| Anthropogenic eutrophication of surface waterways is a major environmental concern as it leads to the decline of aquatic resources and degradation of water. Phosphorus is the most limiting nutrient in freshwater systems and, therefore, an important factor in the control of eutrophication. Identifying the natural P thresholds in soils using the relationship between soil P and potential P losses can help determine areas with a high risk of P loss to waterways via leaching. Twenty-three sites covering a range of land uses were sampled by the USDA-NRCS in the Potomac Watershed in the northeast United States. Four pedons described and sampled at each site were used in this analysis. Samples from horizons in the plow layer (0 to 15 cm) were analyzed and Mehlich 3 P and water extractable P determined. A threshold in the relationship between CaCl2 P and Mehlich 3 P was identified using segmented linear regression (change point analysis) for each land use type. The change point identified for forest lands was 42 mg P kg-1, and for agricultural lands it was 64 mg P kg-1. For the pooled land uses, the change point was 66 mg P kg-1. The change point relates to the P level in soil above which an increase in soil test P is expected to lead to a large rise in P loss to waterways. Using environmental covariates such as soil pH, parent material, and elevation, a prediction surface was generated to predict Mehlich 3 P and Mehlich 3 P sorption ratio levels using various kriging techniques. The modeled surface can be used by land managers as additional information to help prevent P losses that lead to eutrophication. |
