An additive effect of elevated atmospheric CO2 and rising temperature on methane emissions related to methanogenic community in rice paddies

Both elevated atmospheric carbon dioxide (CO2) and rising temperature can alter soil methane (CH4) fluxes, leading to a feedback to climate change. However, predicting this feedback needs to understand the microbial mechanisms involved in CH4 emissions driven by climate change. A 3-year field measurement of CH4 fluxes from rice paddies was taken in 2012-2014 to examine their responses to elevated CO2 (enriched up to 500 μmol mol−1) and rising canopy air temperature (above ambient 1.5-2.0 °C) using a free-air CO2 enrichment (FACE) system. Using real-time PCR and Illumina MiSeq sequencing of 16S rRNA genes, we measured the abundance and composition of methanogenic community in rhizosphere soil of rice paddies in 2014. Elevated CO2 and rising temperature showed additive effects on CH4 fluxes and methanogen abundances, where CH4 fluxes were correlated with methanogen abundances. Elevated CO2, rising temperature and their combination increased seasonal CH4 emissions by 28-120%, 38-74% and 82-143%, respectively. Either elevated CO2 or rising temperature did not significantly alter the diversity of methanogenic community, and methanogenic genera Methanosaeta, Methanosarcina, Methanobacterium, Methanocella and Methanoregula dominated in rhizosphere soils for all treatments. However, elevated CO2 induced a shift from acetoclastic to hydrogenotrophic methanogens in their relative abundances. Rising temperature stimulated CH4 emissions by increasing CH4 production per individual predominant methanogen genus. Besides the enhancement of soil C substrates and rhizosphere methanogen abundances as previously reported, an additive effect of elevated CO2 and canopy warming on CH4 emissions is also associated with elevated CO2-induced changes in the composition of methanogenic archaea and warming-stimulated the activity of methanogenic archaea in rice paddies.