Effects of Microbial Inoculant Application on the Decomposition of Organic Materials and the Priming Effect in Sandy Soils

The priming effect (PE) plays a critical role in regulating soil organic carbon (SOC) sequestration. Whether microbial inoculant combined with organic materials (e.g., crop straw) can reduce PE and enhance the fertility of coarse-textured sandy soils by accelerating straw decomposition remains poorly understood. We conducted a 90-day incubation experiment, applying 13C-labeled maize straw and/or microbial inoculant to coarse-textured sandy and fine-textured clay loam soils. Our results showed that microbial inoculant accelerated straw mineralization in both soils. Notably, microbial inoculant suppressed PE by 8.98% and 43.98% in the clay loam and sandy soil compared to straw addition alone, driven by adjusted soil available, C:N:P stoichiometry, and a shift in microbial community, as evidenced by a decrease in the 13C-G+ to 13C-G− ratio from 2.68 to 2.60 in clay loam soil and from 3.2 to 1.9 in sandy soil. Microbial inoculant further increased net SOC gain (clay loam soil: +22.1%, sandy soil: +641%), with sandy soils exhibiting superior responsiveness due to reduced organo-mineral associations and enhanced microbial carbon use efficiency. In conclusion, our study provides evidence for the mechanism by which microbial inoculants suppress the PE, particularly in sandy soils. This finding establishes a theoretical basis for further exploring microbial regulation strategies in soil carbon cycling under conditions of low organo-mineral associations.