soil fertility and nutrient bioavailability sponsored by the dest program china higher education...
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Soil Fertility and Nutrient Bioavailability
Sponsored by the DEST program China Higher Education Strategic Initiatives
© The University of Adelaide
Soil Fertility and Nutrient Bioavailability
characteristics of soil that enables it to:• provide nutrients• in adequate amounts• in appropriate balance• for growth of particular plant(s)
depends on the:• form of the nutrient in soil• processes of nutrient release to soil solution• movement of nutrients to the absorbing surfaces
(plant/mycorrhiza)• mechanism of absorption by roots
Topics
• cycling of different elements• soil processes and the supply of nutrients
– relative importance of organic and inorganic pools/biological activity
– effects of pH on nutrient availability– effects of soil moisture on nutrient availability and
movement
• movement of nutrients in soil to roots• importance of soil structure in soil fertility
Plants absorb nutrients from the soil solution
concentrations in soil solution (µM) at sites in UK
Na K Mg Ca Fe S P
grassland/arable
(24 soils)465 390 135 2120 3.4 327 64
woodland
(5 soils)335 384 104 592 24.1 398 17
from Tinker and Nye, 2000
values for P can be much lower than those quoted < 10 µm
Major nutrient pools and pathways of nutrient transfers
soil solution
stableinorganic
labileinorganic
microbes
plants
stableorganic
labile organic
leaching erosion
crop atmosphere
The importance of different pools and transfers varies between nutrients
• relative importance of global pools • significance of inorganic and organic pools and
biological cycling• solubility of inorganic forms of nutrients• buffering capacity• movement of nutrients in soil solution• replenishment of nutrients in soil solution• concentrations of nutrients in solution close to
roots
Importance of pools varies with nutrients
Nitrogen (N)– almost entirely in organic form in soil– large inorganic pool in atmosphere (N2); very inert (unreactive)– soil solution: anion and cation (NO3
- and NH4+); very soluble; low
buffering capacity
Potassium (K)– inorganic pools in soil and plants– soil solution: cation (K+); soluble– rapid exchange between pools– atmospheric pool negligible; organic pools negligible– organic pool negligible
Phosphorus (P) – both inorganic and organic pools in soil– atmospheric pool negligible– soil solution: anions (H2PO4
-/HPO42-,depends on pH); insoluble; high
buffering capacity
Global terrestrial N cycling: estimates of pools and transfers
(transfers in 106 tonnes)
atmosphere (mainly N2) 39 x1015 tonnes
N2 fixation biological 139 non-biological 50
‘nitrate’ 32
rivers and then oceans
denitrification ~150
Nitrate leaching 18-33
plants, animals, microorganisms 1.3 x 1010 tonnes,
soil, peat and litter 30 x 1010 tonnes
N is dominated by biological processes
erosion
soil solution
stableinorganic
labileinorganic
microbes
plants
stableorganic
labile organic
leaching (NO3-)
crop atmosphere
N2 fixationsoil
Main forms of organic N in soil
• amino acids – used quickly by soil microorganism
• proteins– variable availablity to soil micro-organisms
• complex polymers– chitin, lignin– not very available
decomposition of organic N produces NH4
organic N
NH4
ammonifying organisms
NO3
nitrifying organisms
NO2/NO/N2
dentrifying organisms
atmosphere
nitrogen fixation
soil solution
N availability
• NO3- and NH4
+ are available to plants
• low buffering capacity in soil• biological cycling of N between organic matter
and inorganic forms in soil solution• mineralisation depends on soil moisture and
temperature and C:N ratio of organic inputs
• biological fixation of gaseous N2
• (fertilizer applications)
K+ pools and pathways 1
• almost entirely inorganicexcept:– organic matter holds K+ because it is negatively
charged
– living organisms act as sink because they need K+
• redistribution between available inorganic pools is rapid so solution K+ is replenished rapidly
• K released only slowly during weathering of minerals
K+ pools and pathways 2
soil solution
stableinorganic
labileinorganic
microbes
plants
stableorganic
labile organic
leaching erosion
crop atmosphere
P cycling
• Plants can use soluble inorganic P
• P is scarce and insoluble
• P is present in soil solution as H2PO4- and HPO4
2-
• P cycling involves both inorganic and organic pools
• P often limits productivity of ecosystems and crops
• fertiliser applications are often required to attain good crop yields
• replacement of P lost to oceans is very, very slow
• accessible deposits of phosphate rocks will run out in ~ 80 years;
P pools and pathways
soil solution
stableinorganic
labileinorganic
microbes
plants
stableorganic
labile organic
leaching erosion
crop atmosphere
Main forms of P in different pools
organic PP in organisms (biomass)
organic P
sugar phosphates
nucleic acids
phytate (~ 80%)
inorganic P
soil solution (H2PO4
-/HPO42-)
insoluble
Ca, Al and Fe phosphates
Effects of soil pH on soil fertility 1
Usually indirect due to effects on:• nutrient availability• toxicities• biological activity
