Pathway of CH₄ production, fraction of CH₄ oxidized, and ¹³C isotope fractionation in a straw-incorporated rice field

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.