Chloride, Soil Solution Osmotic Potential, and Soil pH Effects on Nitrification

Abstract

Abstract Laboratory incubation studies designed to measure chloride (Cl ‐ ), soil solution osmotic potential, and soil pH effects on nitrification of added NH + 4 were conducted in order to help explain effects of Cl‐containing fertilizers and liming on soil NH + 4 and NO ‐ 3 concentrations measured in field experiments. Samples of an Aquultic Argixerolls (pH 5.5, 6.0, 6.4, and 6.7) and a Xeric Haplohumults (pH 4.9, 5.5, and 6.2) were amended with 100 mg NH + 4 ‐N kg −1 , maintained at −10‐kPa matric potential and incubated at 25°C. Ammonium was added as either NH 4 Cl or (NH 4 ) 2 SO 4 salt solutions with four osmotic potential levels ranging from −96 to −692 and −90 to −669 kPa, respectively. Osmotic potential gradients were obtained by adding KCl to the NH 4 Cl solutions and K 2 SO 4 to the (NH 4 ) 2 SO 4 solutions. At 0, 3, 9, 14, and 28 d, samples were extracted with 2 M KCl and analyzed for NH + 4 , NO ‐ 3 , and NO ‐ 2 . At 15 d, soil solutions were recovered by centrifugation and analyzed for Al, Ca, Mg, K, Na, Mn, NH 4 , NO 3 , PO 4 , SO 4 , Cl, pH, and osmotic potential. Soil solution osmotic potential was lower in soils amended with Cl ‐ than with SO 2‐ 4 salt solutions. In both soils, nitrification was inhibited by Cl ‐ and by decreasing soil solution osmotic potential. Chloride inhibition of nitrification was greatest (⋍30%) at low soil pH (4.9‐5.5) but disappeared or decreased markedly in magnitude above pH 6.0 to 6.2. Because of soil solution osmotic potential and pH by Cl ‐ interaction effects, inhibition of nitrification would be expected to be greatest where Cl‐containing fertilizers are applied in a band on moderately acid (pH 5.0‐5.5) soils.