Leaching Methods Can Underestimate Mineralization Potential of Soils

Aerobic incubations to estimate net nitrogen (N) mineralization typically involve periodic leaching of soil with 0.01 M calcium chloride (CaCl₂), so as to remove mineral N that would otherwise be subject to immobilization. A study was conducted to evaluate the accuracy of leaching for analysis of exchangeable ammonium (NH₄ ⁺)-N and nitrate + nitrite (NO₃ ⁻+ NO₂ ^–)-N, relative to conventional extractions using 2 M potassium chloride (KCl). Ten air-dried soils were used, five each from Illinois and Brazil, that had been amended with NH₄ ⁺-N (1 g kg⁻¹) and NO₃ ^–-N (0.6 g kg⁻¹). Both methods were in good agreement for inorganic N analysis of the Brazilian Oxisols, whereas leaching was significantly lower by 12–48% in recovering exchangeable NH₄ ⁺-N from Illinois Alfisols, Mollisols, and Histosols. The potential for underestimating net N mineralization was confirmed by a 12-wk incubation experiment showing 9–86% of mineral N recoveries from three temperate soils as exchangeable NH₄ ⁺.