Background/Question/Methods   Microorganisms are integral components of all ecosystems. This is especially true in the sediments of freshwater marshes, where these communities are associated with biogeochemical processes that can influence elemental cycles at a global scale. Despite their importance, relatively little research has been conducted to determine the environmental parameters that constrain the distribution of microorganisms in freshwater marshes. This study examined temporal changes in microbial communities in an emergent marsh along the James River (Virginia), and sought to determine: (1) How much do sediment microbial communities change during early marsh succession? and (2) To what extent do these changes correlate with environmental parameters? Plots were established in two distinct areas of the marsh, which differed dramatically in terms of hydrology and vegetation, and monitored for two years. Terminal Restriction Fragment Length Polymorphism (T-RFLP) DNA fingerprinting was used to separately profile the bacterial and archaeal portions of the microbial community. Results were compared to several soil properties (e.g., moisture, pH, redox, and organic matter) and attributes of the plant community.

Results/Conclusions   Strong seasonal patterns were observed for both the soil parameters and the wetland vegetation, and both types of data were correlated with successional changes in the soil microbial community. However, the nature of the relationships was dependent on sampling depth and location within the marsh. In the top 10 cm of soil at the wet site, bacterial community composition was strongly correlated to changes in the diversity of the aboveground vegetation, while the bacterial community at the dry site varied primarily in response to soil moisture and redox status. In the deepest soil profile samples, bacterial communities at both sites were correlated to changes in redox status and pH. The archaeal communities were more diverse at the wet site than at the dry site, and this diversity was correlated to soil moisture and redox status. Additionally, archaeal communities exhibited greater diversity with depth at both sites. This study reinforces the importance of understanding temporal patterns and environmental controls on microbial community structure and function, which are essential to preservation of overall ecosystem function in marsh habitats.