Nitrogen fertilisation reduces grass-induced N2 fixation of tree seedlings from semi-arid savannas

Coexistence of trees and grasses in nutrient-poor arid savannas may result in competition for soil N. While grasses may be more effective than woody plants in acquiring N from the soil, some leguminous woody species rely on N-2 fixation. We assessed the role of N-2 fixation in the N-budget of Acacia mellifera seedlings by varying N supply and grass competition. The contribution of N-2 fixation to the N-budget of Acacia mellifera seedlings with varying N supply and grass competition was determined by measuring growth, nutrient concentrations, and N-15 values. Tree seedlings were 4-fold taller and had 20-fold more biomass in the absence of grass. Tree foliar delta N-15 was lower with (-0.25 +/- 0.2aEuro degrees, n = 9) than without grasses (5.2 +/- 0.1aEuro degrees, n = 64). The contribution of N-2-fixation to the N budget decreased with increasing N supply. Greater reliance on N-2-fixation by trees in the presence of grasses did not result in greater biomass accumulation or tissue [N] relative to tree seedlings grown without grass competition. Tree seedlings competing with grass had significantly more negative delta C-13 (-29.5 +/- 0.6aEuro degrees) than seedlings without grass competition (-28.8aEuro degrees aEuro parts per thousand +/- 0.5aEuro degrees). Induction of N-2-fixation by grass may have resulted from competition for nutrients. N-2-fixation enables tree seedlings to compensate for limited soil N and survive grass competition at a critical and vulnerable developmental stage of germination and establishment.