Response of onion plants to arbuscular mycorrhizae - 2. Effects of nitrogen fertilization on biomass and bulb firmness

The effects of N fertilization on growth and root colonization of preinocul ated onion (Allium cepa L. cv. Improved Autumn Spice) were studied. Onion t ransplants, inoculated with either Glomus intraradices, G. versiforme or no thing at sowing, were grown under three levels of N in soils which had eith er been irradiated, irradiated and amended with nonmycorrhizal microflora, or not irradiated. Interactions between inoculation and soil treatment had a significant effect on dry biomass and final bulb diameter. Control plants cultivated in non-irradiated natural soil grew normally because of the pre sence of indigenous arbuscular mycorrhizae, but control plants in irradiate d soils were stunted. There was no such difference among inoculated plants. In non-irradiated natural soil, bulbs of onions inoculated with G. intrara dices or G. versiforme were significantly firmer than bulbs of control plan ts. Bulb firmness decreased as N fertilization level increased. In non-irra diated natural soil, tissue P concentration of onion plants preinoculated w ith either fungus was significantly higher than that of control plants. In all soil types, N, P, and Zn concentrations were higher in onion plants col onized by G. versiforme than in those colonized by G. intraradices. The opp osite was true of Mn tissue concentration.