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

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Abstract
A respirometer system was developed to study the dynamics of partial anaerobiosis and denitrification in unsaturated soil. The system enables one to measure simultaneously the distribution of water, oxygen, nitrate, nitrite, ammonium, and pH as a function of space and time in an unsaturated, artificially made, 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 respirometer system was especially designed to generate coherent data sets to evaluate a simulation model that calculates the development of denitrification products as a function of environmental conditions. Nondestructive measurements during an experiment involve gamma-ray attenuation, gas chromatography, and polarography. Destructive measurements are executed at the end of an experiment in the form of chemical analyses of soil. The reported experiment shows that hysteresis in the soil water characteristic strongly affects the water distribution in the aggregate. As a result, the oxygen supply to the interior of the aggregate is decreased to such an extent that anaerobiosis is maintained there after the oxygen is consumed. The respiratory quotient and the release of denitrification products are underestimated in the partially wet soil because of the high solubilities of carbon dioxide and nitrous oxide in soil moisture. Large amounts of nitrite have been found. Therefore, assessment of denitrification through the measurement of nitrate alone will overestimate nitrogen losses, while the measurement of nitrous oxide and molecular nitrogen alone will give an underestimation. The consumption rate of oxygen and the production rates of carbon dioxide, nitrous oxide, and molecular nitrogen compare well with field data. This is the result of the pretreatment of the soil, which aimed at avoiding the flush of microbial activity upon wetting. The results support the thesis that denitrification will occur in soil when at a certain place and time oxygen is absent and bacteria capable of denitrification, water, nitrate, and decomposable organic compounds are present. The respirometer system yields valuable data to evaluate the simulation model. However, full account of the interrelationships among the generated data can be achieved only by the same simulation model, because the measured variables reflect the integrated effect of biological activity and transport processes. The objective of this paper is to describe the respirometer system and its measuring devices and to report some of the measurements.