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 Alphosphate 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 X P interactions on plant growth.