Growth and nitrogen acquisition strategies of Acacia senegal seedlings under exponential phosphorus additions

There remains conflicting evidence on the relationship between P supply and biological N-2-fixation rates, particularly N-2-fixing plant adaptive strategies under P limitation. This is important, as edaphic conditions inherent to many economically and ecologically important semi-arid leguminous tree species, such as Acacia senegal, are P deficient. Our research objective was to verify N acquisition strategies under phosphorus limitations using isotopic techniques. Acacia senegal var. senegal was cultivated in sand culture with three levels of exponentially supplied phosphorus [low (200 mu mol of P seedling(-1) over 12 weeks), mid (400 mu mol) and high (600 mu mol)] to achieve steady-state nutrition over the growth period. Uniform additions of N were also supplied. Plant growth and nutrition were evaluated. Seedlings exhibited significantly greater total biomass under high P supply compared to low P supply. Both P and N content significantly increased with increasing P supply. Similarly. N derived from solution increased with elevated P availability. However, both the number of nodules and the N derived from atmosphere, determined by the N-15 natural abundance method, did not increase along the P gradient. Phosphorus stimulated growth and increased mineral N uptake from solution without affecting the amount of N derived from the atmosphere. We conclude that, under non-limiting N conditions, A. senegal N acquisition strategies change with P supply, with less reliance on N-2-fixation when the rhizosphere achieves a sufficient N uptake zone. (C) 2010 Elsevier GmbH. All rights reserved.