Department of Earth System Science

Rice paddies contribute significantly to the atmospheric burden of CH₄ but may also sequester atmospheric CO₂. Previous studies based on putative relationships between net CO₂ exchange and CH₄ emissions have concluded that globally significant amounts of carbon can be stored in rice paddy soils. However, the annual ratio of CH₄ emissions to net CO₂ exchange has not previously been measured. We simultaneously measured the net exchange of CO₂ (F _CO2) by eddy covariance and the CH₄ emission rate (F _CH4) using a combination of a flux gradient technique and weekly chamber sampling. During rice growth, F _CH4 was 1.9% to 2.4% of net carbon uptake (mole per mole). F _CO2 closely followed biomass accumulation. In contrast, F _CH4 increased during vegetative rice growth and decreased over the ripening and reproductive phases of rice growth, suggesting that the plants release substrate for methanogenesis early in the season. CH₄ emissions represented 4.8% to 5.6% of the net CO₂ uptake when summed over an entire year (including a 20-week period over which the field was unplanted and flooded). Assuming harvested rice is remineralized within 1 year, the remaining 0.67 t C ha⁻¹ that was sequestered by the paddy potentially offsets the radiative forcing of the emitted CH₄ by 26% to 31%. The ratio of F _CH4 to F _CO2 varied widely over the course of a year depending on management practices in a specific field. The results reported here emphasize the importance of year-round measurements to obtain a reliable estimate of CH₄/CO₂ exchange stoichiometry.