• direct effects of H+ or OH- are only observed at extreme pH values
Effects of soil pH on soil fertility 2
low pH • deficiency of K, Mg• toxicity of Al (Al3+) and Mn
High pH– deficiency of Fe, Mn– toxicity of B and Na
P and N most available at moderate pH – biological activity important for N & P mineralisation is
inhibited at very low and very high pH– P is immobilised at both high and low pH
Effects of pH on retention of inorganic P in soil
4 5 6 7 8 9
high
med
low
insoluble Fe & Al phosphates
sorption to clays and oxides
insoluble Ca phosphates
pH
P retention in soil P most available
Biological activity involved replacement and removal of nutrients from soil solution
• biologically active soil is important
• requires organic matter
• depends on moisture, temperature, aeration, pH
soil solution fungi
bacteria
immobilisation = removal
mobilisation = replacement
Concentration in solution at root surface
depends on:• rate of uptake (later lectures)• rate of replacement in solution and movement
to roots
• uptake>replacement depletion at root surface P, Zn, NH4
+
• uptake<replacement accumulation at root surface SO4
2-
Replacement of nutrients in soil solution
replacement in solution
N P
mineralisation from organic pool +++ ++
dissolution (inorganic sources) - ++
desorption (inorganic) - ++
Movement of nutrients to roots
• root growth towards nutrients– influenced by soil structure and conditions and by
nutrient concentrations
• nutrient movement through soil– mass flow and diffusion
both are influenced by the physico-chemical properties of soil and by soil structure
Mass flow of nutrients
• soil solution (containing dissolved nutrients) moves down gradients of water potential
• wet soil dry soil• all nutrients move in the same direction• rate of nutrient movement depends on
– concentration in solution - affected by uptake and replacement– volume of solution - affected by soil moisture and by soil pore
sizes– rate of flow - affected by transpiration, evaporation and
drainage
NO3- > K+ > P
Diffusion of nutrients
• movement in solution but independent of direction of flow of solution
• nutrient moves down concentration gradient• rate for each nutrient depends on
– concentration gradient – replacement/uptake– diffusion in soil – buffering capacity of soil– tortuosity of pathway in soil– soil moisture (continuity of water-filled pores)
Ds = rate of diffusion of ions in soil (m2 s-1)
ion wet soil
-10 kPa
dry soil
-1000 kPa
NO3-
(low buffering capacity)
10-9 10-11
K+ 10-11 10-13
H2PO4-
(high buffering capacity)
10-13 10-15
values are much lower than for diffusion in pure water due to: • tortuosity of pathway • increased viscosity close to surfaces• exclusion of ions by surface charge on particles
Processes involved in nutrient replacement
replacement at root surface N P
diffusion rapid slow
mass flow +++ (+)
N has low buffering capacity and high concentration in soil solution
N is VERY MOBILE (easily gets to roots; easily leached out of soil)
P has high buffering capacity and low concentration in soil solution (<10µM)
P is VERY IMMOBILE
Soil structure and pore-size distribution influence many aspects of fertility
pore-size distribution• air filled pore space• water filled pore space• nutrient movement• aeration• biological activity• accessibility of pores to
roots, microorganisms and animals
arrangement of soil particles in aggregates
large particles - large pores
small particles - small pores
after Oades, 1993
Water and air-filled pore space
wet soil• larger pores filled with water• continuity of water-filled pore space• air-filled porosity lower• tortuosity lower
dry soil• small pores filled with water• low continuity of water filled pore-
space (tortuosity higher)• air-filled porosity higher
from Griffin 1972
Effects of compaction on pore-size distribution and continuity
Uncompacted soil
Soil compacted by traffic
pore
dia
met
er (
µm
) >300
100-200
30-100
10-30
3-10
1-3
0.2-1
<2
contribution to total porosity (%)
20 400
1.1 Mg m-3
1.6 Mg m-3
from data of Habib Nadian and Liz Drew
Summary
• Plants absorb nutrients from the soil solution• There are major differences between N, K and P in
– cycling in the biosphere– inorganic and organic pools in soil – processes involved in replenishment of soil solution
• Nutrients reach roots by– root interception– mass flow– diffusion
• Nutrient availability and movement in soil are influenced by– pH– water content– pore-size distribution– organic matter
Useful references
Griffin, D.M. 1972. The ecology of soil fungi. Chapman Hall.
Killham, K. 1994. Soil Ecology. Cambridge University Press, Cambridge, UK.
Oades, J. M. 1993 The role of biology in the formation, stabilization and degradation of soil structure. Geoderma. 56: 377-400.
Paul, E.A. and Clark F.E. 1996. Soil microbiology and biochemistry.2nd ed.
Academic Press, San Diego, USA.
Tinker, P.B. and Nye, P.H. 2000. Solute Movement in the Rhizosphere. Oxford University Press, Oxford.
Comerford, N. B. 1998. Soil phosphorus bioavailabiilty. pp 136-147 in Phosphorus in Plant Biology Vol 19, J. P. Lynch, J. Deikman J. Eds. American Society of Plant Physiologists, Rockville, Maryland, USA.