Sizable pool of labile organic carbon in peat and mineral soils of permafrost peatlands, western Siberia

Abstract

In contrast to good knowledge of dissolved organic matter (DOM) adsorption on mineral soils in temperate climate, the behavior of DOM in frozen mineral horizons located under peat soils of permafrost-affected regions remains poorly characterized. Yet, these regions contain sizeable and potentially highly labile pools of organic (peat) carbon (C) that may migrate downwards across mineral layers in case of massive thaw in frozen peatlands induced by on-going climate warming. To quantify these pools and the lability of DOM in permafrost peat soils, we performed experiments focusing on dissolved organic carbon (DOC) desorption from, and adsorption onto, mineral horizons (iron-poor and iron-rich sands as well as silt loam) from the largest frozen peatland in the world, the Western Siberia Lowland (WSL). Desorbed DOC ranged between 0.1 and 0.6 mg C gsoil−1 depending on type of mineral substrate. The adsorption of peat leachate DOM ranged between 0.1 and 0.5 mg C gsoil−1 being highest in Al-Fe-rich mineral horizons.Field measurements of C pools in peat and underlying mineral horizons over 1 m depth in the discontinuous permafrost zone yielded 47 and 15 kg C m−2, respectively. The organic carbon (OC) adsorption capacity of the 1 m – thick mineral layers represented <2% of total amount of OC containing in the 1 m – thick peat layer. However, this adsorption capacity is comparable to the amount of DOC that can be leached from overlaying peat horizons (18%). On average, out of 1.38 ± 0.13 kg C m−2 capable of being initially released from the upper 0–100 cm of peat, 0.25 ± 0.19 kg C m−2 can be adsorbed by the underlying 100–200 cm of Fe- and Al-rich sands and clays. The remaining 1.13 kg C m−2 can be exported to lakes and rivers. Therefore, DOC released during peat thaw in upper soil horizons in permafrost regions can be sizably attenuated via adsorption on mineral layers. This should be taken into account when modeling the feedback of permafrost thaw on C export and CO2 emissions.