Microbial activity and hydroxyl radical oxidation combine to induce CO2 release from winter biocrusted soils.

Soil CO2 release under winter snow in arid regions significantly impacts the carbon balance. Enzymatic decomposition is believed to be the primary driver of CO2 release, however, continuous snow cover promotes the formation of the hydroxyl radical (•OH). Under snow-covered conditions, whether •OH oxidation constitutes an alternative organic matter decomposition pathway to microbial activity, and how these processes interact, remains poorly understood. In this study, we investigated the differences in the CO2 release rates at various depths in biological soil crusts under snow cover and examined the impacts of microbial activity and •OH oxidation on CO2 release. The findings indicate that snow cover has an insulating effect on soil temperature, and that temperature fluctuations decrease with increasing depth. However, this insulating effect did not significantly alter the relative contribution of different soil layers to CO2 release. The crust layer and the 0-5 cm soil layer are the primary zones of CO2 release, while the 5-10 cm layer contributes less. In addition, prolonged infiltration of snowmelt promotes •OH generation, which may play a significant role in regulating CO2 release during winter. Microbial activity primarily influences CO2 production in the surface layer, whereas CO2 emissions from deeper soil are mainly driven by •OH oxidation. This study highlights that under winter conditions when microbial activity is suppressed, both biotic and abiotic processes contribute to CO2 release across different soil depths. Given that snow cover is widespread in terrestrial ecosystems, •OH-mediated CO2 release from deeper soil may represent a previously overlooked carbon emission pathway.