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aWith permission from Voroney et al. (1981).

bPool sizes shown refer to those produced from the added substrate alone. Actual total values in soil are much larger.

aWith permission from Voroney et al. (1981).

bPool sizes shown refer to those produced from the added substrate alone. Actual total values in soil are much larger.

the middle of the soil aggregate). The model also assumes that cultivation of the soil will increase the fraction of decomposable and recalcitrant pools in the non-physically protected fraction (soil aggregate disruption). Clay type, such as allo-phane, and the soil silt and clay content control stabilization of SOM. The size of the microbial biomass is largely controlled by the total soil C content, recent substrate additions, and clay content.

The Century Model

The Century model (Fig. 16.6), originally developed to model SOM dynamics in the High Plains of the United States (Parton et al., 1987, 1994), divides fresh organic residues on the basis of the lignin to N ratio. Structural components comprising lignin, cellulose, and hemicellulose have a decomposition rate of 0.011 day-1, metabolic components have a k of 0.03 day-1. Lignin content is also used to control the decay rates of the structural components where high lignin results in slower decay rates because lignin is closely associated with the cellulose and hemicellulose components and has a protective effect on them. Surface litter is shown to enter a surface microbial pool. Buried litter and roots feed into an active C pool, which comprises microbial biomass and microbial metabolites. The slow C pool (k = 0.00054 day-1) has an intermediate turnover rate and receives some C directly from the lignin plant components. The arrows showing the CO2 evolved during each transformation are indicative of the microbial growth efficiencies discussed earlier. The first-order decay rates for each of the pools, given in Fig. 16.6, correspond to turnover times of roughly 3 and 0.5 years for the structural and metabolic components, 1.5 years for the active fraction, 25 years for the slow, and 1000 years for the passive pools. The Century model can utilize either monthly or daily precipitation and air temperature data and soil-specific values such as sand, silt, and clay contents; bulk density; soil depth; and total soil C and N contents.

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