than on NPP and the other stores (C, organic N) which are primarily controlled by NPP. This important point is taken up later (Section 4.4).

Figure 16: a) mean total plant-available N (including organic N in litter and soil pools, and mineral N). b) mineral N. Both panels with current climate and current agricultural inputs.

Plant-available P stores: Figure 17a shows the steady-state store of total plant-available P (organic plus soluble mineral P) and Figure 17b the soluble mineral P component. • These maps have similar features (and for similar reasons) to Figure 16 for the N stores. • There is an additional important factor for P: the plant-available P stores shown in Figure

17 are far from the total P store in the landscape, because much of the mineral P in the soil is chemically tightly bound to the soil matrix and is therefore only weakly available for plant growth (secondary P) or unavailable in the absence of specific release processes (occluded P). This complication does not arise for N because (to a good approximation) all landscape N stores are plant-available. The properties of the various P stores are discussed in more detail in Report 2.

a b

22

Figure 17: a) mean total plant-available P (organic P in litter and soil pools and dissolved P, but excluding secondary and occluded P). b) dissolved P store. Both panels with current climate and current agricultural inputs.

Dissolved N and P Concentrations in Soil Water: Figures 18a and 18b show, respectively, the dissolved concentrations of N and P in soil water (respectively in mgN/kgWater and mgP/kgWater). These were calculated by assuming that the plant-available stores of mineral N and mineral P are in solution, so that the dissolved N concentration is the ratio of mineral N store to soil water store, and likewise the dissolved P concentration is the ratio of the dissolved P store to the soil water store.

Figure 18: Concentrations of mineral N (a) and dissolved P (b) in soil water, with current climate and current agricultural inputs.

Key features are: • The maps of the dissolved N and P concentrations look quite different to the maps of NPP

(Figure 12) and the C, N and P stores (Figures 13, 16, 17).• The reason (considering N as an example) is that the N concentration in soil water is the

ratio of the mineral N store to the soil water store. Rainfall has comparable influences on both these stores, and hence does not appear as a strong modulator of their ratio, the N concentration in soil water. The dominant rainfall signal in the maps of NPP and the C, N and P stores therefore does not appear in the N concentration in soil water. However, more subtle signals from external drivers do appear in this map. In particular, the effect of saturation deficit on the mineral N store (which is mainly controlled by NPP) is greater

a b

a b

23

than the effect of saturation deficit on the soil water store (which is mainly determined by rainfall or energy limitation). Hence, it is predicted that the N concentration in soil water decreases as the climate-average saturation deficit increases from temperate to semi-arid tropical environments.

• Hence, the moderate to high N concentrations in arid areas are a consequence of very low soil water stores, not of high N stores.

• Similar reasoning applies to the dissolved P concentration (Figure 18b).

This result (a model prediction only at this stage) leads to a conjecture: the soil water concentrations of N and P may be significant drivers for dissolved N and P concentrations in rivers. If so, then there is a climate control on average dissolved riverine concentrations of N and P, through saturation deficit, and these riverine concentrations will show a similar south-north gradient to that evident in Figure 18. However, there are several additional factors in the relationship between landscape and riverine stores and concentrations of N and P which complicate (and possibly dominate) this picture: • Several processes affect the dissolved N and P concentrations in subsurface flow from

landscapes into rivers, including chemical transformations and interactions with potentially large stores not included in the present work. Examples are denitrification in riparian zones (leading to loss of N to the atmosphere) and sorption of dissolved P onto soil particles as secondary or occluded P (effectively acting as a P sink).

• Other processes act on the dissolved N and P concentrations in surface flow from landscapes to rivers. An obvious consideration here is that the water involved in rapid surface runoff is unlikely to equilibrate its N and P concentrations with soil water except in a very shallow layer through which surface runoff and soil water are mixed, thus effectively diluting the soil water concentrations.

• Sediment-borne contributions to riverine N and P (organic N, organic P, sediment-bound P) involve different dynamics to those represented in this work (Prosser et al. 2001, Young et al. 2001).

