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A set of equations governing oxygen diffusion and consumption in soils has been developed to include microbial and plant-root sinks. The dependent variable is the transformed oxygen concentration, which is the difference between the gaseous concentration and a scaled value of the aqueous oxygen concentration at the root-soil interface. The results show how, as the air-filled porosity decreases, the reduced oxygen flux causes the depth of extinction to decrease. The results also show how the depth of extinction at a particular value of soil water content decreases with increasing temperature, due to increased microbial respiration. The critical value of water content at which the oxygen concentration goes to extinction at a finite depth was compared with alternative calculations with only a microbial sink. By ignoring the feedback of oxygen concentration on root uptake, the alternative calculations yielded substantially higher critical values of water content at all temperatures. Two soil oxygen diffusion coefficient functions from the literature were compared and shown to give significantly different critical values of water content for fine-textured soils, one more realistic than the other. A single relationship between the extinction depth and the ratio of the water content to the critical value was shown to apply for all temperatures and soil textures. The oxygen profiles were used along with a function relating redox potential to oxygen concentration to generate redox potential profiles. This application of the model could be useful in explaining soil biochemical processes in soils. For one such process, denitrification, the depth at which a critical oxygen concentration is reached was calculated as a function of the air-filled porosity and temperature of the soil. The implications of the critical value of soil water content in terms of water-filled pore space and matric potential are discussed in relation to the diffusion coefficient functions and recent literature. Additional keywords: air-filled pore space, denitrification, diffusion coefficient function, oxygen transport, redox potential, soil respiration., Introduction The transport of oxygen into soil and its distribution within the profile are fundamental to the understanding of aeration, and biochemical and chemical reactions in soil. The oxygen status [...]

An analytical solution for steady-state oxygen transport in soils including 2 sink terms, viz roots and microbes with the corresponding vertical distribution scaling lengths forming a ratio p, showed p governed the critical air-filled porosity, [θ.sub.c], needed by most plants. For low temperature and p, [θ.sub.c] was < 0.1 but at higher temperatures and p = 1, [θ.sub.c] was >0.15 [m.sup.3]/[m.sup.3]. When root length density at the surface was [10.sup.4] m/[m.sup.3] and p > 3, [θ.sup.c] was 0.25 [m. sup.3] [m.sup.3], more than half the pore space. Few combinations of soil and climate regularly meet this condition. However, for sandy soils and seasonally warm, arid regions, the theory is consistent with observation, in that plants may have some deep roots. Critical [θ.sub.c] values are used to formulate theoretical solutions in a forward mode, so different levels of oxygen uptake by roots may be compared to microbial activity. The proportion of respiration by plant roots increases rapidly with p up top ≅ 2. Synthesis of vertical root biomass density, L [= [L.sub.0] exp (-z/[Z.sub.r]), z is the depth positive down (m)] (m/[m.sup.3]), data using an exponential function to represent the distribution suggested that, on average, 70 ± 10% of fine roots in 10 terrestrial biomes were located in the upper 0.1 m of soil. Integrated over the root-zone, [L.sub.T] is given by the product of the function's 2 parameters, the surface value of L, [L.sub.0] (m/[m.sup.3]), and length scale, [Z.sub.r] (m). As postulated, negative correlations were obtained between [L.sub.0] and [Z.sub.r]. For a maize (Zea mays L.) crop, significantly different distributions were measured during relatively dry and wet seasons and predicted by our model. For woody and herbaceous plants, [Z.sub.r] (the value determines the rate of decrease in L with depth) averaged 0.3 and 0.2 m, respectively, while the corresponding averages for R,n0 [= [L.sub.O].Pr, Pr is root density (kg/m)] were 2.7 and 1.1 kg/[m.sub.3]. Additional keywords: oxygen, roots, root respiration, soil respiration, soil aeration., Introduction For most plants to grow in soil, a proportion of the pore space needs to be filled with air (Glifiski and Stepniewski 1985). This allows for the exchange of [...]

In general, the water content (θ) form of Richards' equation is not used when modeling water flow through layered soil since θ is discontinuous across soil layers. Within the literature, there have been some examples of models developed for layered soils using the θ-form of Richards' equation. However, these models usually rely on an approximation of the discontinuity at the soil layer interface. For the first time, we will develop an iterative scheme based on Newton's method, to explicitly solve for θ at the interface between 2 soils within a numerical scheme. The numerical scheme used here is the Method of Lines (MoL); however, the principles of the iterative solution could be used in other numerical techniques. It will be shown that the iterative scheme is highly effective, converging within 1 to 2 iterations. To ensure the convergence behaviour holds, the numerical scheme will be tested on a fine-over-coarse and a coarse-over-fine soil with highly contrasting soil properties. For each case, the contrast between the soil types will be controlled artificially to extend and decrease the extent of the θ discontinuity. In addition, the numerical solution will be compared against a steady-state analytical solution and a numerical solution from the literature. Additional keywords: heterogeneous soils, unsaturated zone, water flow, numerical solution., Introduction The study of water flow through layered soils has been an important environmental problem, which has relevance in various fields such as contaminant transport, agricultural science, and waste disposal [...]

