From Deficiency to Enrichment: Diverging Soil Phosphorus Trajectories Across China's Croplands (1980-2018)

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.