Long-term fertilization and crop rotation can influence both organic C sequestration as well as the C composition of soils and the more resistant organic C compounds contained in humic acid(HA).This study examined the effects of fertilization and cropping type(monoculture corn(MC)and Kentucky bluegrass sod(KBS)and corn-oat-alfalfa-alfalfa rotation(RC))on the HA composition of soil from a 52-year field study in southern Ontario,Canada.Humic acid samples were extracted from soil,and elemental analysis,infrared spectroscopy,solid state 13C nuclear magnetic resonance spectra,and electron paramagnetic resonance methods were used to determine the influence of the cropping type on the characteristics of HA.Both fertilization and cropping type affected the chemical characteristics of HA.Fertilization led to a 5.9％increase in C,a 7.6％decrease in O,and lower O/C and(N+O)/C ratios in HA as compared to the corresponding non-fertilized treatments.Rotation resulted in a lower proportion of C(48.1％)and a greater(N+O)/C ratio(0.7)relative to monoculture cropping.Infrared spectroscopy analysis showed that HA contained more C-O groups in fertilized soil than in non-fertilized soil under MC and KBS.Fertilization increased the O-alkyl-C,phenolic-C,and free radical contents of HA relative to non-fertilization treatments.Rotation decreased the aliphatic and carboxyl groups and increased the O-alkyl,carbohydrate,aryl,and phenolic groups and free radicals,relative to MC and KBS.Both long-term crop rotation and fertilization dramatically modified the soil HA composition.Significant relationships were observed between the molecular composition of HA and soil organic C.Hence,humic acid characterization could be used as an indicator of the long-term sustainability of crop management practices.