Responses of the methanogenic pathway and fraction of CH4 oxidization in a flooded paddy soil to rice planting

Rice planting (RP) is significant to methane (CH₄) emissions from paddy fields, but its effect on the relative contribution of the acetoclastic methanogenesis to total CH₄ production (F_ac) and the fraction of CH₄ oxidized (F_ox) is poorly understood. To quantify the responses of the F_ac and F_ox to RP, we investigated CH₄ fluxes, CH₄ production and oxidation potentials, dissolved CH₄ concentrations, and their stable carbon isotopes in a flooded paddy soil. The mcrA and pmoA gene copies were also determined by quantitative polymerase chain reaction (qPCR). Compared with the unplanted soil (control, CK), the seasonal CH₄ emissions from the planted soil were significantly enhanced, 13.6 times, resulting in large decreases in the CH₄ concentrations in the soil solution. This indicated that much more CH₄ was released into the atmosphere by the RP than was stored in the soils. Acetoclastic methanogenesis became more important from the tillering stage (TS) to the ripening stage (RS) for the CK, with F_ac values increased from 17%–20% to 46%–55%. With RP, the Fac values were enhanced by 10%–20%, and it significantly increased the copy numbers of the mcrA gene at the four rice stages (TS, booting stage (BS), grain-filling stage (GS), and RS). Furthermore, the effect of the RP on the abundance of the mcrA gene was highly concurrent with the effect on the F_ac values. At the TS, the F_ox values at the soil-water interface were around 50%–75% for the CK, being 15%–20% lower than those of the RP in the rhizosphere. It increased to 65%–100% at the GS, but was reduced by 20%–30% after the RP. These differences might be because the copy numbers of the pmoA gene were significantly raised at the TS while lowered at the GS by the RP. This was further demonstrated by the strong correlations between the effect of the RP on the abundance of the pmoA gene and the effect on the F_ox values. These findings suggest that RP markedly impacts on the abundances of the mcrA and pmoA genes, affecting the pathway of CH₄ production and the fraction of CH₄ oxidization, respectively.