Methyl halide and chloroform emissions from a subsiding Sacramento-San Joaquin Delta island converted to rice fields

Net fluxes of methyl halides and chloroform were measured from recently converted rice fields on Twitchell Island, California, using field- and laboratory-based incubations. The rice paddies were found to be a net source for methyl halides and chloroform with an average of 450 ± 250 nmol m-2 d-1 for CH3Cl, 58 ± 31 nmol m-2 d-1 for CH3Br, 6700 ± 1920 nmol m-2 d-1 for CH3I and 128 ± 68 nmol m-2 d-1 for CHCl3. A stable isotope tracer method was used to demonstrate that the net emissions of CH3Cl and CH3Br during the growing season were predominantly the result of large gross production rates, with gross consumption rates being relatively minor. In agreement with prior studies, the production rates for methyl halides differed significantly at different rice plant growth phases, with the highest fluxes at the reproductive phase. The Twitchell Island rice field, however, had production rates of CH3Cl during the growth phases that were higher than rates reported at other rice fields, presumably because of higher soil chloride concentrations. Laboratory soil incubations showed that the rice field soils acted as a small net sink for CH3Cl (-12 ± 21 nmol m-2 d-1) and as small net sources for CH3Br (0.5 ± 0.7 nmol m-2 d-1), CH3I (7.6 ± 3.6 nmol m-2 d-1) and CHCl3 (10.0 ± 0.8 nmol m-2 d-1); halocarbon production rates were higher from post-harvest drained soil than from tillering stage water-logged soil. Despite the higher CH3Cl emissions during the growing season, the overall emissions of halomethanes from the conversion of all potential islands in the San Joaquin Delta to rice paddies is predicted to be an insignificant source of halomethanes.