Soil organic carbon mineralization with fresh organic substrate and inorganic carbon additions in a red soil is controlled by fungal diversity along a pH gradient

Straw return and liming are important agronomic practices to enable farming in red soils of China. Organic substrates and inorganic carbon have an important effect on soil pH in red soils, but the mechanisms underlying soil organic carbon (SOC) mineralization by soil microorganisms due to pH change have been little studied. Therefore, we measured SOC mineralization, the priming effect, microbial biomass carbon (MBC), and bacterial and fungal community abundance, diversity, and composition during a 100-day incubation experiment. The experimental treatments included no exogenous addition (control), addition of C-14-labeled rice straw, addition of C-14-labeled CaCO3, and a combination of C-14-labeled rice straw and CaCO3. C-14-labeled CaCO3 significantly increased soil pH after 100 days of incubation, while C-14-labeled rice straw significantly decreased soil pH after 30 days of incubation. The priming effects per exogenous unit carbon with the additions of C-14-labeled rice straw and C-14-labeled CaCO3 were 0.057% and 0.021%, respectively. MBC was significantly higher under the C-14-labeled CaCO3 treatment than under the C-14-labeled rice straw treatment and combination treatment. There was no significant difference between C-14-labeled rice straw and the combination treatment. Soil bacteria and fungi were significantly more abundant in all the treatment groups than in the control, but both gradually decreased after 100 days of incubation. Bacteria and fungi showed higher diversity after 100 days than after one day of incubation. SOC mineralization was significantly directly affected by fungal diversity and indirectly affected by pH and bacterial diversity. It suggests that liming significantly increases soil pH and decreases fungal diversity, which benefits to SOC mineralization. These results also suggest that soil fungal communities dominate SOC turnover after liming to change soil pH, and soil fungi have more important effects on soil carbon cycling than do bacteria in acid soil.