• Likewise, riverine N and P sourced from local pollution (for example from effluent or heavily fertilised crops) is subject to different controls.

Changes in Nitrogen and Phosphorus Stores Brought About by European-Style Agriculture:Figure 19a shows the ratio of the steady-state store of total plant-available N (organic plus mineral) with current agriculture to the "base" case without agriculture, as for Figure 15 in the case of NPP. Likewise, for each of the other N and P stores and concentrations in Figures 16, 17, 18, the ratio of that quantity with current agriculture to the same quantity without agriculture is shown in Figures 19, 20 and 21. In the case of mineral N, dissolved P and the N and P concentrations in soil water, this ratio can be as high as 5. It is lower for total plant-available N and P, reaching largest values of around 2 for these quantities.

The ratios shown in Figures 19 to 21 reflect both base and agricultural conditions, for instance in the sandy soils of the WA wheatbelt where there is little natural N and P (see also Figure 15 for the NPP ratio). However, the ratios may be indicators of ecological impacts.

Figures 22 and 23 provide two regional views of the response of the N and P concentrations in soil water to agriculture. Figure 22 shows these concentrations spatially averaged over 12 ANRA drainage divisions defined in Figure 10, without agriculture (upper panel) and with current agricultural practices (lower panel). The largest increases in the soil-water N and P concentrations occur in the SW Coast (6), the Murray-Darling Basin (4) and the SA Gulfs (5).

24

Figure 19: Ratios of mean N stores with current agricultural inputs (irrigation, N and P inputs and offtakes) to mean N stores without agricultural inputs. a) mean total plant-available N (including organic N in litter and soil pools, and mineral N). b) mineral N.

Figure 20: Ratios of mean P stores with current agricultural inputs (irrigation, N and P inputs and offtakes) to mean P stores without agricultural inputs. a) mean total P (including organic P in litter and soil pools, and dissolved P, but excluding some secondary and occluded P). b) dissolved P.

Figure 21: Ratios of concentrations of mineral N (a) and dissolved P (b) in soil water, with current agricultural inputs, to concentrations without agricultural inputs.

a b

a b

a b

25

Figure 22: Nitrogen (a) and phosphorus (b) concentrations in soil water with (black) and without (grey) European-style agriculture, for 12 drainage divisions (Figure 10).

Figure 23: Spatially averaged nitrogen and phosphorus in soil water versus mean annual rainfall for 245 ANRA drainage basins: a) concentrations in mgN and mgP per kg of water with no agriculture; b) Change in nitrogen concentration (%) due to European-style agriculture; c) Change in phosphorus concentration (%) due to European-style agriculture

Con

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Figure 23 provides a different perspective by plotting the N and P concentrations in soil water (without agriculture in the upper panel and with current agriculture in the lower panel) against mean annual rainfall. In this figure all quantities are averaged spatially across the 245 ANRA Drainage Basins (hydrological units which are subsets of the Drainage Divisions). Consistent with the discussion of Figure 18, there is little overall trend with rainfall, but the points separate into a lower branch for the northern catchments and an upper branch for southern catchments. This occurs both because of the link between saturation deficit and soil N through NPP noted above, and (for current agriculture) the large N inputs occurring in southern catchments (Figures 19-22).

Nitrogen Fluxes and Total Nitrogen Balance: According to the total nitrogen balance in Box 1, the fluxes contributing to the total landscape store of nitrogen (including N in plant, litter, soil and mineral stores) are: • Fertilisation: the N input from applied nitrogenous fertiliser; • Fixation: the input from N fixation by legumes, including both natural fixation by native

legume species and agriculturally enhanced fixation by sown crop and pasture legumes;• Atmospheric deposition: the input of N from the atmosphere, by dry deposition (via

particulates and gases) and wet deposition (via rainfall);• Gaseous Loss: the loss of N from the landscape to the atmosphere in the form of

nitrogenous gases, including NO, N2O, N2 and NH3;• Leaching: the loss of N from the plant-available mineral store by transport in dissolved

form, mainly via deep drainage of water from the soil store;• Particulate Transport: horizontal transport of N in particulate form by water or wind

erosion (which can be a sink or source of landscape N depending on whether the net erosion process is depleting or depositing particulate material);

