Greenhouse Gas Emissions within Seasonally Flooded Tropical River Deltas

Abstract:

Soils contain the largest terrestrial carbon pool on Earth, and approximately one-third of soil carbon is stored in the tropics. Gas exchange between soil and the atmosphere occurs largely as a result of microbial degradation (mineralization) of organic carbon. The rate of soil organic matter (SOM) mineralization is determined by a combination of climatic factors and soil ecosystem properties, which dictate the dominant metabolic pathway(s) within soil at a given time; major changes in metabolic rate are particularly pronounced between aerobic and anaerobic mineralization. Here we assessed the impact of soil moisture, a major factor determining soil anaerobiosis, on greenhouse gas fluxes in a tropical, seasonally flooded wetland in the Mekong Delta. We monitored CO₂, CH₄, and N₂O gas fluxes, porewater chemistry, and soil moisture content in a seasonal wetland. Additionally, we collected wetland soil cores (10 cm diameter) and manipulated them in the laboratory, allowing us to control soil moisture and drying rates, and to simulate multiple periods of wetting and drying.

During drying, CH₄ fluxes within the wetland initially increase to a maximum before decreasing as soil moisture decreases and oxygen diffusion into the soil increases. Maximum CH₄ fluxes vary with moisture content, but the wettest sites produced fluxes >1000 mg C m^-2 d^-1 for short periods of time. As drying continues, CH₄ fluxes decrease to nearly zero, and N₂O fluxes begin to increase to ~3 mg N m^-2 d^-1 but do not appear to have reached a maximum before sampling ceased.

Gas flux from soil core incubations (n=5) exhibit trends and values similar to field measurements. CH₄ fluxes initially increase and reach >1000 mg C m^-2 d^-1 in cores while N₂O fluxes reach up to 10 mg N m^-2 d^-1 and decrease with continued drying. CO₂ fluxes in both field and laboratory are sustained until near desiccated conditions.

Seasonal wetlands are characteristic of large tropical deltas. Our findings provide a means to assess the effects of changing moisture regimes, which are being altered by upstream dam installation and local land use change, on greenhouse gas emissions.