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.

氮（N）循环是河流系统中最重要的营养循环之一，在维持生物地球化学平衡和全球气候稳定方面发挥着重要作用。人类改变河流生态系统的主要方式之一是建造水电大坝，这对生物地球化学循环有重大影响。以往大多数关于大坝对氮循环影响的研究都集中在河流沿线的整体预算或通量，以及河流作为全球或集水区氮源或汇的作用。然而，目前还缺乏对大坝蓄水影响水库氮循环及其控制机制的全面系统总结。在这次审查中，我们首先总结了沿纵向河流-过渡-湖泊梯度和垂直分层的表层-温跃层-低层梯度的N循环过程。具体而言，我们强调了多营养微生物群及其在氮循环中的相互作用的直接和间接作用，并讨论了控制这些生物过程的主要因素。此外，总结了将多营养水平纳入生物评估、环境流量设计以及水库调节和恢复的未来研究方向和挑战。该综述将有助于未来研究沿坝河流的氮通量，并为满足生态需求的水库管理提供重要参考。我们强调了多营养微生物群及其在氮循环中的相互作用的直接和间接作用，并讨论了控制这些生物过程的主要因素。此外，总结了将多营养水平纳入生物评估、环境流量设计以及水库调节和恢复的未来研究方向和挑战。该综述将有助于未来研究沿坝河流的氮通量，并为满足生态需求的水库管理提供重要参考。我们强调了多营养微生物群及其在氮循环中的相互作用的直接和间接作用，并讨论了控制这些生物过程的主要因素。此外，总结了将多营养水平纳入生物评估、环境流量设计以及水库调节和恢复的未来研究方向和挑战。该综述将有助于未来研究沿坝河流的氮通量，并为满足生态需求的水库管理提供重要参考。