• Offtakes: net removal of N in harvested product, either plant or animal (which can be negative if harvested product from elsewhere is used as an agricultural input, as in use of off-site hay for stock feed);

• Disturbance: two major disturbance fluxes are lumped into this last category: fire and herbivory. Fire causes a loss of N to the atmosphere as biomass is burned. The effect of herbivory (other than that accounted for in animal offtakes) is to accelerate the gaseous loss of N to the atmosphere as plant N is excreted by animals in a rapidly volatilisable form.

Of these fluxes, all except for disturbance and particulate transport have been estimated explicitly by methods outlined in Report 2. The sum of the disturbance and particulate transport fluxes appears as a residual in the closed, steady-state total landscape N balance, which requires that all the above fluxes sum to zero in the long-term average.

Two views of the total landscape N balance are presented. Figure 24 shows the spatial distributions (for current agricultural practices) of five N fluxes: fertilisation, fixation, gaseous loss, leaching and offtakes. (Atmospheric deposition is not shown in this figure because it is small and its modelled spatial distribution is like that for rainfall). Figure 25 compares the spatial averages across the 12 ANRA Drainage Divisions of all the above N fluxes, without agriculture (upper panel) and with current agriculture (lower panel). This provides an overview of the relative magnitudes of the fluxes, their large-scale patterns and the ways that the N budget has changed in response to agriculture.

27

Figure 24: Spatial distributions of flux terms in the total nitrogen budget: fertilisation, fixation, gaseous loss, leaching, offtakes. All panels with current climate and current agricultural inputs. Note that scales are different between panels; to compare overall magnitudes of the flux terms, see Figure 25. Note also that the gaseous loss and leaching fluxes are shown as negative in Figure 25.

28

Figure 25: Comparison of spatially averaged flux terms in the steady-state mineral N budget for 12 drainage divisions with and without European-style agriculture. N flux terms are fertilisation (+), atmospheric deposition (+), fixation (+), gaseous loss (-), leaching (-), and disturbance (-).

North-East Coast (1)

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South-West Coast (6)

Indian Ocean (7)

Timor Sea (8)

Gulf of Carpentaria (9)

Lake Eyre (10)

Bulloo-Bancannia (11)

Western Plateau (12)

With AgricultureNo Agriculture

29

Taking all results together, the emerging picture is: • Before the advent of European-style agriculture, the N balance was dominated by input of

N from natural fixation. The contribution of atmospheric N deposition as an input flux was (and remains) small.

• Pre-agricultural losses of N occurred through a mixture of gaseous loss, leaching and disturbance (herbivory and fire). Our present estimates indicate that leaching was the largest loss flux, but there is substantial uncertainty about the relative magnitudes of these three loss terms in the N budget (see below).

• The spatial distributions of all N major fluxes prior to European-style agriculture were closely connected with the NPP distribution.

• With the advent of European-style agriculture, the N budget changed substantially. On the input side, the largest term remains fixation, but this has been greatly enhanced in agricultural areas by sown legumes, both crop and pasture. N input from fertilisers is a contributor to the continental N balance but is much smaller than the N input from sown legumes (over the whole continent, agricultural N inputs from sown legumes exceed those from fertilisers by a factor of about 7).

• In the current N budget, losses occur through disturbance, leaching and gaseous loss to the atmosphere. The disturbance flux has increased dramatically in comparison with the pre-agricultural budget. We conjecture that this is mainly attributable to grazing by stock, which causes accelerated N loss to the atmosphere through volatilisation of N from excreta. However, this process has not been estimated directly.

• Through the combination of these disturbance fluxes and gaseous loss as major loss terms, the bulk of the N being applied agriculturally (either through sown legumes or through fertilisation) is being lost to the atmosphere rather than to water bodies through drainage.

