Biogeochemical reservoirs in the ice: Microbial, DOC, and TN storage and export from retreating Tibetan glaciers

Glaciers on the Tibetan Plateau (TBP) function not only as critical freshwater reservoirs but also as significant reservoirs of biogeochemically important constituents, including microorganisms, dissolved organic carbon (DOC), and total nitrogen (TN). Driven by climate warming, rapid glacier retreat accelerates the release and downstream export of these constituents, with profound implications for regional biogeochemical cycles. However, current knowledge of their storage and mass balance remains limited. This study provides the first estimates of microbial, DOC, and TN storage in TBP glaciers based on ice-core data. In 2015, these values were estimated to be 5.73×1022 cells for microorganisms, 2874 Gg for DOC, and 813 Gg for TN. Under the SSP1-2.6 climate scenario, the annual mass balances of microorganisms, DOC, and TN were −3.81×1020 cells yr−1, −15.15 Gg yr−1, and −4.35 Gg yr−1. Within the plateau, glaciers in westerly-dominated regions exhibit markedly larger storage and lower mass loss rates than those in monsoon-dominated regions. As englacial ice comprises the majority of glacier volume, estimates based on ice-core data provide a more robust representation than previous surface-ice-based assessments. Furthermore, glacier runoff was estimated to export 3.89×1021 cells yr−1 of microorganisms, 71.34 Gg yr−1 of DOC, and 20.16 Gg yr−1 of TN. These estimations quantify the potential biogeochemical inputs from deglaciating regions to downstream ecosystems, providing essential data for elucidating how glacier retreat influences nutrient cycling, microbial dispersal, and carbon-nitrogen coupling under a changing climate.