Century is a model of SOM and nutrient dynamics that emphasizes the decomposition of SOM, and the flux of C and N within and between different components (Parton et al., 1987). The grassland/crop and forest systems have various plant production submodels that are linked to a common SOM and nutrient cycling submodel (Figure 9.2) that has been fully described before (Metherell et al., 1993; Parton et al., 1994; Paustian et al., 1997; Kirschbaum and Paul, 2002).
In brief, the model includes two fractions of litter (metabolic and structural) and three SOM pools (active, slow, and passive), which differ in their potential decomposition rates (Figure 9.2). In addition, there are residue pools representing different size fractions of woody debris. The active pool represents microbial biomass and metabolites that turn over relatively rapidly (annual time scales), and the slow pool consists of partially stabilized SOM constituents with an intermediate turnover time (on the order of decades), while
the passive pool represents recalcitrant materials that turn over on time scales of centuries. Separate pools for surface vs. soil locations are maintained for the two litter fractions and the active pool, while the slow and passive pools are represented only within the soil. Various environmental factors (e.g., temperature, moisture), litter quality, soil texture, and management activities affect the parameters controlling decomposition rates and coefficients governing the flow of C and N between the SOM pools.
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