Shifts in composition and function of bacterial communities reveal the effect of small barriers on nitrous oxide and methane accumulation in fragmented rivers.

Rivers are often blocked by barriers to form different habitats, but it is not clear whether this change will affect the accumulation of NO and CH in rivers. Here, low barriers (less than 2 m, LB) increased NO concentration by 1.13 times and CH decreased by 0.118 times, while high barriers (higher than 2 m, less than 5 m high, HB) increased NO concentration by 1.19 times and CH by 2.76 times. Co-occurrence network analysis indicated LB and HB can promote the enrichment of and , further limiting complete denitrification and increasing NO accumulation. The LB promotes methanotrophs (, and ) to compete with denitrifiers () in water, and reduce CH accumulation. While the HB can promote the methanotrophs to compete with nitrifiers () in sediment, thus reducing the consumption of CH. LB and HB reduce river velocity, increase water depth, and reduce dissolved oxygen (DO), leading to enrichment of -type denitrifiers and the increase of NO concentration in water. Moreover, the HB reduces DO concentration and gene abundance in water, which can increase the accumulation of CH. In light of the changes in the microbial community and variation in NO and CH accumulation, the impact of fragmented rivers on global greenhouse gas emissions merits further study.