CARBON TURNOVER IN SOIL PHYSICAL FRACTIONS

Soil organic matter (SOM) as represented in mathematical simulation models involves several hypothetical pools of differing resistance to decay. These conceptual pools satisfy requirements of modeling, but usually have little in common with existing information on physical and chemical properties of SOM. Using our data on turnover times for soil C in fractions of natural aggregates and primary particles, we attempted to relate age of C in physical fractions with that in widely accepted theoretical pools. Soil from a field experiment with C-14-labeled soybean residues was sampled periodically and separated into physical fractions. The amounts of C-14 associated with these fractions at different times provided data for calculation of decay rates and turnover times. The most labile fraction of SOM was plant fragments with turnover time ranging from 1 to 3 yr, which was inversely related to fragment size. Soil aggregates were found to be enriched in C compared with whole soil. This was most pronounced for coarser aggregates whose construction apparently involved the relatively labile plant fragments in some progressive state of decay. The macroaggregates with partially processed C showing turnover from 1 to 3 yr contrasted with microaggregates that included more highly humified C having a longer residence time of almost-equal-to 7 yr. Various soil fractions differing in residence time of associated C were assembled into several groups that demonstrated consistency with conceptual pools of two widely accepted simulation models. Data from C-13 natural abundance studies of soil and of primary fractions were in harmony with models requiring at least two pools of stable SOM.