Dynamics, Chemistry, and Preservation of Organic Matter in Soils

This chapter introduces to the function of organic matter in soils and its chemistry in different soils. Organic carbon enters the soil via root and litter deposition and is composed of a vast range of different precursor molecules. These compounds are only partly mineralized and new microbial structures are synthesized, leading to an alteration of the chemical composition of soil organic matter (SOM). The underlying rates determine the turnover of SOM in soil, with some compounds degrading faster (labile C pools) while other being protected from potential rapid decay (stable C pools). Biomarker and stable isotope techniques are applied as tools for elucidating both the origin and residence time of specific SOM components. In general, the residence time of a given compound is lower if it is either not bioaccessible or not bioavailable for decay, that is, when stabilization processes delay its rapid decomposition. Whereas the major proportion of SOM is mineralized within months, stabilized SOM fractions may reside in soil at timescales of decades to several millennia. These processes contribute to the open soil C cycle, with feedback mechanisms to the atmosphere, hydrosphere, and lithosphere. It is important for plant growth, climate regulation, weathering, and soil formation as well as soil fertility.