Understanding how species interaction and assembly processes structure the abundant and rare bacterial biospheres in soils is crucial for predicting how biodiversity influences ecosystem functioning. Here, we profiled the bacterial communities across a salt marsh ecosystem gradient to investigate the co-occurrence patterns across taxa and the relative influence of ecological processes mediating the assembly of the abundant and rare biospheres in soil. Our results revealed abundant taxa to be ubiquitous across all sites, whereas the distributions of the rare taxa were relatively more site specific. The α-diversity indices and β-diversity of rare subcommunities were significantly higher than those of the abundant subcommunities. Besides, both the taxonomic and functional composition of soil bacterial communities differed significantly between the two biospheres. Furthermore, the influence of stochasticity differed in each subcommunity. In particular, stochastic processes were relatively more important in constraining the assembly of rare taxa. Co-occurrence network analysis revealed that a few abundant taxa occupy central nodes within the networks, possibly indicating crucial roles as keystone taxa. Collectively, these findings suggest that abundant and rare bacterial biospheres have distinct distributions underpinned by a dynamic interplay of ecological processes and taxon co-occurrence patterns. IMPORTANCE Estuarine salt marshes are highly productive ecosystems subjected to regular disturbances by hydrodynamic exchange. However, little is known about how distinct assembly processes and co-occurrence of taxa influence the structure of the abundant and rare bacterial biospheres in these soil systems. This study aims at unravelling these intricacies by studying a typical estuarine salt marsh located in Hangzhou Bay, China. Our study provides important pieces of evidence on the diverse distribution of rare and abundant bacterial biospheres. We show that a few abundant taxa are central nodes in species co-occurrence, potentially playing important roles as keystone species in the system. In addition, we highlight a dynamic interplay of assembly processes structuring these two subcommunities.

Bacterial communities play essential roles in estuarine marsh ecosystems, but the interplay of ecological processes underlying their community assembly is poorly understood. Here, we studied the sediment bacterial communities along a linear gradient extending from the water-land junction toward a high marsh, using 16S rRNA gene amplicon sequencing. Bacterial community compositions differed significantly between sediment transects. Physicochemical properties, particularly sediment nutrient levels (i.e., total nitrogen [TN] and available phosphorus [AP]), as well as sediment physical structure and pH (P < 0.05), were strongly associated with the overall community variations. In addition, the topological properties of bacterial cooccurrence networks varied with distance to the water-land junction. Both node- and network-level topological features revealed that the bacterial network of sediments farthest from the junction was less intense in complexity and interactions than other sediments. Phylogenetic null modeling analysis showed a progressive transition from stochastic to deterministic community assembly for the water-land junction sites toward the emerging terrestrial system. Taken together, data from this study provide a detailed outline of the distribution pattern of the sediment bacterial community across an estuarine marsh and inform the mechanisms and processes mediating bacterial community assembly in marsh soils.IMPORTANCESalt marshes represent highly dynamic ecosystems where the atmosphere, continents, and the ocean interact. The bacterial distribution in this ecosystem is of great ecological concern, as it provides essential functions acting on ecosystem services. However, ecological processes mediating bacterial assembly are poorly understood for salt marshes, especially the ones located in estuaries. In this study, the distribution and assembly of bacterial communities in an estuarine marsh located in south Hangzhou Bay were investigated. The results revealed an intricate interplay between stochastic and deterministic processes mediating the assembly of bacterial communities in the studied gradient system. Collectively, our findings illustrate the main drivers of community assembly, taking into consideration changes in sediment abiotic variables and potential biotic interactions. Thus, we offer new insights into estuarine bacterial communities and illustrate the interplay of ecological processes shaping the assembly of bacterial communities in estuarine marsh ecosystems.

Soil microbiome are extremely complex and diverse, including archaea, bacteria, fungi, protists, and so forth (Bardgett & Van Der Putten, 2014). They participate in a variety of ecosystem functions and services, such as primary production, nutrient cycling, decomposition, climate regulation and pathogen control, which are closely linked to global food supply in agricultural ecosystems (Bardgett

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Predicting the distribution patterns of soil microbial communities requires consideration of more environmental drivers. The effects of soil micronutrients on composition of microbial communities are largely unknown despite micronutrients closely relating to soil fertility and plant communities. Here we used data from 228 agricultural fields to identify the importance of micronutrients (iron, zinc, copper and manganese) in shaping structure of soil microbial communities (bacteria, fungi and protist) along latitudinal gradient over 3400 km, across diverse edaphic conditions and climatic gradients. We found that micronutrients explained more variations in the structure of microbial communities than macronutrients in maize soils. Moreover, micronutrients, particularly iron and copper, explained a unique percentage of the variation in structure of microbial communities in maize soils even after controlling for climate, soil physicochemical properties and macronutrients, but these effects were stronger for fungi and protist than for bacteria. The ability of micronutrients to predict the structure of soil microbial communities declined greatly in paddy soils. Machine learning approach showed that the addition of micronutrients substantially increased the predictive power by 9–17% in predicting the structure of soil microbial communities with up to 69–78% accuracy. These results highlighted the considerable contributions of soil micronutrients to microbial community structure, and advocated that soil micronutrients should be considered when predicting the structure of microbial communities in a changing world.