Chemical Structure of Organic Matter in Water-Stable Macroaggregates of Agrochernozems of Different Positions on the Slope

The chemical structure of organic matter (OM) pools in the 2–1-mm water-stable macroaggregates isolated from air-dry aggregates of the same size in arable horizons of noneroded, eroded, and depositional agrochernozems was studied with solid-state 13C-NMR spectroscopy. The changes in their chemical structure in the denudation–accumulative landscape are assessed. The overwhelming majority of water-stable macroaggregates in the erosional zone are newly formed due to dynamic replacement of OM in situ, which is clearly demonstrated by the integral chemical structure indicators in all OM pools in macroaggregates. Analytical data suggest the prevalent transport of newly formed macroaggregates. The destruction of macroaggregates during the transport phase is accompanied by the release of previously physically protected aggregated OM, which undergoes partial mineralization. Note that its most labile (hydrolyzable) part is predominately mineralized, whereas its stable part remains weakly changed or intact. Mineral-associated OM (clay and residue) weakly changes or does not change at all, retaining relative freshness, which indirectly suggests the prevalent migration of newly formed macroaggregates from the erosional zone. A greater degree of freshness of LFfr (free OM) in macroaggregates of depositional agrochernozem results from the abundance of fresh crop residues of the depositional zone together with the residues transported from the erosional zone.