Pedotransfer functions: State of the art, problems, and outlooks

The potential, state of the art, and outlooks of using the pedotransfer functions concept in soil science were analyzed. Current methods of developing the pedotransfer functions and their statistical and functional testing were considered. Problems related to the spatially distributed estimates of soil properties and parameters and their use in predictive modeling and soilscape assessment are discussed.     

Experimental study on the relation between the fractal characteristics and solute transport parameters of sandy soil

Journal of Soils and Sediments - Assessing solute transport in porous media is important for better understanding groundwater contamination. Transport parameters are closely related to the soil...     

Models for radionuclide transport in soils

Abstract. Mathematical models describing radionuclide transport in soil developed for radiological assessment have increased in complexity over the last decade. In particular fairly simple 'black box', equilibrium approaches have given way to more complex, time-dependent, process-orientated methods. The increase in complexity of these models has outstripped the available data to specify, test and validate them. Current issues in model development include those that are associated with times up to a million years. Further development requires new laboratory and field research to provide adequate data to justify the inclusion or omission of known soil processes.     

Simulation of solute leaching in soils of differing permeabilities

Abstract. The model described divides the soil into layers and considers two categories of water, mobile and immobile, in each layer. It has two main parameters, one a measure of the soil's capacity to hold water and thence to retain solutes against leaching, and the other a measure of the ease with which water can pass through the soil and carry solutes with it. These are, in effect, capacity and rate parameters, and the model is unusual in having both. They can be estimated from the percentages of clay and other soil components. The rate parameter varied appreciably between plots in the field but in a consistent manner. The model has been validated against field experiments following the vertical movement of solute applied to the soil surface and allowed to leach, and the paper includes one such test.     

Electrical parameters of soils and pedogenesis

A concept of the relationship between the electrical parameters of soils (the electrical resistivity and the natural electrical potential) and their pedogenesis was advanced on the basis of the known classical laws of electromagnetism and the general theory of pedogenesis. It was shown that the electrical parameters of soils primarily depend on the density of the mobile electric charges, which include cations of the soil exchange complex and ions of the soil solution. Model notions were developed to explain the laws of the changes in the electrical parameters in the main genetic soil types for the catenary and zonal levels of the soil cover organization.     

Multi-domain model for pore-size dependent transport of solutes in soils

A multi-domain model for the transport of chemicals in soils is developed. The solute flux is related to the microscopic water flux, which is modelled using concepts to estimate the hydraulic conductivity of porous media. The pore space of the soil is divided into an arbitrarily large number of domains each representing an equivalent pore radius. The domains are arranged on a structural coordinate, perpendicular to the direction of mean water flow. Transport in the flow direction takes place in each domain by convection and diffusion with pore-size specific velocities. Solute mixing between the domains is simulated as convective-dispersive transport along the structural coordinate. The model is solved numerically for one-dimensional steady-state water flux under unit-gradient conditions. Required input parameters are the unsaturated conductivity function of a soil and a pore interaction coefficient which characterizes the solute exchange between the pore domains. Simulations show a gradual change from convection dominated transport (isolated tube model) to convective-dispersive transport. The length scale where this change takes place depends on the lateral mixing intensity, pore-size distribution of the medium, and saturation degree.     

Determination of chemical transport properties for different textures of undisturbed soils

Agrichemicals usually contaminate groundwater via preferential flow, therefore determination of the preferential flow characteristics of soil is needed. One model that predicts solute transport due to preferential flow is the mobile–immobile (MIM) solute-transport model, which partitions total water content (θ; m³ m^?3) into mobile (θ_m) and immobile fractions (θ_im). In undisturbed soils, a method is proposed for determining the MIM model parameters, i.e. immobile water fraction (θ_im), mass transfer coefficient (α) and hydrodynamic dispersion coefficient (D _h). Breakthrough curves were obtained for five different soil textures in three replicates, by miscible displacement of Cl^? in undisturbed soil columns. Cl^? breakthrough curves were evaluated in terms of the MIM model. Analysis suggests that the values of D _h and α increased with lighter soil textures and θ_im increased with heavier soil textures. The values of θ_im ranged from 5.31 to 14.28% in different soil textures. Furthermore, values of θ_im were found to be related to soil clay content. Values of α ranged from 0.0257 to 0.32 h^?1 and values of D _h ranged from 0.36 to 11.2 cm² h^?1 in different soil textures. A significant linear correlation was obtained between α, θ_im, D _h and soil saturated hydraulic conductivity (K _s) and pore water velocity (v). A multivariate pedotransfer function was developed to estimate α, θ_im and D _h based on the geometric mean (d _g) and the standard deviation (σ_g) of the diameter of soil particles and soil organic matter content. The pedotransfer functions for D _h, θ_im and α were validated by independent data sets from other investigators.     

