Jd. Jastrow et al., CARBON DYNAMICS OF AGGREGATE-ASSOCIATED ORGANIC-MATTER ESTIMATED BY C-13 NATURAL-ABUNDANCE, Soil Science Society of America journal, 60(3), 1996, pp. 801-807
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.