A sustainable approach for bioremediation of secondary salinized soils: Studying remediation efficiency and soil nitrate transformation by bioaugmentation.

Nitrate is the main nitrogen source for plant growth, but it can also pollute the environment. A major cause of soil secondary salinization is the rising level of nitrates in the soil, which poses a threat to the sustainability and fertility of global greenhouse soils. Herein, Bacillus megaterium NCT-2 was used as a microbial agent to remove nitrate by bioaugmentation, and the remediation efficiency of secondary salinized soil in different degrees was evaluated. The findings showed that the highest nitrate removal rate of 62.76% was in a medium degree of secondary salinized soil. Moreover, the results of 16S rRNA high-throughput sequencing and quantitative real-time PCR (qPCR) demonstrated that NCT-2 agent reduced the microbial diversity, increased the microbial community stability, and changed the composition and function of the microbial community were changed by NCT-2 agent in all districts soil. Further analysis demonstrated that the NCT-2 bacterial agent significantly increased the key enzyme genes of the assimilation pathway (nitrite reductase gene NasD, 87-404 times, and glutamine reduction enzyme gene GlnA, 13-52 times) and dissimilatory reduction to ammonium (DNRA) (nitrate reductase gene NarG, 14-56 times) in different degrees of secondary salinized soils. This proved that NCT-2 agent could promote the nitrate assimilation and the dissimilation and reduction to ammonium in secondary salinized soil. Thus, the current findings suggested that the NCT-2 agent has a significant potential for reducing excessive nitrate levels in secondary salinized soil. The remediation efficiency was related to the microbial community composition and the degree of secondary salinization. This study could provide a theoretical basis for the remediation of secondary salinized soil in the future.