• The direct contribution of agricultural offtakes to the national N mass budget is small, though it is likely to be significant in cropping areas (see below for a discussion of the uncertainties surrounding this issue).

• The determination of a continental N balance is a difficult exercise requiring a number of major assumptions which are spelt out in detail in Report 2. In the concluding section of this Report (under "strengths and weaknesses") we assess the uncertainties in the picture and the ways that it may change as more information becomes available.

Phosphorus Fluxes and Total Phosphorus Balance: Our budget applies to plant-available P, the P store that interacts directly with the C cycle. This includes P in plant, litter, soil organic matter and the soluble mineral store (primary P). As indicated above, plant-available P is only part of the total P store in the landscape, the remainder being only weakly available for plant growth because it is chemically bound to the soil matrix (secondary P) or effectively unavailable because it is both chemically bound and physically protected within soil particles (occluded P). From Box 1, the fluxes in the landscape balance of plant-available P are: • Fertilisation: the P input from applied fertiliser; • Atmospheric deposition: the input of P from the atmosphere, mainly by dry deposition via

particulates (as for N, except that gaseous deposition and wet deposition are probably negligible for P);

• Weathering: mobilisation of P from inert soil and rock stores by physico-chemical or biological processes;

• Leaching: loss of P from the plant-available mineral store by transport in dissolved form in deep drainage (as for N);

• Particulate Transport: horizontal transport of P in particulate form by water or wind erosion (as for N);

30

• Occluded P sink: The return of P to the inert soil store, via the secondary P store (the opposite process to weathering);

• Offtakes: net removal of P in harvested product, either plant or animal (as for N);• Disturbance: the major disturbance flux for P is probably fire, through transport in

airborne particulate ash. The P fluxes due to herbivory (the other major disturbance flux for N) are likely to involve local recycling on the landscape through plant and soil stores, and therefore are unlikely to be major contributors to the overall landscape P balance.

In contrast with the situation for N, it is not possible even to estimate a steady-state landscape balance for plant-available P because of the very slow exchanges with the inert P stores (on time scales up to millenia or longer). This source is postulated to balance the major loss of plant-available mineral P, the leaching of dissolved P in deep drainage. Hence, the balance of total landscape P (including organic plant-available mineral, secondary and occluded stores) is non-steady on these long time scales.

In this work we only estimate a few of the major fluxes in the balance, as shown in Figure 26. These estimates suggest that with present agricultural inputs, P fertilisation is of the same order of magnitude as leaching in the agriculturally managed parts of the country. Given the lack of knowledge of other terms in the P budget, it is not possible to draw robust conclusions about long-term trends in plant-available P stores.

Figure 26: Comparison of spatially averaged flux terms in the steady-state dissolved P budget for 12 drainage divisions (Figure 10) with and without European-style agriculture. P flux terms are fertilisation (+) and leaching (-).

Fert Leach

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31

4.4 Continental-Aggregate Stores and Fluxes

It is useful to use a whole-continent overview of the C, N and P cycles and their responses to external forcing, to assess the ways that they have changed in the past (mainly in response to agricultural land management) and may change in the future (in response to probable changes in climate as well as land management). Therefore, Table 2 presents continental averages of seven key quantities: the NPP, the C stores in biomass and the aggregate of litter and soil; total plant-available N and mineral N; total plant-available P and dissolved P. The reference values apply to present agriculture (nutrient inputs, nutrient offtakes and irrigation) and present climate (the mean over the period 1980-1999).

