Towards a better and more complete understanding of microbial nitrogen transformation processes in the rhizosphere of subsurface flow constructed wetlands: Effect of plant root activities

•Nitrification is boosted and denitrification is inhibited due to plant activities.•Boosted nitrification is positively correlated to ROL and higher HA-like content.•Inhibited denitrification is negatively correlated to ROL and phenol-like content.•Inhibited anammox is correlated to ROL and insufficient NO2−-N in rhizosphere.•Higher relative abundance of nrf in the nonrhizosphere promoted DNRA process. Plant rhizosphere is the most active hotspot for microbial nitrogen (N) removal in constructed wetlands (CWs), but the effects of plant root activities (i.e., radial oxygen loss, ROL and root exudates, REs) on different microbial N transformation processes, including nitrification, denitrification, anaerobic ammonia oxidation (anammox) and dissimilatory nitrate reduction to ammonia (DNRA) have not been comprehensively studied. In this study, N removal performance and transformation processes in the rhizosphere of CWs were investigated using rhizobox, planar optode system, and metagenomic approaches. Results suggested nitrification was promoted in the rhizosphere, due to the high dissolved oxygen (DO) concentration caused by ROL and the humic acid-like substance in REs, making the rhizosphere a higher removal efficiency of NH4+-N than in the nonrhizosphere. Out of expectation, the denitrification process was not promoted by the REs, the potential “carbon source”, but inhibited by the high DO concentration in the rhizosphere and the phenol-like substance (a kind of biological denitrification inhibitor) in REs. In addition, both anammox and DNRA processes were, for the first time, found to be inhibited in the rhizosphere. The humic acid-like substance in REs and ammonia-oxidation production hydroxylamine inhibited the activity of anammox microorganisms and genes, and there was a symbiotic relationship between anammox microorganisms and DNRA microorganisms. This study provides better and more complete insights into the biogeochemical cycle of N occurred in the rhizosphere of CWs.