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 C-13 NMR analyses. Materials and methods Three aggregates (macro-aggregates, micro-aggregates, and silt + clay) were separated and analyzed for SOC contents, C-mineralization, and C-13 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 CO2-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, C-13 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.