Objective  Effects of the widely distributed, important plant growth and stress resistance regulating arbuscular mycorrhizal (AM) fungi on sorghum growth under varied nitrogen (N) deposition in soil were studied on the terrestrial ecosystem.

  Method  Growth of Sorghum hicolor (L.) Mocrnch seedlings inoculated with either Glomus mosseae or inactivated G. mosseae (CK) in pots under varied N addition of 0, 200, 400, or 500 mg·kg^-1 (refer to as N0, N1, N2, and N3, respectively) in a greenhouse was monitored. Mycorrhizal colonization in rhizosphere soils was determined after two weeks, while the biomass, N, and P of the plants in 16 weeks of cultivation.

  Result  ① The AM fungi inoculation significantly increased the mycorrhizal colonization on sorghum roots (P＜0.001) at a decreasing trend with increasing N addition. ② Both the aboveground and total biomasses of the sorghum plants grown under N0 were significantly enhanced by the presence of the AM fungi but inhibited by it under N3 (P＜0.05). ③ Similarly, AM fungi increased the N and P contents and N/P ratio in the plant tissues by N0 but did the opposite under either N2 or N3 (P＜0.05). However, under N2 the AM fungal inoculation exerted no significant effect on P content (P＞0.05). And ④ the mycorrhizal growth effect (MGR), mycorrhizal N-uptake effect (MNR), and mycorrhizal P-uptake effect (MPR) on sorghum changed from positive to negative gradually as the N addition increased.

  Conclusion  Inoculating AM fungi on sorghum seedlings and adding N in soil interactively affected the biomass and N-P uptake of the plants significantly. Without N addition, the AM fungi enhanced the accumulation of biomass, N, and P, but N addition maximized the mycorrhizal effect at certain level. High levels of N in soil could be detrimental to the growth of AM fungi and altered the symbiosis.