Nitrogen (N) deposition is a key factor that affects terrestrial biogeochemical cycles with a growing trend, especially in the southeast region of China, where shortage of available phosphorus (P) is particularly acute and P has become a major factor limiting plant growth and productivity. Arbuscular mycorrhizal fungi (AMF) establish a mutualistic symbiosis with plants, and play an important role in enhancing plant stress resistance. However, the response of AMF to the combined effects of N deposition and P additions is poorly understood. Thus, in this study, a field experiment was conducted in 10-year Chinese fir forests to estimate the effects of simulated nitrogen (N) deposition (low-N, 30 kg ha −1 year −1 and high-N, 60 kg ha −1 year −1) and phosphorus (P) addition treatments (low-P, 20 mg kg −1 and high-P, 40 mg kg −1) on AMF since April 2017, which was reflected in AMF root colonization rates and spore density of rhizosphere soil. Our results showed that N deposition significantly decreased AMF root colonization rates and spore density. In N-free plots, P addition significantly decreased AMF root colonization rates, but did not significantly alter spore density. In low-N plots, colonization rates significantly decreased under low P addition, but significantly increased under high P addition, and spore density exhibited a significant decline under high P additions. In high-N plots, colonization rates and spore density significantly increased under P additions. Interactive effects of simulated N deposition and P addition on both colonization rates and spore density were significant. Moderate N deposition or P addition can weaken the symbiotic relationship between plants and AMF, significantly reducing AMF colonization rates and inhibiting spore production. However, a moderate addition of P greatly enhances spore yield. In the case of interactive effects, the AMF colonization rates and spore density are affected by the relative content of N and P in the soil. Arbuscular mycorrhizal fungi (AMF), a heterogeneous group of diverse fungal taxa and the most widespread fungal symbionts of plants, can establish mutualistic associations with the roots of over 80% of all terrestrial plant families, and plays a critical role in plant nutrient acquisition, growth, and ecosystem sustainability 1-3. The network of AMF extraradical mycelium in the soil supports water and mineral nutrition of the host plant, especially for enhancing the supply of phosphates, by effectively absorbing and translocating mineral nutrients (e.g., N and P) beyond the depletion zones of the plant rhizosphere 4,5. In return, plants provide carbon to AMF (as a carbon (C) source) by transferring hexose produced by photosynthesis via roots 6,7 , or in form of lipids. Typically, 5-10% of photosynthetically fixed C is allocated to the fungal partner 8. Owing to their filamentous organization, fungi exploit diverse substrates on the basis of their nutritional strategy 9 and significantly contribute to the uptake of soil nutrients, increase plant biomass, a...