Drivers of Organic Carbon Preservation Within Soil Aggregates from An Ultisol Treated with Twenty-Year Fertilizations

Purpose: Soil aggregates are crucial for soil structure and organic carbon (OC) preservation. Fertilizations are an efficient pattern to improve crop productivity and OC storage. This study aimed to explore the driving factors regulating fertilization-induced OC conservation within soil aggregates. Methods: Soil aggregates of LM (large macro-aggregates, > 2 mm), SM (small macro-aggregates, 0.25–2 mm), and MI (micro-aggregates, < 0.25 mm) were obtained from an Ultisol treated with twenty-year no (CK), chemical (CF), and organic (OF) fertilizations. Aggregate mass proportions, OC components, and iron/aluminum oxides of each aggregate were investigated to evaluate their roles in aggregate-associated OC preservation. Results: The CF only enhanced the OC content (g kg^− 1 soil) in LM by 27.4%, and the OF increased the OC contents in all aggregates by 14.2-60.8%. The fertilizations were conducive to the formation of large-size aggregates and the stimulation of soil aggregate stabilization. The order OF > CF > CK was observed in the concentrations (g kg^− 1 aggregate) of labile and recalcitrant OC within each aggregate. The fertilizations, particularly the OF, significantly improved the concentrations (g kg^− 1 aggregate) of chelated iron and aluminum oxides by 5.9-117.6%. The aggregate mass proportions, OC components, and iron/aluminum oxides displayed various roles in the OC preservation within every individual soil aggregate. Furthermore, the OC conservation was dominantly controlled by the recalcitrant OC within all soil aggregates. Conclusions: Fertilizations stimulated the OC preservation mostly driven by the recalcitrant OC within soil aggregates from an Ultisol. This study provided a mechanistic understanding of OC preservation within soil aggregates under long-term chemical and organic fertilizations.