The generation of acidity from oxidation of pyrite in acid sulfate soils requires the transport of oxygen into the soil profile. The sink for this oxygen will not only be the chemical reaction with pyrite but the biological processes associated with both microbial and plant respiration. The biological sinks in burning the oxygen ([O.sub.2]) will release C[O.sub.2]. The respiratory quotient which is the molar volume ratio of [O.sub.2] : C[O.sub.2] varies between 1.3 and 0.7 depending on the source of the organic matter being oxidised, but is generally 1.0. The oxidation of pyrite by oxygen will, by comparison with the biological processes, produce minor amounts of C[O.sub.2] (if any) by reaction with intrinsic carbonate minerals. Gas samplers were installed into the soil at various depths and samples collected from these at approximately fortnightly intervals. The samples were analysed by gas chromatography and the C[O.sub.2] and [O.sub.2] profiles obtained. The flux of these gases was calculated and the difference between these attributed to the oxidation of pyrite. The flux difference varied over the period of sampling and on average gave an in situ oxidation rate of 11.5 tonnes [H.sub.2]S[O.sub.4]/ha.year. This is considerably more that the rate of export of acidity from this site and would explain the considerable actual acidity storage in these soils. A model is developed for steady state transport of oxygen into soils with an exponentially decreasing biological sink with depth and an exponentially increasing chemical (pyrite) sink with depth. The model is developed in non-dimensional variables, which allows the relative strengths and rates of increase or decrease in sink terms to be explored. This model does not explicitly treat the flow of oxygen in macropores. Other models that do explicitly calculate macropore flow are compared and found to give similar results. These results suggest that the use of biological or other sinks near the soil surface could be a useful method for reducing the oxidation rate of pyrite in acid sulfate soils., Introduction The rate of oxidation of pyrite is a topic where there are few measurements and much speculation. This is because there are many different pathways via which pyrite can [...]

A model that couples the diffusion of [O.sub.2] to plant roots at the microscale to diffusion of [O.sub.2] through the soil at the macroscale is derived. The solution is complex, but if the length scales ([Z.sub.r] and [Z.sub.m]; see APPENDIX for list of symbols) are equated a simple analytical expression can be obtained. This model is used to investigate relationship between a critical air-filled porosity ([[theta].sub.m]) and the other parameters; viz temperature (T), [O.sub.2] concentration in the bulk soil (C*), [O.sub.2] concentration at the root surface ([C.sub.r]), root length density (L), the ratio of root radius (a) to the water film radius (R), microbial respiration ([M.sub.o]), and length scales ([Z.sub.m] and [Z.sub.r]) related to the depth to which microbial and plant respiration are active in the model using sensitivity analysis. The model shows that [[theta].sub.m] is not very sensitive to the [O.sub.2] concentration at the root surface ([C.sub.r]), or the ratio of root radius (a)/ water film radius (R), but is sensitive to all the other parameters in some part of their range. The results indicate that indices used to define soil aeration; [O.sub.2] diffusion rate (ODR) or [O.sub.2] flux, [O.sub.2] concentration, or air-filled porosity, which have been previously used, are related and a single critical value for these is unlikely. If a constant critical value exists for one of these indexes it cannot exist for the other two. It is also shown that it is highly unlikely that a universal critical parameter related to soil aeration exists for any of these parameters. It is concluded that more parameters than ODR, [O.sub.2] concentration, or air-filled porosity need to be measured if progress in soil aeration research is to be made.

The FILTER (Filtration and Irrigated cropping for Land Treatment and Effluent Reuse) technique was developed to provide a sustainable system for treatment of saline sewage effluent on naturally occurring saline and/or sodic soils. Potentially, it can also be used to ameliorate soils that are salinised by inappropriate application of saline effluent on soils with impeded drainage. The FILTER technique involves using the nutrient-rich effluent for irrigated cropping combined with removal of excess water from the root-zone through a subsurface drainage system, during wet weather and winter periods when evapotranspiration demand is low. This paper describes the changes in salinity and sodicity in FILTER plots used for land application of saline sewage effluent on a heavy clay soil with restricted drainage, at the Griffith City Council sewage works site. The field experiments consist of trials conducted on four 1-ha plots, over an 18-month period. The pre-FILTER soil chemical characteristics and their changes with FILTER operations were measured. In addition, the volumes and the chemical properties of the effluent applied and subsurface drainage water passing through the soil were monitored. These data are used to explain the salinity and sodicity changes within the FILTER soils, and their potential effects on soil stability. Management options to minimise salinity and sodicity to provide a sustainable system are suggested. Additional keywords: wastewater treatment, SAR, ESP, soil hydraulic properties, controlled subsurface drainage., Introduction Conventional systems of land application of wastewater on soils with restricted internal drainage, which occur extensively around urban areas in south-eastern Australia, could lead to soil degradation through waterlogging [...]

Fitting of experimental data to models for flow from disc infiltrometers is discussed. Application of the infiltration model for estimating soil hydraulic properties is also explored.

FILTER is an innovative, CSIRO developed system for treating effluent using high rate land application and subsequent effluent recapture via a closely spaced, subsurface drainage network. We report on the summer performance of a FILTER system established in a subtropical environment on a relatively impermeable swelling clay soil underlain by a deep regional water table. Using secondary treated sewage effluent, the FILTER system produced effluent of tertiary nutrient standards (h5 mg/L TN; h1 mg/L TP), with salinity levels suitable for subsequent irrigation reuse (EC h2.5 dS/m). Removal of faecal coliforms was considerably less effective. The hydraulic loading rate achieved was about two and a half times larger than conventional irrigation demand, but this was associated with high deep percolation losses (c 3 mm/day). Comparisons are made with the original FILTER system developed and tested by Jayawardane et al. in temperate Australia. Suggestions are made for modifications to, and further testing of FILTER in a subtropical environment. [ABSTRACT FROM AUTHOR]