Phosphorus accumulation in soil after successive applications of swine manure: a long-term study in Brazil

Successive applications of swine manure (SM) in agriculture may contribute to environmental contamination by excess of phosphorus (P). In this study, we analyzed the dynamics of P fractions in soil cultivated with Ilex paraguariensis, following successive applications of SM over a long period (> 40 years) in southern Brazil. Soil samples in the 0–5, 5–10 and 20–40 cm layers were collected in areas subjected to 35 (A35) and 45 (A45) years of SM application and an area with no anthropogenic intervention (M). The content of organic and inorganic P in the soil samples was quantified by sequential extraction with NaHCO3 0.5 mol L−1 (labile), H2SO4 (moderately labile), and NaOH 0.5 mol L−1 (recalcitrant). The potential of P adsorption was assessed using the solution of KH2PO4 which remained in contact with the soil for a night. Data were compared by Tukey test at 5% probability. The results showed that 35 and 45 years of successive SM application did not increase total P content in soil. In contrast, an increase in inorganic form of P was observed. In comparison to the forest area, SM application for 45 years increased the content of the labile inorganic P fraction by 168%, 162% and 290% in the 0–5, 5–10 and 20–40 cm soil layers, respectively. The accumulation of this fraction promoted the reduction of P adsorption in soil by 99%, 180% and 175% in the 0–5, 5–10 and 20–40 cm soil layers, respectively, increasing the risk of contamination of groundwater by P excess.