A major factor controlling soil organic matter dynamics is believed to be the differing degrees of protection from decomposition afforded by the spatially hierarchical organization of soil aggregate structure. Changes in the natural 13C content and in the concentration of soil organic C resulting from the growth of C3 pasture grasses (low δ13CPDB) on former C4 cropland (high δ13CPDB) were used to investigate the turnover and inputs of organic C in water-stable aggregates of different sizes. After removal of free and released particulate organic matter (POM) in aggregate size separates (POM with a density ≤ 1.85 g cm−3 that was either exterior to aggregates in situ or released from unstable aggregates by slaking), organic C concentrations were greater in macroaggregates (>212 µm) than in microaggregates (53–212 µm). The turnover time (1/k) for C4-derived C was 412 yr for microaggregates, compared with an average turnover of 140 yr for macroaggregates, indicating that old C associated with microaggregates may be both biochemically recalcitrant and physically protected. Net input rates of C3-derived C increased with aggregate size (0.73–1.13 g kg−1 yr−1), supporting the concept of an aggregate hierarchy created by the binding of microaggregates into increasingly larger macroaggregates. The net input rate for microaggregates, however, was equal to the rates for small macroaggregates, suggesting that the formation and degradation of microaggregates may be more dynamic than is predicted by their stability in cultivated soils or by the observed turnover times for old C.

Carbon Dynamics of Aggregate-Associated Organic Matter Estimated by Carbon-13 Natural Abundance