title: Plasticity in nitrogen uptake among plant species with contrasting nutrient acquisition strategies in a tropical forest.
abstract: Nitrogen (N) availability influences the productivity and distribution of plants in tropical montane forests. Strategies to acquire soil N, such as direct uptake of organic compounds or associations with root symbionts to enhance N acquisition in exchange for carbon (C), may facilitate plant species coexistence and ecosystem N retention. Alternatively, rapid microbial turnover of soil N forms in tropical soils might promote flexible plant N-uptake strategies and mediate species coexistence. We tested whether sympatric plant species with different root symbiont associations, and therefore potentially different nutrient acquisition strategies, partition chemical forms of N or show plasticity in N uptake in a tropical pre-montane forest in Panama. We traced the movement of three15N forms into soil pools, microbes, and seedlings of eleven species differing in root traits. Seedlings were grown in a split-plot field transplant experiment, with plots receiving equimolar mixtures of ammonium, nitrate, and glycine, with one form isotopically labeled in each block. After 48 h, more15N was recovered in microbes than in plants, while all pools (extractable organic and inorganic N, microbial biomass, and leaves) contained greater amounts of15N from nitrate than from ammonium or glycine. Furthermore,13C from dual-labeled glycine was not recovered in the leaves of any seedling, suggesting the studied species do not directly take up organic N or transform organic N prior to translocation to leaves. Nitrogen uptake differed by root symbiont group only for nitrate, with greater15N recovery in plants with arbuscular mycorrhizal (AM) associations or proteoid roots compared to orchids. Some root trait groups differed in15N recovery among N forms, with greater nitrate uptake than ammonium or glycine by AM-associated and N2-fixing plants. However, only five of eleven species showed differences in uptake among N forms. These results indicate flexibility in uptake of N forms in tropical plants across root trait groups, with only a few species displaying weak preferences for a specific N form.
authors.affiliation: Smithsonian Tropical Research Institute, Apartado Postal 0843-03092, Balboa, Republic of Panama.|College of Life and Environmental Sciences, University of Exeter, Exeter, EX4 4RJ, UK.
authors.forename: Kelly M
authors.initials: KM
authors.lastname: Andersen
authors.affiliation: Smithsonian Tropical Research Institute, Apartado Postal 0843-03092, Balboa, Republic of Panama.|Department of Biology, University of Florida, Gainesville, Florida, 32611, USA.
authors.forename: Jordan R
authors.initials: JR
authors.lastname: Mayor
authors.affiliation: Smithsonian Tropical Research Institute, Apartado Postal 0843-03092, Balboa, Republic of Panama.
authors.forename: Benjamin L
authors.initials: BL
authors.lastname: Turner
mesh_terms: D065928:Forests; D009584:Nitrogen / Q000378:metabolism*; D010176:Panama; D018517:Plant Roots; D010944:Plants / Q000378:metabolism*; D012987:Soil
chemical_list: D012987:Soil; D009584:Nitrogen
keywords: ammonium; co-existence; glycine; nitrate; nitrogen partitioning; nutrient acquisition strategy; organic nitrogen; plant-soil interactions; plasticity; root traits; stable isotopes; tropical forest