What are some nitrogen-fixing fruit trees that I can incorporate into the alleyways of my citrus orchard in central Florida as a permaculture method?

Answer: Worldwide, about 650 tree species are known to be, and several thousands are suspected to be, nitrogen fixing, according to the World Acroforestry Centre. For more information, see www.worldagroforestry.org/units/Library/Books/Book%2032/an%20introduction%20to%20agroforestry/html/17_nitrogen.htm?n=94.Here is a list of nitrogen-fixing trees that should be suitable to your zone (“sub-tropical” with lows of 25 to 30 degrees F):Acacia?some speciesLeucaena?Leucaena leucocephala produces edible pods and is native to Mexico and Central AmericaPongamia?Millettia pinnata has marginal hardiness and is being grown around MiamiCassia?Cassia spp. will tolerate short-lived frostsCarob?Ceratonia siliqua is native to the Mediterranean and might not tolerate the wet periods of central Florida Honey locust?Gleditsia triacanthos is popular in permaculture though its nitrogen-fixing ability is disputedRosewood or tipu tree?Tipuana tipuAlbizia (mimosas)Moringa?Moringa oleifera, the most commonly cultivated species of moringa, is native to India and the leaves and seed pods are used as food.However, of these, only carob is really commonly used for its “fruit.” Moringa, though, is widely used for its highly nutritious leaves.These are trees that are or have been used by permaculturists as recorded in the writings of Bill Mollison and David Holmgren. Here’s a link to a great chart of (mostly) tropical leguminous trees used in permaculture: http://agroforestry.net/pubs/speciescharac.html.If you’re not too attached to the fruiting aspect, it does appear that there are several good candidates for nitrogen-fixing leguminous trees in the alleyways of citrus plantings. A word of caution, though: many of these species are extremely fast-growing under the right conditions and may be considered invasive pests in certain areas.For brief descriptions of the taxonomy, ecology, characteristics, and management of over 50 multi-purpose trees used in agroforestry and intercropping systems, including over 30 nitrogen-fixing species, see the list at www.worldagroforestry.org/units/Library/Books/Book%2032/an%20introduction%20to%20agroforestry/html/12_appendix.htm?n=69. The trees on this list are mostly, if not exclusively, native to the tropics and/or subtropics.