DIFFERENTIAL-EFFECTS OF SOIL-WATER CONTENT AND TEMPERATURE ON NITRIFICATION AND AERATION

Environmental concerns have stimulated increased interest in NO3- accumulation in soils. The aeration status of the soil, which is mainly governed by the water content and temperature, is a central factor. The biological process responsible for NO3- accumulation, nitrification, was measured to estimate the combined effects of water content and temperature and determine their joint effect on soil aeration. The effects of temperatures of 15, 20, 25, 30, and 35 degrees C and water contents equivalent to 0.35, 0.42, 0.50, 0.52, 0.57, and 0.60 relative water content (volumetric water content/total porosity) on the nitrification activity of soil samples containing 2-mm sieved soils taken at 0 to 20 or 20 to 40-cm depth were determined by measuring NO3- accumulation for 17 h. A descriptive model including three biological parameters, maximum nitrification rate (Nr(max)) optimal relative water content (Theta(opt)), and temperature, was developed. Maximum Nr(max) occurred at 25.5 degrees C in 0- to 20-cm soil and at 20 degrees C in 20- to 40-cm soil, suggesting an adaptation of soil nitrifying populations to the temperature regime of the soil. The Nr, value was negatively related to Theta(opt), and Theta(opt) was dependent on temperature (T). This Theta(opt) (T) relationship was parabolic in nature, with Theta(opt) being at a minimum between 20 and 25 degrees C. It could be simulated using O-2 diffusion and respiration rates, inferring that these processes influenced Theta(opt) and T correlation. The ranges of O-2 concentrations favorable to maximum nitrification within an aggregate volume fraction were estimated for different temperatures. Nitrification was generally maximum when the intraaggregate pore spaces were saturated with water, with no water in the interaggregate pore space (i.e., 0.44 relative water content at 25 degrees C and 0.36 at 20 degrees C at 0- to 20- and 20- to 40-cm depths).