Implications for Soil Properties of Removing Cereal Straw: Results from Long-Term Studies

Bioenergy developments could lead to large-scale removal of cereal straw from fields, with consequences for soil organic carbon (SOC) and related properties. In 25 experiments of 6 to 56 yr duration there was a trend for SOC and total soil N content to increase where straw was incorporated annually. However the increases were only significant in six experiments and were <10% in the majority of cases. Increases in microbial biomass C or N were always proportionately greater than for SOC or N. In simulations of annual straw incorporation using the RothC model, 90% of the microbial biomass C increase in 100 yr was reached within 20 yr as biomass C moves toward a new equilibrium value more rapidly than total SOC. Simulations also showed that if straw was removed in 50% of years, SOC and biomass C increases were about 50% of those with annual straw incorporation. There is considerable evidence that small changes in total SOC have disproportionately large impacts on soil physical properties such as aggregate stability, water infiltration rate, and plow draft and that microbial activity is crucial in the formation of stable aggregates. We conclude that, although changes in SOC resulting from addition or removal of straw are small, it would be unwise to remove straw every year as this is likely to lead to deterioration in soil physical properties. Local assessments are required to determine the frequency of straw removal that is acceptable for soil functioning; this will influence the capacity of bioenergy installations.