Growth of N-2-fixing African savanna Acacia species is constrained by below-ground competition with grass

P>1. The coexistence of leguminous trees and C-4 grasses in many African savannas forces competition between these two distinct plant life-forms. The nature of tree-grass coexistence and the consequent competition for resources remains poorly understood. 2. We hypothesized that grass suppression of tree seedling growth was due to competition for N in plants not capable of fixing N-2 and due to the costs of N-2 fixation in those capable of nodulation. 3. Growth and N-2 fixation of nodulating (Acacia karroo, Acacia nilotica, Acacia tortilis and Acacia nigrescens) and non-nodulating (Acacia ataxacantha Acacia brevispica and Acacia schweinfurthii) Acacia spp. in response to soil characteristics and fertilizer variations in irrigated field experiments were compared. The extent of N-2 fixation was determined from delta 15N values. 4. Grass suppressed the growth of all species, but particularly of the non-nodulating species. The N-2-fixing tree seedlings had high tissue N and high water-use efficiencies (WUE, based on delta 13C) relative to non-nodulating legumes, making the N-2-fixing species strong competitors with C-4 grasses, which have low tissue N contents and high WUE. The suppression of seedling growth by grass was smaller for plants grown in a soil with 66 mg kg-1 than in a soil with 9 mg kg-1 of Bray II P. The tissue N : P ratios (15.1) indicated that P-limitation constrained growth of seedlings in soils with low Bray II P when grass was present, rather than the energetic costs of N-2 fixation. 5.Synthesis. In Acacia savannas and in the absence of water limitation, competition for N is the main constraint imposed by grass on growth of Acacia seedlings and that the ability to fix N-2 overcomes this limitation. However, the growth of N-2 fixing Acacias may be additionally limited by grass competition for P.