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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]

Microcalorimetry and BIOLOG are common tools in the study of soil microbial metabolism. When used combined, they may reveal further details about soil microbial metabolic diversity than individually. Through this study, we demonstrated the advantages of such a combinatorial methodology by comparing soil samples from two locations in China, each with (OM samples) and without (control) organic fertilization. We used BIOLOG and microcalorimetry to study soil microbes' ability to metabolize different C substrates. Microcalorimetric measurements helped us further reveal the differences in the microbial growth kinetics under different BIOLOG-identified C substrates. Results showed that soils differed in the preferred C substrates, as denoted by the thermodynamic parameters. Some C substrates stimulated the active microbial biomass, while some stimulated microbial growth rate. Most interestingly, certain C substrates (e.g., l-arginine for Shandong soil and glycogen for Henan soil) showed stimulating effects on both OM and control soils, which could be attributed to the pH value and P availability in soil. Hence, we believe microcalorimetry could be potentially used to explore the soil microbial metabolic diversity by combining BIOLOG measurement, especially in determining how microbes assimilate different nutrient sources. [ABSTRACT FROM AUTHOR]

Organic C has many benefits for soil, but it is depleted by tillage and crop harvest, and especially so for biofuel crops. Accordingly, strategies such as partially retaining stover or planting a cover crop can help ameliorate the negative effect of C removal. We used a long-term field experiment to study the impacts of stover retention and planting a cover crop on soil organic matter (SOM), its extractable components, and the soil microbial community. SOM chemical composition characterization was determined by electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in sequential water, methanol (MeOH), and chloroform (CHCl 3) extracts. The characteristics of the soil bacterial community were measured by phospholipid fatty acid (PLFA), real-time quantitative PCR, and 16S rRNA gene sequence. The variations in total SOM content, total microbial biomass, and bacterial population were slight among treatments, but SOM chemical compounds, arbuscular mycorrhizal fungi (AMF) biomass, and bacterial structure changed significantly, and especially so in the coupled application of stover retention and cover crop. Specifically, stover retention enriched more lignin-like compounds in soil, whereas cover crop enriched more condensed hydrocarbons, and had more compounds with an aromaticity index (AI) > 0.5. The bacterial community was not altered by the cover crop, but the corn stover retention increased the relative abundances of Myxococcales (Deltaproteobacteria) and decreased that of Actinobacteria. Redundancy analysis (RDA) further revealed that the bacterial community in the stover treatments had a significant positive association with CHCl 3 -extracted chemical classes, i.e. unsaturated hydrocarbons and lipids, with the coupled application (stover and cover crop), and lignin and proteins with the corn stover only treatment. Taken together, our study shows how different C addition practices influence the molecular composition of SOM and the structure of soil microbial communities. • Soil carbon was characterized by ion cyclotron resonance mass spectrometry. • Corn stover and rye cover crop changed composition of extractable soil organic C. • Corn stover retention enhanced lignin-like compounds. • Cover crop enriched more condensed hydrocarbons. • Soil bacterial community composition corresponded to certain SOM compositions. [ABSTRACT FROM AUTHOR]