『Abstract
A model of the void space of soil is presented, and used for
the a priori biophysical simulation of denitrification. The model
comprises a single critical percolation channel through a 5 cm
stack of four unit cells of a dual-porous void structure. Together,
the micro- and macro-porous structures closely replicate the full
water retention characteristic of a sandy clay loam soil from
the Woburn Experimental Farm operated by Rothamsted Research,
UK. Between 1 and 10 micro-porous hot-spot zones of biological
activity were positioned at equally spaced distances within 5
cm from the surface, and at either 10 μm or 100 μ 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 pore network was fully
saturated following addition of an aqueous ‘amendment’ of nitrate
and glucose which started the reactions, and which mirrored an
established laboratory protocol. Diffusion coefficients for Fickian
and Crank-Nicolson calculations were taken from the literature,
and were corrected for the tortuosity of the micro-porosity. The
model was used to show the amount of carbon dioxide, nitrous oxide
and molecular 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-spot zones 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 soil compaction and saturation on the emission of nitrous oxide.
Keywords: Nitrous oxide; Denitrification; Soil structure; Void
network model; Anaerobic respiration; Diffusion』
1. Introduction
2. Modelling
2.1. The pore-network model
2.2. Larger scale simplified model
2.3. Reaction and diffusion calculations
3. Results and discussion
4. Sensitivity analysis
4.1. Effect of the reaction rate of N2
production
4.2. Effect of the values of O2 thresholds
5. Summary and conclusions
Acknowledgements
References