Cadmium reduced methane emissions by stimulating methane oxidation in paddy soils.

Flooded rice paddy fields are a significant source of anthropogenic methane (CH4 ) emissions. Cadmium (Cd) is one of the most common and toxic contaminants in paddy soils. However, little is known about how the soil microbial communities associated with CH4 emissions respond to the increasing Cd-stress in paddies. In this study, we employed isotopically 13 C-labelled CH4 , high-throughput sequencing analysis, and gene quantification analysis to reveal the effect and mechanism of Cd on CH4 emissions in paddy soils. Results showed that 4.0 mg kg-1 Cd addition reduced CH4 emissions by 16-99% in the four tested paddy soils, and significantly promoted the transformation of 13 CH4 to 13 CO2 . Quantitative polymerase chain reaction (qPCR) demonstrated that Cd addition increased the abundances of pmoA genes, the ratios of methanogens to methanotrophs (mcrA/pmoA) showed a positive correlation with CH4 emissions (R2  = 0.798, p < 0.01). Furthermore, the composition of the microbial community containing the pmoA gene was barely affected by Cd addition (p > 0.05). This observation was consistent with the findings of a pure incubation experiment where methanotrophs exhibited high tolerance to Cd. We argue that microbial feedback to Cd stress amplifies the contribution of methanotrophs to CH4 oxidation in rice fields through the complex interactions occurring among soil microbes. Our study highlights the overlooked association between Cd and CH4 dynamics, offering a better understanding of the role of rice paddies in global CH4 cycling.