Lignin Preservation and Microbial Carbohydrate Metabolism in Permafrost Soils. Issue 1 (27th January 2022) - Dao, Thao Thi
Abstract: Permafrost‐affected soils in the northern circumpolar region store more than 1, 000 Pg soil organic carbon (OC), and are strongly vulnerable to climatic warming. However, the extent to which changing soil environmental conditions with permafrost thaw affects different compounds of soil organic matter (OM) is poorly understood. Here, we assessed the fate of lignin and non‐cellulosic carbohydrates in density fractionated soils (light fraction, LF vs. heavy fraction, HF) from three permafrost regions with decreasing continentality, expanding from east to west of northern Siberia (Cherskiy, Logata, Tazovskiy, respectively). In soils at the Tazovskiy site with thicker active layers, the LF showed smaller OC‐normalized contents of lignin‐derived phenols and plant‐derived sugars and a decrease of these compounds with soil depth, while a constant or even increasing trend was observed in soils with shallower active layers (Cherskiy and Logata). Also in the HF, soils at the Tazovskiy site had smaller contents of OC‐normalized lignin‐derived phenols and plant‐derived sugars along with more pronounced indicators of oxidative lignin decomposition and production of microbial‐derived sugars. Active layer deepening, thus, likely favors the decomposition of lignin and plant‐derived sugars, that is, lignocelluloses, by increasing water drainage and aeration. Our study suggests that climate‐induced degradation of permafrost soils may promote carbon losses from lignin and associated polysaccharides by abolishing context‐specific preservation mechanisms. However, relations of OC‐based lignin‐derived phenols and sugars in the HF with mineralogical properties suggest that future OM transformation and carbon losses will be modulated in addition by reactive soil minerals. Plain Language Summary: Permafrost thawing and subsequent decomposition of large parts of the soil organic carbon (OC) currently stored in the northern circumpolar permafrost region are projected to cause a positive feedback on global warming. To understand the potential consequences of climate change for organic matter (OM) decomposition in permafrost soils, we determined the concentration and degree of decomposition of two dominating constituents of soil OM, lignin and non‐cellulosic carbohydrates by using CuO oxidation and TFA hydrolysis, respectively, in density fractionated soils covering a longitudinal gradient of northern Siberia (from east to west: Cherskiy; Logata; Tazovskiy). We found a stronger degradation of lignin and neutral sugars at Tazovskiy with its shallower active layer, probably due to better aeration, as compared to the other sites. Our study, hence, suggests that climate‐induced degradation of permafrost soils will promote lignin and carbohydrate transformation and carbon loss. In addition, larger contents of clay and Fe and Al oxides at the Cherskiy site with appear to favor accumulation of lignin and neutral sugars, likely suggesting the extent of OM transformation is further modulated by soil mineralogical properties. Key Points: The deepening of active layer favors the decomposition of lignin and plant‐derived sugars in permafrost soils Soil clay and mineral phases appear to favor accumulation of lignin and neutral sugars in permafrost soils Magnitude of climate‐induced degradation of lignin and carbohydrates will depend on soil texture and mineralogical properties