Soil total nitrogen (TN) loss, driven by long-term soil erosion and cultivation, is a critical yet poorly quantified component of biogeochemical processes in agroecosystems. However, quantifying in situ erosion at a point scale remains challenging, limiting our understanding of the mechanisms driving variations in TN fractions, such as coarse particulate organic nitrogen, fine particulate organic nitrogen, and mineral-associated organic nitrogen. Here, we investigated the mechanisms driving soil TN fractions in agroecosystems with different cultivation histories (50 years on convex slopes and 100 years on straight slopes) in the Black Soil Region of Northeast China, as well as quantified the important role played by erosion based on the 137Cs method. Results showed that compared to uncultivated land, TN decreased by 11.52% and 22.48% on convex and straight slopes, respectively. The absolute decreasing ratio showed that the mineral-associated organic nitrogen (86.13%) was the main decreasing fraction in the straight slope, while mineral organic nitrogen (52.97%) and particulate organic nitrogen (47.03%) were equivalent in the convex slope. The contributions of erosion to the TN decreases on convex and straight slopes were 61.69% and 27.05%, respectively. Erosion exerted a more pronounced impact on mineral-associated and fine particulate organic nitrogen than on coarse particulate organic nitrogen, leading to spatial redistribution and even loss of these fractions. As the erosion amount increased, all the content of TN fractions decreased, while the proportion of mineral-associated organic nitrogen increased. Overall, erosion caused both the depletion of TN fractions and the regulation of TN fraction proportions in situ soils, potentially affecting the nitrogen cycle within agroecosystems. These findings highlight the critical role of erosion in regulating the content and proportion of TN fractions and underscore the need to integrate erosion control measures into agricultural nitrogen management strategies to ensure sustainable agroecosystem functioning.