Nitrogen accumulated and biologically fixed by uninoculated Anadenanthera peregrina (L.) Speg trees under monospecific stands in the Atlantic Forest biome

Abstract

Nitrogen-fixing legumes are widely used to improve ecosystems. However, N accumulation in biomass, N2 amounts biologically fixed, and the effects on soil N status are poorly understood for plantations with slow-growing N2-fixing legume trees in seasonally dry environments. In this study, we assessed these questions in monospecific stands of uninoculated Anadenanthera peregrina (L.) var. peregrina (angico) trees established in the Atlantic Forest biome. Nine experimental plots with a stand density of 1111 tree ha−1 were examined about five to six years after tree planting. N content in the leaves, branches, bark, and wood was estimated using allometric equations and N concentration in the tissues. To assess the N content, the topsoil layer was sampled in each plot and in one pasture area. The annual N2 fixation rate at the stand scale was estimated by the natural abundance of 15N, using leaves of angico and a mix of leaves of herbs and shrubs growing in the understory in each plot. N accumulation in aboveground biomass was estimated at 260 kg ha−1 and N allocated in the leaves and branches was four-fold that in the tree stem. N derived from biological N2 fixation was estimated at 50% on average, with an annual rate of N fixed in the aboveground biomass of 22 kg ha−1 year−1. Soil N storage was similar between A. peregrina stands and the pasture area at six years after planting. Long-term gains of N2 fixation may be greatly improved by the adoption of breeding programs for this slow-growing species and the use of an adequate rhizobium strain.