Interacting effects of nutrient enrichment and simulated herbivory on methane emissions and methane associated microbial communities from four freshwater wetland plant species

Background/Question/Methods

Present day concentrations of methane (CH₄) in the atmosphere are unprecedented throughout the past 420,000 years of our planet. This poses a threat to environmental stability because of the power of CH₄ to alter climactic patterns. Wetland ecosystems are the largest natural source of global atmospheric CH₄ emission, emitting 100-231 Tg of CH₄ to the atmosphere per year. This represents 20-39% of global emissions, even as wetlands only constitute 5-8 % of worldwide land area. Large fluctuations in CH₄ emissions from wetlands are observed from year to year, yet the underlying factors controlling this variability remain unknown. Plant community disturbances such as herbivory have the potential to explain some of this variation as plant-mediated transport is the dominant means of methane release from wetlands. We tested the effects of simulated herbivory under three differing nutrient and clipping levels on methane emissions in freshwater microcosms using static flux chambers placed over small monospecific stands of four differing plant species (Sagittaria lancifolia , Panicum hemitomon, Echinochloa walteri , and Eleocharis macrostachya). Sediment samples were analyzed for the functional genes mcrA of methanogens and pmoA of methanotrophs by PCR-DGGE to detect any observable changes in methane associated microbial communities.

Results/Conclusions

CH₄ emissions were variable throughout the experiment ranging between 0.04 - 15.25 kg CH₄ ha^-1 d^-1. A significant three-way interaction between species and the two treatments of nutrient addition and simulated herbivory was detected via a mixed effects model. Of the four species tested, S. lancifolia and P. hemitomon responded to treatments in terms of methane emission. The interaction between nutrient and clipping level suppressed methane emission at mid-levels of N-enrichment, while high-level N-enrichment produced responses similar to that of untreated controls. Treatments where plants were clipped below the water line increased in mean methane emission after 3 days. Mean above and belowground biomass for all species did not differ significantly amongst nutrient treatment levels, but there was a strong negative linear relationship between belowground biomass and increasing nutrient treatment level. Diversity indices based upon relative intensity of bands in DGGE gels suggest that methanotrophic diversity may increase under simulated herbivory and with increasing N-enrichment. Suppression or facilitation of methane emission through plants induced by herbivory and modified by the available nutrient pool warrants further investigation as invasive and native herbivores occupy wetlands globally. Experiments investigating control over methane variability in wetlands may prove invaluable for future climate prediction models.