Biological nitrogen fixation (BNF) is the main form of introducing nitrogen into systems with low fertilizer input, which prevail in the semi-arid region of Brazil. BNF was evaluated in leucena and sabiá grown in samples of a soil collected under different vegetation cover: native vegetation; capoeira (area abandoned for the restoration); agricultural use; sabiá grove; and leucena grove. Nodulation was abundant in soil under all the types of cover. Leucena grown in soil under capoeira exhibited the highest dry biomass of nodules, while the same cover provided the lowest dry biomass of nodules for sabiá. Growth and shoot biomass were higher in plants grown in soil under caatinga. BNF was high in leucena and sabiá, reaching rates higher than 97% in plants grown in soil under capoeira. The highest amounts of fixed nitrogen (N) were found in leucena plants grown in soil under caatinga. It is the first estimate of the potential to fixed N for sabiá and leucena in soils of the semi-arid region, providing an initial estimate of the amounts of N that can be fixed in the field by these tree legumes in the Brazilian semi-arid.
Key words: Indigenous rhizobia, isotope, Leucaena leucocephala (Lam.) de Wit, Mimosa caesalpiniifolia Benth, N-15 natural abundance method, tropical dry forest.
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