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

Abstract

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. © 2012 Springer-Verlag.