Influence of different mineral nitrogen sources (NO₃^--N vs. NH₄⁺-N) on arbuscular mycorrhiza development and N transfer in a Glomus intraradices-cowpea symbiosis

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 (NO₃^- vs. NH₄⁺) 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, NO₃^- and NH₄⁺. However, the amount of N transferred from the FC to the plant was higher when NO₃^- 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 ¹⁵NH₄⁺ compared with ¹⁵NO₃^-. The ¹⁵ N shoot/root ratio of plants supplied with ¹⁵NO₃^- was much higher than that of plants supplied with ¹⁵NH₄⁺, indicative of a faster transfer of ¹⁵NO₃^- from the root to the shoot and a higher accumulation of ¹⁵NH₄⁺ in the root and/or intraradical mycelium. It is concluded that hyphae of the arbuscular mycorrhizal fungus may absorb NH₄⁺ preferentially over NO₃^- but that export of N from the hyphae to the root and shoot may be greater following NO₃^- uptake. The need for NH₄⁺ to be assimilated into organically bound N prior to transport into the plant is discussed.