The influence of no-tilled direct seeding cultivation on greenhouse gas emissions from rice paddy fields in Okayama, Western Japan : 5. Annual emission of CH_4, N_2O and CO_2 from rice paddy fields under different cultivation methods and carbon sequestration into paddy soils

In order to clarify any difference in the emission of methane (CH_4), nitrous oxide (N_2O) and carbon dioxide (CO_2), from two rice paddy fields under the continuation of no-tilled direct seeding cultivation (ND), two rice paddy fields under the conventional tilled transplanting cultivation (TT), and one rice paddy field under the tilled direct seeding cultivation (TD), we measured CH_4, N_2O and CO_2 fluxes from these fields with gray lowland paddy soils in Sanyoh of Okayama Prefectural General Agriculture Center, by using a closed chamber method for 2-5 years (1998-2002). We estimated the Net Ecosystem CO_2 Exchange (NEE) during rice-growing seasons by using a model based on the eddy covariance flux measurement made at a rice paddy field in Hachihama of Okayama University, a 23km-distance from Sanyoh. Consequently, the following results were obtained. 1) A clear trade-off relationship was shown between CH_4 and N_2O fluxes in all the rice paddy fields. 2) The sum of annual CO_2 equivalent emission of CH_4 and N_2O was a little bit higher in ND than in TT but not significantly different between ND and TT, possibly due to the long-term continuation of ND, although the emission of CH_4 was much lower in ND than in TT during a few or several years after TT was converted to ND. 3) The averaged annual emission ratio of N_2O to the sum of CH_4 and N_2O on the CO_2 equivalent basis was 9.0%, 7.3%, and 1.8% in ND, TD and TT, respectively. The highest ratio in ND was caused by sporadic enhancements of N_2O flux in fallow seasons due to an organic matter layer only formed on the surface soil of ND. 4) As a result, the N fertilizer-induced emission factor of N_2O in a three-year average was 0.48% and 2.5% in TT and ND, respectively.5) During fallow and no-flooded seasons, the CO_2 flux from the surface soil to the atmosphere was higher in ND than in TT. An annual NEE in TD was estimated to be -294 and -311g CO_2m^<-2>y^<-1> in 1998 and 1999, respectively. 6) The organic matter layer formed on topsoil increased with the continuation of ND, and the carbon sequestration rate to the surface soil was 86.2g C m^<-2>y^<-1>, equal to about 30% of the total annual CO_2 equivalent emission rate of CH_4 and N_2O. 7) According to the carbon neutral principle, the annual net CO_2 emission in ND was -268g CO_2m^<-2>y^<-1> by adding the CO_2 emission through the slow decomposition of soil organic matter to the carbon sequestration rate. 8) Hence, the total CO_2 equivalent greenhouse gas emission (the sum of CH_4, N_2O, and CO_2) in ND was 811 and 648g CO_2m^<-2>y^<-1> in 1998 and 1999, respectively, and which were lower by 20% than those in TT, mainly due to carbon sequestration to soils in ND 9) The conversion to TD or TT from ND with the continuation for 8 years, by incorporating the surface organic matter into the plowed layer, did not show any increase in the emission of CH_4 or N_2O during the following two years. 10) A possible and effective option for mitigation of greenhouse gas emissions in ND is that ND fields should be converted to TT or TD fields after the continuation of ND for 4-5 years.