Risk of phosphorus losses from Andosols under fertilized pasture

Fertilization of grassland pastures may be a non-point pollution source in the Azores archipelago, despite the high phosphorus (P) retention of Andosols. To evaluate the risk of P desorption, representative Andosols samples (0-15 cm) were subdivided in five layers and different P pools were measured. The risk of P unloading into waters was assessed by the degree of phosphorus saturation (DPS), and by the P concentration in equilibrium solutions (0.01 M CaCl₂). The higher contents in the superficial layers suggest P accumulation due to pasture overfertilization. The organic P represented about 54% of the total P, with an overall average of 2.66 g P_t/kg. Despite being above the agronomic threshold, the soil with the highest average mean values of extractable inorganic P, 77 mg P_Olsen/kg and and 73.7 mg P_AL/kg, is still below environmental thresholds and none of the soils had DPS values above 25%, which is the critical value associated with eutrophication of surface waters. Similarly, all the P concentrations in the equilibrium CaCl₂ solutions were below the critical limits. Therefore, P desorption from these soils did not seem to be the main process responsible for effective waterbodies eutrophication in the Azores. Since mineral fertilizers are applied superficially, the hypothesis of their direct runoff during rainfall events, even before their complete dissolution and interaction with the soil matrix, must be considered. Consequently, P fertilization with deep-banding systems may be the alternative to the interdiction of fertilizers in the most sensitive and hilly areas of the watersheds.