MATHEMATICAL-MODELING OF NITROUS-OXIDE EVOLUTION DURING NITRIFICATION

There is a need for a process-based model of N2O evolution during nitrification as part of larger models used to study trace gas exchange between terrestrial ecosystems and the atmosphere. The model proposed here for N2O evolution is based on the hypothesis that NO2- is used as an alternative acceptor for electrons not accepted by O-2 during C oxidation for growth by NH3 oxidizers. Rates of N2O evolution simulated using this hypothesis are thereby sensitive to any physical or biological attribute of the soil that controls the demand for, or the supply of, O-2 during nitrification, such as substrate concentration, temperature (T) or waiter content (theta). These rates were compared under a common range of T (10, 20 and 30 degrees C) and theta (0.10, 0.20 and 0.30 m(3)m(-3)) to ones reported in the literature that were measured during incubation of an NH4+ amended soil. Simulated rates of N2O evolution reproduced a sensitivity to T and theta that increased with both T and theta, although these rates were overestimated at theta = 0.20 m(3)m(-3). This overestimation is probably caused by uncertainty in parameterizing the model equation in which rates of gas transfer between gaseous and aqueous phases are calculated. Ratios of simulated N2O evolution to NO2- +NO3- production increased with both T and theta through a range of 1-5 x 10(-3) mu g N2O-N mu g(-1) NO2- + NO3--N in a way that was consistent with ratios of measured evolution to production reported from the NH: amended soil as well as with those reported from other soils and pure cultures. As part of the larger ecosystem model ecosys, this model hypothesis will make a useful contribution towards the estimation of N2O evolution from terrestrial ecosystems under different climates and fertilizer managements.