Soil organic matter: its importance in sustainable agriculture and carbon dioxide fluxes

Soil organic matter is important in relation to soil fertility, sustainable agricultural systems, and crop productivity, and there is concern about the level of organic matter in many soils, particularly with respect to global warming. Long-term experiments since 1843 at Rothamsted provide the longest data sets on the effect of soil, crop, manuring, and management on changes in soil organic matter under temperate climatic conditions. The amount of organic matter in soil depends on the input of organic material, its rate of decomposition, the rate at which existing soil organic matter is mineralized, soil texture, and climate. All four factors interact so that the amount of soil organic matter changes, often slowly, toward an equilibrium value specific to the soil type and farming system. For any one cropping system, the equilibrium level of soil organic matter in a clay soil will be larger than that in a sandy soil, and for any one soil type the value will be larger with permanent grass than with continuous arable cropping. Trends in long-term crop yields show that as yield potential has increased, yields are often larger on soils with more organic matter compared to those on soils with less. The effects of nitrogen, improvements in soil phosphorus availability, and other factors are discussed. Benefits from building up soil organic matter are bought at a cost with large losses of both carbon and nitrogen from added organic material. Models for the buildup and decline of soil organic matter, the source and sink of carbon dioxide in soil, are presented.