NO2 − and NO3 − leaching and solubilization of Al in variable charge soils treated with sewage sludge

The use of sewage sludge (SS) for agricultural purposes provides macronutrients and micronutrients and increases the soil organic matter. This study was conducted after 5 years of agricultural sludge application to a variable charge soil (clay loam-textured dark red dystroferric oxisol) in an experimental field at the Brazilian Federal Agriculture Research Institute in Jaguariúna, São Paulo. The mineralization of SS organic matter forms sulfate (SO₄ ²⁻) and ammonium (NH₄ ⁺) or ammonia (NH₃). Subsequently, nitrification of NH₄ ⁺ or NH₃ occurs, resulting in the formation of nitrite (NO₂ ⁻) and NO₃ ⁻ which release H⁺ and increases the redox potential of the solution. In studied area, the SS organic matter decomposition favored the anion exclusion, which led to SO₄ ²⁻, NO₂ ⁻ and NO₃ ⁻ leaching along the profile. At 2 m, the soil solution showed pH and Eh values of 4.2 and 600 mV, respectively. In addition, the highest soil NO₂ ⁻ (27 mg/L), NO₃ ⁻ (1220 mg/L) and Al (20 mg/L) concentrations were observed at 2 m. The low pH and high Eh at 2 m potentially resulted from the oxidation of leached NO₂ ⁻. Furthermore, the H⁺ released in solution due to nitrification was partially consumed during the solubilization of soil Al compounds, such as kaolinite and amorphous Al oxide. Geochemical modeling (GM) indicated that Al³⁺ in solution results from the solubilization of kaolinite and amorphous Al oxide. GM also highlights the importance of soil oxide protonation in buffering the soil solution acidity and in controlling the solubility of Al compounds and free Al in solution.