Objective To understand the influences of arbuscular mycorrhizal fungi (AMF) inoculation and different forms of nitrogen (N) addition on soil nitrogen (N) and phosphorus (P) nutrients, soil N and P enzyme activities and their enzyme stoichiometry in the rhizosphere of Chinese fir (Cunninghamia lanceolata) for providing theoretical basis of sustainable management of Chinese fir plantations.

Method In the present study, a pot experiments were carried out to investigate the AMF inoculation (G. mosseae, Gm) and different forms of N (NH₄⁺ -N, NO₃⁻-N) addition on soil N and P nutrients, related enzyme activities and their stoichiometry of one-year-old Chinese fir seedlings.

Result The results showed that: (1) AMF inoculation increased soil available P content, but decreased the NO₃⁻-N, NH₄⁺ -N, DON and total P content. Compared with NH₄ ⁺ -N treatment, AMF under NO₃⁻-N treatment had more significant effect on soil N and P nutrients (P<0.05). (2) AMF inoculation and different forms of N addition increased the activities of soil acid phosphatase (AP), urease (URE), and N-acetyl-β-D glucosidase (NAG). Inoculation of AMF under NH₄⁺ -N treatment was more conducive to improve urease activity, and inoculation of AMF under NO₃⁻-N treatment was more conducive to improve N-acetyl-β-D glucosidase activity. (3) AMF inoculation reduced the ratios of soil URE: AP, PRO: AP and NAG: AP (p<0.05), and the decrease of soil URE: AP, PRO: AP stoichiometry under NO3-N treatment was higher than that under NH4-N treatment.

Conclusion AMF inoculation can improve soil P availability, and increase the transfer of soil available N to the plants by increasing the activities of nitrogen and phosphorus-related enzyme in the soil and reducing the stoichiometry of nitrogen and phosphorus-related enzyme to maintain soil N and P balance. Moreover, the effects of NO₃⁻-N treatment on soil nitrogen and phosphorus balance is larger than that under NH₄ ⁺ -N treatment.