DYNAMICS OF PARTIAL ANAEROBIOSIS, DENITRIFICATION, AND WATER IN A SOIL AGGREGATE

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Abstract
A simulation model was developed to study the dynamics of partial anaerobiosis and denitrification in unsaturated soil. The model enables one to calculate simultaneously the distribution of water, bacteria, oxygen, carbon dioxide, nitrous oxide, molecular nitrogen, neon, absolute soil atmospheric pressure, nitrate, nitrite, and glucose as a function of space and time in an unsaturated, homogeneous, cylindrical aggregate and the changes in atmospheric composition as a function of time in the chamber that contains the aggregate. Except for water transport, these processes are caused by microbial activity, because roots are not present in the aggregate. The simulation model is the theoretical counterpart of the experimental "soil aggregate system" as studied in a previously described respirometer setup. The simulated results showed a satisfactory agreement with experimental data: part of the experimental results could be described quantitatively, whereas other data that deviated from the experimental data could be understood by studying the dynamic behavior of the model. Hysteresis in the soil water retention curve resulted in low values of the gas-filled porosities in the outer shell of the partially wetted aggregate, permitting only gaseous exchange through the water phase of soil. As a result anaerobiosis and denitrification occurred. A major conclusion was that appropriate model parameterization was needed first. To that purpose the model will be used to plan repirometer experiments, to help interpret the experimental data so obtained, and to investigate the relative importance of a number of parameters in a sensitivity analysis. Furthermore, it was concluded that only the interaction of experiment and theory will ultimately lead to a full understanding of the complex soil biological system described. The objective of this paper is to describe the simulation model, to discuss its parameterization, and to compare some of the simulated results with those of the experimental system described previously.