Abiotic correlates of microbial community structure and nitrogen cycling functions vary within wetlands

We investigated patterns in denitrifier and NH3-oxidizer community structure and function along environmental gradients in midwestern USA wetlands. Denitrifier and NH3-oxidizer functional groups have contrasting sensitivities to O-2 levels and other soil factors. Thus, variable environmental tolerance among and within functional groups can result in a range of microbial responses to the same environmental gradient. We compared microbial structure function relationships in 2 restored wetlands in Illinois and 2 natural wetlands in Michigan. At each site, we established 4 transects perpendicular to the shore and placed 4 plots along each transect. We analyzed denitrifier and NH3-oxidizer community structure (assessed by molecular analysis of functional genes nosZ and amoA) and potential denitrification and nitrification activities (function) across the soil hydrologic gradient. Soil moisture and physicochemical measurements, including organic-matter content, pH, inorganic N levels, and texture, varied along the gradient at each site. Denitrifier and NH3-oxidizer community composition and activity differed between upland and wetland plots at all sites. Denitrifier and NH3-oxidizer community composition were relatively constant through time, whereas microbial activity decreased at most sites from late spring to mid-August. Local environmental factors (e.g., soil moisture, texture) contributed more to microbial function than microbial community composition or functional gene abundance. NH3-oxidizer community composition influenced potential nitrification rates more than denitrifier community composition influenced potential denitrification rates. This result suggests one way that microbial community composition could affect microbial function. Our ability to predict microbial responses to environmental change can be improved by understanding the context-dependence of microbial structure function relationships.