Snow-driven nitrogen cycling and microbial networks in desert biocrusts.

Biological soil crusts (biocrusts) are essential for nitrogen (N) cycling in arid ecosystems, but how snow cover modulates these processes remains unclear. This study investigated the effects of variable snow cover on N fixation, ammonia oxidation (AO), and microbial networks across moss-, lichen-, and cyanobacteria-dominated biocrusts in China's Gurbantunggut Desert. Snow manipulations double snow (DS), ambient snow (CK), and removed snow (RS) were applied to biocrust plots in winter. Nitrogen fixation and AO rates, soil properties soil water content (SWC), organic carbon (SOC), inorganic N, and microbial co-occurrence networks were analyzed in spring. DS enhanced nitrogen fixation rates, particularly in cyanobacteria-dominated crusts (up to 0.4 μg N g^-1 day^-1), while moss crusts showed elevated fixation under RS conditions (rates can up to 0.4 μg N g^-1 day^-1), which correlated with increased SWC (r = 0.72) and SOC (r = 0.65). Conversely, RS reduced AO rates by 50-67 % across biocrusts, with cyanobacteria experiencing the steepest decline (20 μg N g^-1 day^-1). Microbial networks under DS exhibited higher modularity in moss crusts (modularity = 0.85), indicating structured, cooperative communities, while RS fragmented networks, especially in cyanobacteria (modularity = 0.41). Soil salinity (EC) and pH were negatively correlated with microbial activity under RS. Moss crusts showed resilience, maintaining stable N fixation and microbial diversity under varying snow cover, whereas cyanobacteria showed increased sensitivity to drought. These results highlight the critical role of snow cover in maintaining N cycling during the spring season via moisture retention and SOC accumulation. Declining snow cover, as predicted under climate change, may disproportionately affect cyanobacteria-dominated crusts and thus destabilize soil fertility. Conservation strategies that prioritize bryophyte biocrusts could enhance ecosystem resilience in arid landscapes. This study highlights the interplay between snow regimes, biocrust type, and microbial functionality, and provides insights for predicting N cycling dynamics in warming drylands.