Effects of Phragmites invasion on soil microbial activity and structure in a brackish marsh

Background and aims: The spread of invasive plants in wetlands associated with human activity has become a serious environmental problem because of the negative effects of these species on biodiversity and biogeochemistry in ecosystems. Unlike their impacts on aboveground biodiversity, the responses of soil microbial communities and related soil characteristics to invasive plants are largely unknown. In this study, we assessed the structural and functional responses of soil microorganisms and belowground biogeochemistry to the invasion of Phragemites australis, which has heavily invaded in wetland areas globally, in brackish marsh areas in Korea. Methods: We measured soil biogeochemical characteristics including extracellular enzyme activities and microbial community structure (t-RFLP) in the marsh, both undisturbed and invaded areas over a year period. Results: We found higher extracellular enzyme activity in invaded areas compared to the undisturbed region dominated by the native species Scirpus planiculmis, and this response was profound during the growing season. Fungal and bacterial community structure, analyzed by terminal-restriction fragment length polymorphism, indicated that invasion by Phragmites had little effect on these communities. However, significantly higher microbial diversity was found in intermediately invaded areas in which Scirpus and Phragmites were co-dominant. Conclusions: This result suggests that microbial diversity was affected by plant diversity, rather than invasion by or presence of a particular species. Our results suggest that physicochemical conditions related to dominant plant species alter microbial activity, while plant diversity is a more important regulator of microbial community structure and diversity.