Soil physical properties often regulate aeration-dependent microbial activities important to nutrient cycling, soil fertility and environmental quality. Microbial activity depends on soil water content and is maximum at a water content where the limiting effects of substrate diffusion and O 2 supply are equal. The mechanism whereby this occurs and predictions of the soil water content where aerobic microbial activity is a maximum were the objectives of this study. In particular, this study predicted the shape of the microbial activity vs. water content function from soil physical concepts. Soil physical processes are assumed to influence microbial activity by limiting the steady flux of a required substrate or O 2 to sites of microbial activity. Steady-state flux relations are used to define the activity function. The dependence of diffusion coefficient on water content or air-filled porosity is assumed. With these assumptions, it is possible to show that a maximum in the activity function exists. The predicted shape of the activity curve is consistent with experimental observations. The relationship between aeration-dependent microbial activity and soil water content facilitates evaluating the indirect effects of soil management practices, such as tillage, on microbial activity.
A theory of solute movement is presented for those soils where the liquid‐filled pores can be partitioned into two distinct pore size classes. One region represents macro‐ or interaggregate porosity, and the other represents matrix or intraaggregate porosity. The regions may differ in dispersion coefficient, porosity, and flow velocity. In addition, an interaction coefficient characterizes the linear transfer between regions. A regular perturbation method is used to solve the model equations for small interaction coefficients. It is shown that if the interaction coefficient is large, the model approaches the classical dispersion equation. Esitmates of the interaction and its dependence on flow rate are presented along with the influence of interaction on the shape of the breakthrough curves.
Analysis of time‐dependent processes in soils has been the subject of recent interest. Examples of such interest include the demonstration of nonequilibrium conditions during transport, slow release of plant nutrients, and a fundamental interest in reaction mechanisms. No unified approach to these problems exists in the literature. The present study was undertaken to provide uniformity in terminology and a methodology for solving a variety of problems. The interrelationship of chemical kinetics and transport processes is examined. Particular attention is paid to potential ambiguities and alternative interpretations of concentration histories. A variety of rate law formulations are presented. Microscopic and macroscopic approaches to transport process are described. The implications of these descriptions of transport upon chemical reactions are discussed, it is concluded that reaction kinetics and transport may be readily confounded. Furthermore, different kinetic mechanisms may result in concentration histories that are not distinguishable experimentally.
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