There are several reasons for the difference between our estimations and those of the GIO (2007). Our framework includes disposal of livestock excreta but not burning of excreta and crop by-products. N volatilization rate during excreta handling also differed greatly (50% to 85% in this study and 10% to 30% on GIO). These factors would have made our estimation of indirect emissions by N deposition that account as indirect emission from livestock production larger than that of GIO, although GIO did not indicate indirect emissions. Because the emission factor for indirect emissions is larger than that for direct emissions from farmland (except tea), estimated N2O emissions would be larger than GIO. Because our estimation of the basic unit of livestock N excretion was larger than that of the GIO (2007), our estimation of N excreta-related N2O emissions was greater than that of the GIO's. Because N volatilization was much higher than GIO, residual N as manure was less than half of GIO. Chemical N fertilizer application was sometimes estimated to be lower than chemical N fertilizer demand (e.g., about "10% in 2005). Lower N input to farmland caused lower direct emission than GIO and N infiltration, and lower indirect emissions than the GIO's estimation on crop production. These differences in estimations resulted from differences in the basic concept and framework established to reflect real Japanese agricultural practices.
Our estimation contains many uncertainties, such as the emission factor itself. Akiyama et al. (2006) reviewed the Japanese native emission factor, therefore these values might relatively be reasonable than IPCC default value. Because little research has been done on indirect emissions, we used the IPCC default emission factor. The other factors, such as N leaching rate (30% of chemical N fertilizer and manure N input), were also uncertain. Because paddy rice fields have an infiltration soil layer and are waterlogged during summer, N leaching by water would be low. Because tea is generally grown on sandy soil, N leaching would be high. Methods of livestock excreta handling related to direct N2O emissions might have changed during the period, but we used data from the 2001 surveys by Japan Livestock Technology Association, because no other appropriate data were available. These uncertainties should be resolved by future research.
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