Decoupling natural and anthropogenic drivers of soil nutrient evolution in black soils: evidence from a 40-year observation in Northeast China

The black soil region of Northeast China (NEC) is a critical grain-producing area facing severe soil degradation characterized by topsoil thinning, nutrient depletion, and acidification. However, the long-term spatiotemporal evolution of soil nutrients and the quantitative attribution of their drivers over the past four decades remain under-explored. Focusing on typical state farms in the Sanjiang Plain (SJP), this study utilized a dataset spanning the 1980s, 2010s, and 2020s, combining historical records with field profile sampling. Piecewise Structural Equation Modeling (PSEM) was employed to disentangle the relative contributions of climatic conditions, geographic factors, environmental attributes, and agricultural management to soil nutrient dynamics. Results show: (1) From the 1980s to the 2020s, significant acidification was observed, with soil pH decreasing by 0.54 units. Soil organic matter (SOM), total nitrogen (TN), and total potassium (TK) declined by 52.67%, 8.05%, and 3.07%, respectively, whereas total phosphorus (TP) increased by 16.11%, indicating a nutrient imbalance. (2) Temporal heterogeneity was evident: SOM and TK depletion dominated the first phase (1980s–2010s), while rapid acidification and TP accumulation characterized the second phase (2010s–2020s). Spatially, acidification was most severe in the northwestern SJP, while the central region experienced a rapid SOM decline. (3) The PSEM analysis revealed divergent driving mechanisms: Climatic conditions were the primary direct drivers of pH, TP, and TK variations. In contrast, geographic factors and agricultural management were the dominant drivers for TN. SOM dynamics were jointly regulated by climate, environmental attributes, and agricultural management. The results suggest a gradual transition from reclamation-induced nutrient depletion to degradation processes increasingly influenced by coupled climatic and anthropogenic factors.