Phosphorus gradient yields threshold MAOC response governed by microbial and stoichiometric controls

Phosphorus (P) availability is a key factor that mediates the formation of mineral-associated organic carbon (MAOC) in soils, yet reported responses to P fertilization are often inconsistent. 2. We conducted a 45-day soil incubation across a P addition gradient and performed a meta-analysis of 13 studies in order to investigate whether the responses of MAOC to P fertilizer are nonlinear and mediated by microbial communities and nutrient stoichiometry. 3. In incubation, the highest MAOC occurred at an intermediate rate of P addition within the tested gradient. At Day 25, variation in POC loss and the corresponding MAOC gain was mainly explained by microbial properties (e.g. fungi:bacteria ratio), whereas by Day 45, MAOC gain reflected the combined effects of POC loss, shifts in bacterial composition and stoichiometric ratios. The meta-analysis showed that the effect of P fertilization on MAOC increased with P-induced increases in soil C:N at application rates <10 g P m−2 year−1, but this positive association disappeared at ≥10 g P m−2 year−1, which is consistent with the emerging P-induced N limitation. 4. Together, these results suggest that there is an optimal window of P availability below which MAOC formation is promoted, and above which stoichiometric imbalance and microbial restructuring constrain C stabilization. This implies that P management should consider both application rate and C:N-related indicators to enhance soil C sequestration.