WATER-MOVEMENT, OXYGEN-SUPPLY AND BIOLOGICAL PROCESSES ON THE AGGREGATE SCALE

An experimental respirometer system and a theoretical simulation model were developed to study the dynamics of water movement, oxygen supply and biological processes on the aggregate scale in unsaturated soil. The experimental system enables one to measure simultaneously the dist ribution of water, oxygen, and chemical substances as a function of sp ace and time in an unsaturated, artificially made, homogeneous, cylind rical aggregate and the changes in atmospheric composition as a functi on of time in the chamber that contains the aggregate. Non-destructive measurements during an experiment involve gamma ray attenuation, gas chromatography and polarography. Destructive measurements are performe d at the end of an experiment in the form of chemical analyses of soil . The theoretical model enables one to calculate simultaneously the di stribution of water, bacteria, gases (e.g. oxygen, carbon dioxide and molecular nitrogen), absolute soil atmospheric pressure and solutes (e .g. nitrate and glucose) as a function of space and time for exactly t he same cylindrical geometry as in the experimental set up. Also, the changes in atmospheric composition as a function of time in the chambe r that contains the aggregate are calculated. Except for water transpo rt, all processes are caused by microbial activity, since roots are ab sent in the aggregate. The respirometer system was designed to generat e coherent data sets to evaluate the simulation model, while the model was needed to more completely interpret the data obtained from the ex periments. The reported experiment shows that hysteresis in the soil w ater characteristic strongly affects the water distribution in the agg regate. As a result, the oxygen supply to the interior of the aggregat e is decreased to such an extent, that anaerobiosis is maintained ther e after the oxygen is consumed. The respiratory quotient (RQ) was abou t one in the experiment, although one would expect larger values in pa rtially anaerobic soil. Therefore, the RQ does not seem a sensitive me asure to decide whether a soil is partially anaerobic. Nevertheless, t he consumption rate of oxygen and the production rate of carbon dioxid e compared well with field data. This is the result of the pretreatmen t of the soil, which aimed at avoiding the flush of microbial activity upon wetting. The simulated results showed a satisfactory agreement w ith the experimental data: part of the experimental results could be d escribed quantitatively, whereas other data that deviated from the exp erimental data could be understood by studying the dynamic behaviour o f the model. Hysteresis in the soil water retention curve resulted in low values of the gas-filled porosities in the outer shell of the part ially wetted aggregate, permitting gaseous exchange only through the w ater phase of soil. As a result anaerobiosis occurred. The model is ve ry sensitive for the so-called critical gas filled porosity below whic h no ps-continuous pores exists. The simulated respiratory quotient wa s seen to be strongly affected by transport processes. Therefore, also on theoretical grounds it was concluded that RQ is not a sensitive in dicator of partial anaerobiosis in soil. The objective of this paper i s to describe both the experimental respirometer system and the theore tical simulation model, and to report some of the measurements.