Contemporary Trend in Erosion of Arable Southern Chernozems (Haplic Chernozems Pachic) in the West of Orenburg Oblast (Russia)

The contemporary trend in the degradation of arable southern chernozems (Haplic Chernozems Pachic) in the steppe zone of south-eastern European Russia under the impact of water erosion has been evaluated based on a field study of changes in the deposition rate of eroded products on the bottoms of the currently stable negative landforms within a small catchment (1.92 km² in area) with almost completely tilled slopes in the west of Orenburg oblast, in the basin of the Samara River (a left tributary of the Volga River). The dating of deposited sediments and the analysis of their temporal dynamics have been performed using the radioactive isotope ¹³⁷Cs as a chronomarker. The results of a thorough analysis of catchment topography, grain size distribution data on soils and sediments, hydrometeorological observations, and satellite data have been used. It is found that the mean accumulation rate of chernozem erosion products on the bottom of a small catchment valley was 1.9–2.0 cm/year (16.5–28.4 kg/m² annually) during the period of 1959–1986 (4.2–4.8 cm/year, or 30.4–83.5 kg/m² annually in 1959–1963) compared to only 0.52–0.68 cm/year, or 6.6–11.9 kg/m² annually, during the period of 1986–2016; i.e., the thickness of deposited sediments decreased at least by 3.0–3.6 times, and their mass decreased by 2.0–4.3 times (2.9 times on the average). It is shown that the main reason for the presumed significant decrease in the erosion rate of southern chernozems in the region during the last decades was the reduction in surface water runoff from slopes during the spring snowmelt period, as well as the probable change in the structure of crop rotation toward some increase in the share of perennial grasses, and erosion control measures.