Straw-mediated CH4 production via distinct methanogenic pathways in two paddy soils

Straw returning provides large carbon sources to paddy soils, stimulating CH₄ production and emissions. However, the effects of straw types (rice and maize, corresponding to C3 and C4 plants) on the methanogenic pathway and microbial mechanisms are still unknown. Here, two paddy soils [rice-wheat rotation soil (RW) and permanently flooded soil (PF)] were anaerobically incubated with rice straw (RS) and maize straw (MS) additions to investigate the response of methanogenic pathways and archaeal communities to straw addition. Our results showed that the produced CH₄ with MS addition was significantly enriched in ¹³C than that of others, with δ¹³CH₄-values ranging from −39.22 ‰ to −44.99 ‰ and − 48.31 ‰ to −53.19 ‰, respectively. Moreover, CH₄ production potential was increased by 95 %–100 % in RW soil and 161 %–207 % in PF soil with straw addition, compared with no straw addition (13.1–16.6 μg CH₄ g⁻¹ d⁻¹). This stimulation in RW soil was probably associated with the increase in hydrogenotrophic methanogenesis by straw addition. Indeed, the contribution of hydrogenotrophic methanogenesis to total CH₄ production (1– f_ac) increased from 55 % in control (CK) to 75 % in RS addition, and to 85 % in MS addition. Meanwhile, the relative abundance of hydrogenotrophic Methanobacterium increased from 10 % to 20 % (RS) and 25 % (MS), respectively. Straw addition significantly increased acetate consumption in RW soil, indicating that acetate might first transfer to CO₂/H₂ and then to CH₄ rather than directly to CH₄. For PF soil, the significant increase in CH₄ production by RS addition was also likely ascribed to the rise of 1– f_ac and hydrogenotrophic Methanobacterium. In contrast, MS addition mainly promoted acetoclastic methanogenesis to stimulate CH₄ production, with f_ac-values increased from 70 % to 80 % and relative abundance of acetoclastic Methanosarcina increased from 60 % to 70 %. The findings show that the differences in methanogens and carbon isotopic caused by straw types are key factors affecting methanogenic pathways, and further provide valuable insights into the methanogenic mechanisms under straw addition in different paddy soils.