Department of Soil Science,and
Centre for Soils & Ecosystem Function
Rothamsted Research, UK
Soil organic matter in the Rothamsted plots
David Powlson Andy Macdonald, Margaret Glendining,
Andy Whitmore, Kevin Coleman, Dudley Christian
Is it OK to remove straw from arable land for use as bioenergy?
or …
No …. but …. Yes …. but …..
A “perfect storm” :
Professor John Beddington FRSUK Government Chief Scientific Adviser
Climate change
Food security
Energy security
Food security
Food production
Well functioning
soil
Soil organic matter
TransportAffordabilityPolicies, ….
Nutrients:• Recycling• Fertilizers
Management
Ways of maintaining SOM in arable cropping
1. Ley-arable farming – i.e. intermittent pasture
2. Add crop residues
3. Add manures or other organic “wastes” …………………………………………………..
4. Minimise tillage• small effect, mainly redistribution• but useful to concentrate SOM near surface
5. Grow plants with larger root input (breeding)
6. Grow larger crops by using fertilizers (small effect)
7. Utilise “black carbon” or biochar?
So appropriate to be cautious about residue
removalBut ….
• 50% of above-ground residues returned to soil in stubble + chaff (winter wheat, UK conditions)
• 1.9 t C/ha returned (stubble, chaff, roots, exudates) even when straw is removed (calculated from Broadbalk data)
Soil CContent
Time
Increaseinputs (or slow down
decomposition)
Initial Equilibrium
Transition
FinalEquilibrium
Total SOM content changes between different equilibrium levels …. slowly
Long term experiments –
Valuable to quantify changes caused by different managements
In practice (non-experimental situations) –
Equilibrium rarely achieved;one management changesuperimposed on another
Broadbalk continuous wheat experimentData modelled by RothC-26.3 (solid lines)
0
20
40
60
80
100
1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 2020
Year
Organic C in soil(t C ha-1) Farmyard manure annually
Unmanured
NPK
Straw removed in all treatments
Broadbalk- Winter wheat (continuous & rotation)Started 1843
• Straw incorporated in one section since ????
• But this section also has higher clay content than rest of field
• Difficult to resolve effects
Results from Roth and Woburn straw expts
Even if changes in total SOC are small/slow, changes in specific fractions may be
occurring.
So
il o
rgan
ic C
(%
)
0
1
burnt
incorporated
So
il to
tal N
(%
)
0.00
0.05
0.10
Bio
mas
s C
(kg
ha
-1)
0
50
100
150
200
250
300
350
400
Bio
mas
s N
(kg
ha-
1)
0
15
30
45
60
75
%C %C %N BC BC BN BN%N
Straw incorporation experiment, Denmark
(18 years, spring barley)
Powlson et al (1987) Soil Biology & Biochemistry 18, 159-164
No measurable effect on soil total C or N
40% increase in microbial biomass
Can a small change in SOM have large effects on
soil properties?
Draught Forces & Energy
Draught Force
Strain gauged frame(to measure draught forces)
Laser proximity sensors(depth & front furrow width)
Doppler radar sensor(forward speed)
Broadbalk- Winter wheat (continuous & rotation)Started 1843
0 m 5 0 m 1 0 0 m0 m
5 0 m
1 0 0 m
1 5 0 m
2 0 0 m
2 5 0 m
3 0 0 mS pecific
5 0
5 5
6 0
6 5
7 0
7 5
8 0
8 5
9 0
9 5
1 0 0
1 0 5
1 1 0
1 1 5
1 2 0
1 2 5
1 3 0
1 3 5
1 4 0
D raught, kPa
Continuous wheat
1
6
Continuous wheat
Sections
0
(straw incorporated)
Continuous wheat
2Rotation (2nd wheat)
3Rotation (3rd wheat)
4Rotation (forage m aize)
5Rotation (w inter oats)
Continuous wheat(restricted fungicides)
7Rotation (1st wheat)
8
(no herb icides)
9Continuous wheat
S trip N um bers2 0
1 9 1 7 1 5 1 3 1 1 0 9 0 7 0 5 2 . 2 0 1
1 8 1 6 1 4 1 2 1 0 0 8 0 6 0 3 2 . 1
Watts, Clark, Poulton, Powlson, Whitmore. Soil Use and Management 22,334-341 (2006)
Specific draught measurements; Broadbalk Experiment, Rothamsted
Treatment SOC
%
Specific
draught, S
kPa
Nil 0.84 88
FYM 2.80 (↑233%) 77 (↓15%)
NPK 1.08 (↑29%) 75 (↓12%)
Watts, Clark, Poulton, Powlson, Whitmore. Soil Use and Management 22,334-341 (2006)
Broadbalk – SOC and specific draught
“Labile C” – easily oxidisable – about 10% of total C (microbial biomass + metabolites)
• Increased by straw incorporation and N fertilizer application (larger yields, larger residue returns)
• “Labile C” – correlated with:
–Increased aggregate stability
–Increased water infiltration rate
Blair, Faulkner, Till, Poulton. Soil & Tillage Research 91, 30-38 (2006)
Blair, Faulkner, Till, Poulton. Soil & Tillage Research 91, 30-38 (2006)
Rothamsted, Broadbalk Experiment
Water infiltration rate related to “labile C” – increased by straw and N fertilizer
Labile C Total C
Labile C Total C
Blair, Faulkner, Till, Poulton. Soil & Tillage Research 91, 30-38 (2006)
Rothamsted, Broadbalk Experiment
Aggregate stability related to “labile C” – increased by straw and N fertilizer
The benefits of SOM may not be directly proportional to
total SOC content
Arable 45t C ha-1
Managed grass80t C ha-1
Bare fallow40t C ha-1
ESRC transdisciplinary seminar, 20ESRC transdisciplinary seminar, 20thth April 2004 April 2004
Concluding comments• Maintaining SOC content is vital for soil
functioning (“soil quality”)
• So unwise to regularly remove crop residues – one of few ways to add OM
• But considerable OM inputs from roots, stubble, chaff – these continue even if straw removed
• A suggestion – only remove straw every 2nd or 3rd year
Concluding comments• Maintaining SOC is essential for soil functioning (“soil
quality”)– Crop production– Run-off, erosion
• So unwise to regularly remove crop residues – one of few ways to add OM to soil
• But considerable OM inputs from roots, stubble, chaff – these continue even if straw is removed
• A suggestion: only remove straw every 2nd or 3rd year – Region specific modelling can provide guidance– Implications for straw availability for biofuel
• But care – small SOC changes may have disproportionately large impacts on soil (physical) properties
Sanguesa, Spain
200 GWhr/yrUses 160,000 t cereal straw per year
Electricity for 50,000 homes
Putting a value on SOM