The effects of closed circuit microbial fuel cells on methane emissions from paddy soil vary with straw amount

•Microbial fuel cells (MFCs) increase soil CH4 emission when straw amount exceeds 1%.•The effects of MFCs on CH4 emission vary with soil organic carbon content.•MFCs exhaust oxygen, decrease methanotrophs abundance and change their diversity.

Straw return increases methane emissions from paddy fields due to carbon input. In closed circuit microbial fuel cells (CC-MFCs), because exoelectrogenic bacteria can compete with methanogens for soil organic carbon, CC-MFCs should reduce methane emissions. We aimed to study the effects of CC-MFCs on methane emissions from paddy soil that was amended with different amounts of straw. Paddy soil was packed into the CC-MFCs and flooded after amendments with 0.5%, 1% and 2% straw or without straw addition. The methane flux from the soil was assessed every seven days. Forty-five days after the CC-MFCs were put into operation, the dissolved oxygen concentration in the overlaying water was determined. Soil was collected from the CC-MFCs, the chemical properties of the soil were measured and DNA was extracted. This was followed by the quantification and sequencing of the methanogenic mcrA and methanotrophic pmoA genes. The results revealed that with 0.5% straw, CC-MFCs generated less methane than open circuit MFCs (OC-MFCs) which were set as references. However, the CC-MFCs generated more methane than the OC-MFCs under 1% and 2% straw conditions. The dissolved oxygen concentration, which was lower in CC-MFCs than in OC-MFCs, decreased as the straw amount increased. The abundance of methanotrophs in CC-MFCs was significantly lower than that in OC-MFCs with 2% straw. Methylococcaceae, which belong to type II methanotrophic bacteria, were more abundant in CC-MFCs than in OC-MFCs. Thus, we suggest that the effects of CC-MFCs on methane emissions vary with the straw amount. With 0.5% straw, the competition for organic carbon between methanogens and exoelectrogenic bacteria mitigated the production of methane. However, CC-MFCs with 2% straw decreased the concentration of dissolved oxygen, inhibiting the activity of methanotrophs and increasing the emissions of methane.