Aspects of transport processes in aggregated soils

The way in which water and solutes move in aggregated soils depends on the mode of saturation of the pore space that is made up of the micropore region within the aggregates and the macropores surrounding them. When both regions are saturated, a hydraulic-head gradient causes water to flow preferentially in the macropores with little flow within the aggregates, so that movement of solutes into or out of the aggregates is mainly by diffusion caused by the difference between the solute concentrations of the water in the two regions. When macropores full of water surround unsaturated aggregates, water is imbibed by the aggregates giving rise to convective movement of solutes with the moving water. When the macropores are empty, the aggregates become almost isolated so that redistribution of water and solutes occurs only within the aggregates with very little transport of water and solutes between them. The movement of water and solutes in the micropore region within the aggregates can be considered to behave as if in a continuum, and can be described by Darcy's law and the dispersion equation, with boundary conditions imposed by conditions in the macropores. These physical considerations of transport behaviour in aggregated soils can be used to give guidance on soil management practices concerning drainage and leaching.     

Measurement of solute fluxes in macroporous soils: techniques, problems and precision

Abstract. Preferential flow has been increasingly recognised as a major component of water movement in many soils, particularly clays. This paper reviews problems in the measurement of solute fluxes in these soils, and discusses the solutions that have been adopted in UK studies of cracking clay soils. The estimation of solute fluxes is subject to many sources of error, which are best reduced by replicated measurements, such as those available in multi-plot experiments.     

Fractal character of some physical parameters of soils

The concept of soil fertility was defined in agrophysical terms. It was shown that this concept is related to the geometric parameters of soil, and, hence, fractal geometry can be used for its characterization. The main concepts of fractal geometry were considered in this aspect. Analysis of the dependence of the aggregate density and porosity of some soils on their size and water retention curve in the pF range 2–5 showed that these functions can obey the laws of fractal geometry in the entire or specific range or not obey them at all.     

Determination of salt-transport model parameters for leaching of saturated superficially salted soils

Analytical solutions were given for the problem of dissolution and leaching of salts in waterlogged regions with deep and shallow water tables under surface salinization and with account for equilibrium sorption (characterized by a linear exchange isotherm) and the problem of nonequilibrium irreversible sorption (characterized by biological transformation in the soil solution, which follows the first-order kinetics). A method was developed for determining the hydrochemical parameters (mixing step and dissolution rate coefficient of solidphase salts) from the average salt contents of water-saturated soils of given thickness before and after leaching determined under laboratory and field conditions.     

Kinetic parameters of the rhodanese reaction in soils

Studies to determine the kinetic parameters of the rhodanese-catalyzed reaction in soils showed that the K_m values of thiosulfate and cyanide for this enzyme are similar to those for the same enzyme isolated from other biological systems. Application of the three linear transformations of the Michaelis-Menten equation indicated that the apparent K_m constants of thiosulfate and cyanide varied among the soils used, but the results obtained by the three plots were similar. By using the Lineweaver-Burk plot. the K_m values of S₂O₃^?2 and CN^? for rhodanese activity in five soils ranged from 1.20 to 10.3 (av 5.46) and from 2.48 to 10.20 (av 5.81) mM, respectively. The V_max values ranged from 511 to 1431 (av 759) nmoles SCN^? produced · g^?1 soil · h^?1. The activation energy values ranged from 21.6 to 34.0 (av 28.0) kJ · mole^?1, and the average Q₁₀ for temperatures ranging from 10 to 60 C ranged from 1.25 to 1.45 (overall average. 1.37).     

The Langmuir parameters of orthophosphate and pyrophosphate sorption for ammoniated tropical soils

Abstract

The Langmuir parameters of orthophosphate (OP) and pyrophosphate (PP) sorption for the ammoniated tropical soils were determined. Positive linear relationships between OP and PP sorption maxima and amounts of anhydrous NH₃ added were noticed. Indexes of bonding energy of OP and PP increased exponentially as ammoniation level of the soils increased from 33 to 100% of ammonia retention capacity.     

The variation of critical-state parameters with water content for two agricultural soils

Triaxial tests were conducted on 436 reconstituted samples of two agricultural soils (a sandy loam and a loam) at moisture contents from air dryness to close to the respective liquid limits. Volume-change behaviour was largely consistent with the critical-state concept, the role of soil-moisture content being expressed in the values of the measured critical-state parameters. The most important parameters, and their inherent variability, have been specified. Most of the parameters varied systematically with soil-moisture content, but differently for the two soils. Some of the parameters were linearly correlated. The specific volume (v) was a linear function of the logarithm of the mean normal stress (In p) during normal compression and along the critical-state line (CSL) with significantly different gradients (λ and λ*, respectively) at most moisture contents except close to the liquid limits. The standard errors of λ* were typically of the same magnitude as the gradients found during hydrostatic unloading from two different v-levels whereas the standard errors of λ were up to a factor of ten smaller. The linear projections of the CSLs on the deviatoric stress (q)–mean normal stress plane showed intercepts on the q-axis. Generally, precompaction leading to brittle failure had very little influence on the shear strength as compared with the strength in critical state.     

