Nitrogen release potential and ecological environmental effects during particulate matter deposition in sub-deep reservoirs of Southwest China

The study focuses on two sub-deep reservoirs (PD and BH) in southwestern China. Suspended particulate matter (SPM) is one of the most important sources of nitrogen (N) for maintaining the trophic state and stimulating primary production in lakes and reservoirs. The research on the migration and transformation of N during SPM sinking is much limited, especially in the sub-deep reservoirs. This study deployed SPM traps to systematically analyze the nitrogen forms, concentrations, and fluxes of settling particles at different depths. Additionally, a flooding incubation experiment was conducted to simulate the mineralization and release processes of particulate nitrogen under natural conditions. The results demonstrated significant mineralization and release of particulate N during SPM settling in both reservoirs, with released N primarily in the form of NH 4 + . Compared to PD Reservoir, BH Reservoir, which has a higher eutrophic status, exhibited stronger particulate N mineralization and release. Thermal and oxygen stratification significantly influenced N migration and transformation. Due to high primary productivity and thermal stratification, BH Reservoir experienced bottom hypoxia, leading to ammonia accumulation and elevated NH 4 + /NO 3 - ratios in surface water. Consequently, for eutrophication control in sub-deep reservoirs, besides reducing external pollutant inputs, enhancing dissolved oxygen (DO) levels is crucial for improving aquatic ecology. • Nitrogen undergoes significant mineralization and release during particle settling. • Nitrogen release is stronger in eutrophic reservoirs. • Particles have higher nitrogen release potential than sediments. • DO regulates water NH 4 + /NO 3 - , influencing phytoplankton composition.