Environmental variability shapes microbial community response to altered hydrology in an Illinois wetland

Background/Question/Methods

Soil microbial communities carry out ecological processes such as the transformation of nitrogen and carbon. Hydrology is known to affect the rates of these processes, but it is unknown how communities adapt to water level fluctuations. Climate change is expected to alter precipitation patterns, which may affect microbial processes. To study the stability of microbial ecosystems, floodplain soil from the La Grange Wetland Mitigation Bank in central Illinois was subjected to artificial water level manipulations. Soil samples collected from across the floodplain (i.e., upland or bottomland) were transferred to mesocosms. A subset of each sample was subjected to prolonged saturation or drying, or repeated cycles of saturation and drying to simulate hydrological regimes. Potential rates of denitrification, nitrification, and methanogenesis were quantified before and after, and we used culture-independent approaches to monitor changes microbial community composition. Communities were defined as stable if the structure remained constant, and no change was observed in the rates of nutrient cycling. Communities from regions with variable conditions were expected to be the most stable in response to disturbance because the organisms present should already be adapted, while those from stable (constantly flooded or dry) regions should be most easily disturbed, coupled with decreased function.

Results/Conclusions

We found that most soils originating from regions of the floodplain with variable flood frequency showed unchanged nitrification rates in response to fluctuating conditions, but were more sensitive to prolonged altered conditions. In contrast, the same soils showed increased potential denitrification rates with fluctuating hydrology. Nitrification and denitrification rates in the dry upland soils appear to be sensitive to altered environmental conditions because the rates decreased following both the prolonged saturation treatment and the alternating cycles of saturated and dry. Microbes in typically saturated floodplain soils were also sensitive to altered conditions, with decreases microbial activity following prolonged drying. Preliminary results show the highest methane production from dry upland soils following two months of saturation, and the majority of treatments resulted in higher potential methanogenesis rates from all soils. Shifts in soil response following each treatment was correlated with shifts in both the composition and relative abundance of microbial functional groups involved.  These data support the assertion that changed precipitation patterns have the potential to disrupt soil nutrient cycling. Understanding how microbial communities respond to this change has implications for predicting microbial processes that impact water quality and greenhouse gas production from soils.