Soil Organic Matter in the Rothamsted Plots.

Results from the Rothamsted long-term experiments demonstrate that soil organic carbon (SOC) content moves towards a new equilbrium value following changes in management. If crop residues are removed rather than incorporated organic carbon content will move towards a lower equilbrium value, but under temperate climatic conditions changes in total SOC are slow and often difficult to detect even after many years. In experiments in south-east UK in which cereal straw was either removed or incorporated for 17-22 years, SOC averaged 6% higher in the incorporated treatments, though differences ranged from undetectable to 20% higher depending on soil type and cultivation method. However, even small changes in total SOC may have a disproportianately large influence on soil physical properties. In the long-term Broadbalk Experiment, different rates of N fertilizer have been applied to winter wheat for over 160 years, thus influencing C inputs in roots and stubble, and causing small differences in SOC. But these small changes had as large an impact on specific plough draught (an indication of the energy required for soil tillage) as a much larger increase in SOC resulting from organic manure applications. Small changes in SOC also had large impacts on water infiltration rate and aggregate stability. Microbial biomass content also changes more rapidly than total SOC. So caution is required in making decisions on straw removal, even if changes in total SOC content appear to be small. But even when straw is removed, inputs of organic C to soil from cereals are substantial: for winter wheat under UK condition, about 50% of above-ground residues are returned to soil in stubble and chaff. Calulations based on Rothamsted long-term experiments show that, for a typical winter wheat crop, 1.9 t C per ha are returned in residues (including roots and exudates) even when straw is removed.