Methane emissions from inland waters are constrained by salinity

<title>Abstract</title> <p> Inland waters are a significant source of atmospheric methane (CH ₄ ), yet the regulatory role of salinity in arid and semi-arid regions remains poorly understood. Here, we conducted a large-scale field investigation across 90 water bodies in Ningxia and the eastern and northern regions of the Qinghai-Tibet Plateau. We consistently observed a significant negative correlation between salinity and both CH ₄ concentration and emission flux. Further analysis of major ionic composition revealed that elevated concentrations of sodium (Na ⁺ ), calcium (Ca ²⁺ ), and sulfate (SO ₄ ²⁻ ), were key ionic factors associated with suppressed methanogenesis. Our results demonstrated that increased salinity significantly reduced dissolved organic carbon (DOC) concentration ( <italic>p</italic>  &lt; 0.05) and also led to a markedly depleted in proteins, lignin, and lipids, thereby reducing the overall substrate quality and availability for microbial decomposition. Furthermore, elevated salinity suppressed the diversity and abundance of key microbial taxa involved in carbon cycling, such as <italic>Clostridium</italic> , <italic>Acidovorax</italic> , and <italic>Novosphingobium</italic> . Functional gene predictions further indicated a downregulation of methanogenic pathways. Collectively, our findings suggested that salinity acts as a critical environmental filter, limiting DOM availability and reshaping methanogenic microbial communities, thereby suppressing CH ₄ production. This study provided mechanistic insights into how salinity regulates CH ₄ emissions in inland waters of arid and semi-arid regions and underscored the necessity of incorporating salinity effects into regional and global carbon budgets. </p>