Spatial distribution of soil erosion and its impacts on soil productivity in Songnen typical black soil region

Soil erosion is an important reason for land degradation. The black soil region of Northeast China is the main grain production base in China, however, the black soil productivity in this region is at risk and unsustainable now because there was few soil conservation measures in nearly 100 years of cultivation. To assess the impact of erosion on land productivity, both 210 sampling points for 137Cs measurement and 112 sampling points for soil profile features were laid out on cultivated land of Songnen typical black soil region in combination with the 2010 China land use map. The current soil erosion/deposition rate and land productivity index PI were calculated over the study area, and then the PI values were predicted erosion by 20, 50, 100, and 200 years without soil and water conservation measures based on the current erosion rate and soil profile characteristics. The results showed that the average soil erosion rate of cultivated land in the study area was 705.8 t/(km2⋅a), and 77.0% of the cultivated land had soil erosion rate larger than the soil loss tolerance of 200 t/(km2⋅a). Spatially, the soil erosion rate in the northern region was greater than that in the southern region significantly, which was mainly affected by slope steepness. The relationship between soil erosion rate (SL) and slope steepness (θ) in degree was established according to the results of 137Cs measured at 210 sample points: SL = 556.4∗θ or SL = 0.464∗θ in mm/a (given soil bulk density is 1200 kg/m3). The current mean value of PI for cultivated land was 0.59, decreasing significantly from north to south, which was mainly influenced by organic matter content. The predicted PI values decreased significantly with the increase of erosion years in the northern area. This is related to the topography of rolling and hills in the northern area with a greater soil loss rate, leading to a rapid decrease of organic matter content and the increase of sand content in the topsoil in the future. The spatial distributions of the current soil erosion rates and predicted PI values after years of erosion for cultivated land would be helpful for soil conservation policy-making.