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

Environmental concerns have stimulated increased interest in NO3- accu mulation in soils. The aeration status of the soil, which is mainly go verned by the water content and temperature, is a central factor. The biological process responsible for NO3- accumulation, nitrification, w as measured to estimate the combined effects of water content and temp erature and determine their joint effect on soil aeration. The effects of temperatures of 15, 20, 25, 30, and 35 degrees C and water content s 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 2 0 or 20 to 40-cm depth were determined by measuring NO3- accumulation for 17 h. A descriptive model including three biological parameters, m aximum nitrification rate (Nr(max)) optimal relative water content (Th eta(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-c m 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 T heta(opt) (T) relationship was parabolic in nature, with Theta(opt) be ing at a minimum between 20 and 25 degrees C. It could be simulated us ing O-2 diffusion and respiration rates, inferring that these processe s influenced Theta(opt) and T correlation. The ranges of O-2 concentra tions favorable to maximum nitrification within an aggregate volume fr action were estimated for different temperatures. Nitrification was ge nerally maximum when the intraaggregate pore spaces were saturated wit h water, with no water in the interaggregate pore space (i.e., 0.44 re lative water content at 25 degrees C and 0.36 at 20 degrees C at 0- to 20- and 20- to 40-cm depths).