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

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.