Denitrification and fluxes of nitrogenous gases from soil under steady oxygen distribution

Abstract

Transport equations for O2, NO3−, NO2−, N2O and N2 were formulated to investigate NO3− stability, denitrification and formation of gaseous nitrogen compounds in a soil profile under different moisture and temperature conditions. The source-sink terms of the transport equations, including those for O2, were based upon competitive Michaelis-Menten type kinetics of denitrification. The equations were solved under various soil and seasonal temperature conditions typical of the Prairie Region of Canada in order to explore the effects of these parameters upon predicted NO3− stability, denitrification product distribution in a soil profile and gaseous N fluxes from the soil surface. The depth to the aerobic-anaerobic interface from the soil surface was controlled by temperature, moisture and microbial activity distributions in the soil. The kinetic expressions predicted that NO3− was generally stable under aerobic conditions since the affinity coefficient of O2 for electrons is much greater than that of NO3−. However, nitrate in the anaerobic zone was subject to denitrification to produce N2O and N2. The N2O produced in the anaerobic zone diffused into the aerobic zone where it was stable and eventually emitted to the atmosphere. As the NO3− concentration decreased, a greater proportion of N was emitted to the atmosphere as N2. Thus, the ratio of N2O to N2 emitted from the soil decreased with decreased NO3−. We found that the potential for denitrification to take place in a soil profile was actually greater in late summer than in mid-summer since subsoil temperatures were higher later on in the season. The ratio of fluxes of N2O to N2 was a function of time, moisture content distribution, the depth at which the maximum reduction of NO3− took place, and NO3− concentration. Key words: Transport, denitrification, gas flux, nitrous oxidenot available