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