Fixation of CO2 by soil fungi: contribution to organic carbon pool and destination of fixed carbon products

Dark microbial CO2 fixation plays a crucial role in soil organic carbon (SOC) sequestration, but the fungus contributions to this process in varying agroecosystems remain unclear. This study investigated fungal CO2 fixation in 29 soil samples collected from the major agricultural regions across China. Phospholipid fatty analysis-stable isotope probing revealed variations in fungal fixation by 2.2 to 65.5% of the total microbial CO2 fixation. CO2 assimilation of ten fungal strains belonging to the phyla Ascomycota and Basidiomycota was determined in three different media using an isotope labeling experiment. Trichocladium uniseriatum had the highest CO2 fixation capacity. T. uniseriatum had the reductive tricarboxylic acid cycle (rTCA) for CO2 assimilation associated with sulfite metabolism. T. uniseriatum was thus selected for use in soil inoculation experiments, aimed to trace the fractionation of its fixed carbon in SOC. The CO2 fixation rate of T. uniseriatum was 0.07-0.09 & mu;g C per g of soil per day. Notably, 77-82% of the fixed C was partitioned into the mineral-associated soil organic carbon. This study highlights the significance of fungal CO2 fixation in soil carbon sequestration.