Response of mycorrhizal Norway spruce seedlings to various nitrogen loads and sources

Norway spruce seedlings were grown under greenhouse conditions in Rootrainers with a vermiculite-peat moss mixture under various N-regimes for 6 months. Either ammonium or nitrate was applied in loads of 100 or 800 kg N ha(-1) year(-1) to seedlings which were either non-mycorrhizal or inoculated with the mycorrhizal fungi Hebeloma crustuliniforme or Laccaria bicolor. The use of increasing N loads enhanced shoot and total biomass, whereas root/shoot ratio, number of short roots and mycorrhization decreased. A significant enhancement of the concentration and content was obvious for the element N, whereas a significant decrease was obvious for P and Zn concentrations. The use of ammonium, as opposed to nitrate, significantly enhanced the biomass and the numbers of short roots, and reduced the root/shoot ratios, but did not influence the mycorrhization. It further significantly enhanced the N concentrations in roots and shoots. Fungal inoculation with H. crustuliniforme or L. bicolor compared to noninoculated controls significantly enhanced shoot and total biomass, but reduced root/shoot ratios. The mycorrhization further significantly enhanced N and P concentrations and contents, but reduced Mn. Overall, the mycorrhization improved the P nutrition of the seedlings independently on the applied N loads or N sources. Dose response curves using ammonium nitrate as N source with a maximum load of 1600 kg N ha(-1) year(-1) applied on seedlings associated with H. crustuliniforme revealed that the maximum growth was reached at a load of 800 kg N ha(-1) year(-1) with a simultaneous decrease of the mycorrhization. In both shoots and roots, N concentrations increased constantly with increasing N loads, while P, Ca, and Zn concentrations decreased constantly. (C) 2001 Elsevier Science Ltd. All rights reserved.