ИСТИНА

European Russia is considered one of the most productive agricultural areas worldwide, in particular due to high organic carbon content of the Chernozems soils. One of the most intensively cultivated area of the Russian Chernozem zone is the Kursk Region with extended areas of row crops, such as sugar beet, potato and corn. The high frequency of rainstorm events and the steep slope gradients of the cultivated fields cause high soil erosion rates ranging between 5 and 15 t/ha/year 1. The highest erosion rates are observed during summer rainstorms, particularly on fields under fallow or row crops. For instance, in the Sovetsky District of the Kursk Region, a major event was observed in 1976, on 20-21 August. About 190 mm of precipitation was recorded, causing a soil loss of 200 t/ha during that single rainfall event 2. The Vorobzha River basin is a representative river basin for the chernozem zone of Central Russia. It is located in the Kursk Region 15-20 km south-west from the regional centre Kursk. It has an area of 228 km2 with about 85% of cultivated land. The area is characterized by a continental climate with relatively cold winters and hot summers. Average annual precipitation is 585 mm (for a 100-year period of observation), varying between 400 and 800 mm. Only 30% of precipitation falls during cold months mostly as a snow. The most typical precipitation events are rainstorms with total precipitation of 10-40 mm occurring commonly from May to October. The most intensive erosion is associated with rainstorms which occur after a few days of antecedent rainfall and take place most frequently in May or late August – September, when essential percentage of cultivated fields do not have any vegetative cover. The Vorobzha River basin was contaminated by radionuclide fallout after the Chernobyl accident in 1986. Level of the Chernobyl-associated contamination is about 10 times higher than the bomb-derived 137Cs contamination which took place during 1950-70s. Detailed study was undertaken in the Vorobzha River basin for evaluation of soil redistribution along pathways from cultivated slopes through dry valley bottoms into the main river valley. Integrated approach (including erosion model calculations, soil morphological method, 137Cs budget, 137Cs sediment sequence dating, large-scale geomorphic mapping) was applied for quantitative assessment of soil loss/gain on arable hillslopes and in different order catchments for several time intervals for a period since middle of 20th century. In addition, detailed information about land-use and crop rotation changes and metheorological data for 100-year period were collected for the study area. Principles of the integrated approach are based on application of at least two independent methods for assessment of each component of sediment budget and possibility to extrapolate the results of more detailed evaluation of soil redistribution obtained for relatively small catchment in order to cover larger areas. Assessments of soil redistribution rates were undertaken for the Gracheva Loschina catchment (area 1.98 km2) and the Lebedin catchment (area 15.2 km2), which are the typical first and third Hortonian order catchments within the Vorobzha River basin. Both catchments have the earthen dam in their outlets which were constructed in 1986 and 1956 respectively. In addition system of soil conservation measures was introduced on 70% of the Gracheva Loschina catchment. At the first stage of the study, large-scale geomorphic mapping was applied for area evaluation of different morphological units within the Gracheva Loschina catchment. Soil loss/gain within each morphological unit were determined using 137Cs techniques, soil morphological method and erosion model calculation. As a result it was possible to check correctness of each technique applied. It allowed us to calculate the Gracheva Loschina catchment sediment budget based on evaluation of soil losses from cultivated lands and sediment redeposition within cultivated slopes, aggradation of uncultivated valley banks and dry valley bottoms, including deposition in small reservoir upstream from the earthen dam. Values of soil redistribution rates and sediment delivery ratios determined for different geomorphic units within a smaller catchment were used for evaluation of sediment budget for larger catchment based on distinguishing similar types of morphological units and estimation of erosion rates by the model. Resulting values of sediment delivery into the main valey of the Lebedin catchment were tested by comparison with sedimentation rates and volumes obtained from analysis of 137Cs-based valley bottom sediment stratigraphy, including the dry reservoir infill. It was found that mean annual gross erosion rates from arable lands did not considerably changed for period since beginning of intensive cultivation (1850-1860) until 1986, and they were in range 8-12 t/ha per year for both sites. Part of eroded sediments redeposited within cultivated slopes. All other eroded sediments were delivered to the valley banks and bottoms. Before the dam construction part of sediment were transported by permanent flow to the Vorobza river valley, but essential part of sediment deposited within the valley bottom. After dam construction in 1956 the only about 10% of sediment were transported outside the Lebedin catchment 3. Gross erosion rates for periods 1956-1964, 1964-1986 and 1986-2008 were calculated as 8.6; 10.6 and 6.8 t/ha per year respectively. Total sediment deposition in valley bottoms was determined based on 137Cs vertical distribution. It was found that for periods 1956-1964, 1964-1986 and 1986-2008, respectively 38%, 42% and 54% of eroded soil were redeposited within the valley bottoms. Increasing of sediment proportion deposited in valley bottom can be explained by active redistribution of sediment by regressive erosion of active bottom gullies promoted transport of certain part of previously deposited sediment downstream. As a result it is possible to evaluate sediment redeposition along the pathway from cultivated slopes to the dry valley bottom in a range of 38-54%. Analyzing the results of 137Cs technique application for the catchment, it is possible to conclude that maximum deposition is observed within cultivated slopes 4. Application of soil conservation measures in the Gracheva Loschina catchment since 1986 has led to decrease of average soil loss rates at least a factor of 2.5-2.8 according to application of the 137Cs budget method. It is in good agreement with results of direct monitoring of snowmelt erosion rates within the Gracheva Loschina catchment 5. In conclusion, the present study shows that the combined effect of climate and crop rotation changes caused significant decrease of soil loss rates from cultivated fields of typical catchments of the Central European Russia during the last two decades. Most of the eroded sediment and sediment-associated 137Cs have been redeposited within cultivated fields and dry valley bottoms. Application of the integrated approach is the most correct choice for detailed evaluation of soil degradation and both localized and off-site soil and nutrient losses and sinks.