Nitrification of Ammonium Sulfate in a Calcareous Soil as Influenced by Combinations of Moisture, Temperature, and Levels of Added Nitrogen

Abstract (NH 4 ) 2 SO 4 at levels of 0, 150, and 450 ppm. N was incubated in a calcareous soil at 2°, 10°, and 22° C. at 10, 1, and 0.3 bars moisture tension using periodic aeration. A second incubation study included 4 other moisture levels (7, 15, 115, and 415 bars moisture tension) and continuous aeration. Good control of the moisture was attained with the continuous aeration by humidifying the incoming air over saturated salt solutions known to give the required relative humidity. The soil used, Millville loam from the Agronomy farm, is a highly calcareous (47% CaCO 3 equivalent) fertile soil with a saturated paste pH of 7.8, and moisture contents at field capacity (0.3 bar tension), permanent wilting percentage (15 bars tension), and air dry of 18.0, 6.9, and 1.6%, respectively. The only evidence of nitrification at 2°C. was the production of NO 2 ‐N which was not converted to NO 3 ‐N in 10 weeks. At 10°C. there was nitrification at ⅓, 1, and 10 bars tension where the lowest moisture (10 bars tension) apparently only delayed the initiation of nitrification and did increase the period of NO 2 ‐N accumulation. At 25°C. nitrification was completely inhibited at 415 and 115 bars tension. At 15 bars tension 50% of the 150 ppm. N was oxidized to NO 3 ‐N in 28 days, while at 10, 7, and 1 bars tension the nitrification was complete in about 28, 20, and 12 days. In this soil, 35°C. was less favorable for nitrification than 25°C. NO 2 ‐N accumulated at this temperature and persisted throughout the 8 weeks. Temperature (both high and low) and lowered moisture had a greater inhibiting effect on NO 2 ‐N oxidizers than on NH 4 ‐N oxidizers. There was a decrease in the total inorganic N recovered when unfavorable nitrification conditions were encountered. This loss of inorganic N was apparently from the NH 4 ‐N pool.