Phosphorus purification in buffer zones in cold climates

The retention of agricultural P by 10-m wide grass buffers (GBZ) and buffers under natural vegetation (VBZ) was studied for 10 years in southwestern Finland. The results were compared with those from 70-m long plots without buffers (NBZ). The GBZs were mowed but the VBZs were not managed. Surface waters were directed into a collector trench on each plot. Soil samples were taken from the buffers to determine easily soluble P by a Finnish method (0.5 M NH₄-acetate–0.5 M acetic acid, pH 4.6) and by the Olsen method.The highest losses of all P fractions were measured in spring, when the buffer vegetation had not yet started to grow. The mean annual total phosphorus (TP) loss from the GBZ and VBZ plots (0.7 kg ha⁻¹) was 40% lower than the TP loss from the NBZs (1.2 kg ha⁻¹). However, the loss of molybdate-reactive P (RP) was 70% higher from the VBZs than from the other plots. The concentration of Olsen-P was high (55.9 mg l⁻¹) on the soil surface, 0–2 cm, in the VBZs. The high loss of RP from the VBZs was most likely due to P leaching from the soil surface and decaying grass residue on the VBZs in spring.