Response mechanism of methanotrophs in dryland farmland soil under in situ mild cadmium stress.

Methanotrophs play an important role in mitigating methane emission in agricultural soils. However, the response of methanotrophs to low concentrations of cadmium (Cd) under actual field conditions remains unclear. This study analyzed samples collected from three fields with gradient Cd concentrations (0.3-1.2 mg kg-1). The results showed that low-concentration Cd had a significant effect on archaeal diversity, but no significant effect on bacteria. The aerobic methanotrophs dominated the methanotrophic community, with Candidatus Methylomirabilis and ANME archaea being the minority. Methylocaldum (Type Ⅰ) showed high relative abundance in high-Cd site. The abundances of proteobacterial pmoA gene and ANME-2d mcrA gene generally declined from the jointing to the filling stage during the wheat growth period, and significant differences were observed among sites. At the filling stage, pmoA and mcrA gene abundances at the high-Cd site were 67.56% and 79.93% lower, respectively, than those at the low-Cd site. Conversely, microcosm experiments showed an opposite trend. Exogenous addition of low-concentration Cd slightly increased the abundances of proteobacterial pmoA, ANME-2d mcrA, and NC10 phylum 16S rRNA genes. Correlation analysis revealed that sampling time and soil physicochemical properties exerted a much stronger influence on methanotrophs than Cd content. This suggests that the effect of in-situ low-concentration Cd on methanotrophs is negligible under actual field management conditions. This study provides a scientific basis for assessing greenhouse gas dynamics in Cd contaminated dryland farmland soils.