**6.2 Effects of soil pH and Eh on methane production in paddy field**

Soil pH is another influential factor in CH4 production. The pH effects on CH4 production in a flooded rice soil. Methanogenic bacteria are acid sensitive. Generally, the optimum pH for methanogenesis is 7.0. Introducing the acidic materials frequently results in a decrease in CH4 production [39]. A slight decrease in soil pH can cause decreases in CH4 production. A slight increase in soil pH (about 0.2 unit higher than the natural soil suspension pH) resulted in an enhancement of CH4 production by 11 to 20% and 24 to 25% at controlled Eh of −250 and − 200 mV, respectively [40]. These results suggested that a small reduction of soil pH could be obtained a decrease in CH4 production in paddy soil.

**Figure 3.**

*Effects of temperature on CH4 production in paddy soil shown in each figure. Periods of soil sampling: (A) continuous flooding (B) intermittent irrigation (C) from harvest to winter [38].*

#### **6.3 Effects of paddy cultivar, root and rhizosphere on methane production**

The decomposition of organic matter by methanogens under anaerobic condition leads to the production of CH4. Rice cultivars have significant effects on CH4 production of planted soil due to large variation in composition and content of root exudates [41]. The roots of rice plants are colonized by methanogenic Archaea, which performs the last step in the production and emission of CH4 [42–45]. It has previously been shown using pulse labeling of rice plants with 13CO2 that plant photosynthesis accounts for more than 50% of total CH4 emission [45, 46] and that a particular group of methanogens, the Rice Cluster I is responsible for the production of CH4 from rice photosynthates [47]. It is well established that the type of rice cultivar affects the CH4 production from rice fields [48]. In the randomly determined CH4 production experiment found the initial production of CH4 was much larger on roots grown in the rice field (RF) than in low-level river bank soil (LL), as shown by factorial ANOVA (P < 0.0001), while the effect of the different rice cultivars was not significant (*P = 5* 0.7401). The randomly determined CH4 production rates during the initial 3–8 days activity of the methanogenic community are shown in **Figure 4a** [49]. The rates of CH4 production also increased

**77**

**Figure 4.**

between 4 and 52% [45].

**6.4 Effects of paddy growing stages on methane emission**

More than 90% of the total CH4 emitted during the cropping season of the rice plants. Emission through rice plants may be expected to show great seasonal variations as a function of changes in soil conditions and variations in plant growth. The CH4 emissions varied during the growth period of the rice plant. The CH4 emission

*Methane Cycling in Paddy Field: A Global Warming Issue*

with incubation time, those of roots grown on LL soil after a prolonged lag phase, but in a late phase, arbitrarily chosen after 17–28 days, reached a similar value in all different root samples assayed (**Figure 4b**) [49]. The root morphophysiological traits were positively and significantly correlated with grain yield, whereas root length, specific root length, root oxidation activity, root total and active absorbing surface area were negatively and significantly correlated with total CH4 emission [10]. However, rice plants can enhance CH4 production by providing substrates for methanogenesis through the production of root litter and root exudates that contain carbohydrates and amino acids [50]. These nutrients stimulate microbial activities and lead to an increase in CH4 production [51, 52]. In terms of the substrates, root exudates, especially in the form of acetate, represented a considerable source of CH4 production at the rice ripening stage [53]. The CH4 emission peak occurring at the rice ripening stage because this stage attributed to the easily decomposable organic matter exuded from roots [54]. The rhizospheric CH4 oxidation at the different rice growth stages in detail. Under laboratory conditions, 29% of CH4 oxidation in the rhizosphere was found one week before panicle initiation and no CH4 oxidation occurred at the rice ripening stage. The CH4 oxidation in the rhizosphere at the harvest stage may be negligible in the field [55]. The rhizospheric CH4 oxidation rate varied with rice growing stages, being lower at the tillering stage (36.5% of CH4 produced in rice rhizosphere) than at the panicle initiation stage (54.7%). This may be related to the oxidizing activity of rice roots varying with the rice growth stage [50]. At the late tillering stage, root exudates dominate CH4 production of planted soil [41, 56]. This would probably be due to the stimulating effect of plant roots on O2 released during the decomposition of soil organic carbon exceeding CH4 oxidation in the rice rhizosphere [41, 57]. The relative contribution of root-associated CH4 production to CH4 emissions could be important in the rice paddies, as it varied

