Groundwater Plays an Important Role in Controlling Riverine Dissolved Organic Matter in a Cold Alpine Catchment, the Qinghai–Tibet Plateau

Abstract

In cold alpine areas, the hydrological control on dissolved organic matter (DOM) export in relation to permafrost distribution remains uncertain on a catchment scale. To address this, the seasonal variations in DOM chemical features of groundwater and stream water were examined from 2013 to 2019 in a representative catchment on the northeastern Qinghai–Tibet Plateau, which is covered by both permafrost (at elevations higher than 3,500 m a.s.l.) and seasonally frozen areas. The results show that the highest dissolved organic carbon (DOC) concentration occurred during the thawing period (April–May), while the highest DOC flux occurred during the thawed period (June–early October). In the permafrost area, the freeze–thaw process significantly influenced the groundwater flow paths and DOM. During the thawing period, with shallow groundwater flow path through the organic soil layer, the stream water DOM recharged by suprapermafrost groundwater was characterized by high aromaticity and high DOC concentrations. During the thawed period, the deepening groundwater flow path through the mineral layer and dilution by overland flow, resulted in low DOC concentrations and DOM with lower aromaticity in stream water. When groundwater flowed from the permafrost area into the porous aquifers in the seasonally frozen area, the groundwater flow path also affected DOM dynamics. The stream water at the catchment outlet, which received groundwater discharge from the porous aquifer, showed a lower peak DOC concentration, a higher proportion of protein‐like DOM, and a higher contribution of cold season DOC flux to annual DOC flux when compared to the water flowing out from the permafrost area. Our work highlights the role of the thick, porous aquifer in the seasonally frozen area in regulating DOM export. The results suggest that the cold alpine area may exhibit a stronger positive feedback to climate warming than that in Arctic areas.

Plain Language Summary

Permafrost degradation has resulted in the release of a large amount of soil carbon into streams, potentially affecting global carbon cycling. The mountainous areas on the Qinghai‐Tibet Plateau are characterized by the coexistence of permafrost and seasonally frozen areas and are sensitive to climate warming. To understand the export pattern of dissolved organic matter (DOM) through streams and its controlling factors on the Qinghai–Tibet Plateau, we analyzed the multi‐year chemical features of DOM in a representative catchment. We found that in the permafrost area, the soil freeze–thaw cycle and dilution of surface flow during warm season control the export of DOM. Compared to the water flowing out from the permafrost area, the stream water at the catchment outlet has a higher cold season dissolved organic carbon flux as percentage of the annual dissolved organic carbon flux, a lower peak concentration of dissolved organic carbon, and a higher proportion of protein‐like DOM. We attribute these differences to the effect of groundwater flow in the thick, porous aquifer in the seasonally frozen area, which has not been previously identified. We infer that this process may increase carbon loss rate when DOM is transported downstream on the Qinghai–Tibet Plateau.

Key Points

• Permafrost and seasonally frozen areas play different roles in driving riverine dissolved organic matter (DOM) export in an alpine catchment

• Groundwater regulates seasonal variations in riverine DOM flux and composition

• Riverine DOM characteristics differ in cold alpine and Arctic regions owing to contrasting hydrological processes