High Salinity Inhibits Soil Bacterial Community Mediating Nitrogen Cycling.

Salinization is considered a major threat to soil fertility and agricultural productivity throughout the world. Soil microbes play a crucial role in maintaining ecosystem stability and function (e.g., nitrogen cycling). However, the response of bacterial community composition and community-level function to soil salinity remains uncertain. Here, we used multiple statistical analyses to assess the effect of high salinity on bacterial community composition and potential metabolism function in the agricultural ecosystem. Results showed that high salinity significantly altered both bacterial alpha (Shannon-Wiener index and phylogenetic diversity) and beta diversity. Salinity, total nitrogen (TN), and soil organic matter (SOM) were the vital environmental factors shaping bacterial community composition. The relative abundance of Actinobacteria, Chloroflexi, Acidobacteria, and Planctomycetes decreased with salinity, whereas Proteobacteria and Bacteroidetes increased with salinity. The modularity and the ratio of negative to positive links remarkedly decreased, indicating that high salinity destabilized bacterial networks. Variable selection, which belongs to deterministic processes, mediated bacterial community assembly within the saline soils. Function prediction results showed that the key nitrogen metabolism (e.g., ammonification, nitrogen fixation, nitrification, and denitrification processes) was inhibited in high salinity habitats. MiSeq sequencing of 16S rRNA genes revealed that the abundance and composition of the nitrifying community were influenced by high salinity. The consistency of function prediction and experimental verification demonstrated that high salinity inhibited soil bacterial community mediating nitrogen cycling. Our study provides strong evidence for a salinity effect on the bacterial community composition and key metabolism function, which could help us understand how soil microbes respond to ongoing environment perturbation. IMPORTANCE Revealing the response of the soil bacterial community to external environmental disturbances is an important but poorly understood topic in microbial ecology. In this study, we evaluated the effect of high salinity on the bacterial community composition and key biogeochemical processes in salinized agricultural soils (0.22 to 19.98 dS m-1). Our results showed that high salinity significantly decreased bacterial diversity, altered bacterial community composition, and destabilized the bacterial network. Moreover, variable selection (61% to 66%) mediated bacterial community assembly within the saline soils. Functional prediction combined with microbiological verification proved that high salinity inhibited soil bacterial community mediating nitrogen turnover. Understanding the impact of salinity on soil bacterial community is of great significance for managing saline soils and maintaining a healthy ecosystem.

Community assembly mechanisms and co-occurrence patterns of nitrite-oxidizing bacteria communities in saline soils.

Nitrite-oxidizing bacteria (NOB) catalyze the second step of nitrification by oxidizing nitrite to nitrate, which is a key process in the biogeochemical nitrogen cycling. However, little is known about the co-occurrence patterns and assembly processes of NOB communities in agricultural soils with different salinities. Here, we explored the effects of salinity on Nitrobacter and Nitrospira community using high-throughput sequencing and multivariate statistical analyses. Our results showed that high salinity significantly inhibited the nitrite oxidation rates and decreased the abundance of Nitrobacter and Nitrospira. Extreme salty conditions significantly altered the diversity and composition of Nitrospira community but had little effect on Nitrobacter community. Nitrobacter network in high salinity soils was more closely connected while the connectivity of Nitrospira network became weak. Nitrobacter and Nitrospira community exhibited distinct assembly processes at different salinity levels. Stochastic processes were dominant in the Nitrobacter community assembly in both low and high salinity soils. Interestingly, the assembly of Nitrospira community was governed by stochastic and deterministic processes in low and high salinity soils, respectively. To our knowledge, our study provides the first description of the co-occurrence patterns and assembly processes of NOB communities in agricultural soils with different salinities. These results can help us understand the NOB ecological roles and improve the nitrite oxidation activity in a high salinity environment.