Effects of dam building on N2O-producing and N2O-reducing community structures in river sediments and the associated N2O emission potential

Abstract

Dam construction is regarded as the greatest anthropogenic disturbance in aquatic ecosystems, and it promotes denitrification, through which large N2O emissions occur. However, the effect of dams on N2O producers and other N2O-reducing microorganisms (especially for nosZ II), and the associated denitrification rates remain poorly understood. This study systematically investigated the spatial variation of potential denitrification rates in dammed river sediments in winter and summer and the microbial processes driving N2O production and reduction. Sediments in the transition zone of dammed rivers were found to be critical for N2O emission potential, with lower potential denitrification rate and N2O production rate in winter than in summer. In dammed river sediments, the dominant N2O-producing microorganisms and N2O-reducers were nirS-harboring bacteria and nosZ I-harboring bacteria, respectively. Diversity analysis showed that diversity of N2O-producing did not differ significantly between upstream and downstream sediments, whereas the population size and diversity of N2O-reducing microbial communities in upstream sediments significantly decreased, leading to biological homogenization. Further ecological network analysis revealed that the ecological network of nosZ II microbes was more complex than that of nosZ I microbes, and both exhibited more cooperation in the downstream sediments than in the upstream sediments. Mantel analysis showed that the potential N2O production rate was mainly influenced by electrical conductivity (EC), NH4+, and TC content, and that higher nosZ II/nosZ I ratios contributed to improved N2O sinks in dammed river sediments. Moreover, the Haliscomenobacter genus from the nosZ II-type community in the downstream sediments contributed significantly to N2O reduction. Collectively, this study elucidates the diversity and community distribution of nosZ-type denitrifying microorganisms influenced by dams, and also highlights the non-negligible role played by nosZ II-containing microbial groups in mitigating N2O emissions from dammed river sediments.