The example guild in Gaia's garden has a ring of alliums or daffodils around the mature
drip line of the tree, and also a smaller ring around the trunk. It has a nitrogen fixing bush like goumi under the tree while it's young, which gets removed as the tree matures. At that point, the nitrogen fixing bushes can be close-by the tree but not under it. The roots have a larger range then to find nitrogen.
Other suggested plants under the tree are insectory plants (lemon balm, dill), soft-leaved mulch plants (comfrey, artichoke), nutrient accumulators (plantain, yarrow). If you don't want an N bush under the tree you could plant something like lupine, vetch or beans.