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This article in SSSAJ

  1. Vol. 59 No. 1, p. 227-233
    Received: Nov 30, 1993

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Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils

  1. Y. Teng and
  2. V. R. Timmer 
  1. Faculty of Forestry, Univ. of Toronto, 33 Willcocks Street, Toronto, ON, Canada M5S 3B3



The cause of N-induced P deficiency in white spruce [Picea glauca (Moench) Voss] seedlings was investigated in a greenhouse pot trial testing factorial additions of ammonium nitrate (AN) and phosphoric acid (PA) to root zone and root-free compartmentalized soils. Plant growth was significantly improved by combined N and P topdressings during the growing season. However, N-only fertilization induced P deficiency symptoms and reduced biomass and P status in shoots, demonstrating an apparent N antagonism on P. Phosphorus availability in this treatment also was 35% lower in the rhizosphere soil than in the root-free bulk soil, reflecting rhizosphere P depletion. Soil acidity and Al activity were increased most by AN-only applications, probably contributing to reduced P availability. A diagnosis of induced Al toxicity by N fertilization based on plant analysis data was supported by symptoms of root injury. Phosphorus capture by ion-exchange resin bags at the base of the pots was lower than that of N with combined N-P applications, suggesting rapid fixation and low mobility of P in the soil compared with N. Nitrogen-induced rhizosphere P depletion in this nursery soil was attributed to restricted root development of seedlings due to Al toxicity, reduced P availability by Al-phosphate precipitation, and low P replenishment because of slow diffusion from the bulk soil. Topdressing with both PA and AN increased P availability in the rhizosphere (320 mg kg−1), reduced soil extractable Al by 40% compared with AN-alone treatments, and increased plant uptake of N and P by 270%, resulting in positive N × P interactions on plant growth.

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