Methane dynamics and carbon dioxide exchange in a California rice paddy

Rice paddies are responsible for a significant fraction of the atmospheric CH₄ burden but may also sequester atmospheric CO₂. Previous studies based on putative relationships between net CO₂ exchange and CH₄ emissions have asserted 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 measured the net exchange of CO₂ by eddy covariance over a period of 2.4 years in a California rice paddy. In the final year of the study, methane emission rates were also measured using a flux gradient technique and weekly chamber sampling. Net CO₂ uptake over a ∼150 day growing season exceeded that of the ∼215 day fallow period by −414 to −466 g CO₂-C m−2. However, when the carbon lost by harvesting the rice and nocturnal stability artifacts were accounted for, this paddy was either close to a carbon balance or a source of atmospheric CO₂. Estimates of annual net CO ₂ exchange (including carbon lost via harvest) ranged from gains of ∼90 ± 10 kg C ha−1 to losses of 550 ± 18.7 kg C ha−1 depending on the integration period. Using an assemblage of all possible integration periods over the course of the study, we estimate the mean annual exchange of carbon to be 190 ± 380 t C ha−1 y−1 (loss to the atmosphere). During rice growth, CH₄ emission rates were negatively correlated to net CO₂ exchange and, on a molar basis, were 2% of the net CO ₂ uptake rate. Over an entire year, CH₄ emissions represented a higher proportion (7%) of the net CO₂ uptake.; In a separate study, we investigated the influence of rice straw management practices on CH₄ dynamics. Measurements of flux rates and δ 13C of emitted CH₄ were related to the size and δ 13C of soil CH₄ and CO₂ pools in fields where, following harvest, rice straw was either incorporated in to the soil or burned. We estimated the relative importance of methanogenesis pathways and methanotrophy on the quantity and δ13C of emitted CH₄. Methane emissions in straw amended plots were 2.8 times greater than in burned plots. High emission rates were associated with an increasing proportion of CH ₄ formed from acetate fermentation rather than H₂/CO ₂ reduction. The fraction of CH₄ oxidized did not exert an over-riding control on the amount of CH₄ released to the atmosphere.