High-carbon bio-organic fertilizer reshapes soil carbon fractions and aggregate structure while maintaining high carbon stocks in red soil.

Introduction: The application of bio-organic fertilizers is key to enhancing soil carbon storage, but their carbon inputs often fall short of microbial demands, limiting their long-term impact. High-carbon bio-organic fertilizers, which contain more than 40% organic matter, are used to improve soil fertility, but their effects on soil aggregate stability, organic carbon fractions, and bacterial communities, and the consequent impacts on soil carbon storage, are not well understood. Methods: This study employed a field experiment with four treatments: chemical fertilizer alone (control, CK), organic fertilizer (OF), bio-organic fertilizer (BOF), and high-carbon bio-organic fertilizer (HBOF). Results: The results showed that organic fertilizer treatments significantly increased organic carbon content across all aggregate size classes, promoted the formation of large aggregates, and improved aggregate stability compared with the CK treatment. The HBOF treatment increased the proportion of soil aggregates larger than 2 mm by 8.58% and 6.87% compared with the OF and BOF treatments, respectively. All organic fertilizers boosted soil carbon storage and active carbon fractions, particularly particulate organic carbon (POC). Under HBOF, POC and microbial biomass carbon (MBC) in active carbon fractions increased by 15.83% and 24.05%, respectively, compared to BOF. The HBOF treatment reduced bacterial diversity but markedly altered community composition, enriching genera such as Conclusion: Overall, HBOF appears to promote a microbially mediated redistribution of SOC and physical protection of newly added labile carbon within macroaggregates, leading to short-term increases in SOC and improved soil structure in red soils. These findings provide a basis for optimizing organic fertilization and carbon management in medium- and low-yield croplands.