N2 fixation by two lupine species under different soil management systems. Transfer of fixed N2 from legume to intercropped eucalyptus

Abstract

The rate and pattern of symbiotic fixation and N credit by white lupine as affected by soil tillage, and yellow lupine affected by soil amendment with eucalyptus harvest residues were evaluated; also, fixed N transferred from yellow lupine to the intercropped eucalyptus was assessed. Lupine species growing in sandy soils with low nutrient status fixed >94% N₂ with the indigenous soil rhizobium, in two sites (Pegões, Óbidos). Soil disturbance and harvest residues incorporation did not affect the fixing capacity of both lupines. Positive N inputs (>+69 kg N ha^-1) were estimated for both lupines, which is relevant in a rotation or association with non-legume crops. Nitrogen concentration in newly-planted eucalyptus was 23% lower in harvest residues amended soil and 36% greater by the addition of mineral-¹⁵N compared with organic fertilization (yellow lupine). Mineral-N use efficiency by eucalyptus seedlings was low (4%) which may be associated with the small root development (shoot:root ratio=4.3). Organically fertilized trees showed ¹⁵N enrichment thirteen times smaller than mineral-¹⁵N fertilized trees.