Influence of vermicompost application on rhizospheric microbial communities and Arbuscular mycorrhizal fungal colonisation of BT and non-BT maize in agricultural soil
Van Wyk, Deidré Alima Bregené
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Increasing crop production to ensure future food security while reducing environmental pressure on agro-ecosystems requires improved nutrient and water use efficiency. The soil microbial community directly and/or indirectly has important consequences on food security since these microorganisms participate in several soil processes. Genetically modified maize, a product of biotechnological advancement that addresses agricultural challenges related to yield losses and pests was adopted as a sustainable solution. Despite these advances, the insecticidal proteins expressed by Bt maize may alter soil functions and microbial communities associated with rhizosphere soil. The application of arbuscular mycorrhizal fungi and vermicompost, due to its innate biological, physiochemical and biochemical properties has been suggested as a possible solution to combat potential negative impact of Bt maize and can be indirectly involved in controlling plant pathogens, nematodes and other pests. This study investigated the impact of vermicompost application on rhizospheric microbial communities and arbuscular mycorrhizal fungal colonisation of Bt and non-Bt maize in agricultural soils. In addition, rhizosphere soil samples were also collected from Bt and non-Bt maize fields and analysed for chemical composition, enzyme activity and community ecology. It was observed that nitrate and phosphorus concentrations were significantly higher in non-Bt maize dryland soils, while organic carbon was significantly higher in non-Bt maize irrigated field soil. Acid phosphatase and β-glucosidase activities were significantly reduced in soils under Bt maize cultivation. The bacterial diversity analysis showed no differences in species abundance or richness between Bt and non-Bt maize treatments for all samples. Evaluation of microbial communities showed Actinobacteria, Proteobacteria, and Acidobacteria to be the dominant phyla. Differences in the abundance of some genera, including Acidovorax, Bacillus, Flavobacterium, Paenibacillus and Pseudomonas, whose species are known plant growth promoting bacteria were observed between Bt and non-Bt maize treatments. Redundancy analyses indicate that chemical properties, enzyme activities and bacterial diversity were mostly related to the different amendments and growth stages rather than the effect from genetic modification of maize. The differences were more pronounced between the diversity and abundance of particular species, rather than the species richness of the maize bacterial community. Investigation of the potential effect of vermicompost application in the elimination or alleviation of the negative impact of genetic modification on the interaction between arbuscular mycorrhizal fungi and Bt maize showed that maize dry matter, chemical properties, enzyme activities and mycorrhizal root colonisation in maize were significantly improved by the co-application of arbuscular mycorrhizal fungi and vermicompost. The findings were in comparison to treatments without the addition of vermicompost. However, caution should be exercised in the interpretation of results obtained in this study because it is possible that the presence of the Cry protein in Bt maize plants could have contributed to the differences observed.