Role of soil organic matter composition and microbial communities on SOC stability: Insights from particle-size aggregates

Purpose Rivers transfer eroded sediment with soil organic carbon (SOC) from terrestrial land to ocean basins, playing a key role in the global carbon cycle. During sediment delivery, the coarse particles can be preferentially settled and the fine particles may be transported to downstream depositional sinks. However, the size-specific quantification of soil microbial communities and SOC composition and their roles in SOC stability have not been fully elucidated. Materials and methods In this study, two soils with similar textures but different aggregate structures were fractionated using a settling tube apparatus into three size classes according to their settling velocities. The individual soil particle-size aggregates and unfractionated soil were analyzed to determine (1) the diversity and composition of the soil microbial communities via 16S rRNA and ITS1, and (2) the SOC composition using mid-infrared diffuse reflectance spectroscopy. Results The results showed that as the soil aggregate size decreased, the ratio of alkyl groups to carboxyl groups (C−H/COO, C−H/C=O) and soil CO 2 emission per gram of SOC significantly decreased; however, the richness and diversity of the soil bacterial and fungal communities increased. Meanwhile, the aggregates of smaller size had lower relative abundances of Proteobacteria and Basidiomycota but higher relative abundances of Actinobacteria and Ascomycota. Rhizobiales was an indicator taxon for bigger soil aggregate communities, while Deinococcales , Capnodiales , and Venturiales were indicator taxa for communities with smaller soil aggregates. SOC composition showed a stronger direct effect on SOC stability in bigger soil aggregates (path coefficient, 0.68). However, SOC composition exerted a greater indirect impact on SOC stability by regulating soil bacterial and fungal community composition in smaller size aggregates (path coefficient, 0.65). Conclusions Overall, our study provides a fundamental understanding of the chemical and microbial control of SOC stability at the aggregate level and highlights the potential impacts of aggregate-scale SOC decomposition on the fate of eroded SOC.