Effects of Fallow Season Water and Straw Management on Methane Emissions and Associated Microorganisms

The effects of fallow season water and straw management on methane (CH₄) emissions during the fallow season and the subsequent rice-growing season are rarely reported, and the underlying microbial mechanisms remain unclear. A field experiment was conducted with four treatments: (1) fields flooded in both the fallow and rice seasons (FF), (2) fields drained in the fallow season and flooded in the rice season (DF), (3) FF with straw retention (FFS), and (4) DF with straw retention (DFS). The CH₄ emissions in fields under different water and straw treatments were monitored using the static closed chamber method. Methanogenic and methanotrophic communities in these fields were examined using terminal restriction fragment length polymorphism (T-RFLP) analysis based on the <i>mcrA</i> gene and <i>pmoA</i> gene encoding methyl coenzyme M reductase and particulate methane monooxygenase, respectively. The results showed that CH₄ emissions were significantly affected by water management, straw retention, season, and their interactions. Over 80% of CH₄ emissions occurred during the rice season. Field drainage during the fallow season reduced CH₄ emissions by 47.0% and 53.8% with and without straw during the rice season, respectively. Water management altered the abundance and composition of methanogens and methanotrophs, whereas the effects of straw retention were less pronounced. The quantitative polymerase chain reaction (qPCR) assay revealed that field drainage in the fallow season decreased the <i>mcrA</i> gene abundance by 30.0% and 23.2% with and without straw in rice season, respectively, and increased the <i>pmoA</i> gene abundance by 108.9% and 213.7% with and without straw in rice season, respectively. CH₄ flux was significantly positively associated with <i>mcrA</i> gene copy number and the ratio of <i>mcrA</i> to <i>pmoA</i> gene copy number, whereas it was significantly negatively correlated with the <i>pmoA</i> gene copy number. Results indicated that fallow drainage greatly decreased CH₄ emission not only during the fallow season but also during the subsequent rice season by altering the community composition of methanogens and methanotrophs. These findings provide scientific insight into the role of water and straw management in controlling CH₄ emissions through microbial community dynamics.