Fractionation soil organic matter, carbon dynamics

C. A. Cambardella and E. T. Elliott, Carbon and nitrogen dynamics of soil organic matter fractions from cultivated grasslands soils. Soil Sci. Soc. Am. J. 58 123 (1994). 

Results of field experiments confirm that carbon becomes increasingly resistant to decomposition with time. This has led some investigators to conclude that the organic component exists in three major fractions when considered on a dynamic basis (1) decomposing plant residues and the associated biomass, which turn over every few years (2) microbial metabolites and cell wall constituents that become stabilized in soil and have a half-life of 5 to 25 years and (3) the smallest fraction, resistant organic matter, ranging in age from 250 to 2500 years or more. 

Soils vary greatly in composition and reactivity. Many complex and dynamic processes occur continuously in most soils composed primarily of mineral and organic matter, water, and air. The soil atmosphere is composed of oxygen, carbon dioxide, nitrogen, and several minor gases whereas the mineral fraction varies in amounts of sand, silt, and clay and in types and amounts of clay minerals. Moreover, hydration and base saturation of the clay minerals also vary considerably. The organic matter and mineral colloids present in the soil contribute directly and indirectly to the extremely active nature of pesticide-soil systems. Since soil water contains many soluble compounds, it serves as an essential medium for many chemical and physical processes. The extreme complexity of these soil systems has been the primary reason that so few fundamental studies have been undertaken involving the ultimate fate of pesticides in soils. 

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