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 N(2)O and CH(4) in rivers. Here, low barriers (less than 2 m, LB) increased N(2)O concentration by 1.13 times and CH(4) decreased by 0.118 times, while high barriers (higher than 2 m, less than 5 m high, HB) increased N(2)O concentration by 1.19 times and CH(4) by 2.76 times. Co-occurrence network analysis indicated LB and HB can promote the enrichment of Cyanobium and Chloroflexi, further limiting complete denitrification and increasing N(2)O accumulation. The LB promotes methanotrophs (Methylocystis, Methylophilus, and Methylotenera) to compete with denitrifiers (Pseudomonas) in water, and reduce CH(4) accumulation. While the HB can promote the methanotrophs to compete with nitrifiers (Nitrosospira) in sediment, thus reducing the consumption of CH(4). LB and HB reduce river velocity, increase water depth, and reduce dissolved oxygen (DO), leading to enrichment of nirS-type denitrifiers and the increase of N(2)O concentration in water. Moreover, the HB reduces DO concentration and pmoA gene abundance in water, which can increase the accumulation of CH(4). In light of the changes in the microbial community and variation in N(2)O and CH(4) accumulation, the impact of fragmented rivers on global greenhouse gas emissions merits further study.