Water-Extractable Organic Matter of Soils with Different Degrees of Erosion-Induced Degradation and Sedimentation in a Small Catchment in the Central Forest-Steppe Part of the Central Russian Upland: Tilled Soils

Dissolved organic matter is the most mobile part of soil organic matter; however, its change and transformation occurring during soil erosion are insufficiently studied. We assess the optical properties of water-extractable organic matter (WEOM) in arable soils with different degrees of degradation caused by erosion and sedimentation in a small arable catchment in the Kursk oblast, namely, the WEOM of arable Haplic Chernozems (noneroded and moderately eroded) and their analog with soil matter sedimentation, Fluvic Chernic Phaeosem (Loamic, Pachic). WEOM is isolated from the aggregates of 2-1 mm and >10 mm. In water extracts, the contents of organic carbon and nitrogen were measured. Optical properties were assessed according to absorption spectra and three-dimensional fluorescence spectra. The eroded and aggraded soils emerge to be similar to each other but significantly differ from noneroded chernozem in terms of the main quantitative characteristics of soil organic matter-the content of organic carbon and nitrogen, and pH. However, both the quantitative and qualitative WEOM characteristics show a different trend: the WEOM of Fluvic Chernic Phaeosem (Loamic, Pachic) significantly differs from eroded and moderately eroded Haplic Chernozems. In addition, some characteristics of WEOM (nitrogen content, SUVA(254), S350-400, and S-R) depend on the size of the aggregates from which WEOM is isolated (2-1 or >10 mm). The fluorescent properties of WEOM also depend on the size of the aggregates. Our data suggest that the properties of the WEOM in a small arable catchment of the central forest-steppe zone are variable and are largely determined by the destruction of water-unstable aggregates and consolidation of their particles, as well as by the leaching of dissolved organic matter. When aggregates are destroyed by water, their particles migrate with flows along the slope, and organic matter is decomposed. However, particles accumulate in depressions to become consolidated into blocky structural units, while the properties of soil WEOM are significantly altered, both due to degradation of organic matter and as a result of its leaching.