CARBON TURNOVER IN SOIL PHYSICAL FRACTIONS

Soil organic matter (SOM) as represented in mathematical simulation mo dels involves several hypothetical pools of differing resistance to de cay. These conceptual pools satisfy requirements of modeling, but usua lly have little in common with existing information on physical and ch emical 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 soy bean residues was sampled periodically and separated into physical fra ctions. The amounts of C-14 associated with these fractions at differe nt times provided data for calculation of decay rates and turnover tim es. The most labile fraction of SOM was plant fragments with turnover time ranging from 1 to 3 yr, which was inversely related to fragment s ize. Soil aggregates were found to be enriched in C compared with whol e soil. This was most pronounced for coarser aggregates whose construc tion apparently involved the relatively labile plant fragments in some progressive state of decay. The macroaggregates with partially proces sed 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 t ime of associated C were assembled into several groups that demonstrat ed consistency with conceptual pools of two widely accepted simulation models. Data from C-13 natural abundance studies of soil and of prima ry fractions were in harmony with models requiring at least two pools of stable SOM.