Spatial distribution and environmental risk of soil phosphorus under a long-term fertilizer strategy in a rice-wheat rotation system

Straw returning and livestock manure reuse are effective nature-based solutions (NBS) for nutrient cycling and sustainable crop production. However, it remains unclear how these agricultural practices affect phosphorus (P) transformation, movement, and the risk of loss in long-term crop production. The present study assessed the 30-year effects of the following five fertilizer treatments on rice yield, P transformation, movement, and loss in the soil profile: control (no fertilizer), NPK (chemical fertilizer), NPK + S (NPK with straw returning), NPK + M (NPK with manure), and NPK1.5 + S (1.5-folds NPK with straw returning). Compared with the NPK treatment, the NPK + S, NPK + M, and NPK1.5 + S treatments demonstrated no further increase in rice grain yield but significantly increased the seasonal soil P surplus by 3.3, 6.0, and 16.8 kg P ha-1, respectively. Compared with NPK, the soil Olsen-P concentration in the 0-0.2 m soil layer increased by 24.5 and 89.7% respectively for NPK + S and NPK1.5 + S. The environmental soil P threshold for the rice-wheat rotation system in Purpli-Udic Cambisol is 49.9 mg kg-1. The straw returning treatment (NPK + S and NPK1.5 + S) continually maintained a higher P index (9.5-13.5) in the 0-0.3 m soil layers. These results suggested that an integrative P strategy should be provided to manage soil P surplus and P environmental loss risk in the chemical combined with organic fertilizers of rice-wheat rotation, ensuring sustainable food security and achieving NBS goals.