Soil CO2-fixation by cbbL-harboring microorganisms in different types of permafrost peatlands

Permafrost peatlands serve as crucial reservoirs of soil carbon (C). Clarifying the mechanisms of microbial-driven soil C-fixation within the different types of permafrost peatlands is essential for understanding their C sink function. In this study, 13 C isotopic labeling DNA stable isotope probing ( 13 C-DNA-SIP), and high-throughput sequencing techniques were used to investigate the microbial C-fixation rate and the C-fixation microbial community in different soil layers of continuous permafrost (CP), discontinuous permafrost (DP) and seasonally frozen (SF) peatlands. The measured microbial C-fixation rates were in the range of 2.76 to 74.86 mg C m −2 day −1 . The results revealed that microbial C fixation was significantly affected by both permafrost types and soil depth. In CP and DP peatlands, soil microbial C-fixation rate and RubisCO activity in the active layer (0–10 cm) were significantly higher than those in the transition (40–50 cm) and permafrost (70–80 cm) layers, reflecting permafrost degradation will increase the soil microbial C-fixation. However, in SF peatland, microbial C-fixation rate was highest in the soil depth of 40–50 cm. Furthermore, high-throughput sequencing revealed the involvement of cbb L taxa including Comamonadaceae, Sphaerotilaceae, and Beijerinckiaceae families in 13 CO 2 assimilation in DP, CP, and SF peatlands, respectively. The relative abundance of these families exhibited a significant positive correlation with the microbial C-fixation rate. Structural equation modeling analysis showed the soil microbial C-fixation rates were primarily controlled by RubisCO activity. Whilst, the soil water content (SWC), total C, and the microbial biomass C served as the main positive factor via an indirect pathway, which was unexpected. Our findings quantitatively reveal the soil microbial derived C fixation in permafrost peatlands and highlight the driving mechanism of soil microbial C-fixation, offering key insights for increase C sequestration in permafrost peatlands. • Microbial C fixation was significantly affected by permafrost types and soil depth. • Soil microbial C-fixation rate in permafrost peatland was lower than in other peatlands. • Soil RubisCO activity had the greatest positive effect on microbial C fixation rate. • Permafrost degradation will increase the soil microbial C-fixation.