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The concept of the soil phosphorus storage capacity (SPSC) was successfully used to evaluate the phosphorus (P) loss risk and the P retention capacity of acidic soil. This study extended the concept of SPSC from acidic soil to neutral or alkaline soil. A total of 95 surfaces (0–10 cm) soil samples were collected from the Yellow River Delta (YRD) for use in this study. Batch sorption experiments, correlation analysis, stepwise regression, and a split-line model were used to calculate the threshold value of the degree of P saturation (DPS). The SPSC was developed based on the DPS threshold value. Based on a DPS threshold value of 11.5%, we developed the following equation for calculating the SPSC: SPSC = (11.5% – soil DPS) × (0.113 × SOM (soil organic matter) + 1.343 × CaCO₃). In the continuous system in this watershed, from wetland to farmland, the SPSC for vegetable fields (−94.7 ± 79.1 mg/kg) was lowest and that of the restored wetland (76.3 ± 26.1 mg/kg) was the highest. Along the transition zone in the YRD, both the natural soil development and human alternations significantly affected the soil P loss/retention capacity. In terms of P storage, the restored wetlands are the highlands for P retention and the vegetable fields contribute significantly to the P loss in the YRD. As a result, we strongly recommend that the restored wetlands be fully utilised for P retention and that P fertiliser no longer be applied to the vegetable fields to prevent P loss into the watershed.

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