Dissolved reactive phosphorus in runoff assessed by soil extraction with an acetate buffer

Agronomic soil test phosphorus (STP) data is, in addition to fertility studies, increasingly utilised in environmental risk assessment. We compared relationships between soil P extracted by acid ammonium acetate (AAAc-P) and water-soluble P (Pw) in laboratory, and AAAc-P and dissolved molybdate-reactive P (DRP) in field runoff. The laboratory study suggested a close relationship (R-2 = 0.87, n = 64) between AAAc-P and soluble P concentration in 1:100 (w/v) soil-to-water extracts, described by a linear equation: Pw (mg 1(-1)) = 0.021 x AAAc-P (mg 1(-1) soil) - 0.015 (mg 1(-1)). In Lake Rehtijarvi cathcment, dominated by clayey soils, the AAAc-P content of field Ap horizon in a similar manner influenced the flow-weighted DRP concentration in surface runoff and drainflow: a 1 mg 1(-1) increase in soil AAAc-P corresponded to 0.015 and 0.018 mg 1(-1) increase in surface runoff and drainflow DRP, respectively. When the AAAc-P vs. Pw relationship obtained in the laboratory test was used to predict the average DRP concentration in edge-of-field runoff, the precision of the DRP estimates inferred from STP data was in 95% of the cases +/- 0.10 mg 1(-1). In the L. Rehtijarvi catchment, 50% of the diffuse DRP loading risk was assigned to an area that corresponded to less than 20% of the fields and the situation may be similar in the national scale.