Impact of Phalaris arundinacea on wetland methane emissions and microbial community

Background/Question/Methods

Wetlands are significant sources of greenhouse gases, producing 20-39% of global methane emissions. A mechanistic understanding of wetland ecosystem functioning in the context of anthropogenic changes is essential to assess their impact on global climate. Among the least resolved factors likely to influence wetland functioning are plant abundance and species composition. We investigated the impact of Reed Canary Grass (RCG, or Phalaris arundinacea) on wetland microbial communities and methane emissions. RCG is a particularly aggressive invasive wetland plant in the Midwest, and little is known of its impact on wetland ecosystem function. We conducted experiments on an ephemeral prairie pothole and a permanent pond in central Minnesota.  RCG was abundant in both wetlands, but density varied substantially near the permanent wetland. We sampled soil and plant biomass from three sites of increasing distance from water around the ephemeral wetland, and from three sites of varying RCG cover around the permanent wetland. We measured soil methane production via anoxic laboratory incubations immediately upon sampling and after soils were saturated and incubated for four weeks. We ran PCR amplification on soil DNA extracts and used agarose gel electrophoresis to assess presence/absence of genetic markers specific to methanogen and methanotroph taxa.

Results/Conclusions

In the ephemeral wetland, methane production was high, averaging 17 µg CH4/ kg dry soil/hr in the initial incubation and increased significantly after incubation, with rates averaging 122 µg CH4/ kg dry soil/hr. Methane production also significantly increased with ambient soil moisture, likely due to fluctuating water levels, leading to more frequent occurrence of anoxic conditions. The number of methanogen and methanotroph taxa did not differ between sites around the ephemeral wetland. Methane production in sites near the permanent wetland was undetectable in the initial incubation. However, after incubation, the sites containing RCG showed a large increase in methane production, with rates averaging 110 µg CH4/ kg dry soil/hr. In contrast, rates in the barren sites changed very little with incubation and methane production remained nearly undetectable. Furthermore, the abundance of methanotroph taxa was significantly lower in sites with high RGC biomass. Our results suggest that RCG has the potential to increase methane emissions in permanent wetlands when conditions are favorable for methanogens, and may in fact reduce the number of methanotrophs. Given that RCG is an aggressive invasive wetland plant, further study is warranted to understand and quantify the effects RCG has on methane production and microbial community composition.