Departure from reference (present conditions) (%)Quantity Reference value No Offtakes No Irrig No Agric Rain -20% Temp+3°C CO2+200ppmNPP 0.961 [GtC/yr] 0.0 -0.3 -5.0 -7.6 -6.0 16.7Biomass C 26.584 [GtC] 0.1 -0.2 -5.7 -6.4 -27.4 15.4Litter+soil C 31.959 [GtC] 0.1 0.2 -5.3 1.2 -24.7 16.3Total N 2.657 [GtN] 0.1 0.2 -5.7 0.4 -24.9 16.3Mineral N 0.033 [GtN] 0.0 -0.3 -13.2 -7.7 -11.4 16.9Total P 0.389 [GtP] 0.0 0.0 -6.5 -2.8 -20.6 16.7Labile P 0.006 [GtP] 0.0 -0.3 -7.9 -7.4 -11.3 16.9

Table 2: Continental aggregates of the NPP and major stores of C, N and P. The reference values apply to present agriculture (nutrient inputs, nutrient offtakes and irrigation) and present climate (mean of 1980-1999). The first three "departure" columns give changes from reference values (1) without nutrient offtake; (2) without nutrient offtake and irrigation; (3) without nutrient offtake, irrigation and agricultural nutrient inputs. The other three "departure" columns give changes in reference values under three climate perturbations: (4) all rainfall reduced by 20%; (5) all temperatures increased by 3 degrees; (6) all CO2 concentrations increased by 200 ppm.

Significant features of these reference values are: Conclusions from Table 2 are: • The mean continental estimate of 0.96 GtC/year is in agreement with a recent estimate by

Barrett (2001), but is higher than several earlier estimates (for instance Gifford et al.1992).

• Nearly 60 Gt of carbon is stored on the continent in biomass and soil. • Of the plant-available N and plant-available P, only a small fraction (between 1 and 2%)

is in mineral form. The rest is "in use" in biomass, or "on return" to the mineral stores, through litter and soil organic matter.

An assessment has been made of the extent to which these quantities have been changed by European-style agriculture, including nutrient inputs (fertiliser inputs of N and P and sown legume inputs of N), irrigation, and nutrient offtakes in product. To do this, three statistical-steady-state situations have been considered in addition to the reference state of present agriculture and climate. These three states differ from the reference state in being (1) without nutrient offtake; (2) without nutrient offtake and irrigation; (3) without nutrient offtake, irrigation and agricultural nutrient inputs. Table 2 shows the departure of each of these three from the reference state. It is clear that:

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• On a continental basis, the effect of the inclusion or otherwise of nutrient offtakes in the balances is negligible.

• Irrigation continentally has a small effect on the NPP and the C, N and P stores. Naturally, however, its local effect is much larger and its economic impact is larger still because irrigated areas tend to produce high-value commodities. Young (2002) has estimated that although less than 1% of land used for agriculture in Australia is irrigated, it contributes roughly half of total agricultural profits.

• The effect of agricultural nutrient inputs is large. These have increased the continental NPP by 5%, the mineral N store by 13% and the mineral P store by 8%.

We have also assessed the effects of changes in climate, using the simple methodology of comparing steady-state predictions under different kinds of forcing. The climate changes considered (Table 2) are (4) all rainfall reduced by 20%; (5) all temperatures increased by 3°C; (6) all CO2 concentrations increased by 200 ppm. These changes are broadly indicative of climate scenarios for the next 100 years (IPCC 2001). Each change in forcing was considered in isolation, in the spirit of a linear perturbation analysis in which different external forcing variables are perturbed in turn while keeping all else fixed at the reference state. The results (Table 2) suggest a decrease in NPP of around 7% in response to rainfall and 6% in response to temperature, offset by an increase due to the "CO2 fertilisation effect" (the tendency for plants to grow faster as the ambient CO2 concentration increases). The temperature responses of all stores are larger than that for NPP, because of the tendency for higher temperatures to decrease store turnover times as noted in Section 4.2. However, all these results for the effects of climate changes must be treated as indicative only, because the model is simple (strictly, too simple for this purpose) and the imposed climate-change scenarios are crude.

4.5 Uncertainties

To this point we have not attached formal uncertainties to any of the above numerical predictions. A formal uncertainty analysis is quite difficult because apparently large uncertainties in individual fluxes are constrained by overall mass balance requirements and by links between balances for different entities. Hence, an informal assessment is offered on the basis of levels of scatter in calibration plots (Report 2), experience with sensitivities of the calculations to variations in model parameters, judgements about the robustness of the various flux parameterisations and assumptions, and the uncertainties in the input data.

