Reservoir hydrological fluctuations induce rhizosphere N-cycling divergent patterns: integrating root multi-adaptive...

Dam regulation-induced water level fluctuations (WLFs) significantly impact plant distributions and their interactions with microbes in drawdown zones, driving ecosystem functionality and nutrient dynamics. However, an integrated understanding of how periodic WLFs affect root adaptive traits and rhizosphere microbial dynamics to regulate nitrogen cycling remains limited. To address this, we compared WLF-affected zones (Zones I-II) with an unflooded zone (Zone III) to examine root multi-adaptive strategies, microbial structure and assembly, and nitrogen-cycling divergences. Root economics space (RES) results indicated the root nutrient foraging strategy across Zones I-III. Crucially, WLFs promoted convergent resource acquisition strategies (community-weighted mean-based and functional dispersion-based) and shifted root trait networks toward higher path length, diameter, modularity, but lower edge density. We further tested whether these adaptive strategies are related to rhizosphere microbial dynamics. We found that WLFs resulted in diverse and stochastic rhizobacterial distribution, which was induced by 'outsourcing' traits (distributed on one side of the 'collaboration gradient' of RES) and key environmental drivers. Given the reduced linkage in trait networks and destabilized N-functional microbial co-occurrence networks, structural equation modeling indicated that WLFs enhanced symbiotic root-rhizobacteria relationships. Furthermore, functional traits (Root N and root length) and key soil properties jointly suppressed N-transformation in Zone I, while divergently regulated N-fixation, denitrification, and dissimilatory nitrate reduction to ammonium in Zone II. The reduction of N-transformation was linked to resource scarcity in Zone III. These findings establish that WLFs foster root-microbe cooperation to regulate N-cycles, providing a theoretical basis for managing reservoir operations and riparian ecological functions.