Microbial processes influencing methane emission from rice fields

Irrigated rice fields are an important source of atmospheric methane. In order to improve our understanding of the controlling processes, we measured in situ CH₄ emission and CH₄ oxidation in an Italian rice field in 1998 and 1999, and studied CH₄ production in soil and root samples. The CH₄ emission rates were correlated with diurnal temperature variations and showed pronounced seasonal and interannual variations. The contribution of CH₄ oxidation to total CH₄ flux, determined by specific inhibition with difluoromethane, decreased from 40% at the beginning to zero at the end of the season. The stable carbon isotopic composition of the emitted CH₄ also decreased. The CH₄‐oxidizing bacteria probably became limited by nitrogen as indicated by the seasonal decrease of NH₄⁺. Thus, CH₄ oxidation had little effect on CH₄ emission. Methane production on rice roots was relatively constant over the season. Methane production in soil slowly increased after flooding and was highest in the middle of the season. Pore water concentrations of CH₄ showed a similar seasonal pattern. In 1999, CH₄ production increased later in the season and reached lower rates than in 1998. An additional drainage in 1999 resulted in higher ferric iron concentrations, higher soil redox potentials and lower acetate concentrations. As a result, acetate‐utilizing methanogens were probably out‐competed by iron‐reducers so that a larger percentage of 2–¹⁴C]acetate was converted to ¹⁴CO₂ instead of ¹⁴CH₄. The residual CH₄ production was relatively low and was mainly due to H₂/CO₂‐dependent methanogenesis. Experiments with radioactive bicarbonate and with methyl fluoride as specific inhibitor showed that the theoretical ratio of 7:3 of methanogenesis from acetate vs. H₂/CO₂ was only reached later in the season when total CH₄ production was at the maximum. In conclusion, our results give a mechanistic explanation for the intraseasonal and interannual differences in CH₄ emission.