This assessment gives the following uncertainties at a scale of a large region (100 km by 100 km or greater): • NPP: 30% • Organic stores of C, N and P: 50% • Mineral stores of C, N and P 100% • Current / pre-agricultural ratios: 50% • Leaching and drainage fluxes: large uncertainty

Uncertainties over smaller areas increase. The predictions are designed to determine large-scale patterns, not the behaviour of individual farms or paddocks, and should never be interpreted at single-cell (5 km) scale.

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5 Discussion and Conclusions

This report is essentially an extended summary of the results of the project, with a briefer version provided in the Executive Summary. Therefore, we do not recapitulate the main results here but concentrate instead on an assessment of the implications of the work, its strengths and weaknesses, and the next steps.

5.1 Implications

Australian agriculture is set against the backdrop of a natural landscape in which plant growth is limited mainly by rainfall and air dryness (because the water use efficiency of plants decreases as humidity falls, for basic physiological reasons). In addition to these limitations imposed by average climatic conditions, Australian agriculture has to cope with very high climate variability. This means that efficient use of natural rainfall, supplemented where possible by irrigation for crops and pastures, is of paramount importance in sustaining productivity and environmentally sound water balances in Australian agricultural landscapes.

Likewise, the use of nutrient inputs is essential to overcome innate nutrient deficiencies in Australian landscapes and to replenish nutrients lost during agricultural production, either through export as product or through leakage to water bodies or the atmosphere.

These additional resources of water and nutrients have been employed extensively in Australian agriculture. Because of them, the Australian landscape is sustaining a substantially higher productivity (NPP) than it was before the advent of European-style agriculture. This is being done primarily through nutrient inputs. Overall, we have increased landscape nutrient stores by much more than we have increased landscape production (NPP). This is especially evident in the 400-700 mm southern agricultural belts, where increases in plant-available mineral N and P are up to a factor of 5 (current level / pre-agricultural level), while in the same areas the landscape yield (measured by NPP) has increased only by a factor of about 2.

The implication is that, on large scales, nutrient application rates exceed those required to achieve optimum production levels and are approaching diminishing returns. There is anecdotal evidence from industry sources (for example, cotton) to support this view.

Because of these higher overall nutrient levels, landscapes are leaking more nutrients into the atmosphere and into soil water and waterways than they were before the advent of European-style agriculture.

It is important to note that the production benefits and the environmental costs of agricultural nutrient application behave quite differently as functions of the nutrient input. As sketched schematically in Figure 27, production benefits approach a plateau or a point of diminishing returns as nutrient levels increase. This is for the fundamental reason that plants can only use a finite amount of nutrient before other resources such as water or light become limiting. Environmental costs, on the other hand, tend to increase progressively more steeply as nutrient inputs increase, because the resulting damage (for instance eutrophication in waterways or estuaries) often has a threshold character: exceedance of the threshold causes an undesirable change in the state of the system which is usually expensive and sometimes impossible to reverse.

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Figure 27: Schematic diagram showing the responses of landscape production and environmental costs to nutrient inputs. Point A: maximum sustainability, with acceptable leakage but less than maximum production; Point B: maximum production, at the cost of high leakage and ecological damage.

Because of the contrary behaviour of the benefits and costs of nutrient application to landscapes as sketched in Figure 27, there is an optimum point (A) at which the net benefit (total benefit less cost) is maximum. This point occurs at a nutrient input below that required to achieve maximum production, because at input rates beyond A the benefits rise progressively more slowly (diminishing returns) while the costs increase at least linearly and probably more steeply as suggested in Figure 27.

An important issue concerning Figure 27 is that the production benefits and environmental costs are usually borne by different groups (benefits accruing on-farm and within farm industries, and costs accruing to users of environmental resources such as waterways, and to the entire community through loss of environmental amenity).

