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

Abstract

Natural 13C abundance of different organic matter (OM) pools in water-stable macro- and free microaggregates of Haplic Chernozem in contrasting land use variants (steppe and long-term bare fallow) are described. Fractionation of 13C at certain stages of the formation of OM pools is relatively constant, regardless of the level of structural organization. This is demonstrated by the presented conceptual scheme, which allows one to quantify the fluxes of carbon (C) in the system of aggregate/OM pool. It was revealed that the main C fluxes in the OM pools go from the free OM (LFfr) to the Residue fraction through microaggregates within water-stable aggregates (mWSA), the components of which are the occluded OM (LFocc) and the Clay. With a high degree of probability, C flows from macroaggregates (WSAma) to free microaggregates (WSAmi). At the same time, the higher probability of the C flux into the Residue from the mWSA as compared to the direct C fluxes from the LFocc and the Clay, testifies in favor of the hypothesis that the Residue is represented, to a large extent, by parts/fragments of disintegrated mWSAs of 50–1 μm in size. Regardless of the size, the WSA contains a labile OM (as the components of mWSA, along with LFfr – only in macroaggregates) and a stable OM (Residue). Labile OM (LFocc and Clay) within WSAmi is characterized by a lower degree of microbial processing (“lighter” isotopic signature) compared to that within WSAma, which is due to its greater “physical” protection against microbial attacks. However, the most stable OM pool, concentrated in the Residue within WSAmi is enriched in 13C compared to that within WSAma. Considering that the Residue determines the total isotopic composition of C in WSA, the organic matter of free microaggregates is characterized by a higher degree of microbial processing in comparison with that of macroaggregates. Free microaggregates are parts of disintegrated macroaggregates.