Characteristics of nitrogen and phosphorus migration at sediment-water interface in seasonal frozen lakes and the mechanism of microbial driven cycling: a case study of Lake Daihai.

Nitrogen and phosphorus play pivotal roles in determining the eutrophic conditions and nutrient provision in lakes. However, the mechanisms and processes of nutrient release at the sediment-water interface of shallow lakes in cold regions remain unclear, especially under the complex environmental conditions of freezing and open-water periods. Therefore, Diffusive Gradients in Thin-films (DGT) and High-resolution Peeper technologies (HR-Peeper) were used to investigate the nitrogen and phosphorus characteristics of the sediment water interface, and the process of bacteria affecting the nitrogen and phosphorus cycle was clarified by the high-throughput sequencing technology. The results indicated that sediment phosphorus (PO) flux ranged from -1.39 to 3.6 mg/m·d, with the interstitial water-Soluble Reactive PO presenting notable fluidity and potential bioavailability. The ammonia nitrogen (NH-N) flux varied from -4.71 to 3.65 mg/m·d. The nitrate nitrogen (NO-N) flux varied from -11.64 to 1.18 mg/m·d, exhibiting an opposite trend to NH-N, which was released into water bodies during the freezing period and migrated to the sediments in the open water period. Common metabolic pathways and functional genes for nitrogen and phosphorus were identified in Methylomicrobium, Marinobacter, and Psychrobacter. The dissimilatory nitrate reduction to ammonium (DNRA) facilitated the transformation of polyphosphates and the release of phosphorus. Water temperature indirectly regulated the fluxes of nitrogen and phosphorus at the sediment-water interface (SWI) by modulating the microbial abundance and dissolved oxygen (DO) content.