Effects of plant invaders on rhizosphere microbial attributes depend on plant identity and growth stage

Invasive species are considered a serious threat to local biodiversity and ecosystem functioning. Invasive success is often dictated via a plant's capacity to influence belowground processes. We considered the role of two invasive plants - Solanum elaeagnifolium Cav, a perennial, and Conyza bonariensis L, an annual species - on the rhizosphere soil microbial communities and soil functionality. This study included the cultivation of each of the invasive species and bare soil as a control, in a mesocosm experiment. Rhizosphere soil was collected at three samplings during plant growth. Phospholipid fatty acids and the activities of beta-glucosidase, N-acetylglucosaminidase, urease, peroxidase, polyphenol oxidase and acid phosphomonoesterase were analysed. Soil planted with invasive plants exhibited different enzyme activities and microbial biomasses compared with controls. Also, the plant species had soil enzyme profiles that differed from each other, mostly at the vegetative stage, lesser at the maturity stage, with no difference recorded at the middle of the annual cycle. The differences between plant species were significant for microeukaryotes, actinomycetes and the fungi/bacteria and Gram-positive/Gram-negative bacteria ratios but not significant for total microbial biomass and biomasses of Gram-positive and -negative bacteria and fungi in all samplings. The network of interactions among the microbial community and enzymes was affected by plant presence, plant species as well as plant growth stage. This study suggests that life-cycle stage is important in relation to invasive species influence on belowground processes.