Antagonistic effect of changing salinity and dissolved organic carbon on NO production via different pathways in saline lakes.

Saline lakes are experiencing significant changes in salinity and organic matter content due to climate change. However, the specific impacts of these environmental changes on the production processes of nitrous oxide (NO)-particularly nitrification and denitrification-in saline lake sediments are still poorly understood, leading to significant uncertainty in current estimates of greenhouse gas (GHG) emission from these ecosystems. Here, we employed N-isotope labeling, functional gene quantification, and structural equation modeling to elucidate NO production pathways and rates in surface sediments along a salinity gradient (0.7-149.3 g/L) within Qinghai-Tibet Plateau (QTP) lakes undergoing rapid desalination and organic carbon enrichment. The results identified saline lake sediments as hotspots for NO production, with nitrification contributing an average of 43.51 % to NO flux and reaching up to 91.73 % in specific high-salinity habitats, highlighting its previously underestimated significance. Salinity was found to limit NO production through both nitrifying and denitrifying processes in lake sediments, although dissolved organic carbon (DOC) in the sediment could counteract the limitation caused by salt. Low-salinity systems (<35 g/L) exhibited predominant salinity-related inhibition of denitrification, whereas high-salinity systems (>35 g/L) displayed DOC-mediated counteraction of salinity stress, stimulating both denitrification and heterotrophic nitrification through alterations in microbial community structure (e.g., reflected by nir/nos ratios). This finding illustrates that climate-driven freshening and organic carbon loading synergistically exacerbate NO emissions in saline lakes. While denitrification remains dominant, heterotrophic nitrification pathways are increasingly significant under saline conditions. This highlights susceptibility of cryosphere-affected ecosystems to hydrological disturbances, and emphasizes the necessity of refining global GHG inventories by incorporating context-dependent NO source partitioning.