The mobilisation and fate of soil and rock phosphate in the rhizosphere of ectomycorrhizal Pinus radiata seedlings in an Allophanic soil

Phosphorus (P) characteristics and the fate of a finely divided Sechura phosphate rock (SPR) applied to a P deficient Allophanic Soil (Andosol) were investigated in the rhizosphere of 1-year-old Pinus radiata seedlings grown for 10 months in pots. The rates of SPR (13.9% P) application were 0, 50, 100 and 150 mug P g(-1) soil. Increased rate of application of SPR increased seedling growth and P uptake by R radiata, but SPR rates had no consistent effect on mycorrhizal tip density. The highest mycorrhizal tip density (tips per metre root) was observed at the P rate of 50 mug P g(-1) soil. Increased seedling growth with increased SPR rates was due to increased SPR dissolution. The SPR dissolution at plant harvest was high (> 80%) but was significantly higher in the rhizosphere soil than in the bulk soil, probably due to increased soil acidification and oxalate production in the rhizosphere. The seedling roots induced significantly higher acid and alkaline phosphatase activities in the rhizosphere soil. This was associated with significant decreases in the concentration of labile organic P (0.1 M NaOH-P-o) in the rhizosphere soil. Rhizosphere acidification and decrease in 0.1 M NaOH-P, resulted in accumulation of 0.1 M NaOH extractable inorganic P in the rhizosphere soil, but no difference was recorded in resin-P concentration between the rhizosphere and bulk soils. These results suggest that mycorrhizal root-induced conversion of organic P to inorganic P is a major process of P supply to P radiata seedlings in P deficient Allophanic Soils.