Mechanisms of phosphorus release from coastal sediments driven by organic matter enrichment at high nitrogen...

Coastal pollution, characterized by high nitrogen (N) inputs and organic matter (OM) enrichment, is one of the most pervasive environmental stressors that may significantly alter phosphorus (P) cycling in marine ecosystems. Although extensive research has elucidated mechanisms of P release mediated by OM in terrestrial ecosystems, these findings exhibit limited applicability to coastal environments with steep environmental gradients. To address this critical knowledge gap, we performed a three-month mesocosm experiment using sediments from Xiangshan Port (East China Sea), employing six OM content gradients (6-24 g kg^-1) under a high N loading. The results demonstrated that OM enrichment within this high-N context enhanced coastal sediment P release, most pronouncedly in the higher OM treatment. This increased P release was primarily attributed to: i) intensified sediment resuspension, stimulated by benthic invertebrate disturbance under the ample OM availability; ii) reduced redox potential, accompanied by anoxic desorption of redox-sensitive P, due to the oxygen consumption during OM mineralization and nitrification; iii) enhanced organic P mineralization, caused by the upregulation of the organic P mineralization gene (phoD) and increased alkaline phosphatase activity; and iv) the rapid diffusion of P from sediment into overlying water, driven by an elevated phosphates concentration gradient (ΔPO₄^3-) at the sediment-water interface. Collectively, these factors explained 92 % of the variation in P increase as suggested by piecewise structural equation modeling. These findings highlight that N and OM enrichment can increase P availability, which may underlie the complex mechanisms driving persistent eutrophication in anthropogenically impacted coastal zones. This underscores the importance of integrated management strategies for N, P, and OM inputs.