Effects of nitrogen fertilization on CH 4 emissions from rice fields

Nitrogen fertilization is essential for achieving high rice yields and is widely practiced in rice cultivation. There is an ongoing discussion on the possible effects of N application on CH4 emission from rice fields. CH4 emission is a net consequence of CH4 production, oxidation and transport from the soil in which the CH4 is produced to the atmosphere, and the interactions among these processes. Nitrogen influences all the processes of CH4 emission from rice fields either directly or indirectly and the effects are either negative or positive at the ecosystem, microbial and biochemical level. Because of this complexity and counter-balance among the effects, it is difficult to assess the net N effect on CH4 emissions from rice fields on a national or global scale. Field measurements also show a contradiction in that positive, negative and no effects of N application on CH4 emissions have been observed. Nevertheless, it is clear that the effect of N application on CH4 emission is N-form dependent. Nitrate-based fertilizers are able to mitigate CH4 emission, but they are rarely applied to rice fields and generally not practicable to mitigate CH4 emission because of their low use efficiency and stimulatory effect on N2O emission. In contrast, the application of organic N stimulates CH4 emission because additional organic carbon is supplied for CH4 production. However, it is not sufficient to conclude that CH4 emission intensity would have been decreased by the replacement of organic N, which dominated traditional rice cultivations, with chemical N fertilizers, which are used in current rice cultivations, because the fertilizer replacement has also enhanced rice yields, which in turn affects CH4 production, oxidation and transport. Establishing quantitative relationships between N status in soil and CH4 production, oxidation and transport is essential to assess the effects of chemical N fertilizer application on CH4 emissions from rice fields.