Selective Leaching of Dissolved Organic Matter From Alpine Permafrost Soils on the Qinghai‐Tibetan Plateau

Abstract

Ongoing global temperature rise has caused significant thaw and degradation of permafrost soils on the Qinghai‐Tibetan Plateau (QTP). Leaching of organic matter from permafrost soils to aquatic systems is highly complex and difficult to reproduce in a laboratory setting. We collected samples from natural seeps of active and permafrost layers in an alpine swamp meadow on the QTP to shed light on the composition of mobilized dissolved organic matter (DOM) by combining optical measurements, ultrahigh‐resolution Fourier transform ion cyclotron resonance mass spectrometry, radiocarbon (14C), and solid‐state 13C nuclear magnetic resonance spectroscopy. Our results show that even though the active layer soils contain large amounts of proteins and carbohydrates, there is a selective release of aromatic components, whereas in the deep permafrost layer, carbohydrate and protein components are preferentially leached during the thawing process. Given these different chemical characteristics of mobilized DOM, we hypothesize that photomineralization contributes significantly to the loss of DOM that is leached from the seasonally thawed surface layer. However, with continued warming, biodegradation will become more important since biolabile materials such as protein and carbohydrate are preferentially released from deep‐layer permafrost soils. This transition in DOM leachate source and associated chemical composition has ramifications for downstream fluvial networks on the QTP particularly in terms of processing of carbon and associated fluxes.

The optical characteristics and sources of chromophoric dissolved organic matter (CDOM) in seasonal snow of northwestern China

“…Moreover, the filters for samples in this region were in yellow due to the high loading of soil dust. The average a CDOM (280) was the highest among all five regions (2.30 ± 0.52 m −1 ), and the S 275−295 fell in the range of 0.0170-0.0212 nm −1 (0.0188 ± 0.0015 nm −1 ), which shows similar values of leaching for permafrost on the Tibetan Plateau (Wang et al, 2018).…”

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

“…5). This difference is likely attributed to the selective release of aromatic components from AL and carbohy- drate/protein components from the deep PL during the soil thawing process, which was observed in our previous study (Wang et al, 2018). Since chemical composition impacts the reactivity of DOM (Kellerman et al, 2015), the differing chemical composition between the AL and PL leachates that enter the stream may influence bioavailability (Vonk et al, 2013) and photolability (Stubbins et al, 2017).…”

“…3850 m above sea level; m a.s.l. ); detailed description on the collapse can be found in Wang et al (2018). The top 60 cm is AL that comprises abundant grass litter and roots, underlain by dark PL without visible plant debris.…”

“…As a climate-sensitive region, the QTP has experienced significant warming since the 1950s (Qiu, 2008), with the mean annual air temperature rising at a rate of 0.36 • C per decade from 1961 to 2007 (Wang et al, 2008). Consequently, permafrost soils on the QTP have begun to thaw and collapse, causing abundant organic carbon loss from in situ degradation (Mu et al, 2016) and relocation (e.g., selective leaching in different soil layers; Wang et al, 2018). However, compared with an abundance of studies on Arctic permafrost, biogeochemical studies on QTP permafrost are scant.…”

Disentangling the Effects of Climate, Vegetation, Soil and Related Substrate Properties on the Biodegradability of Permafrost‐Derived Dissolved Organic Carbon

“…Actually, DOC released from permafrost soils contains large amounts of low‐molecular‐weight, ancient organic acids and other labile C fractions, which can be rapidly mineralized on daily to weekly time scales (Drake et al, ; Vonk et al, ; Wang, Spencer et al, ). Permafrost‐released DOC would then suffer from substantial biodegradation and release greenhouse gases before reaching nearby aquatic systems (Spencer et al, ), with DOC leached from permafrost soils having higher biodegradability than that from nearby streams or rivers (Mann et al, ; Wang, Xu et al, ). In addition to the lack of DOC observations from permafrost soils, previous studies also revealed conflicting evidence about the relative biodegradability of permafrost‐derived DOC versus DOC from active‐layer overlying permafrost.…”