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Long-term monoculture of watermelon results in inhibited growth and decreased crop yields, possibly because of imbalance in microbial ecology caused by accumulation of the pathogen in soil. This results in serious problems in the economics of watermelon production. We investigated the build-up of Fusarium in soil under watermelon cultivation and changes over 3 yr of fallow in a microcosm. We focused on changes in the microbial community of Fusarium-infected soil, including the diversity of the microfloral species composition, and species abundance. Long-term monoculture of watermelon leads to changes in microbial diversity and community structure. The microbes most readily cultured from infested soil were suppressed by watermelon wilt pathogen Fusarium oxysporum f. sp. niveum (FON). Of 52 isolated and identified culturable microbes, 83.3% of bacteria, 85.7% of actinomycetes, 31.6% of fungi and 20.0% of Fusarium sp. were inhibited by FON on bioassay plates. Prior to fallowing, infested soil was a transformed 'fungus-type' soil. After 3 yr of fallow, the infested soil had remediated naturally, and soil microbial diversity recovered considerably. Abundance of dominant bacterial populations was increased by 118-177%, actinomycetes, fungi and FON were decreased by 23-32, 33-37 and 50%, respectively.
As atmospheric nitrogen (N) deposition elevates, extensive researches into the consequences of N deposition on litter decomposition process have been triggered; yet, responses of aboveground macro‐detritivores to atmospheric N deposition and their interactive effects on saprotrophic microorganisms and litter decomposition process are not thoroughly understood. By soil incubation, we assessed the influence of N deposition on the decomposition process of broad leaf (Quercus acutissima) and needle (Pinus massoniana) litter mediated by macro‐detritivore isopods (Armadillidium vulgare) and soil microorganisms. Changes in litter chemical composition (total carbohydrate and N), litter mass loss, soil pH values, soil microbial biomass and the activities of degradative enzymes were determined during a 6‐month laboratory incubation. Results showed that N addition enhanced Q. acutissima litter decomposition in the absence of A. vulgare, but decreased that when A. vulgare presence. N addition had no significant effect on P. massoniana litter decomposition regardless of A. vulgare presence or absence. However, N addition decreased isopod feeding contributions, with litter mass loss of 1.83–2.92 times lower than those of only isopod addition treatments in the two litter types. N addition inhibited soil microbial biomass and enzymatic activities related to N and phenolic metabolism of needle litter when isopods presence. Our findings suggest that N addition likely weakens the feeding activity of soil fauna and slows down the litter decomposition in broad‐leaved forests. This implies that a long‐term consequence of N deposition may induce the soil C accumulation and affect the balance of ecosystem nutrient flux in subtropical broad‐leaved forests.
Biofilm plays important roles in the life cycle of Bacillus species, such as promoting host and object surface colonization and resisting heavy metal stress. This study utilized transcriptomics to evaluate the impacts of cadmium on the components, morphology, and function of biofilms of Bacillus subtilis strain 1JN2. Under cadmium ion stress, the morphology of the B. subtilis 1JN2 biofilm was flattened, and its mobility increased. Moreover, differential gene expression analysis showed that the main regulator of biofilm formation, Spo0A, decreased in expression under cadmium ion stress, thereby inhibiting extracellular polysaccharide synthesis through the SinI/SinR two-component regulatory system and the AbrB pathway. Cadmium ion treatment also increased the SigD content significantly, thereby increasing the expression of the flagella encoding and assembly genes in the strain. This promoted poly-γ-glutamic acid production via the DegS/DegU two-component regulatory system and the conversion of biofilm extracellular polysaccharide to poly-γ-glutamic acid. This conferred cadmium stress tolerance in the strain. Additionally, the cadmium ion-mediated changes in the biofilm composition affected the colonization of the strain on the host plant root surface. Cadmium ions also induced surfactin synthesis. These findings illustrate the potential of Bacillus species as biocontrol strains that can mitigate plant pathogenic infections and heavy metal stress. The results also provide a basis for the screening of multifunctional biocontrol strains.
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