Zhen Xu

Decades of fertilizer intensification have reshaped phosphorus (P) cycling across China's croplands, driving a widespread transition from P deficiency to enrichment while leaving spatially heterogeneous legacies of accumulated soil P. However, the long‐term trajectories of soil available P, their regional divergence, and the drivers regulating P accumulation or depletion remain poorly resolved at the national scale, limiting the development of sustainable and region‐specific P management strategies. Here, we integrate nearly 190,000 soil samples from harmonized national soil surveys with long‐term fertilization experiments to quantify long‐term soil available P dynamics across China at 1 km resolution and to assess the importance of key environmental and management drivers using an interpretable, spatially explicit machine‐learning framework. Soil available P increased sharply from 1980 to 2018, with an overall increase of 20.6 mg kg ⁻¹ , shifting from widespread deficiency to pronounced enrichment, particularly in eastern croplands. Long‐term experimental evidence demonstrates that sustained fertilizer inputs play a central role in driving soil P accumulation. Interpretable machine learning revealed spatial heterogeneity in the dominant drivers of soil available P change, with management effects strongest in Central China, and climatic and environmental influences more pronounced in the Northwest and Northeast. Sensitivity‐based future projections indicated divergent potential trajectories to 2060, with climatic changes reducing soil available P, whereas management intensification increased soil available P by approximately 13%. These findings show that long‐term soil available P dynamics across China are governed by the combined effects of fertilization legacies, soil‐environmental context, and climate, providing a national basis for region‐specific and sustainable P management.