Differential Effects of Soil Water Content and Temperature on Nitrification and Aeration

Environmental concerns have stimulated increased interest in NO⁻₃ 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 NO⁻₃ 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°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 NO⁻₃ accumulation for 17 h. A descriptive model including three biological parameters, maximum nitrification rate (Nrₘₐₓ), optimal relative water content (Θₒₚₜ), and temperature, was developed. Maximum Nrₘₐₓ occurred at 25.5°C in 0- to 20-cm soil and at 20°C in 20- to 40-cm soil, suggesting an adaptation of soil nitrifying populations to the temperature regime of the soil. The Nrₘₐₓ value was negatively related to Θₒₚₜ, and Θₒₚₜ was dependent on temperature (T). This Θₒₚₜ (T) relationship was parabolic in nature, with Θₒₚₜ being at a minimum between 20 and 25°C. It could be simulated using O₂ diffusion and respiration rates, inferring that these processes influenced Θₒₚₜ and T correlation. The ranges of O₂ 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°C and 0.36 at 20°C at 0- to 20- and 20- to 40-cm depths). Financial support was provided by the CNRS PIR Environment Program.