Microbial ecology of restored floodplains

Background/Question/Methods: Freshwater flooding of terrestrial soils occurs naturally, but residential and commercial land use can exacerbate flooding and sedimentation of rivers and lakes, causing environmental and economic damage. Restoring private and public property to natural floodplains can help alleviate the damage due to flooding events, but little is known about the effect that periodically waterlogged soils have on microbial activity and the resultant greenhouse gas emission to the atmosphere. Because flooding these soil systems generates an anaerobic state due to heterotrophic oxygen consumption, the resulting change in microbial community structure and metabolism leads to CH₄ flux to the atmosphere. Substantial carbon (C) release from waterlogged soils to the atmosphere impacts atmospheric concentrations of carbon dioxide (CO₂) and methane (CH₄) (which has ~25x the greenhouse capacity of CO₂, and has steadily increased in the atmosphere for the last 200 years). Hence, we must quantify these interactions to understand their net effect on atmospheric radiative forcing of methane. The present study utilizes extensive 16S rRNA amplicon sequencing data to correlate floodplain sites at different ages since restoration (<5 years, 5-10 years, >10 years) to the microbial ecology particular to that site. Additionally, moisture, depth, terminal electron acceptor concentrations, pH, and methane flux are all examined to ascertain the complexities of microbial activity in response to land-use change. 

Results/Conclusions: Results from this study indicate that the variability of the microbial communities could largely be explained by site location, soil depth, C:N ratio, and pH. Microbial diversity was higher at older floodplains and decreased with deeper sampling depths. The latter correlated with a decrease in moisture, total carbon, and total nitrogen with depth. Additionally, Deltaproteobacteria OTUs were significantly enriched at lower depths compared to Verrucomicrobia and Bacteroidetes at higher soil depths. Furthermore, each site had certain taxa that were significantly enriched, laying the foundation for building site-specific microbial signatures. We propose that these signatures could eventually be used for determining floodplain health and predictive modeling of greenhouse gas emissions from waterlogged soils.