Wetland hydroperiod affects soil microbiome structure and function

Background/Question/Methods

Methane is a potent greenhouse gas with 100 times the climate impact of carbon dioxide; however, methane emissions are difficult to predict given the high level of spatial and temporal variability. Wetland ecosystems are responsible for nearly 30% of global methane release, with small, inland freshwater wetlands producing proportionately more methane than other wetland types. Methane emissions are a balance between production by methanogenic archaea and consumption by methanotrophic bacteria. These organisms are part of an interacting community of microorganisms within wetland soils and sediments that compete for electron donors and acceptors. In this study we examine the effects of wetland hydroperiod (length of time with standing water) on microbial community structure and porewater methane concentration. Surficial soil (0-5 cm depth) and porewater methane samples were collected seasonally from 15 wetlands over a three-year period. These wetlands fall into three broad hydroperiod classes (permanently flooded, seasonally flooded, and ephemerally flooded; n=5 each). Microbial community structure was examined using 16S sequencing. DNA was extracted using Qiagen DNEasy Soil kits and sequenced following Earth Microbiome protocols. Porewater methane concentrations were analyzed using gas chromatography with a flame ionization detector.

Results/Conclusions

Microbial community composition was significantly different among wetlands with different hydroperiods (ANOSIM R=0.56, p=0.001). Shannon’s diversity was also significantly different between seasonal and ephemeral wetlands (p=0.04). Further, examination of a non-metric multidimensional scaling ordination showed that microbial communities from seasonal wetlands were more distinct from communities from either ephemeral or permanently flooded wetlands than communities from those wetland types were from each other. Finally, seasonal wetlands showed the greatest within site variation in microbial community composition across sampling dates. Porewater methane concentrations were consistently higher in wetlands with longer hydroperiods and little methane was present in soil porewater when seasonal and ephemeral wetlands were wet but not flooded. Microbial communities were affected by hydroperiod, with wetlands experiencing both a long flood and long dry period having the highest Shannon’s diversity and greatest variation in species composition across sampling dates. This impact of hydroperiod was measured examining the overall community (i.e. not just active community members), suggesting that shifts in microbial community composition in response to dynamic redox conditions are both rapid and pronounced. Methane production was limited in wetlands during periods without standing water, demonstrating the sensitivity of both the microorganisms and processes to hydroperiod.