Global assessment of soil methanotroph abundances across biomes and climatic zones: The role of climate and soil properties

The pmoA gene serves as a biomarker of the methane-oxidizing communities in soils. Therefore, much effort has been directed at quantifying the pmoA gene to quantify the abundances of methanotrophs at the local or regional scale, however, the abundances of methane-oxidizing bacteria as well as factors shaping the abundance of methane-oxidizing bacteria at the global scale remains poorly understood. To fill this knowledge gap, we per-formed a quantitative analysis of peer-reviewed publications to assess the distribution of the pmoA gene abundances across various biomes, climatic zones, land use, and between forest and grassland biomes, with 114 observations collected from 27 peer-reviewed articles. Results showed that the abundance of pmoA genes increased significantly with latitude, whereas a higher pmoA gene abundance was observed in the boreal forest than in cold grassland, dry grassland, temperate forest, and temperate grassland, while tropical grassland and the tropical forest did not differ significantly from other biomes in pmoA gene abundance. The PCA indicated that pmoA abundance was positively correlated with MAT and MAP but negatively correlated with pH. The TOC was as a key driver of pmoA gene abundance at a global scale. The abundance of the pmoA gene in tropical and boreal forests increased with TOC, while the pmoA gene abundance in temperate forests and grasslands decreased with TOC. The pmoA gene abundance in the tropical and subtropical zones increased with TOC while pmoA gene abundance in the temperate climatic zones decreased with TOC. The path model revealed a direct effect of vegetation cover on the pmoA gene abundance, but indirect effect of climate and soil properties on pmoA gene abundance via their direct effect on vegetation cover. We suggest that the changes of TOC and C:N ratio due changes of vegetation and climate across biomes may serve as a possible mechanism underlying the idiosyncratic effects of TOC on the abundance of methanotrophs in soils.