*Rates of CH4 production on excised roots from eight different rice cultivars retrieved from microcosms consisting of RF soil (open bars) or LL soil (hatched bars). The rates of CH4 production were determined* 

*(a) after 3–8 days of incubation and (b) after 17–28 days of incubation [49].*

*DOI: http://dx.doi.org/10.5772/intechopen.94200*

*Methane Cycling in Paddy Field: A Global Warming Issue DOI: http://dx.doi.org/10.5772/intechopen.94200*

**Figure 4.**

*Rates of CH4 production on excised roots from eight different rice cultivars retrieved from microcosms consisting of RF soil (open bars) or LL soil (hatched bars). The rates of CH4 production were determined (a) after 3–8 days of incubation and (b) after 17–28 days of incubation [49].*

with incubation time, those of roots grown on LL soil after a prolonged lag phase, but in a late phase, arbitrarily chosen after 17–28 days, reached a similar value in all different root samples assayed (**Figure 4b**) [49]. The root morphophysiological traits were positively and significantly correlated with grain yield, whereas root length, specific root length, root oxidation activity, root total and active absorbing surface area were negatively and significantly correlated with total CH4 emission [10]. However, rice plants can enhance CH4 production by providing substrates for methanogenesis through the production of root litter and root exudates that contain carbohydrates and amino acids [50]. These nutrients stimulate microbial activities and lead to an increase in CH4 production [51, 52]. In terms of the substrates, root exudates, especially in the form of acetate, represented a considerable source of CH4 production at the rice ripening stage [53]. The CH4 emission peak occurring at the rice ripening stage because this stage attributed to the easily decomposable organic matter exuded from roots [54]. The rhizospheric CH4 oxidation at the different rice growth stages in detail. Under laboratory conditions, 29% of CH4 oxidation in the rhizosphere was found one week before panicle initiation and no CH4 oxidation occurred at the rice ripening stage. The CH4 oxidation in the rhizosphere at the harvest stage may be negligible in the field [55]. The rhizospheric CH4 oxidation rate varied with rice growing stages, being lower at the tillering stage (36.5% of CH4 produced in rice rhizosphere) than at the panicle initiation stage (54.7%). This may be related to the oxidizing activity of rice roots varying with the rice growth stage [50]. At the late tillering stage, root exudates dominate CH4 production of planted soil [41, 56]. This would probably be due to the stimulating effect of plant roots on O2 released during the decomposition of soil organic carbon exceeding CH4 oxidation in the rice rhizosphere [41, 57]. The relative contribution of root-associated CH4 production to CH4 emissions could be important in the rice paddies, as it varied between 4 and 52% [45].

#### **6.4 Effects of paddy growing stages on methane emission**

More than 90% of the total CH4 emitted during the cropping season of the rice plants. Emission through rice plants may be expected to show great seasonal variations as a function of changes in soil conditions and variations in plant growth. The CH4 emissions varied during the growth period of the rice plant. The CH4 emission

#### *Agrometeorology*

low during the early growth stages of the rice plant [58]. This may be due to low levels of methanogenesis in this stage. And the high amounts of CH4 emissions were measured during the reproductive and ripening stages. This was happened probably due to the higher availability of fatty acids and sugars in this stage [58]. A significantly high amount of CH4 emission occurred at the reproductive stage of the rice plants in the paddy field (**Figure 5**) [59].

**Figure 5.** *CH4 emission pattern of different stages of rice growth [59].*
