Organic manure rather than chemical fertilization improved dark CO₂ fixation by regulating associated microbial...

Dark microbial fixation of CO₂ is an indispensable process for soil carbon sequestration. However, the whole genetic information involved in dark CO₂ fixation and its influence on dark CO₂ fixation rates under diversified fertilization regimes were largely unclear. Here, revealed by ¹³C-CO₂ labeling, dark CO₂ fixation rates in upland red soils ranged from 0.029 mg kg^-1 d^-1 to 0.092 mg kg^-1 d^-1, and it was 75.49 % higher (P < 0.05) in organic manure (OM) soil but 44.2 % decline (P < 0.05) in chemical nitrogen fertilizer (N) soil compared to unfertilized (CK) soil. In addition, the normalized abundance and Chao1 index of dark CO₂ fixation genes (KO level) were significantly different between OM and N soils, showing the highest and lowest, respectively. And they were positively (P < 0.05) correlated with dark CO₂ fixation rate. Besides, among the identified CO₂ fixation pathways in this study, the DC/4-HB cycle (M00374) was enriched in OM soil, yet the 3-HP cycle (M00376) was enriched in N soil, and their relative abundances were positively and negatively correlated (P < 0.05) with dark CO₂ fixation rate, respectively. The PLS-SEM analysis revealed that dark CO₂ fixation-related functional traits (i.e. normalized abundance, Chao1 index and gene composition) were directly and positively associated with dark CO₂ fixation rate, and organic manure could exert a positive effect on soil dark CO₂ fixation rate through enhancing soil properties (e.g., pH and soil organic carbon) and further altering associated microbial functional traits. These results have implications for explaining and predicting the soil CO₂ fixation process from the perspective of microbial functional potential.