Mechanisms of Nitrogen Cycling Driven by Salinity in Inland Plateau Lakes, Based on a Haline Gradient Experiment Using Pangong Tso Sediment

Pangong Tso, a typical plateau lake exhibiting an east-to-west gradient from freshwater to saline conditions, was used to simulate the migration and transformation of nitrogen compounds under different salinity conditions. This study systematically investigates the effects of salinity on nitrogen cycling and transformation in Pangong Tso sediments from 12 sites through controlled laboratory experiments and field monitoring across 120 sites. The data analysis method includes correlation analysis, ANOVA, structural equation modeling (SEM), and mixed-effects modeling (MEM). The results demonstrate that salinity significantly affects nitrogen cycling in plateau lakes. Salinity inhibits nitrification, resulting in an accumulation of ammonium nitrogen (NH4+-N), while simultaneously suppressing gaseous nitrogen emissions through the inhibition of denitrification. Salinity has a significant negative effect on nitrite nitrogen (NO2−-N), which is attributable to enhanced redox-driven transformations under hypersaline conditions. A salinity threshold of approximately 9‰ was identified, above which nitrite oxidation was strongly inhibited, consistent with the known high salinity sensitivity of nitrite-oxidizing bacteria (NOB). Higher salinity levels correlated positively with increased NH4+-N and total nitrogen (TN) concentrations in overlying water (p < 0.01), and were further supported by observed increases in dissolved organic nitrogen (DON) and dissolved total nitrogen (DTN) along with rising salinity, and vice versa.