GB Zhang Y Ji J Ma G Liu H Xu and K Yagi. Pathway of CH4 production fraction ofCH4 oxidized and 13C isotopefractionation in a straw incorporated ricefield. Biogeosciences Discuss. 2012 9: 14175–14215.
Abstract
Straw incorporation generally increases CH4 emission from rice fields but its effects onthe mechanism of CH4 emission especially on the pathway of CH4 production and thefraction of CH4 oxidized are not well known. To investigate the methanogenic pathwaythe fraction of CH4 oxidized as well as the stable carbon isotope fractionation during theoxidation and transport of CH4 as affected by straw incorporation production and oxidationof CH4 in paddy soil and rice roots and 13C-values of produced CH4 and CO2and emitted CH4 were observed in incubation and field experiments. Straw incorporationsignificantly 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 methanogenesiswith Fac-value being 6–30% higher. Straw incorporation highly decreased the fractionof 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 largerextent. There was little CH4 formed during aerobic incubation and the produced CH4was more 13C-enriched relative to that of anaerobic incubation. Assuming 13C-valuesof CH4 aerobically produced in paddy soil to be the 13C-values of residual CH4 afterbeing oxidized Fox-value still appeared to be 45–68% lower when straw was incorporated. Oxidation fractionation factor ( ox) was higher with straw incorporation (1.033)than without straw incorporation (1.025). The 13C-values of CH4 emitted after cuttingof the plants (−50–−43 ‰) were more positive than those of before (−58–−55 ‰)suggesting a transport fractionation factor (“transport) was −8.0‰ with straw incorporationand −12.0‰ without straw incorporation. Reasons for this difference may be related to the decrease in growth of the rice crop as a result of straw incorporation.The experiment shows that straw incorporation increases the contribution of acetate tototal 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 fractionationwhile reducing transport fractionation.
