Spatiotemporal transformation of dissolved organic matter along an alpine stream flow path on the Qinghai–Tibet Plateau: importance of source and permafrost degradation

Yinghui Wang,David C Podgorski,Wenjie Xiao,Yunping Xu,Dandan Wei,Robert G M Spencer,Yuanhe Yang,Anne M Kellerman,Harunur Rashid,Phoebe Zito

doi:10.5194/bg-15-6637-2018

Abstract

Abstract. The Qinghai–Tibet Plateau (QTP) accounts for approximately 70 % of global alpine permafrost and is an area sensitive to climate change. The thawing and mobilization of ice-rich and organic-carbon-rich permafrost impact hydrologic conditions and biogeochemical processes on the QTP. Despite numerous studies of Arctic permafrost, there are no reports to date for the molecular-level in-stream processing of permafrost-derived dissolved organic matter (DOM) on the QTP. In this study, we examine temporal and spatial changes of DOM along an alpine stream (3850–3207 m above sea level) by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), accelerator mass spectrometry (AMS) and UV–visible spectroscopy. Compared to downstream sites, dissolved organic matter (DOM) at the headstream site exhibited older radiocarbon age, higher mean molecular weight, higher aromaticity and fewer highly unsaturated compounds. At the molecular level, 6409 and 1345 formulas were identified as unique to the active layer (AL) leachate and permafrost layer (PL) leachate, respectively. Comparing permafrost leachates to the downstream site, 59 % of AL-specific formulas and 90 % of PL-specific formulas were degraded, likely a result of rapid in-stream degradation of permafrost-derived DOM. From peak discharge in the summer to low flow in late autumn, the DOC concentration at the headstream site decreased from 13.9 to 10.2 mg L−1, while the 14C age increased from 745 to 1560 years before present (BP), reflecting an increase in the relative contribution of deep permafrost carbon due to the effect of changing hydrological conditions over the course of the summer on the DOM source (AL vs. PL). Our study thus demonstrates that hydrological conditions impact the mobilization of permafrost carbon in an alpine fluvial network, the signature of which is quickly lost through in-stream mineralization and transformation.

Highlights

The amount of organic carbon stored in permafrost is roughly twice as much as that in the atmosphere and represents the largest component of the terrestrial carbon pool (Zimov et al, 2006; Tarnocai et al, 2009)
The mobilization of organic carbon from permafrost soils where it has been relatively stable for thousands of years into dissolved carbon could increase greenhouse gas emissions (Cory et al, 2013; Vonk et al, 2013; Mann et al, 2015; Ward and Cory, 2016; Selvam et al, 2017) and exacerbate climate warming via a positive feedback loop (Koven et al, 2011; Schuur et al, 2015)
We focused on a continuous system that starts with a thermo-erosion gully (> 200 m long), which forms a stream that flows into Qinghai Lake, the largest lake in China with a surface area of ca. 4500 km2

Summary

Introduction

The amount of organic carbon stored in permafrost is roughly twice as much as that in the atmosphere and represents the largest component of the terrestrial carbon pool (Zimov et al, 2006; Tarnocai et al, 2009). Y. Wang et al.: Spatiotemporal transformation of dissolved organic matter has changed watershed hydrology, topography and ecosystem biogeochemistry (Frey and Smith, 2005; Abbott et al, 2015; Vonk et al, 2015). The mobilization of organic carbon from permafrost soils where it has been relatively stable for thousands of years into dissolved carbon could increase greenhouse gas emissions (Cory et al, 2013; Vonk et al, 2013; Mann et al, 2015; Ward and Cory, 2016; Selvam et al, 2017) and exacerbate climate warming via a positive feedback loop (Koven et al, 2011; Schuur et al, 2015)

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Discussion

Conclusion