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Nitrogen vs. phosphorus limitation across an ecotonal gradient in a mangrove forest
Authors:Ilka C Feller  Karen L McKee  Dennis F Whigham  John P O'Neill
Institution:(1) Smithsonian Environmental Research Center, Edgewater, 21037, USA;(2) Wetland Biogeochemistry Institute, Louisiana State University, Baton Rouge, 70803, USA
Abstract:Mangrove forests are characterized by distinctive tree-height gradientsthat reflect complex spatial, within-stand differences in environmentalfactors,including nutrient dynamics, salinity, and tidal inundation, across narrowgradients. To determine patterns of nutrient limitation and the effects ofnutrient availability on plant growth and within-stand nutrient dynamics, weused a factorial experiment with three nutrient treatment levels (control, N,P)and three zones along a tree-height gradient (fringe, transition, dwarf) onoffshore islands in Belize. Transects were laid out perpendicular to theshoreline across a mangrove forest from a fringe stand along the seaward edge,through a stand of intermediate height, into a dwarf stand in the interior ofthe island. At three sites, three trees were fertilized per zone for 2yr. Although there was spatial variability in response, growth byR. mangle was generally nitrogen (N) -limited in thefringe zone;phosphorus (P) -limited in the dwarf zone; and, N- and/or P-limited in thetransition zone. Phosphorus-resorption efficiency decreased in all three zones,and N-resorption efficiency increased in the dwarf zone in response to Penrichment. The addition of N had no effect on either P or N resorptionefficiencies. Belowground decomposition was increased by P enrichment in allzones, whereas N enrichment had no effect. This study demonstrated thatessential nutrients are not uniformly distributed within mangrove ecosystems;that soil fertility can switch from conditions of N to P limitation acrossnarrow ecotonal gradients; and, that not all ecological processes respondsimilarly to, or are limited by, the same nutrient.
Keywords:Decomposition  Mangrove ecosystem  N and P fertilization experiment  Nutrient availability and cycling  Nutrient limitation  Nutrient resorption efficiency
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