Microbial driven nitrogen transformation in the plant rhizosphere of a saltmarsh wetland: From functional genes to activity and contribution

Abstract

Rhizosphere soil microbes played a crucial role in regulating nitrogen transformations in saltmarsh wetlands. Nevertheless, studies that quantitatively determined bacterial metabolic clusters to predict the biological and environmental impacts were limited. In this study, stable isotopic and molecular biological analyses were utilized to detect bacterial biodiversity, community structure, abundances and activities in the rhizosphere. Results showed that the absolute copy numbers of nitrogen functional genes (amoA, nirS, nosZ, etc.) and 16S rRNA were 7.57 × 10⁴-6.68 × 10⁷ and 7.65 × 10⁸-8.21 × 10⁹ copies·g⁻¹, and the relative abundances of nitrifying genera and dissimilatory nitrate reduction processes (anammox, DNRA and denitrification) varied from 0.02 % to 0.10 % and from 47.95 % to 63.16 %, respectively. The Sphingomonas, Lysobacter, Massilia and Pseudarthrobacter were the predominant genera related to nitrogen loss by denitrification process, and DNRA (Pseudomonas, Paracoccus and Bacillus), anammox (Candidatus Scalindua and Candidatus Kuenenia) and nitrification (Nitrosomonas and Nitrospira) co-existed with denitrifying organisms. The potential rates of nitrification, denitrification, anammox and DNRA (dissimilatory nitrate reduction to ammonium) were 69.08–170.01, 219.04–325.67, 15.87–37.06 and 29.94–51.21 nmolN₂·g⁻¹·d⁻¹, respectively; and the nitrification was the crucial pathway of NH₄⁺ oxidation, while denitrification played a vital role in NO_X⁻ reduction and N₂ production. In the meantime, the rhizosphere soil physicochemical properties could affect the microbial distributions, and NH₄⁺, NO₂⁻, NO₃⁻, TOC (total organic carbon), TS (total sulfur) and TN (total nitrogen) were the most crucial factors. The microbial functional profiles were predicted by FAPROTAX analysis, and several functions related to nitrogen metabolisms were annotated, such as nitrate reduction and ammonia oxidation. Overall, these findings provided significant insights into microbial driven nitrogen cycles in rhizosphere soil of saltmarsh wetlands.