Exogenous carbon addition reduces soil organic carbon: the effects of fungi on soil carbon priming exceed those of bacteria on soil carbon sequestration

Abstract

Abstract Aims Soil organic carbon (SOC) forms the largest terrestrial organic C pool, which is regulated by the complex connections between exogenous C input, microbial activity, and SOC turnover. Few studies have examined the changes in SOC due to microbial activity after exogenous C inputs in karst lime soils in China. We aimed to quantify the responses of soil organic carbon to exogenous carbon. Methods the 13C isotope tracer technique was employed to investigate the priming effect on typical lime soil of 13C-litter and 13C-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. Results 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 associated 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 associated 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). Conclusions Our results reveal that after adding litter or CaCO3, soil fungi stimulate SOC mineralization and decomposition and that 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.