Eh increases to an upper limit. The repetition of this routine is the method of Eh control. The lower limit is established to inhibit CH4 production and emission. The upper limit is established to inhibit the excessive soil drying and also not to inhibit rice growth and yield. The change in soil Eh by Eh control is similar to that in soil moisture tension by AWD (Fig. 14.7).
The values for the lower and upped limit are variable. Generally, CH4 production begins at approximately -150mV and stimulated at lower values. Therefore, the lower limit has been empirically determined to be —150 mV. On the other hand, the upper limit has no correct solution. From the results of the preliminary pot experiment as described below, it is recognized that soil Eh should not be kept at over +500 mV for several days, which causes the excessive drying of soil and thus the reduction in rice yield. Moreover, soil Eh sharply increases after the drainage because of the crack formation on the surface soil, while the relatively slow decrease under the flooded conditions. Therefore, the upper limit has been expedientially determined to be +100 mV, in order not to reach the soil Eh +500 mV or so. It is desirable that the values of lower and upper limit are determined to prevent soil Eh from reaching the critical ones.
Soil Eh is measured by a common method using a portable Eh meter and platinum-tipped electrodes. Usually, the electrodes are inserted to a depth of 5 cm in the soil with 3-5 replications at a site. Although there is a variation within replications, soil Eh finally converges to almost similar value under the flooded conditions. On the other hand, during drainage, the crack formation causes a wide variation in soil Eh among replications. However, these are natural phenomena, and thus it is necessary to establish replications for monitoring the adequate values of soil Eh by the present method.
Surface soil level of the flooded field is usually uneven if puddled uniformly, and the surface is in a concave shape. Therefore, flooded water can not be drained entirely, indicating that soil Eh is also variable depending on the site within a field. The first drainage randomly forms the crack on the soil surface, and the following drainages promote the crack formation, where soil Eh is drastically fluctuated. Therefore, it is also necessary to select an adequate site for obtaining the representative value of soil Eh in the field.
14.4.3 Effects on CH4 Emission and Rice Yield 220.127.116.11 Practicability of Eh Control
Can Eh control decrease CH4 emission as compared to the conventional water management? Minamikawa and Sakai (2005) examined the effects of different kinds of water management on CH4 emission under the pot conditions with two paddy soils. The early stages of Eh control had only the lower limit (-150 mV), and re-flooding was conducted 1-2 days after drainage. Figure 14.9 shows the seasonal changes in soil Eh and CH4 flux with four kinds of water management at the Fluvisol pot. The Eh control had the distinctive change in soil Eh (Fig. 14.9a). Between the two soils, there was a wide difference in the frequency of the routine (the Fluvisol with high reductive potential > the Andosol with low reductive potential). Methane emission, measured by a closed chamber method, with Eh control was significantly lower than that with continuous flooding or mid-season drainage, and as low as that with intermittent irrigation (Table 14.3). Therefore, Eh control is effective to decreasing CH4 emission. However, Eh control caused the excessive soil drying, and thus significantly reduced rice yield (Table 14.3), indicating that the necessity of considering rice growth while deciding the upper limit of soil Eh.
Fig. 14.9 Seasonal changes in (a) soil Eh and (b) CH4 flux with four kinds of water management (modified from Minamikawa and Sakai 2005). MD -midseason drainage; FD -final drainage; IM -intermittent irrigation
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