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Aims: The effects of shrub encroachment on plant and soil properties have been well studied. However, little is known about how shrub encroachment influences soil bacterial communities. We investigated the effects of shrub encroachment on grassland soil bacterial communities along a soil depth gradient in the Inner Mongolian region of China.Methods: The belowground bacterial communities were examined using high-throughput sequencing of the 16S rRNA gene (V4-V5 region, Illumina MiSeq).Results: Bacterial alpha-diversity was higher in shrub-encroached soils than in control grassland soils. Bacterial OTU richness was highest at 0-20 cm soil depths, while phylogenetic diversity was greatest at 10-20 cm soil depths. At each soil depth layer, shrub encroachment was associated with a significant shift in bacterial community composition. Change in soil pH was the factor most strongly related to change in bacterial community composition associated with shrub encroachment at all four depth horizons in the soils. Shrub encroachment appears to alter the distribution of bacterial life history strategies in the surface soil (i.e., showing an enrichment in copiotrophs and a depletion in oligotrophs) and shrubs are associated with an increase in nitrification potential in deeper soil horizons.Conclusions: Our results indicate that the influence of shrub encroachment on bacterial community composition extends deep into the soil. The intensity of shrub encroachment at this study site suggests that this ecosystem is undergoing dramatic succession towards shrub-dominance, which will likely trigger shifts in ecosystem function. [ABSTRACT FROM AUTHOR]

• Hairy vetch and peanut straw altered soil bacterial community structure. • Hairy vetch rather than peanut straw altered soil bacterial potential function. • Hairy vetch increased the relative abundance of certain plant-beneficial bacteria. • Hairy vetch increased nutrient levels offsetting the baleful effect of soil acidification. Empirical studies have intensively clarified the effects of livestock manure on soil fertility, crop yield and microbial community, with little information about their response to application of peanut straw and green manure in dryland of Ultisols. In this study, quantitative polymerase chain reaction (qPCR) and next-generation sequencing were used to compare bacterial abundance, community structure and predicted metabolic function under four agricultural practices in Ultisol: without fertilization (control), mineral fertilization (NPK, conventional regime), mineral fertilization with returning composting peanut straw (NPKS) and with planting hairy vetch (NPKG). Compared to the control, the NPKS and NPKG significantly decreased soil pH. The NPKG was associated with the highest soil nutrient contents and peanut yield. Compared to control, the NPKS and NPKG increased bacterial abundance and decreased bacterial alpha-diversity. Fertility improvement and soil acidification were responsible for increasing bacterial abundance and decreasing alpha-diversity, respectively. Phylogenetic clustering was found in soil bacterial community, with the stronger clustering in NPKS and NPKG, suggesting that soil acidification might induce stronger niche-based filtering to decrease bacterial alpha-diversity. Compared to control, the NPKS and NPKG altered soil bacterial community composition, but only NPKG changed the predicted metabolic functions. The NPKG triggered larger changes in bacterial community composition than NPKS, implying that dramatic shifts in community composition might lead to modification of bacterial functions. Soil bacterial community composition and predicted function co-varied with soil pH and nutrient contents, but nutrient improvement had stronger (i.e., 23.3% vs. 11.5% in composition; 21.8% vs. 13.6% in function) and contrary effects (i.e., the opposite arrows) than soil acidification. In addition, the NPKG was associated with higher relative abundance of plant-beneficial bacterial taxa (Mycobacterium and Pseudomonas). Overall, mineral fertilization with planting hairy vetch might be an effective practice for improving soil fertility, optimizing bacterial community structure and increasing peanut yield in dryland of Ultisol. However, the further soil acidification should be taken into account following planting hairy vetch longer. [ABSTRACT FROM AUTHOR]

The spatial distribution of bacteria in bulk soil has been well studied, but little is known about the bacterial biogeography in the rhizosphere of crops. Here, we investigated bacterial distribution in bulk soil, loosely- and tightly-bound soils, from wheat fields distributed across 800,000 km 2 of the North China Plain. Bacterial community composition differed dramatically among bulk and rhizospheric soils, and bacterial diversity decreased with the root proximity. Soil pH correlated with bacterial community composition and diversity in three compartments. Bacterial community in tightly bound soil formed a hub-based network topology with higher transitivity and greater number of central nodes compared with loosely bound and bulk soils, potentially as a result of more direct ecological interactions between the members of the tightly bound soil compartment. Bulk and rhizospheric soils maintained similar compositional distance decay patterns (with equal decay rates), but distinct phylogenetic distance decay patterns (with steeper slope of tightly bound soil). Geographical distance described a relatively greater proportion of bacterial spatial distribution in tightly bound soil, compared with loosely bound soil and bulk soil. Deterministic processes dominated the assemblage of bacterial communities in all soil compartments, while phylogenetic clustering was weaker in tightly bound soil. Taken together, our results suggest distinct bacterial network structure and distribution patterns among bulk soil, loosely bound soil and tightly bound rhizospheric soil, which could possibly result in potential functional differentiation. [ABSTRACT FROM AUTHOR]