Response of soil humic substances to snowpack decline across different soil layers in the subalpine forest

Soil humic substances are central to the forest carbon cycle. Climate warming-induced snow cover reduction may alter soil biochemical properties and microbial communities, potentially influencing soil humic substances accumulation in forests. We conducted a snow manipulation experiment with natural snow (CK), 50% snow removal (snow reduction), and 100% snow removal (snow removal) treatments in a subalpine Abies fargesii var. faxoniana forest in western Sichuan. Seasonal changes in soil biochemical properties, microbial communities, enzyme activities, and humic substances content were measured in organic matter (0–10 cm) and mineral (10–20 cm) soil layers. Results showed that humic substances content in both layers declined under snow reduction/removal treatments, with levels generally higher during the snow-covered (SC) period than during the snow-free (SF) period. Microbial communities responded to snow reduction/removal treatment across both soil layers, as reflected primarily in changes in the relative abundances of Acidobacteriota, Actinobacteriota, Basidiomycota, and Mortierellomycota in the organic matter layer and Bacteroidota and Basidiomycota in the mineral layer. Functional groups, such as bacteria involved in genetic information processing and ectomycorrhizal fungi, also underwent dynamic adjustment. Compared to CK, snow removal suppressed β-1,4-glucosidase (BG) activity in the organic matter layer during SC and SF periods, but enhanced polyphenol oxidase (PPO) activity during the SF period. Structural equation models indicate that snowpack decline may directly influence soil humic substances content in the organic matter layer, or indirectly affect it by altering the bacterial community through changes in soil biochemical properties. In the mineral layer, snow reduction/removal likely exerted indirect effects on humic substances. These findings underscore the soil-layer-dependent nature of snow cover impacts on humic substances and provide important insights into humus‑carbon feedback mechanisms under climate warming scenarios. Our study contributes to a better understanding of how climate- or anthropogenically-driven changes in snow cover influence carbon cycling in subalpine forest soils.