Identifying environmental phosphorus risk classes at the scale of Prince Edward Island, Canada

Phosphorus (P) loss from agricultural land poses a major risk to the environment. The main objectives of this study were (i) to adapt a simple P saturation indicator using 141 soils that had contrasting P levels and to deduct critical environmental P values, and (ii) to identify environmental risk classes and their spatial and temporal distribution at the scale of Prince Edward Island, Canada. The P saturation index (PSI) was greatly influenced by the soil acidity, and two critical P saturation indices were identified (i) a PSI (P/Al)(M-III) of 19.2% for very to extremely acidic soils (pH < 5.5), and (ii) a PSI of 14.2% (corresponding to 200 mg PM-III kg(-1)) for slightly to moderately acidic soils (pH > 5.5). Above these critical values, P fertilization should be limited to crop requirements. Six environmental P risk classes from very low to extremely high were identified. Spatial distribution of the identified classes was performed using georeferenced soil data collected between 2003 and 2015. The moderate risk class (P/Al ratio from 7% to 14% for soil pH above 5.5) was the predominant class, covering approximately 70% of the total area. Hot spots in the very high to extremely high range were found in about 10% of the total area, and mitigation strategies are needed to reduce P inputs to control P-related eutrophication risks in surrounding waters.