Abstract: Aims The impact of global warming on soil freeze-thaw cycle in high-altitude area is increasing. However, the contribution of litter-derived carbon to various soil organic carbon fractions and its response to freeze-thaw cycle remains unclear. Methods The study utilized indoor simulated freeze-thaw cycling experiment and stable isotope tracing technique, using 13C-labeled Picea asperate litter (root, twig, leaf), to explore the contribution of litter-derived carbon to dissolved organic carbon (DOC), microbial biomass carbon (MBC), humus carbon (HC), particulate organic carbon (POC) and mineral associated organic carbon (MAOC) in soil under freeze-thaw cycles. Important findings The results showed that: after 30 days of incubation, litter-derived carbon contributed significantly to soil POC and MBC, accounting for 13.1% and 9.0%, respectively. The contribution of litter to different soil organic carbon fractions varied among different organs, with roots exhibiting significantly lower contribution rate to POC, MAOC, and HC compared to twigs and leaves. Under freeze-thaw cycles, litter carbon contributed higher to soil DOC and MBC, while having a lower contribution rate to soil POC, MAOC, and HC. Correlation analysis revealed a significant positive relationship between soil carbon acquisition-related enzymes and the contribution of litter-derived carbon to soil organic carbon. These findings indicate that freeze-thaw processes facilitate the accumulation of litter-derived carbon in active organic carbon fractions such as soil DOC and MBC, but inhibit the sequestration of plant-derived carbon in stable soil organic carbon during the initial litter decomposition period in subalpine forests. The research findings contribute to a deeper understanding of the contribution of forest litter return to soil organic carbon fractions, providing a scientific basis for the management and operation of soil carbon pools in subalpine forests under the backdrop of climate change.

Key words: freeze-thaw cycle, litter decomposition, soil organic carbon, δ13C