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 (F_ac) in the paddy soil by ∼10–30%, but decreased F_ac-value of the rice roots by ∼5–20%. Compared with rice roots, paddy soil was more important in acetoclastic methanogenesis, with F_ac-value being 6–30% higher. Straw incorporation highly decreased the fraction of CH₄ oxidized (F_ox) 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, (F_ox-value still appeared to be 45–68% lower when straw was incorporated. Oxidation fractionation factor (α_ox) 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 (ϵ_transport) 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.