A procedure for isolating soil organic matter fractions suitable for modeling

defined by their rate of decomposition. Hence fraction i is defined as obeying: Fractions of soil organic matter (SOM) were obtained from three soils using alternative physical fractionation procedures, and evaluated i kiYi [1] against the requirements of model pools. We compared two-stage where i is the mineralization rate of fraction i; ki is its density fractionation (isolating free and intra-aggregate fractions, befirst order reactivity; and Yi is its size. Cohort models fore and after dispersion, respectively) with particle-size separation of dispersed soil. For full comparison, the organomineral fraction (Bosatta and A u gren, 1996) are conceptually different residual from density fractionation was also size separated. In stand- from the approach described above. However, they are ardizing the density-based method, we found recovery of intra-aggre- similar mathematically, and may be considered as modgate organic matter highly sensitive to separation density as compared els whose multiple pools encompass a sufficient range with the free. Recovery of the intra-aggregate was also influenced by of reactivity (ki). dispersion energy. The greatest amount was obtained using a combina- Measurements of system-level outputs, such as total