Determination the best method for concentration of phosphorus lost critical threshold in Talkherood Watershed soils, East Azerbayjan

High soil phosphorus (P) can increase the amount of runoff P and cause pollution of P in water bodies. In this paper, to select the best method between laboratory extracting and rainfall simulator method for determining environmental threshold level of P concentration was compared. For this purpose, surface soil was sampledfrom 30 points of Talkherood Watershed inEastern Azerbayjan with widely available P contents. After determining some of general properties, four soil P tests comprising Olsen, Mehlich-3, Iron oxide, and Soltanpour were used. Water soluble P in lab was measured and runoff soluble P of soils was determined with using rainfall simulator by applying 75 mm h-1 rainfallfor 30 minutes. Relationship between P concentrations determined by four methods with  a) water soluble soil P, as P loss index, and b) runoff soluble P, showed that environmental soil P concentration can be estimated by both lab extracts and rainfall simulation methods for lime soils of this region. As the lab extracts method are easier and less time-consuming, this method is recommended for threshold concentration estimation. Since in iron oxide method, slope difference between the two split-lines was greater than other methods and high correlation coefficients were obtained for all of the methods, as a result iron oxide method is better than others for threshold concentration estimation of available P concentration. Since changing point for dissolved reactive P was seen in 0.36 mg l-1 concentration, as a result this concentration is lower than environmental P concentration for eutrophication phenomenon.