(NH4)2SO4 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% CaCO3 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 NO2-N which was not converted to NO3-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 NO2-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 NO3-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. NO2-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 NO2-N oxidizers than on NH4-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 NH4-N pool.