In order to explore the nutrient utilization mechanisms of arbuscular mycorrhizal fungi (AMF) in Karst soil, Cinnamomum camphora seedlings were inoculated with Glomus etunicatum and fertilized with nitrogen in compartment equipment of microcosms separated by double 20μm-nylon mesh or 0.45μm-nylon mesh. Lolium perenne litter labelled isotope of ¹⁵N as organic residue were put into soil of compartments. Some indexes, which are traits of seedlings growth, nitrogen and phosphors uptake, δ¹⁵N value, micro-biomass carbon, micro-biomass nitrogen, hyphae length in soil, were measured after 15 weeks. The results indicated that: the arbuscular mycorrhizal symbiosis of Glomus etunicatum can have saprotrophic capability and it enhanced soil litter decomposition and taked up the ¹⁵N from the adjoining compartment to supply for the host plants; Cinnamomum camphora seedlings gived priority to the use of nutrient in the rhizosphere. Under low nitrogen status, the Cinnamomum camphora utilized more ¹⁵N which was released from the decomposed litter in the adjoining coompartment. Additional nitrogen of rhizosphere increased litter decomposition by AMF in the compartment, but reduced the utilization of organic nitrogen from litter by the seedlings. The plants were able to increase the uptake of nitrogen and phosphors in the rhizosphere under high nitrogen condition. The content of mic-biomass carbon and nitrogen and the hyphae length were enhanced under the high nutrient conditions.