Tracing nitrate sources and transformations using nitrogen and oxygen isotopes in the second largest freshwater lake of China

Nitrate (NO₃^–) is often the dominant nitrogen species in aquatic ecosystems, but tracing its sources and transformations is always challenging. Atmospheric deposition contributes to nitrogen in aquatic ecosystems both directly and indirectly, but the relative contributions of the two pathways remain unclear. Furthermore, triple isotopes (δ¹⁵N-δ¹⁸O-Δ¹⁷O) have recently been promoted as a more reliable way to quantify source contributions to NO₃^–in water than dual isotopes (δ¹⁵N-δ¹⁸O), but their differences in results remain unknown. In this study, we measured the concentration, isotopes of NO₃^–and water in Dongting Lake, investigated the nitrogen transformation process, and quantified the source contributions based on the dual and triple isotopes of NO₃^–using the SIAR model. Our results showed that seasonal differences in ion concentrations and isotopes were generally found in Dongting Lake. The seasonal variations of nitrogen and oxygen isotopes revealed the role of nitrification process and changes in source proportional contributions. The source contributions quantified based on δ¹⁵N-δ¹⁸O and δ¹⁵N-δ¹⁸O-Δ¹⁷O are consistent, showing that soil nitrogen sources contribute the most, followed by agricultural fertilizer application and manure and sewage sources. Atmospheric deposition contributes the least to NO₃^–in Dongting Lake, with an average fraction of less than 9 %. Source contributions quantified on the basis of triple isotopes can effectively reduce uncertainties in the proportional contribution of atmospheric deposition. In Dongting Lake, most of the atmospheric deposition of NO₃^–is indirect, accounting for about 95.2 % and 89.9 % of the total atmospheric deposition to water NO₃^–fluxes in wet and dry seasons, respectively. The study reveals the source contribution and transformation process to NO₃^–in Dongting Lake, and provides an example for quantifying direct and indirect atmospheric deposition to inland lakes. The results provide valuable observational data for modelling the nitrogen cycle in aquatic ecosystems and help to better constrain the nitrogen budget in inland waters.