Environmental Vulnerability and Phosphorus Fractions of Areas with Pig Slurry Applied to the Soil

The application of pig slurry as a fertilizer can cause soil and water contamination. Intrinsic characteristics of the environment may enhance this effect and influence the vulnerability of the agricultural system. The goal of this study was to evaluate the accumulation of soil P fractions in areas treated with pig slurry and in forest areas and to propose an evaluation of the areas’ vulnerability to P contamination. Soil samples were collected from 10 areas with pig slurry applied to the soil and one in forest without a history of pig slurry application, all located in the Coruja and Bonito rivers microbasin at Braço do Norte, Santa Catarina, southern Brazil. Samples were prepared and subjected to P chemical fractionation. Two versions of the P index method, based on soil P forms or only on P extracted by Mehlich‐1, were used to evaluate the environmental risk of the studied areas. Estimated soil losses were lower for the forest and natural pasture and highest in areas with black oat (Avena strigosa Schreb.)–corn (Zea mays L.) crop cultivation. Concentrations of P fractions, especially of organic and inorganic P extracted by 0.1 and 0.5 mol L−1 NaOH and NaHCO3 and of inorganic P extracted by anion exchange resin and HCl, were higher in areas with a longer history and higher frequency of pig slurry applications. Vulnerability to P contamination was mainly influenced by soil P concentrations and soil losses in the studied areas. The P index based on Hedley's fractionation P forms resulted in a more accurate risk scoring of the studied areas than the P index based on the concentration of available P extracted by Mehlich‐1.