Advances in isotopic composition of nitrate research in glaciers: insights into sources, transformations, and environmental implications

The nitrogen cycle in glacier ecosystems is a critical component of the global biogeochemical cycle. Nitrate (NO₃^-)， a key form of bioavailable nitrogen， is essential for the functioning of glacier ecosystems. Understanding its sources， transformations， and export is essential for assessing the impacts of environmental changes and human activities on glaciers， glacial streams， and lakes. The stable isotopic composition of NO₃^- (δ¹⁵N-NO₃^-， δ¹⁸O-NO₃^-， and Δ¹⁷O-NO₃^-) provides a powerful tool to trace its origins and cycling， offering critical insights into nitrogen dynamics in glacier ecosystems. This review synthesizes recent advancements in NO₃^- isotopic composition studies across various habitats in glacier ecosystems， including snow， ice cores， cryoconite， and glacial-fed hydrologic continuums. Key findings highlight that snow photolysis occurs commonly in Antarctica and the Arctic regions， but their impacts on NO₃^- loss vary in degree. The δ¹⁵N-NO₃^- values in ice cores reflect regional nitrogen deposition patterns， while the δ¹⁸O-NO₃^- and Δ¹⁷O-NO₃^- values provide insights into historical atmospheric conditions. Additionally， the stable isotopic compositions of NO₃^- have identified that nitrification-derived NO₃^- is an important source in cryoconite， glacial streams and lakes. Given these findings， we emphasize the Tibetan Plateau， the largest glacier distribution area in the low- to mid-latitudes， as a critical region for future research. With increasing atmospheric nitrogen deposition and its ecological significance， future research should expand research on isotopic composition of NO₃^- studies in ice cores， conduct long-term monitoring of isotopic composition of NO₃^- in the glacier ecosystems of the Tibetan Plateau， and develop a comprehensive nitrogen cycle framework for glacier ecosystems. Such efforts will improve our understanding of nitrogen dynamics in glacier ecosystems， assess climate change impacts， and inform conservation strategies in cryosphere regions.