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

Abstract

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 N2 fixation. We assessed the role of N2 fixation in the N-budget of Acacia mellifera seedlings by varying N supply and grass competition. The contribution of N2 fixation to the N-budget of Acacia mellifera seedlings with varying N supply and grass competition was determined by measuring growth, nutrient concentrations, and 15N values. Tree seedlings were 4-fold taller and had 20-fold more biomass in the absence of grass. Tree foliar δ15N was lower with (−0.25 ± 0.2‰, n = 9) than without grasses (5.2 ± 0.1‰, n = 64). The contribution of N2-fixation to the N budget decreased with increasing N supply. Greater reliance on N2-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 δ13C (−29.5 ± 0.6‰) than seedlings without grass competition (−28.8‰ ± 0.5‰). Induction of N2-fixation by grass may have resulted from competition for nutrients. N2-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.