Abstract: [Objective] The increased atmospheric N sedimentation in recent years has brought the increased N availability and N/P ratio in forest soil, which would impact growth and P efficiency of mycorrhizal symbiosis plants. This study investigated effects of continuous N sedimentation on the ectomycorrhizal symbiosis of Pinus massoniana seedlings under low P stress to provide a theoretical basis for exerting the mycorrhizal potential to improve efficiency of Masson Pine P and growth.[Method] An elite breeding population of P. massoniana was used as test materials, two P conditions (homogeneous low P and heterogeneous low P among soil layers) and two N sedimentation levels were simulated in a pot experiment.[Result] 1) Under different P environments,,there were different responses of 2-year-old mycorrhizal seedlings to sustainable increase of N sedimentation. Under the homogeneous low P condition, N sedimentation significantly reduced the mycorrhizal infection rate and degree of P. massoniana seedlings, and inhibited the growth and P absorption efficiency of P. massoniana mycorrhizal seedlings, resulting in a significant reduction of P. massoniana biomass. Under the heterogeneous low P condition, simulated continuous N sedimentation increased seedlings mycorrhizal infection rate and degree, promoted the plant absorption of N and P, and thereby increased the seedlings biomass accumulation. 2) Under the homogeneous low P condition, N sedimentation significantly increased the plant N/P ratio, by which the seedlings growth was limited by P element, but was not sensitive to the increase of N. Under the heterogeneous low P condition, continuous N sedimentation improved the effectiveness of mycorrhizal effect and promoted the seedlings P and N absorption, and thus improved the seedlings growth. 3) Under the homogeneous low P condition, N sedimentation improved the nutrient status of mycorrhizal soil. Soil N and P levels were increased, but pH values did not change significantly. This was due to increase in the mycorrhizal effectiveness. Under the heterogeneous low P condition, simulated continuous N sedimentation reduced the inhibition degree of inoculation mycorrhiza fungi on soil nutrients, but soil pH value decreased significantly.[Conclusion] The effectiveness of mycorrhizal effect had different responses to increased N sedimentation under two P environment. Under the homogeneous low P condition, the effectiveness of the mycorrhiza effect increased. The status of soil nutrient was improved, but with the steady accumulation of soil effective N, soil P element was relatively serious deficiency. With the significantly increased plant N/P ratio, the limited degree of seedlings growth by P element was increased. Under the heterogeneous low P condition, the continuous N sedimentation reduced the limited degree of soil nutrient by mycorrhizal, thus the mycorrhizal fungi infection of the seedlings was promoted. Ultimately the accumulation of seedling biomass was increased.

Key words: Pinus massoniana, low P stress, continued N sedimentation, ectomycorrhizal symbiosis, P efficiency, soil nutrient