[Methane Production Potential and Methanogenic Pathways in Paddy Soils Under Different Rice-based Cropping Systems]

Differences inmethane (CH₄) production potential in paddy soils under different rice-based cropping systems and especially in the methanogenic pathways (mainly acetate fermentation and CO₂/H₂ reduction) remain unclear. Anaerobic incubations of soil with or without fluoromethane (CH₃F) inhibitor (2% and 0%) were conducted. With the soils from three typical paddy ecosystems (rice-wheat rotation, RW; rice-fallow, RF; double-rice, DR) in China, the cumulative concentration of CH₄ production, CH₄ production potential, dissolved organic carbon (DOC) content, and acetic acid content were determined. Meanwhile, the relative contribution of acetate-dependent methanogenesis (f_ac) was quantified using the stable carbon isotope method. The results showed that the CH₄ production potential was 7.18 μg·(g·d)^-1 in RF, which was significantly lower than that in RW[10.33 μg·(g·d)^-1]and DR[13.42 μg·(g·d)^-1] (P<0.05). Correlation analysis showed that CH₄ production potential was significantly negatively correlated with soil cation exchange capacity and pH (P<0.01); the addition of CH₃F significantly inhibited CH₄ production (P<0.05). The content of DOC and acetic acid in DR were 255 mg·kg^-1 and 7.34 mg·kg^-1, respectively, which were 17%-51% and 22%-23% higher than those in RW and RF, respectively. The δ¹³CH₄ and δ¹³CO₂ values were affected greatly by different rice-based cropping systems, and the highest δ¹³CH₄ value was -43.89‰ in RF, which was more positive than that of RW and DR by 11.06‰ and 8.33‰, respectively (P<0.05). By contrast, the lowest value of δ¹³CO₂ was observed in RF, which was more negative than that of RW (7.63‰) and DR (5.14‰) (P<0.05). The α_(CO values of RW and RF were 1.057 and 1.058, respectively, which were significantly lower than 1.062 in DR (P<0.05). The f_ac values of RF ranged from 84% to 98%, being 34%-38% and 20%-23% higher than those of RW and DR, respectively (P<0.05).