Abstract
The effects of elevated atmospheric CO2 concentration on soil microbial communities have been previously recorded. However, limited information is available regarding the response of methanogenic communities to elevated CO2 in freshwater marshes. Using high-throughput sequencing and real-time quantitative PCR, we compared the abundance and community structure of methanogens in different compartments (bulk soil, rhizosphere soil, and roots) of Calamagrostis angustifolia and Carex lasiocarpa growing marshes under ambient (380 ppm) and elevated CO2 (700 ppm) atmospheres. C. lasiocarpa rhizosphere was a hotspot for potential methane production, based on the 10-fold higher abundance of the mcrA genes per dry weight. The two marshes and their compartments were occupied by different methanogenic communities. In the C. lasiocarpa marsh, archaeal family Methanobacteriaceae, Rice Cluster II, and Methanosaetaceae co-dominated in the bulk soil, while Methanobacteriaceae was the exclusively dominant methanogen in the rhizosphere soil and roots. Families Methanosarcinaceae and Methanocellaceae dominated in the bulk soil of C. angustifolia marsh. Conversely, Methanosarcinaceae and Methanocellaceae together with Methanobacteriaceae dominated in the rhizosphere soil and roots, respectively, in the C. angustifolia marsh. Elevated atmospheric CO2 increased plant photosynthesis and belowground biomass of C. lasiocarpa and C. angustifolia marshes. However, it did not significantly change the abundance (based on mcrA qPCR), diversity, or community structure (based on high-throughput sequencing) of methanogens in any of the compartments, irrespective of plant type. Our findings suggest that the population and species of the dominant methanogens had weak responses to elevated atmospheric CO2. However, minor changes in specific methanogenic taxa occurred under elevated atmospheric CO2. Despite minor changes, methanogenic communities in different compartments of two contrasting freshwater marshes were rather stable under elevated atmospheric CO2.
Highlights
Atmospheric CO2 concentration has increased dramatically over the past 250 years, and is expected to continue to rise in the future (Denman et al, 2007)
The net photosynthetic rates in the C. lasiocarpa and C. angustifolia marshes were increased under elevated CO2 treatment compared with ambient CO2 treatment, irrespective of plant types
There appeared to be a greater abundance of the methanogenic archaeal mcrA gene in the rhizosphere soil than in the bulk soil and roots, especially in the C. lasiocarpa marsh (Figure 2)
Summary
Atmospheric CO2 concentration has increased dramatically over the past 250 years, and is expected to continue to rise in the future (Denman et al, 2007). Elevated atmospheric CO2 concentrations are predicted to affect plant growth and alter soil properties. Atmospheric CO2 enrichment has been found to increase plant photosynthesis (Sasaki et al, 2005) and biomass (Hungate et al, 1997). These changes subsequently regulate the quality and quantity of the labile organic compounds that are secreted (Drigo et al, 2008) and influence downstream consumers. Wetland ecosystems play a pivotal role in global biogeochemical cycles and are a major source of atmospheric methane (CH4), having contributed 32% of the global annual CH4 emissions in the 2000s (International Panel on Climate Change [IPCC], 2013). Understanding the belowground process response to elevated CO2 in a wetland ecosystem may help to accurately evaluate ecosystem feedbacks to global climate change
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