Influence of mineral nitrogen on growth and mycorrhizal colonization of tree seedlings

Abstract

Growth responses of "fedegoso" (Senna macranthera), "cássia-verrugosa" (Senna multijuga), cinnamon (Melia azedarach L.) and "jacarandá-mimoso" (Jacaranda mimosaefolia) to N supply and forins of N were studied. Arbuscular mycorrhizal (AM) and non-mycorrhizal seedlings were transferred to sand culture containing different N sources at concentration of 4 mM, and allowed to grow for 120 days. The experiment was set up as a factorial with four replications, being with and without AM colonization and N-treatments (no N, N-NO₃^-, N-NH₄⁺ and N-NH₄NO₃). It was found that all plant species exhibited positive growth response to N supply. Plants treated with N-NO₃^- grew much better than those with the same amount of N as N-NH₄⁺. Plant height, for instance, was 13, 12, 5 and 4 fold bigger in N-NO₃^- treated plants than those with no N added to nutrient solution for "fedegoso", "cassia verrugosa", cinnamon and "jacarandá", respectively. AM colonization was reduced in N-NH₄⁺ treatead plants when compared to those treated with N-NO₃^-. In addition, N-NO₃^- plants accumulated more Ca and K than those fed with N-NH₄⁺. Nitrate reductase activity was found only on cinnamon and jacarandá roots treated with either N-NO₃^- or N-NH₄NO₃. Implication of these results for reforestation in low-fertility soils is discussed.