Legumes are normal looking plants that have a special kind of bacteria around the roots which are very good at chemically fixing the free-floating nitrogen in the environment. This symbiosis provides nitrogen in form the plant can absorb. Many farmers innoculate legume seeds with beneficial bacteria to be assured off the nitrogen fixation properties, and frequently use legumes as a rotation/cover/forage plants.
Permaculturists use larger legumes for coppicing, a technique of top pruning a bush or tree to promote root die-back. The premise is that if you greatly reduce the photosynthesis capability of the top off the plant then the roots will die down as well as the plant struggles to remain alive. The dead roots and nitrogen fixing bacteria die off, releasing that nitrogen gathered from the environment. Most leguminous plants are survivors, and coppicing properly done rarely kills the plant. Nature naturally coppices here frequently as storms come through and reshape our forests.
There are plenty of native and non-native legumes that can be incorporated into the landscape. Following are a few.
Shy leaf (Aeschynomene viscidula), Leadplant (Amorpha fruticosa), Gray nicker (Caesalpinia crista), Partridge pea (Cassia chamaecrista), Bahama cassia (Cassia chapmanii), Sensitive plant (Cassia nictitans), Coffee senna (Cassia occidentalis), Eastern redbud (Cercis canadensis), Rattlebox (Crotalaria pallida), Dalea (Dalea carnea), Globe headed prairie clover (Dalea feayi), Summer farewell (Dalea pinnata), White prairie clover (Dalea pinnata var. adenopoda), Beggar's tick (Desmodium lineatum), Coral bean (Erythrina herbacea), Florida milk pea (Galactia floridana), White milk pea (Galactia elliottii), Sky-blue lupine (Lupinus diffusus), Sundial lupine (Lupinus perennis), Lady lupine (Lupinus villosus), Wild tamarind (Lysiloma bahamense), White sweet clover (Melilotus alba), Sour sweet clover (Melilotus indica), Blackbead (Pithecellobium keyense), Cat claw (Pithecellobium unguis-cati), Necklace pod (Sophora tomentosa), Southern pencil flower (Stylosanthes hamata), Crimson clover (Trifolium incarnatum), White clover (Trifolium repens), Vetch (Vicia acutifolia)
Some more native legumes : (Rhyncosia reniformis), (Petalostemon pinnatum), (Lespedeza augustifolia).
Some data from Your Florida Backyard