Dam construction alters function and community composition of diazotrophs in riparian soils across an environmental gradient

Abstract

Abstract Dam construction significantly influences riparian ecological functions and environmental conditions in the reservoir region. Despite the importance of diazotrophic mediation of nitrogen cycling in riparian ecosystems, knowledge concerning the impacts of dam construction on diazotrophic function and community composition remains very limited. Here, riparian soils were collected from control sites and dam-affected sites located in six cascade dam-formed reservoirs on the Lancang River, China. Diazotrophic activity was assessed by measuring nitrogen fixation rate, while bacterial abundance and community composition were determined with the nifH gene, using quantitative real-time polymerase chain reaction and MiSeq sequencing, respectively. The results indicate that the activity, abundance, and ɑ-diversity of diazotrophs in the affected sites were significantly higher than those in the control sites. The diazotrophic community composition was differed between the control and affected sites, with the latter showing significantly lower community dissimilarity than the control. Both geographic distance and environmental heterogeneity varied in the diazotrophic community, while environmental heterogeneity was the primary factor controlling diazotrophic assemblage in riparian soils, especially at the affected sites. Moreover, co-occurrence network analyses revealed a much higher possible network connectivity at the affected sites than at the control sites. This study suggests distinct activity, abundance, distribution pattern, and network structure of the diazotrophic populations in riparian soils between dam-affected and control sites; thus, providing a novel insight into the potential ecological effects of dam construction on nitrogen cycling.