Determination of redox-sensitive phosphorus in field runoff without sediment preconcentration

Reduction-induced phosphorus (P) release from particles transported by field runoff has been poorly studied for want of a method that could be used for large surveys. To rectify this shortcoming, we modified the bicarbonate-dithionite (BD) extraction step of a sediment P speciation scheme for analyzing redox-sensitive P in runoff without sample preconcentration. The extraction comprised the addition of bicarbonate (pH buffer) and dithionite (reducing agent) into a runoff sample, 15 min of gentle shaking, filtration, and sample digestion. The samples were greatly reduced (Eh < -200 mV), and Fe and P were solubilized, but Al solubility was not increased. Phosphorus release from rock phosphates (calcium phosphates) was greater in the BD extraction than in water or bicarbonate solution, although no more than 0.2% of the total P was released. For runoff from a very fine Typic Cryaquept, the particulate phosphorus (PP) versus BD-PP relationship was linear up to a PP concentration of about 1.0 mg L(-1), but over the whole PP range studied (up to 2.6 mg L(-1)) somewhat better described by an exponential equation (BD-PP = 0.297 x PP(0.766); r2 = 0.91, n = 79). The minimum detectable value given by the method was relatively low, 0.023 mg L(-1), but reproducibility varied, with the coefficient of variation for 10 samples analyzed with 5 replicates ranging from 1.8 to 28.5%. Considering the variable reproducibility of the results and the lack of suitable reference material, the method needs further refinement and testing if it is to be used for quantitative determination of redox-sensitive P in runoff.