Long-term fertilization alters chemical composition and stability of aggregate-associated organic carbon in a Chinese red soil: evidence from aggregate fractionation, C mineralization, and 13C NMR analyses

Purpose Long-term fertilization is a widely accepted strategy to enhance soil organic (SOC). However, fertilization effects on the stability of aggregate-associated OC remained largely unknown. Thus, stability of aggregate-associated OC was studied through aggregate fractionation, C-mineralization, and 13 C NMR analyses. Materials and methods Three aggregates (macro-aggregates, micro-aggregates, and silt + clay) were separated and analyzed for SOC contents, C-mineralization, and 13 C NMR analysis, for the following fertilization modes: control (CK), inorganic (NPK), and NPK combined with manure (NPKM). Results and discussion Highest contents (12.3–15.4 g kg −1 aggregate) for SOC were obtained in macro-aggregates under NPK and NPKM application which were 47 and 85% higher than CK. Under the applied treatments, the highest CO 2 -C mineralization (mg kg −1 soil) was observed for macro-aggregates and least for silt + clay fractions indicating high stability of OC associated with silt + clay fraction. Moreover, manure combined with inorganic fertilizer (NPKM) considerably lowered C-mineralization (per unit SOC) in aggregates and bulk soil suggesting high potential of manure addition to stabilize SOC through minimizing proportional to total aggregate or bulk soil OC decomposition. Furthermore, 13 C NMR analysis revealed carbonyl-C as the chief C-functional group sequestered. Manure application greatly enhanced SOC stability indices AI, HI, and A/OA which further indicates high SOC stability under manure addition. Conclusion Silt + clay fraction was more capable of protecting SOC against decomposition and manure combined with inorganic fertilizer not only had the potential to sequester more C but could also improve the stability of sequestered SOC associated with different aggregates.