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The wide spread of nanoparticles (NPs) has caused tremendous concerns on agricultural ecosystem. Some metallic NPs, such as zinc oxide (ZnO), can be utilized as a nano-fertilizer when used at optimal doses. However, little is known about the responses of plant development and concomitant soil bacteria community to ZnO NPs. The present pot experiment studied the impacts of different doses of ZnO NPs and bulk ZnO (0, 1, 10, 100 mg ZnO/kg), on the growth of lettuce (Lactuca sativa L.) and the associated rhizospheric soil bacterial community. Results showed that at a dose of 10 mg/kg, ZnO NPs and bulk ZnO, enhanced the lettuce biomass and the net photosynthetic rate; whereas, the Zn content in plant tissue was higher in NPs treatment than in their bulk counterpart at 10 mg/kg dose or higher. For the underground observations, 10 mg/kg treatment doses (NPs or bulk) significantly changed the soil bacterial community structure, despite the non-significant variations in alpha diversity. Taxonomic distribution revealed that some lineages within Cyanobacteria and other phyla individually demonstrated similar or different responses to ZnO NPs and bulk ZnO. Moreover, some lineages associated with plant growth promotion were also influenced to different extents by ZnO NPs and bulk ZnO, suggesting the distinct microbial processes occurring in soil. Collectively, this study expanded our understanding of the influence of ZnO NPs on plant performance and the associated soil microorganisms.

Purpose: The increasing soil microplastics (MPs) pollution and the dry–wet alternation caused by climate change occur widely. Emerging studies have shown their negative impacts on the structure of soil bacteria and plant physiology. However, there is limited research on the effects of MPs exposure under different water regimes, and the influence processes are inconclusive. Therefore, the impacts of MPs under different water regimes on the antioxidant activity of vegetable and the soil bacterial community were studied. Materials and methods: A continuous water supply and a dry–wet cycle were established to compare the effects of two types of MPs (polypropylene (PP); polyethylene terephthalate (PET)) on lettuce (Lactuca sativa L. var. longifolia) growth and soil bacterial community, determined by antioxidant enzyme activity analysis, and by high-throughput sequencing analysis, respectively. Results: The PET influenced the lettuce growth and bacterial diversity largely compared to PP. The dry–wet cycle enhanced the effects of MPs on the activity of the antioxidant enzymes in lettuce, while it reduced lettuce biomass and root activity. Proteobacteria and Actinobacteria were the dominant soil bacteria under all treatments. The addition of MPs increased the relative abundance of nutrient-related bacteria such as Proteobacteria. MPs addition increased intragroup differences in soil bacterial diversity, and the dry–wet cycle reduced intergroup differences caused by MPs. Conclusion: The polymer type of MPs influenced the antioxidant activity of lettuce roots and leaves. The dry–wet cycle intensified the exposure risk of MPs by enhancing the stimulation of MPs on the antioxidant activity of lettuce. The dry–wet cycle significantly reduced the MPs-induced differences in bacterial communities. In short, the dry–wet cycle might alter the ecological risks in plant-soil ecosystem by shifting soil bacterial lineages related to nutrient cycling and metabolic function under MPs exposure. [ABSTRACT FROM AUTHOR]

Purpose: The extract of Stevia residue is an ideal substitute for cultivation of the purple nonsulfur bacterium, like Rhodopseudomonas palustris ( R. palustris). But the influence of R. palustris grown under residue extract on its downstream application is still not well-characterized. The objective of this study was to assess the effect of foliar spray of R. palustris grown under Stevia residue extract on the plant growth and soil microbial properties. Materials and methods: A pot experiment was carried out under the greenhouse condition, consisting of four treatments varying in the sprayed substances: sterilized water (control), R. palustris grown under the chemical medium supplemented with L-tryptophan (SyT), R. palustris grown under Stevia residue extract supplemented with L-tryptophan (ExT), and R. palustris grown under Stevia residue extract supplemented with NHCl (ExT). The net photosynthesis rate of the uppermost leaves was measured with a portable photosynthesis system. Soil microbial activity was analyzed by microcalorimetry. Soil bacterial community components were determined by real-time quantitative PCR (qPCR) and high-throughput sequencing techniques. Results and discussion: Compared with SyT, the R. palustris grown under Stevia residue extract not only improved the plant biomass and the net photosynthetic rate to a large extent, but also increased soil microbial metabolic activity and altered community compositions as well. The treatments receiving R. palustris, especially ExT and ExN, increased the relative abundances of some functional guilds involved in C turnover and nutrient cycling in soil, including Acidobacteria, Actinobacteria, Proteobacteria, Gemmatimonadaetes, Nitrospirae, and Planctomycetes. Conclusions: R. palustris grown under the Stevia residue extract showed advantages over that under the chemical medium on both plant growth and soil microbial properties. One of the possible reasons could result from the increases in microbial activity and several bacterial keystone guilds involved into C and nutrient cycling, both of which potentially contribute to the improved plant growth. The results would be conducive to the downstream application of R. palustris in an economical way. [ABSTRACT FROM AUTHOR]

Due to the ever-increasing worldwide plantation of sweet leaf Stevia rebaudiana (Bertoni), how to efficiently and effectively utilize the huge amounts of leaf residues that contain abundant nutrients after sweetener extraction becomes an eminent issue. One option is to return these residues into soil, as organic manure in the fresh or composted form, in order to both sustain soil fertility and avoid potential environmental pollution. In a field experiment, we studied if the Stevia leaf residue returning affected both plant and soil microbial growths as well as the possible change of soil microbial community composition. In doing so, four treatments were employed: (1) no chemical fertilization and no Stevia residue returning (no-fertilization control); (2) chemical N, P, and K fertilization (NPK); (3) fresh Stevia residue plus NPK (FS + NPK); and (4) composted Stevia residue plus NPK (CS + NPK). Responses of plant and soil microbial communities to Stevia residue input after 1-year fertilizations were investigated by multiple approaches, including soil enzyme assay, real-time quantitative polymerase chain reaction, and PCR-denaturing gradient gel electrophoresis. Our results showed that compared to the sole NPK and no-fertilization control, returning Stevia residues to soil stimulated the enzyme activities of dehydrogenase, invertase, and urease, except neutral phosphomonoesterase; thereby, both the Stevia leaf biomass and sweet glycoside of rebaudioside A were increased. The soil microbial community abundance was increased by the returning of Stevia residues, and their composition was shifted, evidenced by an increase of relative abundance of some genotypic groups, such as Bacillales. Further molecular analysis of Bacillus confirmed that this guild composition was positively influenced by Stevia residue returning, especially for Bacillaceae. Our results demonstrated an effective use of Stevia residues as organic manure for promoting Stevia yield and quality through stimulating soil microbial growth and enzyme activities. [ABSTRACT FROM AUTHOR]