Nitrogen Pools in Tropical Plantations of N₂-Fixing and Non-N₂-Fixing Legume Trees under Different Tree Stand Densities

We investigated the nitrogen pools in monocultures of legume species widely used in reforestation in Brazil that have contrasting growth and nitrogen acquisition strategies. The plantations were established with the slow-growing and N-2-fixing tree Anadenanthera peregrina var. peregrina, and the fast-growing and non-fixing tree Schizolobium parahyba var. amazonicum. The measurements of N pools in the tree biomass and the soil followed standard methods and were carried out on 54 experimental plots. The N-2 fixation pools were evaluated by abundance natural of N-15 and the N accretion methods. The soil N content was of similar magnitude between species and stand densities. The species showed similar amounts of N in the biomass, but divergent patterns of N accumulation, as well as the N-15 signature on the leaves. S. parahyba accumulated most N in the stem, while A. peregrina accumulated N in the roots and leaves. However, the N accumulation in biomass of A. peregrina stand was less constrained by environment than in S. parahyba stands. The percentage of N derived from N-2 fixation in A. peregrina stands decreased with the increase of stand density. The biological N-2 fixation estimates depended on the method and the response of tree species to environment.