Pathway of CH<sub>4</sub> production, fraction of CH<sub>4</sub> oxidized, and <sup>13</sup>C isotope fractionation in a straw-incorporated rice field

Abstract

Abstract. Straw incorporation generally increases CH4 emission from rice fields, but its effects on the mechanism of CH4 emission, especially on the pathway of CH4 production and the fraction of CH4 oxidized, are not well known. To investigate the methanogenic pathway, the fraction of CH4 oxidized as well as the stable carbon isotope fractionation during the oxidation and transport of CH4 as affected by straw incorporation, observations were conducted of production and oxidation of CH4 in paddy soil and rice roots and δ13C-values of produced CH4 and CO2, and emitted CH4 in incubation and field experiments. Straw incorporation significantly enhanced CH4 production potentials of the paddy soil and rice roots. However, it increased the relative contribution of acetate to total CH4 production (Fac) in the paddy soil by ∼10–30%, but decreased Fac-value of the rice roots by ∼5–20%. Compared with rice roots, paddy soil was more important in acetoclastic methanogenesis, with Fac-value being 6–30% higher. Straw incorporation highly decreased the fraction of CH4 oxidized (Fox) by 41–71%, probably attributed to the fact that it increased CH4 oxidation potential whereas CH4 production potential was increased to a larger extent. There was little CH4 formed during aerobic incubation, and the produced CH4 was more 13C-enriched relative to that of anaerobic incubation. Assuming δ13C-values of CH4 aerobically produced in paddy soil to be the δ13C-values of residual CH4 after being oxidized, (Fox-value still appeared to be 45–68% lower when straw was incorporated. Oxidation fractionation factor (αox) was higher with straw incorporation (1.033) than without straw incorporation (1.025). The δ13C-values of CH4 emitted after cutting of the plants (−50 to −43‰) were more positive than those of before (−58 to −55‰), suggesting a transport fractionation factor (ϵtransport) was −8.0‰ with straw incorporation and −12.0‰ without straw incorporation. Causes of this difference may be related to the diffusion process in transport as affected by growth of rice plants and pressure in the rhizosphere. The experiment shows that straw incorporation increases the contribution of acetate to total methanogenesis in paddy soil but decreases it on rice roots, and it significantly decreases the fraction of CH4 oxidized in the field and expands oxidation fractionation while reducing transport fractionation.

Oxidative mitigation of aquatic methane emissions in large Amazonian rivers

“…Therefore, we chose to use α values available in the literature, obtained from experiments from similar environments and temperatures compared to the rivers reported here, where the mean temperature was 29.9 °C. The range of α values (1.025 and 1.033) determined experimentally for different paddy environments at incubation temperatures between 26 and 28 °C (Tyler et al ., ; Zhang et al ., ) was used in both Eqns and . Because the waters studied here are oxic and poor in S and N species, anaerobic MOX in the water column is likely significant.…”

Comparison of isotope methods for partitioning methane production and soil C priming effects during anaerobic decomposition of rice residue in soil

“…Residue incorporation has been widely reported to increase CH 4 emissions in rice fields (Bossio et al, 1999;Chidthaisong and Watanabe, 1997;Sander et al, 2014;Zhang et al, 2013).…”

Seasonal Methane and Nitrous Oxide Emissions of Several Rice Cultivars in Direct-Seeded Systems

“…The site average seasonal CH 4 emissions at three of the four sites (CA-1, AR-1, AR-2) ranged from 34 to 70 kg CH 4 -C ha -1 season -1 , which is within the lower ranges reported for U.S. rice systems (Bossio et al, 1999;Fitzgerald et al, 2000;Sass et al, 2002;Pittelkow et al, 2013) (Table 5). Seasonal CH 4 emissions at CA-2, on the other hand, were particularly low (site average of 13 kg CH 4 -C ha -1 ), likely because this field had been fallow the previous 4 yr and residue inputs into this system were limited to weeds only, reducing C substrate and ultimately lowering CH 4 production potential in this soil (Zhang et al, 2013). Additionally, the field was not flooded during the extended fallow period.…”