Many researchers have reported that mid-season drainage and/or intermittent irrigation decrease CH4 emission without reducing rice yield (Sass et al. 1992; Yagi et al. 1996; Bronson et al. 1997; Wang et al. 1999). Well then, what about the effects on N2O and CO2 emission?
Generally, there is a trade-off relationship between CH4 and N2O emission (Fig. 14.5). Nitrous oxide has the high solubility of itself into water. Accordingly, almost all of N2O dissolves into water if produced in the flooded soil, and then emitted to the percolating water. Furthermore, under the strict reductive conditions, dissolved N2O is further denitrified to N2. Therefore, the direct N2O emission is usually observed at the non-flooded soil where CH4 production and emission have been already interrupted (Figs. 14.2b, 14.2c, and 14.5). Hou et al. (2000) reported that significant N2O emission only occurred when soil Eh was above +200 mV, which corresponds to the oxidative or semi-reductive conditions in the non-flooded soil. From the statistical analysis of available data, Akiyama et al. (2005) estimated the emission factor of fertilizer-induced N2O emission for continuous flooding and mid-season drainage at 0.22 and 0.37%, respectively. This result supports that mid-season drainage increases N2O emission as compared to continuous flooding.
N2O emission was also observed just after fertilization even during the flooded period when the field received nitrogen fertilizer more than 300 kg N ha-1 to Chinese paddy fields (Cai et al. 1997; Zou et al. 2005). It derives from the diffusion through the surface water where dissolved N2O concentration would be extremely high (Fig. 14.2a). On the other hand, with < 100 kg N ha-1 application to Japanese fields, N2O emission is hardly observed during the flooded period (Fig. 14.5).
The direct CO2 emission also occurs during the drainage (Fig. 14.2b). Similar to N2O emission, CO2 emission increases with the duration of drainage. However, CO2 emission (decomposition of soil organic carbon) as the direct and/or dissolved one, is inevitable whether under the flooded conditions or not. There were several cases that the GWP with mid-season drainage was higher than that with continuous flooding, such as when no straw was applied (Bronson et al. 1997) and when a large amount of chemical fertilizer or manure was applied (Cai et al. 1999). Therefore, a series of mid-season drainage and intermittent irrigation are effective in decreasing
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