Temporal variation of methanogenic pathways in rice fields under three different cropping systems.

Taking fresh soil samples from a rice–wheat rotation field (RW), a permanently flooded rice field (PF), and a double-rice cropping field (DR: DRE for the early rice; DRL for the late rice), we measured the CH4 production potential (MPP), the relative contribution of acetate-dependent methanogenesis (ƒac), soil properties, and methanogenic archaeal communities mainly in order to reveal the temporal variation and corresponding influencing factors of methanogenic pathways in paddy soils. Consistent with the change in dissolved organic C (DOC) content, the MPP generally decreased with rice growth in RW and DRL while increasing in PF and DRE. Based on the measurements of stable carbon isotopes, the estimated ƒac-value in PF dropped sharply from 54%-61% at the tillering stage to 30%-35% at the booting stage and rose again at the ripening stage. This variation pattern was positively correlated with that of acetate content, perhaps resulting from the activation of acetoclastic Methanosarcina. In contrast, the ƒac-value in RW and DR rose from 20%-44% at the tillering stage to 49%-59% at the ripening stage, possibly owing to the increase in the relative abundance of acetoclastic Methanothrix. The relative abundance of Methanosarcina in PF was 3%-4% higher than those in RW and DR, whereas that of Methanothrix was 3%-7% lower (P < 0.05). Soil acetate, DOC, and moisture contents significantly affected the methanogenic community composition. Our results demonstrate that the temporal variation of methanogenic pathways was influenced by the relative abundance of acetoclastic methanogens depending on the acetate level. [ABSTRACT FROM AUTHOR]