Effect of soil structure disturbance on erosion and phosphorus losses from Finnish clay soil

No-till (NT) has proved to be an effective method to reduce erosion and losses of particulate phosphorus (PP) from fields to watercourses. However, the accumulation of P in the uppermost soil layer and the increased leaching risk of dissolved reactive phosphorus (DRP) in surface runoff reduce the environmental benefits of NT. The objective of this study was to determine whether the concentration of DRP in percolates could be decreased by mixing surface soil (0–5 cm) and deeper soil layers (5–20 cm) and how this affects erosion. We also tested the impact of high ionic strength on erosion from undisturbed and disturbed soil columns.Soil samples were collected from conventionally tilled (CT) and non-tilled (for 5 years) plots from a clay field (Vertic Cambisol) at 0–2.5, 2.5–10 and 10–20 cm depths. Moreover, undisturbed and disturbed soil columns representing the same plots were saturated with water in the laboratory and allowed to drain. The physico-chemical properties of the percolates were analysed to unravel the impact of cultivation methods and disruption of the aggregate structure. To see the effect of high ionic strength on detachment of soil particles, the soil columns were leached with ammonium acetate solution.The low pH of NT surface soil had enhanced the sorption of P and easily-soluble P had accumulated in the uppermost soil layer. Surprisingly, this enrichment did not increase the DRP concentration in the percolates when water percolated through the 0–5 or 0–20 cm soil layers. Disruption of the aggregate structure increased the turbidity and concentrations of suspended solids and PP in the percolates. In the NT samples, this action increased the concentration of DRP relatively more than in the CT samples. When the disturbed soil columns were leached with a solution of high ionic strength, the turbidity of the eluates was almost as low as before the breakage.To prevent erosion and the leaching of PP and DRP, we have to ensure an even water infiltration by improving the soil structure and by avoiding the disruption of stabilized aggregates. Ploughing the surface layer of NT soil can be recommended only if erosion from the field can be kept under control.