Nodulação e fixação de nitrogênio por forrageiras da caatinga cultivadas em solos do semiárido paraibano

Abstract

The biological nitrogen fixation (BNF) of Desmanthus pernambucanus (pigeon bundleflower), Macroptilium martii and M. lathyroides (wild bushbean), grown in soils from the semiarid region of Paraíba, manured or not, was evaluated in a pot experiment. Nodule number and biomass, N content and the proportions of fixed N (%Ndfa) were estimated, using the natural abundance methodology and buffel grass as a reference plant. The legume plants nodulated abundantly in all three soils, manured or not, except when grown in Fluvic Neosol without manure. The number and biomass of the nodules increased with manure. The BNF was high in all three species and soils: in most of the plants resulting in %Ndfa above 50% and reaching 85%. Using these data, a rough estimate of the fixation capacity of these species in the field indicates amounts of 10 to 30 kg ha-1 of N, taking planting and yielding usual densities. The lowest %Ndfa and fixed quantities were obtained in plants growing in the Regolithic Neosol and they increased with manure application. Among the species, BNF was least important to D. pernambucanus. Neither the proportion nor the fixed quantities were explained by any of the nodule variables measured (number, biomass or biomass/nodule).