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 FRS UK Government Chief Scientific Adviser Climate change Food security Energy security

Food security Food production Well functioning soil Soil organic matter Transport Affordability Policies, …. 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 removal But …. 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 C Content Time Increase inputs (or slow down decomposition) Initial Equilibrium Transition Final Equilibrium 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 change superimposed on another

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.

Straw incorporation experiment, Denmark (18 years, spring barley) Powlson et al (1987) Soil Biology & Biochemistry 18, 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

Watts, Clark, Poulton, Powlson, Whitmore. Soil Use and Management 22, (2006 ) Specific draught measurements; Broadbalk Experiment, Rothamsted

TreatmentSOC % Specific draught, S kPa Nil FYM2.80 (233%) 77 (15%) NPK1.08 (29%) 75 (12%) Watts, Clark, Poulton, Powlson, Whitmore. Soil Use and Management 22, (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, (2006)

Blair, Faulkner, Till, Poulton. Soil & Tillage Research 91, (2006) Rothamsted, Broadbalk Experiment Water infiltration rate related to labile C – increased by straw and N fertilizer Labile C Total C

Labile CTotal C Blair, Faulkner, Till, Poulton. Soil & Tillage Research 91, (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 grass 80t C ha -1 Bare fallow 40t C ha -1 ESRC transdisciplinary seminar, 20 th 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 2 nd or 3 rd 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 2 nd or 3 rd 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/yr Uses 160,000 t cereal straw per year Electricity for 50,000 homes

Putting a value on SOM