Phragmites invasion slows carbon and nutrient turnover and alters microbial communities in a Great Lakes Coastal wetland

Background/Question/Methods Invasive species can dramatically alter ecosystem structure and function through direct and indirect pathways.  Concern has grown in recent years over the invasion of Great Lakes coastal wetlands by the common reed, Phragmites australis, which often replaces cattail-dominated communities.  Both Phragmites and Typha x glauca are large, fast-growing plants, with the potential to strongly control carbon and nutrient cycling.  In this study, we investigated how invasion by Phragmites alters wetland ecosystem structure and function (plant and microbial communities, surface and soil waters, and ecosystem processes related to carbon (C) and nutrient cycling) by comparing pre-invaded Typha stands and stands of Phragmites in an open coastal marsh located within the Detroit River International Wildlife Refuge on the northwestern shore of Lake Erie.  We measured plant diversity, biomass production, and annual rates of plant carbon and nutrient uptake.  We also sampled soil and surface waters bi-monthly throughout the ice-free season for dissolved nutrients, dissolved organic carbon (DOC), and dissolved CO₂ and CH₄.  Carbon turnover was assessed using C substrate utilization profiles and by measuring rates of plant litter decomposition, soil respiration, and extracellular enzyme activities.  We also characterized the soil microbial community (with qPCR and t-RFLP) to determine impacts on soil bacterial communities. 

Results/Conclusions Phragmites plots had lower plant species richness (p<0.05) and diversity (n.s.) and higher rates of plant biomass production and C, nitrogen, and phosphorus uptake compared to pre-invaded Typha communities (p < 0.05).  Dissolved C concentrations (as DOC, CO₂, and CH₄) were significantly lower in Phragmites soil and surface waters (p < 0.05).  The difference in dissolved CO₂ was more dramatic during the drier summer (2011), while the difference in CH₄ was greater during the wetter summer (2010).  There was no difference in specific UV absorbance (SUVA) between Phragmites and Typha soil and surface waters.  A principle components analysis revealed distinct differences in soil bacterial community composition between Phragmites and Typha sites.  Potential enzyme activities for phenoloxidase, beta-glucosidase, and xylosidase were significantly lower in Phragmites soil waters (p < 0.05); however, C substrate utilization profiles were similar between Phragmites and Typha soils, and differences in litter decomposition and soil respiration were not significant.  The results suggest that wetlands invaded by Phragmites may become sinks for C and nutrients, and that while direct effects of the plant community dominate this shift, indirect effects on the soil microbial community may also play a role altering ecosystem function.