Differential contribution of arbuscular mycorrhizal fungi to plant nitrateuptake (N-15) under increasing N supply to the soil

The objective of this study was to determine how the uptake and transport o f nitrate by two species of arbuscular mycorrhizal (AM) fungi is affected b y its concentration in the medium and by the age of the AM symbiosis. Trace r amounts of N-15 nitrate were applied at two plant growth periods to mycor rhizal or nonmycorrhizal lettuce plants, which had been grown in soil suppl ied with nitrate to provide a total of 84, 168, or 252 mg N/kg. At both inj ection times, Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe and Glomus fasciculatum (Thaxter sensu Gerd.) Gerd. and Trappe reached the highest va lues of nitrogen derived from the fertilizer (NdfF) at 84 mg N/kg. Glomus m osseae also reached the highest values of labeled fertilizer N utilization at 84 mg N/kg, whereas G. fasciculatum reached the highest values at 168 mg N/kg in the medium. The highest N level in the medium (252 mg N/kg) had a negative effect on % NdfF and % labeled fertilizer utilization for all myco rrhizal plants. Regarding the time of N-15 fertilizer application, G. fasci culatum-colonized plants had a minimum change in % NdfF and % labeled ferti lizer utilization during the growth period (60 days application vs. 30 days application). In contrast, G. mosseae-colonized plants growing at 168 mg N /kg in the medium, decreased these two values in the latest application. Th e present results confirm that mycorrhizal symbiosis may be particularly im portant for nitrogen nutrition in plants growing in neutral-alkaline soils.