High biodegradability of water-soluble organic carbon in soils at the southern margin of the boreal forest

Abstract. Water-soluble organic carbon (WSOC) is an important component of the organic carbon pool in boreal ecosystems. However, the biodegradability of WSOC across various soil depths in boreal ecosystems remains unclear. Here, based on spectroscopic techniques, we conducted a 28-day laboratory incubation to analyze the molecular composition, biodegradability, and compositional changes of WSOC at different soil depths in a southern region of the boreal forest. The results showed that in the upper 2 m soils, the average content of biodegradable WSOC was 0.228 g/kg with an average proportion of 86.41 % in the total WSOC. In the deep soils below 2 m, the average content of biodegradable WSOC content was 0.144 g/kg, comprising 80.79 % of the total WSOC. Spectroscopic analysis indicates that the WSOC in the upper soils is primarily composed of highly aromatic humic acid-like matter with larger molecular weights than those in deep soils. Both the aromaticity and molecular weight decrease with depth, and the WSOC is mainly composed of fulvic acid-like matter in the deep soils, suggesting high biodegradability of WSOC in the deep soils. Overall, our results suggest that the water-soluble organic carbon in the boreal forests exhibits high biodegradability both in the shallow layer and deep soils.