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Keywords:

  • carbon sequestration;
  • global warming potential;
  • greenhouse gas;
  • methane;
  • redox potential;
  • sea level rise;
  • wetland

Abstract

This study was conducted at three locations in a bottomland hardwood forest with a distinct elevation and hydrological gradient: ridge (high, dry), transition, and swamp (low, wet). At each location, concentrations of soil greenhouse gases (N2O, CH4, and CO2), their fluxes to the atmosphere, and soil redox potential (Eh) were measured bimonthly, while the water table was monitored every day. Results show that soil Eh was significantly (P < 0.001) correlated with water table: a negative correlation at the ridge and transition locations, but a positive correlation at the permanently flooded swamp location. Both soil gas profile analysis and surface gas flux measurements indicated that the ridge and transition locations could be a sink of atmospheric CH4, especially in warm seasons, but generally functioned as a minor source of CH4 in cool seasons. The swamp location was a major source of CH4, and the emission rate was higher in the warm seasons (mean 28 and median 23 mg m−2 h−1) than in the cool seasons (both mean and median 13 mg m−2 h−1). Average CO2 emission rate was 251, 380 and 52 mg m−2 h−1 for the ridge, transition and swamp location, respectively. At each location, higher CO2 emission rates were also found in the warm seasons. The lowest CO2 emission rate was found at the swamp location, where soil C content was the highest, due to less microbial biomass, less CO2 production in such an anaerobic environment, and greater difficulty of CO2 diffusion to the atmosphere. Cumulative global warming potential emission from these three greenhouse gases was in an order of swamp > transition > ridge location. The ratio CO2/CH4 production in soil is a critical factor for evaluating the overall benefit of soil C sequestration, which can be greatly offset by CH4 production and emission.