Resumo

Plant atmospheric biological nitrogen fixation through symbiosis with rhizobia is responsible for the ecological importance of legume species and the main form of N input in native vegetation. Little is known about the importance of this natural process in semiarid of Brazil. Agricultural production in this region is based on slash and burn systems, resulting in large areas covered by native vegetation in different regeneration stages. In these abandoned areas, regeneration depends on the replenishment of the nutrient stocks lost with burning, erosion and product exportation. Therefore, the objective of this paper is to estimate the biomass and N accumulation and fixation in spontaneous herb species growing in caatinga areas in four regeneration stages. (2-, 17-, 39- and >50-years after abandonment). Biomass of all herb plant was estimated as the sum of the highest biomass of each species and biological nitrogen fixation (BNF) was estimated for legume species using the 15N natural abundance methodology. The 2-year regeneration area had the highest total herb (6355 kg/ha) and legume (262 kg/ha) biomass production, N stock (82 kg/ha) and fixed N (5.2 kg/ha). Nodulating legumes (nine species in the whole sampled area) contributed with over 97% of legume biomass in all areas, but the biomasses of legume species were always less than 5% of the total herb biomass. Adequate isotopic condition was found to proper quantification of BNF, from 56% in the 39- year regeneration area to 79% in the 2- year regeneration area. All nodulating legumes obtained above 50 % of their N from the atmosphere, showing their potential to enrich degraded soil and to produce protein rich forage and green manure. However, the amounts of fixed N per unit area were relatively low (0.3 to 5.2 kg/ha, 0.5 to 6.3%). Macroptilium gracile added the highest amount of N (3.5 kg/ha, in the 2- year regeneration area), because of its large biomass production (205 kg/ha), although it was not the species with the highest proportion of fixed N (61%).