Effect of hydrological conditions on nitrous oxide, methane, and carbon dioxide dynamics in a bottomland hardwood forest and its implication for soil carbon sequestration

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 CO₂emission 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.