Molecular analysis of microbial nitrogen transformation and removal potential in the plant rhizosphere of artificial tidal wetlands across salinity gradients

Abstract

This study explored the microbial nitrogen transformation and removal potential in the plant rhizosphere of seven artificial tidal wetlands under different salinity gradients (0–30‰). Molecular biological and stable isotopic analyses revealed the existence of simultaneous anammox (anaerobic ammonium oxidation), nitrification, DNRA (dissimilatory nitrate reduction to ammonium) and denitrification processes, contributing to nitrogen loss in rhizosphere soil. The microbial abundances were 2.87 × 103–9.12 × 108 (nitrogen functional genes) and 1.24 × 108–8.43 × 109 copies/g (16S rRNA gene), and the relative abundances of dissimilatory nitrate reduction and nitrification genera ranged from 6.75% to 24.41% and from 0.77% to 1.81%, respectively. The bacterial 16S rRNA high-throughput sequencing indicated that Bacillus, Zobellella and Paracoccus had obvious effects on nitrogen removal by heterotrophic nitrifying/aerobic denitrifying process (HN-AD), and autotrophic nitrification (Nitrosomonas, Nitrospira and Nitrospina), conventional denitrification (Bradyrhizobium, Burkholderia and Flavobacterium), anammox (Candidatus Brocadia and Candidatus Scalindua) and DNRA (Clostridium, Desulfovibrio and Photobacterium) organisms co-existed with HN-AD bacteria. The potential activities of DNRA, nitrification, anammox and denitrification were 1.23–9.23, 400.03–755.91, 3.12–35.24 and 30.51–300.04 nmolN2·g−1·d−1, respectively. The denitrification process contributed to 73.59–88.65% of NOx− reduction, compared to 0.71–13.20% and 8.20–15.42% via DNRA and anammox, as 83.83–90.74% of N2 production was conducted by denitrification, with the rest through anammox. Meanwhile, the nitrification pathway accounted for 95.28–99.23% of NH4+ oxidation, with the rest completed by anammox bacteria. Collectively, these findings improved our understanding on global nitrogen cycles, and provided a new idea for the removal of contaminants in saline water treatment.