Labeled nitrogen ((15)N) was applied to a soil-based substrate in order to study the uptake of N by Glomus intraradices extraradical mycelium (ERM) from different mineral N (NO(3)(-) vs. NH(4)(+)) sources and the subsequent transfer to cowpea plants. Fungal compartments (FCs) were placed within the plant growth substrate to simulate soil patches containing root-inaccessible, but mycorrhiza-accessible, N. The fungus was able to take up both N-forms, NO(3)(-) and NH(4)(+). However, the amount of N transferred from the FC to the plant was higher when NO(3)(-) was applied to the FC. In contrast, analysis of ERM harvested from the FC showed a higher (15)N enrichment when the FC was supplied with (15)NH(4)(+) compared with (15)NO(3)(-). The (15)N shoot/root ratio of plants supplied with (15)NO(3)(-) was much higher than that of plants supplied with (15)NH(4)(+), indicative of a faster transfer of (15)NO(3)(-) from the root to the shoot and a higher accumulation of (15)NH (4)(+) in the root and/or intraradical mycelium. It is concluded that hyphae of the arbuscular mycorrhizal fungus may absorb NH(4)(+) preferentially over NO(3)(-) but that export of N from the hyphae to the root and shoot may be greater following NO(3)(-) uptake. The need for NH(4)(+) to be assimilated into organically bound N prior to transport into the plant is discussed.