Spatial Variation in Soil Erosion and Potential Pattern of Soil Nutrient Loss in the Southeastern Low Mountains and Hills of the Daxing’anling Mountains

Soil erosion and the nutrient loss it causes are core issues threatening sustainable land use in arid and semi-arid regions. In this study, our aim was to reveal the spatiotemporal differentiation characteristics of soil erosion and soil nutrients in an ecologically fragile area of eastern Inner Mongolia—Tuquan County and clarify the relationship between them in order to provide a scientific basis for the precise management of water and soil resources and ecological construction in this region. Based on four sets of remote sensing images and ground observation data from 2012, 2016, 2020, and 2024, the Revised Universal Soil Loss Equation (RUSLE) was used to evaluate the dynamics of soil erosion, statistical methods were employed to analyze the spatial distribution and grade characteristics of soil nutrients (organic carbon, SOC; total nitrogen, TN, total phosphorus, TP) and pH values, and correlation analysis was conducted to explore their association with environmental factors (rainfall erosivity, R; soil erodibility, K; slope length, LS; vegetation cover and management factor, C). Our results demonstrate the following: (1) From 2012 to 2024, the intensity of soil erosion in the study area showed an increasing trend, with the average annual soil erosion modulus increasing from 551.4 t/(km2·a) to 859.6 t/(km2·a), and the high-intensity erosion areas were mainly distributed in the northwest. (2) The soil nutrient content was generally at medium to low levels, with the SOC and TN in the study area mainly categorized as “deficient” and “adequate”. The SOC ranged from 5.8 to 33.8 g·kg−1, with an average content of about 23.5 g·kg−1, while the TN content ranged from 0.45 to 4.63 g·kg−1, with an average content of about 1.50 g·kg−1, and was significantly affected by soil type. (3) There was a significant negative correlation between the soil erosion modulus and the SOC and TN content (p < 0.05), which was a key driving factor for nutrient loss. This conclusion suggests that soil erosion in Tuquan County is intensifying: the risk of nutrient loss is severe, and its spatial pattern is jointly restricted by topography, vegetation cover, and soil background characteristics. Therefore, future ecological engineering should focus on high-intensity erosion areas and combine the prevention of soil and water loss with the conservation of soil fertility in order to achieve sustainable land use in the region.