METHANE EMISSION FROM A WETLAND RICE FIELD AS AFFECTED BY SALINITY

The impact of salinity on CH4 emission was studied by adding salt to a Philippine rice paddy, increasing pore water EC to approx. 4 dS.m(-1) Methane emission from the salt-amended plot and adjacent control plots was monitored with a closed chamber technique. The addition of salt to the rice field caused a reduction by 25% in CH4 emission. Rates of methane emissions from intact soil cores were measured during aerobic and anaerobic incubations. The anaerobic CH4 fluxes from the salt-amended soil cores were three to four times lower than from cores of the control plot, whereas the aerobic CH4 fluxes were about equal. Measurements of the potential CH4 production with depth showed that the CH4 production in the salt-amended field was strongly reduced compared to the control field. Calculation of the percentage CH4 oxidized of the anaerobic flux indicated that CH4 oxidation in the salt-amended plot was even more inhibited than CH4 production. The net result was about equal aerobic CH4 fluxes from both salt-amended plots and non-amended plots. The data illustrate the importance of both CH4 production and CH4 oxidation when estimating CH4 emission and show that the ratio between CH4 production and CH4 oxidation may depend on environmental conditions. The reduction in CH4 emission from rice paddies upon amendment with salt low in sulfate is considerably smaller than the reduction in CH4 emission observed in a similar study where fields were amended with high-sulfate containing salt (gypsum). The results indicate that CH4 emissions from wetland rice fields on saline, low-sulfate soils are lower than CH4 emissions from otherwise comparable non-saline rice fields. However, the reduction in CH4 emission is not proportional to the reduction in CH4 production.