The effects of drying and re-wetting on methanogenesis in two midwestern wetlands

Background/Question/Methods

Wetlands are crucial parts of the landscape because they are hotspots for anaerobic and aerobic processing of materials, and because they vary widely from highly ephemeral “prairie potholes”, to permanent bodies of water.  The degree of permanence of water in wetlands strongly influences the structure and function of resident microbial communities because of the impacts of drying, re-wetting, and the intermediate exposure to the atmosphere on microenvironmental conditions.    Methanogenesis is particularly sensitive to oxygen and tends to be high only in areas that are rarely dried and exposed to aerobic conditions.  However, it has been shown that methanogens can survive and produce methane in chronically aerobic environments as well as those that exhibit strong temporal fluctuations in the degree of exposure to atmospheric oxygen.  It is important to gain a better understanding of the impacts of aerobic/anaerobic, and dry/re-wetted dynamics for the purposes of understanding sources and sinks of carbon and the production of greenhouse gasses.  The goals of this project were to determine how the magnitude and the spatial patterns of rates of methanogenesis change after drying, rewetting, and the intermediate exposure to the atmosphere within and between wetlands.

Results/Conclusions

We conducted a series of laboratory assays to determine the methanogenic potential of field moist and re-wetted soils from the margins of two wetlands in a restored prairie in South-Eastern Minnesota.   We found that after a period of drying and re-wetting that rates of methanogenesis were at least an order of magnitude higher (p<0.001, ANOVA) at all locations sampled.  We also found that the spatial distribution of methanogenesis in re-wetted soils was different, but not significantly so (p=0.06, ANOVA), especially in a wetland with a stable water level and not in a more ephemeral one.  However, we found that after drying and re-wetting the variation in these rates increased, and while methane produced under field moisture was correlated with the moisture of the soil (p<0.05, linear regression), there was no such relationship between field soil moisture and the rates of methanogenesis observed after drying and re-wetting.  These results suggest that the period of drying and exposure to the atmosphere differentially affects certain controls on the methanogens ability to produce methane such as substrate availability, enhancing them in some cases, and inhibiting in others.