Water-Extractable Organic Matter of the Soils with Different Degrees of Erosion and Sedimentation in a Small Catchment in the Central Forest-Steppe of the Central Russian Upland: Soil Sediments on the Dry Valley Bottom

The composition of the water-extractable organic matter (WEOM) of sedimented soils, namely, dark-humus water-accumulative stratozems (Fluvic Chernic Phaeozems (Loamic, Pachic)) in the nonplowed bottom of a dry valley is studied and compared to the WEOM composition of arable soils (Haplic Chernozems and Fluvic Chernic Phaeozems (Loamic, Pachic) ) on the adjacent cropland of a small catchment. The WEOM composition of stratozems is examined layer by layer from the surface to the depth of 120 cm with a step of 20 cm. Water extracts are analyzed for organic carbon, nitrogen, and pH. The optical properties of WEOM are analyzed by spectrophotometry and fluorescence spectroscopy and compared in these soil types. The specific features of the changes in the properties of stratozems with the depth are also analyzed. The erosion and accumulation processes are shown to considerably influence the composition of soil WEOM. However, the content of dissolved carbon in WEOM does not significantly differ either between arable chernozems and sediments or down along the vertical profiles of the sediments on dry valley bottom. In turn, the content of nitrogen in the WEOM of arable chernozems is generally higher as compared with that in sediments, where it predictably decreases with depth. Presumably, the decrease in the nitrogen content of WEOM in sediments with depth is associated with its uptake by plant roots and an increase with depth in the share of anaerobic zones, with their activated denitrification processes. The top 0–60-cm soil layer in sediments actively retains the nutrients leached from arable soils, primarily, dissolved nitrogen. This process promotes the carbon accumulation in the underlying layers. An increase in the content of total organic carbon in Fluvic Chernic Phaeozem (Loamic, Pachic) layers below 60 cm is explainable with the accumulation of the dissolved organic matter migrating downward. On the one hand, the dissolved organic matter is sorbed by soil and, on the other hand, is preserved as a result of a decrease in the microbial activity caused by the deficiency in nutrients.