Wetlands can store a lot of carbon in soils, but wetland microbial respiration also releases a great deal of carbon dioxide (CO₂) and methane (CH₄). These gases combined are responsible for ~80% of the radiative climate forcing.  Understanding the controls on microbial respiration and the importance of different metabolic pathways has important climate change implications. Plants affect microbial respiration in wetlands by impacting the available soil carbon pool and the redoximorphic conditions of the soil environment. These plant impacts in turn affect microbial competition. In this study we were particularly interested in determining the role of soil carbon quality versus environmental factors in influencing the relative contributions of denitrification, iron reduction, sulfate reduction, and methanogenesis to overall microbial respiration in a freshwater tidal wetland (Jug Bay) and a brackish marsh (Jack Bay) on the Chesapeake Bay, USA. We collected soils from each site, homogenized them, and buried samples at their original location or at the opposite location. A year and a half later samples were collected (October, 2008) and analyzed for the amount of respiration contributed by different metabolic pathways. 

Results/Conclusions

Overall microbial respiration rates were higher in the soil with the higher carbon content (Jack Bay average soil organic matter = 54%; Jug Bay = 18%). However, when normalized to soil carbon content, respiration rates were actually higher for the soil with lower carbon content at both locations (Jack Bay soils total carbon respired = 4.4 µmols per g soil C per day; Jug Bay soils = 7.4 µmols per g soil C per day). These results suggest that carbon quality, more than quantity or environmental factors such as sulfate availability, drives microbial respiration rates. We conclude that plant carbon inputs to soils have a lasting legacy on microbial competition in wetlands.