title: Growth and nitrogen acquisition strategies of Acacia senegal seedlings under exponential phosphorus additions.
abstract: 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 μmol of P seedling(-1) over 12 weeks), mid (400 μmol) and high (600 μmol)] 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 (15)N 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.
authors.affiliation: CIRAD, UMR Eco&Sols-Ecologie Fonctionnelle & Biogéochimie des Sols & Agroécosystèmes (SupAgro-CIRAD-INRA-IRD), Place Viala, F34060 Montpellier, France. marney.isaac@utoronto.ca
authors.forename: M E
authors.initials: ME
authors.lastname: Isaac
authors.forename: J M
authors.initials: JM
authors.lastname: Harmand
authors.forename: J J
authors.initials: JJ
authors.lastname: Drevon
mesh_terms: D000045:Acacia / Q000254:growth & development* / Q000378:metabolism*; D018533:Biomass; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism*; D009586:Nitrogen Fixation / Q000187:drug effects*; D009587:Nitrogen Isotopes / Q000032:analysis; D010758:Phosphorus / Q000032:analysis / Q000378:metabolism / Q000494:pharmacology*; D018515:Plant Leaves / Q000254:growth & development / Q000378:metabolism; D018517:Plant Roots / Q000254:growth & development / Q000378:metabolism; D018547:Plant Stems / Q000254:growth & development / Q000378:metabolism; D058441:Rhizosphere; D036226:Seedlings / Q000254:growth & development / Q000378:metabolism; D013997:Time Factors
chemical_list: D009587:Nitrogen Isotopes; D010758:Phosphorus; D009584:Nitrogen