Nitrogen cycling processes and the role of multi-trophic microbiota in dam-induced river-reservoir systems

The nitrogen (N) cycle is one of the most important nutrient cycles in river systems, and it plays an important role in maintaining biogeochemical balance and global climate stability. One of the main ways that humans have altered riverine ecosystems is through the construction of hydropower dams, which have major effects on biogeochemical cycles. Most previous studies examining the effects of damming on N cycling have focused on the whole budget or flux along rivers, and the role of river as N sources or sinks at the global or catchment scale. However, so far there is still lack of comprehensive and systematic summarize on N cycling and the controlling mechanisms in reservoirs affected by dam impoundment. In this review, we firstly summarize N cycling processes along the longitudinal riverine-transition-lacustrine gradient and the vertically stratified epilimnion-thermocline-hypolimnion gradient. Specifically, we highlight the direct and indirect roles of multi-trophic microbiota and their interactions in N cycling and discuss the main factors controlling these biotic processes. In addition, future research directions and challenges in incorporating multi-trophic levels in bioassessment, environmental flow design, as well as reservoir regulation and restoration are summarized. This review will aid future studies of N fluxes along dammed rivers and provide an essential reference for reservoir management to meet ecological needs.