High proportional nitrification-derived nitrate in glacier runoffs on the Tibetan Plateau indicated by triple oxygen isotopes of nitrate

Abstract

Glacier and Ice sheets are the largest reservoir of freshwater on Earth. Global warming accelerates glacier retreat and enhances the volume of glacier runoff, which heavily impacts the geochemical cycling in downstream ecosystems. Nitrate is the main form of nitrogen in glacier runoff, while little is known about its source in high-altitude regions. Here, we measured complete nitrate isotopes (δ15N-NO3–, δ18O-NO3– and Δ17O-NO3–) in ten geographically separated glacier runoffs on the Tibetan Plateau (TP) with altitudes ranging from 2,409 to 5,564 m. The average NO3– concentration in glacier runoff was 1.0 ± 0.7 mg·L-1, which is lower than that in the river and lake of the TP. This indicates that the glacier runoff on the TP was less influenced by human activities. Nitrification-derived NO3– was the major source of glacier runoff NO3–, which ranged from 57 % to 90 %. The nitrification process could occur in subglacial environments and glacial foreland soils. Furthermore, denitrification process may not occur in glacier runoff over the TP. Our work may shed light on the nitrogen cycle in high-altitude areas.