The dynamics of soil organic matter and nutrient cycling

This chapter discusses the dynamics of soil organic matter and nutrient cycling. Knowledge of the turnover rates of plant and animal residues, microbial bodies, and soil organic matter (SOM) is a prerequisite for understanding the availability and cycling of nutrients such as carbon, nitrogen, sulfur, and phosphorus. Understanding the dynamics of nutrient, plant residue, or SOM transformations in the field requires meaningful mathematical expressions for the biological, chemical, and physical processes involved. Models can be used to gain an understanding of the processes and controls involved in nutrient cycles, to generate data on the size of various pools and the rates at which nutrients are transformed, and to make predictions when experiments are inappropriate. While conceptual models may be sufficient for the first task, only quantitative models can achieve the latter tasks. Quantitative models of SOM and nutrient dynamics are attempts to describe soil biological processes rather than strictly mathematical expressions and statistical procedures used to find best-fitting curves. This chapter presents the Rothamsted, Van Veen and Paul, and Century models as examples with the focus on how they are constructed. It discusses in detail about reaction kinetics, modeling the dynamics of decomposition and nutrient transformations, and establishing pool sizes and kinetic constants.