INFLUENCE OF MINERAL NITROGEN ON GROWTH A ND MYCORRHIZAL COLONIZATIONOF TREE SEEDLINGS

Growth responses of ''fedegoso'' (Senna macranthera), ''cassia-verrugo sa'' (Senna multijuga), cinnamon (Melia azedarach L.) and ''jacaranda- mimoso'' (Jacaranda mimosaefolia) to N supply and forms of N were stud ied. Arbuscular mycorrhizal (AM) and non-mycorrhizal seedlings were tr ansferred to sand culture containing different N sources at concentrat ion 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 co lonization and N-treatments (no N, N-NO3., N-NH4+ and N-NH4NO3). It wa s found that all plant species exhibited positive growth response to N supply. Plants treated with N-NO3.-grew much better than those with t he 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 add ed to nutrient solution for ''fedegoso'', ''cassia verrugosa'', cinnam on and ''jacaranda'', respectively. AM colonization was reduced in N-N H4+ treatead plants when compared to those treated with N-NO3.. In add ition, N-NO3. plants accumulated more Ca and K than those fed with N-N H4+. Nitrate reductase activity was found only on cinnamon and jacaran da roots treated with either N-NO3. or N-NH4NO3. Implication of these results for reforestation in low-fertility soils is discussed.