Interlaboratory Comparison of Iron Oxide‐Impregnated Paper to Estimate Bioavailable Phosphorus

The transport of bioavailable phosphorus (BAP) in agricultural runoff can accelerate the eutrophication of receiving water bodies. However, procedural and theoretical limitations of algal bioassays and chemical extractants have restricted widespread measurement of BAP. Thus, a simple method using iron oxide—impregnated paper (Fe‐oxide strips) as a sink for BAP in runoff samples has been developed. As the method has potential application for BAP extraction at one location and analysis at another, Fe‐oxide strips were sent to Bologna and Rome, Italy, for P extraction of runoff samples and BAP determined. These BAP estimates were not significantly different (p < 0.001) from values determined on duplicate strips returned to the USA for BAP analysis. The close agreement in BAP measured in Italian and U.S. laboratories indicates that the simplicity of the proposed method can give highly reproducible BAP estimates. Prepared Fe‐oxide strips may be sent to a location and BAP extracted using only a 100‐ to 500‐mL bottle in which a strip and runoff sample are shaken overnight. The Fe‐oxide strip may then be air‐dried and returned to an analytical laboratory for subsequent P removal and measurement. Storage of these strips for up to 60 d had no effect on the amounts of P released.