Sea Ice Freezing and Melting Modulate the Estuarine Sediment Nitrogen Removal Process Through Affecting the Microbiome

ABSTRACT Climate change accelerates sea ice dynamics, and its freezing and melting profoundly alter the bottom water environment, affecting microbial ecosystems and biogeochemical processes. However, the holistic effects of sea ice‐induced environmental changes on microbial nitrogen removal processes in estuaries remain unclear. This study conducted a sea ice freezing and melting simulation and investigated the response mechanism of microbial nitrogen removal processes in estuarine sediments to sea ice dynamics by combining 15 N labeling and molecular techniques. Results indicated that temperature was the most critical factor influencing nitrifying microorganisms and nitrification rates during sea ice freezing and melting. The increase in nitrate availability at the early stage of freezing enhanced microbial denitrification, followed by decreased denitrification rates due to the inhibitory effects of high salinity and low temperatures on denitrifying bacteria abundance. Anammox rates decreased during the freezing period, primarily due to the inhibitory effects of increased salinity, and recovered during the melting period. It was estimated that approximately 26%–30% of the annual terrestrial inorganic nitrogen input was removed by estuarine sediments, with approximately 87%–89% of this removal occurring via denitrification and 11%–13% via anammox. The inorganic nitrogen flux removed via sediment during sea ice period accounted for about 7% of the annual total removal flux. Overall, this study reveals how sea ice dynamics regulate microbial nitrogen removal in estuarine sediments, providing valuable insights into predicting and managing nitrogen removal in cold‐region estuaries under global climate change.