Purpose Internal phosphorus (P) input has been proven to be an important cause of eutrophication. The purpose of this study is to explore the process and mechanism of P release from sediments in seasonal hypoxic reservoirs.

Material and methods Six sediment cores were collected from Hongfeng Reservoir, one of the largest reservoirs in southwestern China. Incubation experiments were conducted using the sediment cores under aerobic and anaerobic conditions. The diffusion gradients in thin films (DGT) technique was employed to determine the concentration profiles and release characteristics of labile-P and labile-Fe at the sediment-water interface. The microbial community structure in surface sediments was determined by 16S rRNA sequencing.

Results and discussion Compared with the aerobic condition, the P release flux was similar to 3.75 times under anaerobic condition, which mainly came from BD-P and NaOH-P. In addition, DGT-P and DGT-Fe were significantly positively correlated (R-2 > 0.66, p < 0.001). From 16S rRNA sequencing, SRB and PSB were shown to promote P release through sulfate reduction and P dissolution in sediments. Moreover, the control measures of internal P release are discussed due to the potential risk of it in deep-water reservoirs.

Conclusion Dissolved oxygen is the key control factor of P release; thus, anaerobic conditions promoted the release of P from sediments. Fe-P reduction and dissolution are the main processes. SRB and PSB played an important role in the P cycle of sediments. It is necessary to increase oxygen in seasonal hypoxic reservoirs to reduce the risk of internal P release.