Objectives We studied the impact of nitrogen (N) sources and phosphorus (P) supply levels on the N uptake and transport efficiency through the extraradical mycelium (ERM) of arbuscular mycorrhizal (AM) fungi.

Methods A symbiotic system between AM fungus (Rhizophagus irregularis) and wheat (Triticum aestivum L.) was established using a three-chamber system. Wheat was cultivated in the root chamber for 60 days, before subjected to nutrient starvation for 7 days. On the first experiment, 30 mL 4 mmol/L N sources of KNO₃, (NH₄)₂SO₄, glutamine (Gln), arginine (Arg), and urea were supplied in the extraradical hyphae (ERH) chamber, and pure water was used as control (CK). On days 3, 5, and 7 after the N supply, the AM fungal extraradical hyphae were collected for the analysis of the content of Arg. And wheat plants were harvested for determination of leaf free amino acids and chlorophyll. On the second experiment, P level of 0, 35, and 700 µmol/L were added into ERH chamber, and then ¹⁵N-labeled KNO₃, (NH₄)₂SO₄, and urea, as well as pure water, were added under each P level. N sources and different P levels were added 10 mL every 7 days, and wheat samples were collected after 42 days to measure ¹⁵N abundance, and key enzyme activities in leaves, as well as plant N and P contents.

Results 1) With the increase in nitrogen supply culture time, the Arg content in ERM continuously decreased, reaching its lowest level on the 7th day. Compared to the control (CK), all N source treatments significantly increased the Arg content in ERM on the 3rd day of incubation, with KNO₃, and Arg treatments exhibiting the highest contents. Except for the Arg group, compared to the CK, N source treatments on different days significantly increased the free amino acid content in wheat leaves, with the highest free amino acid content (1.26 mg/g) observed on the 5th day after applying KNO₃. Chlorophyll content of wheat leaves also showed an overall upward trend with the extension of time. 2) Both P supply levels and N forms significantly affected the ¹⁵N abundance in wheat roots. The treatment combining KNO₃ with P 700 µmol/L resulted in the highest ¹⁵N abundance in root, while (NH₄)₂SO₄ showed the highest abundance at P 35 µmol/L. Except for the urea treatment with P 700 µmol/L, N application significantly enhanced the activities of nitrate reductase and glutamine synthetase in leaves. After applying different N sources, nitrate reductase activity decreased with increasing P levels. At different P levels, compared to the no-nitrogen treatment, N and P contents in both the shoot and root of plants treated with various N sources increased to varying degrees.

Conclusions The efficiency of extra-radical AM hyphae in absorbing and transporting nitrate and ammonium N is higher than that of organic N. The absorbed N is transferred to intra-radical hyphae in the form of Arg and then transported to the mycorrhizae to support wheat growth, with the entire transport process taking approximately 7 days. High P levels favor the absorption and transport of nitrate nitrogen by AM fungi, while low P levels favor the absorption and transport of ammonium N.