Lability classification of soil organic matter in the northern permafrost region

Abstract

Abstract. The large stocks of soil organic carbon (SOC) in soils and deposits of the northern permafrost region are sensitive to global warming and permafrost thawing. The potential release of this carbon (C) as greenhouse gases to the atmosphere does not only depend on the total quantity of soil organic matter (SOM) affected by warming and thawing, but it also depends on its lability (i.e., the rate at which it will decay). In this study we develop a simple and robust classification scheme of SOM lability for the main types of soils and deposits in the northern permafrost region. The classification is based on widely available soil geochemical parameters and landscape unit classes, which makes it useful for upscaling to the entire northern permafrost region. We have analyzed the relationship between C content and C-CO2 production rates of soil samples in two different types of laboratory incubation experiments. In one experiment, ca. 240 soil samples from four study areas were incubated using the same protocol (at 5 ∘C, aerobically) over a period of 1 year. Here we present C release rates measured on day 343 of incubation. These long-term results are compared to those obtained from short-term incubations of ca. 1000 samples (at 12 ∘C, aerobically) from an additional three study areas. In these experiments, C-CO2 production rates were measured over the first 4 d of incubation. We have focused our analyses on the relationship between C-CO2 production per gram dry weight per day (µgC-CO2 gdw−1 d−1) and C content (%C of dry weight) in the samples, but we show that relationships are consistent when using C ∕ N ratios or different production units such as µgC per gram soil C per day (µgC-CO2 gC−1 d−1) or per cm3 of soil per day (µgC-CO2 cm−3 d−1). C content of the samples is positively correlated to C-CO2 production rates but explains less than 50 % of the observed variability when the full datasets are considered. A partitioning of the data into landscape units greatly reduces variance and provides consistent results between incubation experiments. These results indicate that relative SOM lability decreases in the order of Late Holocene eolian deposits to alluvial deposits and mineral soils (including peaty wetlands) to Pleistocene yedoma deposits to C-enriched pockets in cryoturbated soils to peat deposits. Thus, three of the most important SOC storage classes in the northern permafrost region (yedoma, cryoturbated soils and peatlands) show low relative SOM lability. Previous research has suggested that SOM in these pools is relatively undecomposed, and the reasons for the observed low rates of decomposition in our experiments need urgent attention if we want to better constrain the magnitude of the thawing permafrost carbon feedback on global warming.

Greenhouse gas production and lipid biomarker distribution in Yedoma and Alas thermokarst lake sediments in Eastern Siberia

“…The study of talik sediments is however highly relevant for climate studies as it allows important insights into the pathways of previously frozen OM upon rapid thaw and thus the potential for GHG production below thermokarst lakes. While many studies suggested a higher biolability of old Yedoma OM upon thaw compared to Holocene thermokarst deposits (Dutta et al, 2006;Jongejans et al, 2018;Lee et al, 2012;Neubauer, 2016;Schuur et al, 2009;Strauss et al, 2015Strauss et al, , 2017Zimov et al, 2006), a few studies showed opposite findings (Kuhry et al, 2020;Schädel et al, 2014). Furthermore, climatic conditions during permafrost formation were shown to play a crucial role for OM decomposition after thaw (Knoblauch et al, 2013;Walz et al, 2018).…”

Organic carbon characteristics in ice-rich permafrost in alas and Yedoma deposits, central Yakutia, Siberia

“…1b). The land surface within the alas basins is covered by grasslands, whereas the boreal forest found on the Yedoma uplands mainly consists of Larix cajanderi with several Pinus sylvestris communities (Kuznetsova et al, 2010;Ulrich et al, 2017b). Central Yakutian alas landscapes are characterized by extensive land use, which mainly consists of horse and cattle herding and hay farming (Crate et al, 2017).…”

From fibrous plant residues to mineral-associated organic carbon – the fate of organic matter in Arctic permafrost soils

“…9) drives the differentiation into large plant-derived, less decomposed POM and mostly microbial-dominated small POM in the studied permafrost-affected soils. The less decomposed fibrous fPOM and oPOM are hotspots for microbial activity and, thus, for the decay of these larger plant structures (Kuzyakov and Blagodatskaya, 2015). These hotspots for the formation of MAOM in Cryosols, plant residues in direct contact with silt-to clay-sized mineral particle surfaces, were already demonstrated on intact Cryosol cross sections using spectromicroscopic imaging (Mueller et al, 2017).…”