Exogenous Carbon Addition Reduces Soil Organic Carbon: The Effects of Fungi on Soil Carbon Priming Exceed Those of Bacteria on Soil Carbon Sequestration

Soil organic carbon (SOC) forms the largest terrestrial organic carbon (C) pool, which is regulated by complex connections between exogenous C input, microbial activity, and SOC conversion. Few studies have examined the changes in natural abundance C due to microbial activity after exogenous C inputs in karst lime soils in China. In this research, the 13C isotope tracer technique was employed to investigate the priming effect of SOC on typical lime soil (0~20 cm) of 13C_litter and 13C_calcium carbonate (CaCO3) through a mineralization incubation experiment. Samples were collected at 5, 10, 20, 40, 60, and 80 days of incubation and analyzed for SOC mineralization, SOC distribution across fractions (>250 μm, 53~250 μm, and <53 μm), and soil microbial diversity. A control consisting of no exogenous C addition was included. SOC mineralization and SOC priming were considerably higher (15.48% and 61.00%, respectively) after litter addition compared to CaCO3. The addition of either litter or CaCO3 reduced the total organic C (TOC) and macroaggregate (>250 μm) and microaggregate (53~250 μm) C fractions by 2150.13, 2229.06, and 1575.06 mg C kg−1 Cbulk on average and increased the mineral particulate C fraction (<53 μm) by 1653.98 mg C kg−1 Cbulk. As the incubation time extended, a significantly positive correlation was apparent between SOC priming and soil fungal diversity, as well as between the mineral particulate C fraction and soil bacterial diversity. The effect of soil fungal diversity on SOC priming (R = 0.40, p = 0.003) significantly exceeded that of bacterial diversity on SOC sequestration (R = 0.27, p = 0.02). Our results reveal that after adding litter or CaCO3, soil fungi stimulate SOC mineralization and decomposition and soil bacteria enhance SOC sequestration, with the effects of fungi being more pronounced. These findings can provide a theoretical basis for understanding C sequestration and emission reduction in karst lime soils.