Impact of long-term inorganic phosphorus fertilization on accumulation, sorption and release of phosphorus in five Swedish soil profiles

The impact of long-term fertilization with inorganic P was studied in soil profiles (0-100 cm) from five sites in Sweden. Accumulation of P was studied by comparing P extracted with ammonium lactate/acetic acid (P-AL) and NaHCO3 (Olsen-P) in non-fertilized and fertilized soil profiles. The fertilized soils had received 42-49 kg P ha(-1) y(-1) for more than 30 years. P-AL and Olsen-P were significantly higher in the fertilized than in the non-fertilized profiles down to 40 cm depth. The P sorption index (PSI2) based on a single-point P addition of 50 mmol P kg(-1) soil was used to estimate P sorption capacity in the soils. The variation in PSI2 with depth was not consistent between the five soil profiles. PSI2 did not vary with depth in one soil, while it decreased in one and increased in the other three, and it was weakly but significantly correlated with the sum of Fe and Al extracted with ammonium oxalate (Fe-ox+Al-ox) (r=0.65**) and with clay content (r=0.69*** ). To estimate P release in the soils, P was extracted with CaCl2 (CaCl2-P) and water (P-w). CaCl2-P and P-w were significantly higher in the fertilized treatment than in the non-fertilized treatment in the top 20 cm. Below 30 cm depth, CaCl2-P was very low in all soils, while P-w was relatively high in two soils and low in the other three soils. To estimate the degree of P saturation, the ratio of P-AL/PSI2 and Olsen-P/PSI2 was calculated. P-AL/PSI2 was significantly higher in the fertilized treatment in the 0-20 cm layer, while Olsen-P/PSI2 was significantly higher in the fertilized treatment in the 0-40 cm layer. P-AL/PSI2 was correlated with CaCl2-P and P-w when all soils and horizons were included (rgreater than or equal to0.78***), but the correlation increased markedly when only 0-40 cm was included (rgreater than or equal to0.94***). Olsen-P/PSI2 was well correlated with CaCl2-P and P-w (rgreater than or equal to0.94***) for all soils and depths. Thus the two indices, P-AL/PSI2 and Olsen-P/PSI2, were comparable in their ability to predict P release in the top 40 cm, whereas Olsen-P/PSI2 was better when all depths were included. The overall conclusion was that P fertilization had an impact on P properties down to 40 cm depth, while the effects were small below this depth.