Phosphorus fertilisation causes durable enhancement of phosphorus concentrations in forest soil

The duration of P fertiliser in acid forest soil was investigated in a Norway spruce (Picea abies Karst.) forest in south-central Sweden. The fertilisation of the soil started in 1967, but no P has been applied since 1988. The N fertilisation is still continuing. Totally, 300 kg P per hectare, as superphosphate, and/or 1090 kg N per hectare, as ammonium nitrate, was applied. Concentrations of both 0.05M Na₂SO₄ + 0.02M NaF extractable P and 0.5M H₂SO₄ extractable P in the Of, Oh, E and top B horizons of fertilised soils were elevated compared to the control. The P fractions considered to be extracted are adsorbed and some Al-bound phosphate, in the case of Na₂SO₄ + NaF, and Ca phosphates, in the case of H₂SO₄. 3–4% of the added P was recovered as Na₂SO₄ + NaF extractable P, and 10–22% was recovered as H₂SO₄ extractable P in the soil profile down through the first 5 cm of the B horizon. Still continuing ammonium nitrate fertilisation has decreased the H₂SO₄ extractable P concentration in this soil. Cumulative P fertiliser application of 300 kg P per hectare has counteracted this decrease.