A precise bio-physical model of denitrification and N2O emission in soil

A precise, dual porous model of the void space of soil is presented, and used for the modelling of nitrification and denitrification. The model comprises a single critical percolation channel through a 5 cm stack of four unit cells of a previous dual-porous void structure model. The previous model closely reproduced the full water retention curve and macroand micro-porosities of an experimental sample, namely Warren soil from the Woburn Experimental Farm operated by Rothamsted Research, U.K.. Between 1 and 10 micro-porous „hot-spots‟ of biological activity were positioned at equally spaced distances from the surface, and at either 10 m or 100 m from the critical percolation channel. Nitrification and denitrification reactions within the hotspots were assumed to follow Michaelis-Menten kinetics, with estimated values of rate coefficients. Estimates were also made of the threshold values of oxygen concentration below which the anaerobic processes would commence. The sample was fully saturated following addition of an aqueous „amendment‟ of nitrate and glucose which started the reactions. Diffusion coefficients for Fickian and Crank-Nicolson calculations were taken from the literature, and were corrected where necessary for the tortuosity of the micro-porosity. The model was used to show the amount of carbon dioxide, nitrous oxide and nitrogen emerging from the simulated soil with time. Adjustment of the rate coefficient and oxygen threshold concentrations, within the context of a sensitivity analysis, gave emission curves in good agreement with previous experimental measurements. Positioning of the hot-spots away from the critical percolation path slowed the increase and decline in emission of the gases. The model and its parameters can now be used for modelling the effect of compaction and saturation on the emission of nitrous oxide. ___________________________________________________________________________ 2