Our work suggests that, on average, Australian agriculture is beyond the point A. There are two implications: first, a requirement for greater precision in agricultural nutrient use. Facilitating technologies already exist (for instance, precision agriculture). Second, there is a need to develop mechanisms for spreading the benefits and costs shown in Figure 27 among the different groups to whom they accrue, so that rational management of the whole environment-production system is possible.

5.2 Strengths, Weaknesses and Next Steps

This has been a very ambitious project in large-scale biophysical science. As such, it has succeeded in some areas and (to date) fallen short of its full potential in others. It has met all of its technical specifications (see Section 1). However, a piece of science like this is judged – especially by the scientists directly involved – not only by its delivery against milestones but also by a set of more intangible criteria. As the authors see them, these criteria include contributing to the evolution of a holistic understanding of Australian landscapes at continental scale, and particularly the biophysical and biogeochemical processes that sustain them, how human management of landscapes is influencing these processes, and how humans can manage their landscapes better.

Nutrient input

Res

pons

e

LandscapeProduction

Environmental costthrough nutrient leakage

A B

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Weaknesses and Next Steps: We first enumerate some of the weaknesses in the work, many of which will be addressed as this work is further developed and written up for the scientific literature.

1. Dynamical model: All the work reported here has been carried out with a quasi-steady-state model (BiosEquil). During this work a dynamical or time-dependent model (BiosEvolve) has been developed but was not used in final delivery for reasons of time and data requirements. There are a large number of important issues to do with transient responses of landscapes (to both climate variability and land management forcing) that by definition can only be tackled with a time-dependent model.

2. Equations for fluxes and scaling issues: We have used a number of assumptions to develop descriptions of water, C, N and P fluxes at large scales in both space and time. Some of these assumptions (especially for drainage fluxes) are rough. Many can be improved by use of small-scale information, through rational averaging of small-scale models in space and time (Raupach et al. 2001). Work on these lines is already in progress in the areas of modelling soil water flows, and also photosynthesis and NPP.

3. Uncertainty and parameter estimation: There are several techniques for rational estimation of uncertainties and parameters from given constraining data, some of which are being explored in companion work by one of the present authors (Barrett 1999, 2001; Barrett et al. 2001). We wish to apply these methods formally here.

4. Process improvements: Some processes were described roughly in this work (for instance drainage terms) and some were not described at all. It is particularly important to introduce descriptions of the disturbance fluxes (herbivory, fire).

5. Connections with river and atmospheric budgets: Predictions from this work provide source terms for other biophysical domains, particularly rivers and the atmosphere. Are measurements and balance assessments in these other domains consistent with our work? How can the consistency be improved, and how can balances in these different domains constrain one another?

Strengths: Against the less tangible criteria hinted at above, we believe the project has broken some new ground. A rational framework, based on the linked balances of water, C, N and P, has provided a synthesis of many important biophysical and biogeochemical processes on landscapes. If detailed process understanding is the bones of a skeleton, these bones are beginning to connect. Unexpected emergent phenomena have appeared in this process, such as the behaviour of the soil-water nutrient concentrations. The connection has also constrained the estimates of many of the important fluxes in the water, C, N and P balances, simply through the requirement that the result of connecting the processes (bones) is a functioning landscape (a working body).

Acknowledgments

We acknowledge with appreciation the financial support for this work from the National Land and Water Resources Audit, and the personal support from the NLWRA Office particularly from Mr Warwick McDonald.

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We have received much benefit from many interactions with colleagues (some working on closely related NLWRA projects and others outside the NLWRA process), including Elisabeth Bui, Helen Cleugh, Dean Graetz, Stefan Hajkowicz, Ray Leuning, Hua Lu, Chris Moran, Ian Prosser, Graeme Priestley, Doug Reuter, David Simon, Chris Smith, Murray Unkovich, Bill Young and Mike Young. Especially, we thank Hua Lu and Chris Moran for detailed reviews of a draft of this report.

Finally, as ever, we cannot overstate the importance for this work of the support and forbearance of our families.

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