Characterization of soil organic matter fractions from grassland and cultivated soils via C content and delta13C signature.

Variations in (13)C natural abundance and distribution of total C among five size and density fractions of soil organic matter, water soluble organic C (WSOC) and microbial biomass C (MBC) were investigated in the upper layer (0-20 cm) of a continuous grassland soil (CG, C(3) vegetation), a C(3)-humus soil converted to continuous maize cultivation (CM, C(4) vegetation) and a C(3)-humus soil converted to a rotation of maize cultivation and grassland (R). The amounts of WSOC and MBC were both significantly larger in the CG than in the CM and the R. In the three soils, WSOC was depleted while MBC was enriched in (13)C as compared with whole soil C. The relative contributions to the total C content of C stored in the macro-organic matter and in the size fraction 50-150 microm decreased with decreasing total C contents in the order CG > R > CM, while the relative contribution of C associated with the clay- and silt-sized fraction <50 microm increased. This reflects a greater stability and physical protection against microbial degradation associated with soil disruption (tillage) of the clay- and silt-associated organic C, in relation to the organic C in larger size fractions. The size and density fractions from the CG soil showed significant differences in (13)C enrichment, indicating different degrees of microbial degradation and stability of soil organic C associated with physically different soil organic matter (SOM) fractions. Delta(13)C analysis of the size and density fractions from CM and R soils reflected a decreasing turnover rate of soil organic C with increasing density among the macro-organic matter fractions and with decreasing particle size.