| Summary: | Arbuscular mycorrhizal fungi (AMF) form mutualistic symbiotic relationships with many land plants and play a key role in nitrogen (N) acquisition. NO<sub>3</sub><sup>−</sup>-N and NH<sub>4</sub><sup>+</sup>-N are the main sources of soil mineral N, but how extraradical mycelial transfer affects the different N forms and levels available to tomato plants is not clear. In the present study, we set up hyphal compartments (HCs) to study the efficiency of N transfer from the extramycelium to tomato plants treated with different N forms and levels of fertilization. Labeled <sup>15</sup>NO<sub>3</sub><sup>−</sup>-N or <sup>15</sup>NH<sub>4</sub><sup>+</sup>-N was placed in hyphal compartments under high and low N application levels. <sup>15</sup>N accumulation in shoots and the expression of <i>LeNRT2.3</i>, <i>LeAMT1.1</i>, and <i>LeAMT1.2</i> in the roots of tomato were measured. According to our results, both <sup>15</sup>NO<sub>3</sub><sup>−</sup>-N and <sup>15</sup>NH<sub>4</sub><sup>+</sup>-N were transported via extraradical mycelia to the shoots of plants. <sup>15</sup>N accumulation in shoots was similar, regardless of the N form, while a higher <sup>15</sup>N concentration was found in shoots with low N application. Compared with the control, inoculation with AMF significantly increased the expression of <i>LeAMT1.1</i> under high N and <i>LeNRT2.3</i> under low N. The expression of <i>LeAMT1.1</i> under high N was significantly increased when NO<sub>3</sub><sup>—</sup>N was added, while the expression of <i>LeNRT2.3</i> was significantly increased when NH<sub>4</sub><sup>+</sup>-N was added under low N. Taken together, our results suggest that the N transfer by extraradical mycelia is crucial for the acquisition of both NO<sub>3</sub><sup>−</sup>-N and NH<sub>4</sub><sup>+</sup>-N by the tomato plant; however, partial N accumulation in plant tissue is more important with N deficiency compared with a higher N supply. The expression of N transporters was influenced by both the form and level of N supply.
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