Natural 13C Abundance in the Organic Matter of Water-Stable Aggregates of Haplic Chernozem under Contrasting Land Uses

Natural C-13 abundances in different organic matter (OM) pools in the water-stable macro- and free microaggregates of Haplic Chernozem under contrasting land uses (steppe and long-term bare fallow) are described. The C-13 fractionation at individual stages in the formation of OM pools is relatively constant regardless of the level of soil structural organization. The proposed conceptual scheme, allowing for quantification of the carbon (C) fluxes in the system of aggregate/OM pool, illustrates this assertion. As is shown, the main C fluxes in the OM pools go from the free OM (LFfr) to the residue fraction (Res) through microaggregates within water-stable aggregates (mWSAs), the components of which are the occluded OM (LFocc) and clay fraction (Clay). With a high degree of probability, C migrates from macroaggregates (WSA(ma)) to free microaggregates (WSA(mi)). However, a higher probability of the C flux from the mWSA into the Res as compared with the direct C fluxes from the LFocc and Clay supports the hypothesis that the Res is to a greater degree represented by pieces/fragments (50-1 mu m in size) of disintegrated mWSAs. Regardless of the size, the WSAs contain labile OM (as the components of mWSAs) along with LFfr (only in macroaggregates) and stable OM (Res). The labile OM (LFocc and Clay) within WSA(mi) displays a lower degree of microbial transformation (a "lighter" isotopic signature) compared to that in WSA(ma), which is a result of its better "physical" protection against microbial attacks. However, the most stable OM pool, concentrated in the Res within WSA(mi), is enriched in C-13 as compared with that in WSA(ma). Considering that the Res determines the total isotopic composition of C in WSAs, the OM of free microaggregates is generally characterized by a higher degree of microbial processing compared to that of macroaggregates. Free microaggregates are pieces of disintegrated macroaggregates.