Isotopic Evidence for Microbial Nitrogen Cycling in a Glacier Interior of High-Mountain Asia

Abstract

Glaciers are now acknowledged as an important biome globally, but biological processes in the interior of the glacier (englacial) are thought to be slow and to play only a minor role in biogeochemical cycles. In this study, we demonstrate extensive, microbially driven englacial nitrogen cycling in an Asian glacier using the stable isotopes (δ 15 N, δ 18 O, and Δ 17 O values) of nitrate. Apparent decreases in Δ 17 O values of nitrate in an 8 m shallow firn core from the accumulation area indicate that nitrifiers gradually replaced ∼80% of atmospheric nitrate with nitrate from microbial nitrification on a decadal scale. Nitrate concentrations did not increase with depth in this core, suggesting the presence of nitrate sinks by microbial assimilation and denitrification within the firn layers. The estimated englacial metabolic rate using isotopic mass balance was classified as growth metabolism, which is approximately 2 orders of magnitude more active than previously known coldenvironment metabolisms. In a 56 m ice core from the interior of the ablation area, we found less nitrification but continued microbial nitrate consumption, implying that organic matter is microbially accumulated over centuries before appearing on the ablating surface. Such englacial microbial products may support supraglacial microbes, potentially promoting glacial darkening and melting. With predicted global warming and higher nitrogen loads, englacial nutrient cycling and its roles may become increasingly important in the future.

Isotopic Constraints on Sources and Transformations of Nitrate in the Mount Everest Proglacial Water

“…An investigation of the NO 3 – isotopic values of ice cores from Urumqi Glacier No. 1 revealed that N deposited in glaciers could undergo in-suit nitrification and microbial consumption . Glacial lakes and glacier-fed rivers in proglacial environments serve as major recipients and distributors of glacier-derived NO 3 – and nitrification-related microorganisms. ,, The highly hydrological connectivity and unique topography of high-mountain glacier regions facilitate the rapid transport of glacier-derived NO 3 – .…”