Clay mineral content modulates biogenic gas production in coal: divergent microbial responses in low- and medium-rank coals revealed by multi-omics.

The influence of clay mineral content on biogenic gas production in coal seams remains insufficiently understood. This study systematically investigated the mechanisms by which clay minerals affect biogas production in low- and medium-rank coals by integrating simulated biogas production experiments with multidimensional analytical techniques, including infrared spectroscopy, X-ray diffraction, scanning electron microscopy, gas chromatography-mass spectrometry, fluorescence spectroscopy, and metagenomic analysis. The results demonstrated that in low-rank coal, increasing the clay content from 2.78 to 4.75 g per 20 g of coal reduced the biogas yield from 6.30 to 3.47 mL/g. Conversely, in medium-rank coal, increasing the clay content from 1.66 to 2.65 g per 20 g of coal enhanced the biogas yield from 3.45 to 5.28 mL/g. These contrasting outcomes are primarily attributed to the distinct mechanistic roles of clay minerals across coal ranks. In low-rank coal, the hydration-induced swelling of clay minerals intensified pore blockage, impeded gas diffusion, decreased the abundance of genes involved in propionate degradation, and suppressed microbial metabolic activity, ultimately limiting methane production. In contrast, in medium-rank coal, clay minerals facilitated the enrichment of key functional microbial taxa, such as Acetobacteroides and Methanoculleus, promoted the degradation of fatty acids, hydroxyls, and amines, and enhanced the activity of acidogenic and methanogenic pathways, thereby increasing methane yield. This study elucidates the microbial mechanisms underlying the regulatory role of clay minerals in biogas production, offering new theoretical insights into the origin of coalbed methane (CBM) and providing a scientific foundation for optimizing biogenic CBM recovery.