Nitrogen cycling mechanisms in aquatic systems of arid areas on the Qinghai-Xizang Plateau, China 后印本

摘要: Arid areas account for approximately one-quarter of the global land surface. Therefore, a comprehensive understanding of nitrogen cycling in arid watershed systems is essential for water environment protection and land use planning in dryland ecosystems. Using the Gasikule Lake Basin on the Qinghai-Xizang Plateau, China, as a representative study area, this study examined the sources, transport, and transformation mechanisms of nitrogen within a hierarchically nested hydrological system, including river water (S1R), groundwater (S2G), spring-fed river (S3R), and lake water (S4H). Dual nitrate isotopes (δ15N-NO3– and δ18O-NO3–) were integrated with a Bayesian mixing models in R (MixSIAR) to quantify external nitrate sources. In addition, δ15N-NH4+ isotopes combined with microbial techniques were applied to trace nitrogen transformation processes in water bodies, where nitrogen inputs were dominated by nitrate. The results indicated that nitrate was the primary form of nitrogen input across the study area, although overall nitrate loading remained relatively low. Atmospheric deposition and soil organic nitrogen were the dominant sources of exogenous nitrate input. Microbial genera associated with nitrate reduction generally showed low relative abundance in groundwater, whereas facultatively aerobic genera were predominant in surface water. In surface water, nitrogen transformation was mainly driven by organic nitrogen ammoniation and subsequent nitrification. In contrast, groundwater systems were characterized by stronger hydrological confinement and oxygen limitation, resulting in suppressed nitrification, incomplete denitrification, and a tendency toward ammonium accumulation. Collectively, these findings define a ''low-input, low-transformation, and low-loss'' nitrogen regime in the Gasikule Lake Basin. This sluggish nitrogen cycling reflects a limited ecological self-remediation capacity, highlighting the inherent biogeochemical fragility of alpine desert ecosystems. This study provides a critical theoretical basis for understanding nitrogen budgets in global dryland systems and offers scientific support for water environment protection and ecological sustainability in high-altitude areas.