CARBON DYNAMICS OF AGGREGATE-ASSOCIATED ORGANIC-MATTER ESTIMATED BY C-13 NATURAL-ABUNDANCE

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 C-13 content and in the concentration of soil o rganic C resulting from the growth of C3 pasture grasses (low delta(13 )C(PDB)) on former C4 cropland (high delta(13)C(PDB)) were used to Inv estigate the turnover and inputs of organic C in water-stable aggregat es of different sizes. After removal of free and released particulate organic matter (POM) in aggregate size separates (POM with a density l ess than or equal to 1.85 g cm(-3) that was either exterior to aggrega tes in situ or released from unstable aggregates by slaking), organic C concentrations were greater in macroaggregates (>212 mu m) than in m icroaggregates (53-212 mu m). 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 micr oaggregates may be both biochemically recalcitrant and physically prot ected. 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 hie rarchy created by the binding of microaggregates into increasingly lar ger 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.