Anion and cation exchange resin membranes to assess the phosphorus status of some Portuguese soils

Abstract

Ion exchange resin methods were applied to 78 different soils to assess their phosphorus (P) status for predicting their response to P fertilization. The techniques used were anion exchange resin membranes eluted with hydrochloric acid (HCl) (AEM) and cation‐anion exchange resin membranes eluted with HCl (CAEM‐HC1), sodium chloride (NaCl) (CAEM‐NaCl) or water with directly color development (CAEM‐H₂O). Greenhouse studies were conducted with the same soils in order to validate laboratory data. Ryegrass was grown with two levels of P: nil and 150 mg P kg^‐1 of soil. Results indicate that soil P levels are significantly correlated (p<0.001) if extracted with AEM or CAEM, both eluted with HCl, although the CAEM technique had extracted larger amounts of P. Concerning the type of elution, results did not show significant differences (p<0.05) between CAEM‐HC1 and CAEM‐NaCl, but both were significantly correlated (p<0.001) with the results obtained with CAEM‐H₂O. All the techniques used to measure extractable P correlated significantly with relative yield and P uptake by ryegrass, showing their ability to predict soil P availability. Nevertheless, CAEM extraction had higher values of r². Among the three techniques for elution, the levels of correlation with the biological parameters were equivalent. From these results, it was concluded that: (i) exchange resins, specially CAEM, is an accurate method to assess the P fertility status of soils, and (ii) the traditional step of elution can be avoided, allowing the process to be less time consuming, thus more suitable for routine use.     

The use of porous ceramic cup water samplers to measure solute leaching on chalk soils

Abstract. This paper reports results from a four year study to investigate the suitability of porous ceramic cups to measure solute leaching on shallow chalk soils. Measurements were carried out in one field following surface applications of nitrate and bromide tracers and in two fields after only bromide was applied. Soil water samples were collected from porous cups at 30,60 and 90cm depth after every 25 mm of drainage, and soil samples from 0–30, 30–60 and 60–90 cm were collected monthly eachwinter. Soil matric suctions andvolumetric moisture content were measured in one winter. Leaching losses, measured with ceramic cups were compared with those measured by soil analysis. Porous cups installed in chalk at 60 and 90 cm depth were only able to collect samples regularly when soil matric suctions were less than 15 kPa. Water held at such low suctions is likely to move quickly through relatively large fissures in the chalk. The slow rate of equilibration between solute concentrations in water moving in macrofissures and those in water moving through micropores of the chalk matrix, means that porous cups may not provide good estimates of leaching losses if they are installed in chalk rock.     

Lead and cesium transport in european forest soils

The downward velocity of Pb and Cs in undisturbed European forest soils is determined from the depth distribution of atmospheric ²¹⁰Pb , and from the penetration depth of the bomb Cs peak in the soil. The downward velocity of Pb and Cs shows no correlation with soil type and pH. The downward migration of Pb and Cs is found to be due to an apparent downward movement of organic material, caused by the sedimentation of soil organic matter and subsequent turnover into CO₂ . Cs migration, however, shows an additional velocity component which is found to depend on the turnover rate of soil organic matter.     

Lateral solute mixing processes — A key for understanding field-scale transport of water and solutes

Lateral mass exchange mechanisms affect the spreading of solutes in the main direction of flow. Modeling vertical solute spreading requires therefore an understanding of the lateral transport across regions of varying velocities. Experimental observations show that the variable extent and rates of lateral mixing cause dramatically different transport regimes, which can neither be predicted nor explained mechanistically in terms of known state variables. In this paper, we show that the various solute flow regimes are sensitive to the relative magnitudes of the vertical and lateral solute particle velocities. Different concepts of lateral mass exchange are discussed, and we postulate that physically meaningful parameters describing lateral mixing would be a clue for deciding a priori which solute transport regime is appropriate for a particular soil or soil horizon.     

Spatial variability of bromide and atrazine transport parameters for a Udipsamment

Variability of soil properties can have major influence on the results of solute transport models when applied to field conducted experiments. The objective of this study was to determine spatial variability of transport and degradation parameters for a bromide tracer and atrazine herbicide in a 0.1 ha field of alluvial soils under no-till management. The soil investigated was a Sarpy (mixed, mesic Typic Udipsamment) with surface texture varying from sand to loam. The field was classified into three areas (Area I, sand; Area II, sandy loam; and Area III, loam) according to surface texture. Atrazine and bromide were applied at rates of 1.8 kg ha^− 1 and 115 kg ha^− 1, respectively. Soil cores were extracted at selected dates, segmented into 75 mm increments, and analyzed for each chemical separately. Soil physical and chemical properties were determined as a function of soil depth from additional soil cores removed when the transport study was completed. The value of the average pore water velocity, v, estimated using bromide concentrations was 6.0 mm day^− 1 which was about 29% lower than the value computed from meteorological information, 8.5 mm day^− 1. The average bromide dispersion coefficient (D), 508 mm² day^− 1, was 80 times higher than that for atrazine, 6.13 mm² day^− 1. The bromide v and D parameters were found to be spatially autocorrelated with ranges between 20 and 23 m and 12 and 24 m, respectively. Atrazine retardation and degradation parameters were found to have spatial structure for only one sampling date each with a range between 18 and 24 m. Soil textural parameters were found to have spatial structure for the first three depths (0–45 cm) of the five depths monitored (0–75 cm). This study found spatial autocorrelation within about 20 m for bromide transport parameters and a few atrazine transport parameters for an alluvial site.