In the last sub-section, the effects of Eh control on CH4 emission and rice yield were reviewed from the results of three experiments. Although there remain some uncertainties of the result for other soil types than Andosol, soil Eh can be kept between the pre-determined range by Eh control disregard to other factors. Therefore, at the experimental level, Eh control is practical and can always decrease CH4 emission with keeping rice yield more than the conventional level.
The N2O emission with Eh control was not monitored. Similar to the conventional water management, N2O emission will occur during the drainage for Eh control. Hou et al. (2000) indicated that an intermediate range of soil Eh between approximately —100 to 200 mV would be sufficient to minimize the direct emission of CH4 and N2O. Is this range the best one for Eh control? This range would be effective in mitigating the direct CH4 and N2O emission. However, with this range, N2O production would occur in the flooded soil, and dissolved into the percolating water. Therefore, further studies are needed to determine the best Eh range that minimizes the direct and dissolved GHG emission from an irrigated paddy field.
Lastly, we will evaluate the effects of Eh control on the soil carbon budget. Table 14.6 shows the soil carbon budget under the different conditions of water management and rice straw application. Quantitatively, CO2 exchanges predominated in the soil carbon budget with a little amount of CH4 emission. The effect of water management on the total budget was fairly low. Of course, water management partially changed the CO2 exchanges, but these were offset by each other. Considering
Oxidative Soil Eh
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