Biological nitrogen fixation in young and old tropical forests under five contrasting edaphoclimatic conditions

Young tropical forests have higher relative plant densities of potentially nodulating Leguminosae species than old forests, but comparisons of their proportions of effective N-fixing plants and amounts of fixed N are scarce. We determined leaf N stocks derived from symbiotic biological N fixation in woody legumes of young and old forests under the following five edaphoclimatic conditions, in Northeastern Brazil: (1) humid lowland coastal; (2) humid montane; (3) subhumid high fertility soil; (4) semiarid high fertility soil; and (5) semiarid montane low fertility soil. N amounts derived from fixation and stocked in the leaves were estimated using the N-15 natural abundance technique. Fixed N stocks in the leaves were low in both young and old forests of the nutrient-poor semiarid montane forests (< 0.3 kg ha(-1)) due to the low relative densities of nodulating legume plants (15 and 10 pl ha(-1), respectively), despite most individuals of the nodulating species fixing N. The relative density of nodulating species in the four other sites was higher in the young (148 to 635) than in the old forests (42 to 327 pl ha(-1)), as was the density of effectively fixing plants (96 to 265 vs. 9 to 140 pl ha(-1)). Leaf N stocks were also higher in the young than in the old forests, especially in the coastal humid forest (15.6 vs. 3.2 kg ha(-1)), except in the humid montane site (2.4 vs. 6.2 kg ha(-1)), due to the large N-fixing legume leaf biomass of the old forest. N fixation seems to provide an ecological advantage over non N-fixing plants in young tropical forests, usually growing on N poor soils, but not in semiarid forests growing on soils which are poor in other nutrients, like P, in which fixation is very low. However, the advantage of N-fixing legumes may also be related to other environmental variables and fixation continues to play a role in old forests.