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.

Automated summary

The study found that under NPK and NPKM applications, the highest SOC content was in macro-aggregates, 47% and 85% higher than CK. Mineralization experiments showed that CO2-C mineralization was highest in macro-aggregates and lowest in silt + clay, indicating high stability of OC in the silt + clay fraction. Additionally, NPKM significantly reduced C-mineralization in aggregates and bulk soil, suggesting that manure addition could stabilize SOC by reducing decomposition proportional to total OC in aggregates or bulk soil.