Pathway of CH 4 formation in anoxic rice field soil and rice roots determined by 13C-stable isotope fractionation

Abstract

In anoxic rice fields methane is produced by either reduction of CO 2 or cleavage of acetate. We measured the δ 13C-values of CH 4 and CO 2, acetate and organic carbon during time course experiments with anoxic methanogenic soil and root samples and used these values to calculate the fractions of CH 4 (and acetate) produced from CO 2 reduction. Comparison with radiotracer and/or inhibitor studies constrained the kinetic fractionation factors used for calculation. The fractionation factors for the conversion of CO 2 to CH 4 and of acetate to CH 4 were on the order of α=1.07 ( ε = −70 ‰) and ε⩾−20‰, respectively. The pathway of CH 4 production changed with time of anoxic incubation. Anoxic slurries of rice field soil first produced CH 4 predominantly (>50%) from CO 2, then predominantly (>80%) from acetate and finally (after about one month) according to the theoretically expected ratio (33% CO 2 and 67% acetate). Anoxic rice roots, on the other hand, initially produced CH 4 exclusively from CO 2, followed by contribution of acetate of about 40–60%. Rice roots also produced acetate that partially originated (⩽30%) from reduction of CO 2 as determined by calculation of isotopic fractionation using fractionation factors from the literature. The results demonstrate that there is quite some variability in pathways of CH 4 production, and also indicate that isotopic fractionation factors may be different in different habitats and change with time.