The effects of ammonium sulfate application on methane emission and soil carbon content of a paddy field in Japan

Decreases in methane (CH4) emission from paddy fields are important for mitigating global warming. Some substrates such as soil carbon are used by methanogens for CH4 production. Thus, changes in CH4 emission may be accompanied by changes in soil carbon content. This study was carried out at an experimental paddy field in Agricultural and Forestry Research Center, University of Tsukuba, Japan. The experimental variables were rates of ammonium sulfate application (AS; 0, 45, 90, and 135 kg N ha−1) and levels of rice (Oryza sativa L., cultivar: Koshihikari) plant density (PD; 18.5 and 30.3 hill m−2). During the rice growing period, we measured CH4 emission by a closed-chamber method, and soil carbon content. Then, we considered the effects of the variables on CH4 emission and soil carbon content, and analyzed the quantitative relationship between them. There were no effects of PD level on CH4 emission and soil carbon content. This was likely due to the compensatory effect of rice plants. The CH4 emission decreased with increases in AS application rate, caused by competition for substrates between sulfate-reducing bacteria and methanogens. Decreases in soil redox potential (Eh) were delayed with increases in AS application rate. This process was another cause for the decreased CH4 emission at higher rates of AS application, but the reason for the higher soil Eh is not clear. Soil carbon content decreased with increases in AS application rate. Thus, there was a positive linear correlation between CH4 emission and soil carbon content (R2 = 0.977). Consequently, although AS application was effective to decrease CH4 emission, it simultaneously stimulated soil carbon decomposition. Carbon is stored in the soil in huge quantities, and is partly decomposed to carbon dioxide, also a greenhouse gas. Thus, it is necessary to consider not only CH4 emission but also soil carbon storage for mitigating global warming.