Salinization is considered as 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. Herein, 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 bacterial both alpha (Shannon-Wiener index and phylogenetic diversity) and beta diversity. Salinity, TN, and 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 indicated 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 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 salinity effect on the bacterial community composition and key metabolism function, which could help us understand how soil microbe responds 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 bacterial network. Moreover, variable selection (61-66%) 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 in managing saline soils and maintaining a healthy ecosystem.

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Summary Microbial diversities are key drivers of soil multifunctionality in terrestrial ecosystems and are important for stability and productivity of ecosystems. However, the relationships among microbial diversity, community assembly and soil multifunctionality in forest ecosystems remained unclear. Here, soil samples were collected from a subtropical forest ecosystem, Lushan Mountain, China. High‐throughput sequencing was employed to reveal the bacterial/fungal community assembly and biodiversity, as well as 10 enzyme activities were measured to assess soil multifunctionality. We found that soil multifunctionality was negatively regulated by bacterial and fungal alpha diversity, implying a higher potential functional redundancy in this forest soil. The null model indicated that deterministic processes (variable selection) and stochastic processes (dispersal limitation) govern bacterial and fungal phylogenetic turnover, respectively. Correlation analysis revealed that bacterial rather than fungal community assembly processes have a significant linkage to soil multifunctionality. These observations projected that soil variables could regulate multifunctionality by shaping the phylogenetic and taxonomic turnover of bacteria rather than fungi. In summary, our study highlighted that soil multifunctionality is mainly driven by bacterial diversity and community assembly processes while not fungal, presenting different views and knowledge of microbial diversity and community assembly processes in ecosystem functioning.

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The elevational distribution patterns of microbial functional groups have long been attracting scientific interest. Ammonia-oxidizers (ammonia-oxidizing archaea [AOA] and bacteria [AOB]), complete ammonia oxidation (comammox) Nitrospira and nitrite-oxidizers (e.g. Nitrobacter and Nitrospira) play crucial roles in the nitrogen cycle, yet their activities and abundances in response to elevational gradients in a subtropical forest ecosystem remain unclear. Here, we investigated the distribution of potential functions and abundances of these nitrifiers in forest soils along elevational gradients on Mount Lu, China. Our results showed that AOA and Nitrospira abundance was higher than that of their counterparts. Only AOA, Nitrobacter and comammox Nitrospira abundances followed a hump-backed-model with altitude. Soil potential ammonia-oxidation activity (PAO) and nitrite-oxidation activity (PNO) ranged from 0.003 to 0.084 and 0.34 to 0.53 μg NO2−-N g−1 dry soil h−1, respectively. The biotic (AOA, Nitrobacter, Nitrospira and comammox Nitrospira abundances) and abiotic factors (soil variables) jointly affected PAO, whereas the abiotic factors were mainly responsible for PNO. Variance partitioning analysis showed that contemporary environmental disturbance is the most important driver for the biogeography of nitrifier assemblages. Overall, our findings indicate that forest soil nitrifier assemblages exhibit a biogeographic pattern largely shaped by soil chemistry along an elevational gradient.