Microbial community responses and functional shifts in carbon degradation driven by water-salt gradients in lakeshore wetlands of semi-arid lakes.

Wetlands, especially lakeshore ecosystems, play an essential role in global carbon (C) cycling and are strongly influenced by dynamic water-salt gradients. However, the mechanisms by which these gradients shape microbial communities and affect soil organic C (SOC) degradation remain incompletely understood. Here, we aimed to elucidate microbial mechanisms driving C degradation shifts along water-salt gradients. We investigated microbially mediated SOC degradation potential in Daihai Lake wetlands by metagenomic profiling of carbohydrate-active enzyme (CAZyme) genes. Our results demonstrated a significant shift in CAZyme gene abundances across the gradients. Genes involved in plant-derived C (cellulose, hemicellulose, lignin) degradation decreased with increasing soil salinity and moisture, while those related to microbial-derived C (chitin, peptidoglycan, glucans) decomposition were significantly enriched. Concurrently, the microbial community composition shifted toward greater abundance of salt-tolerant taxa, notably Proteobacteria and Bacteroidetes, while Actinobacteria and Firmicutes declined under higher water-salt conditions. Statistical analyses revealed strong correlations between key environmental factors (soil salinity (EC), soil moisture (SM), and pH) and both microbial community structure and CAZyme gene abundances (p < 0.001). Random Forest analysis further identified EC and SM as primary factors influencing C-degrading microbial functions. This study highlights the importance of water-salt gradients in regulating microbial-mediated C cycling in wetlands, providing critical insights into microbial adaptation strategies and their implications for wetland C dynamics under environmental change.