The turnover of organic matter in soil

There is little quantitative data on the turnover of soil organic matter even for parts of the world where long-term field experiments have been carried out. Comparisons of turnover under different climatic conditions, under different systems of agriculture, under different plant associations, at different pH's, in soils differing mechanical composition would be of pedological and agricultural interest. Although many turnover problems, particularly gross turnover ones can (and should) be studied by non-isotopic methods, isotopes used in conjunction with long term field experiments are potentially capable of giving quantitative information obtainable in no other way. Isotopes are particular useful under steady state conditions, and experiments under steady state conditions have the additional advantage that many sampling problems, uncertainties about constancy of plant return etc., are avoided. Long term field experiments on turnover are practicable, and should be set up in different parts of the world. Equations of the type dX/dt=A-rX, where X is the the organic matter in the soil, A the annual return of plant material, r the fraction of X decomposed per year and t time, have been used to represent the turnover of organic matter in soil. Their validity and limitations are discussed. There is a formal analogy between turnover in an animal in metabolic balance and turnover in a soil where annual additions of organic matter balance between annual losses., and many of the isotopic techniques used in steady state metabolic studies can be applied to soil problems. Carbon-14 dating is of limited value in measuring turnover in soils but the increase in atmospheric carbon-14 resulting from thermonuclear tests has potential applications in turnover studies