Soil Macropores Research Articles

Soil compaction and poultry litter effects on factors affecting nitrogen availability in a claypan soil

Soil compaction may affect N mineralization and the subsequent fate of N in agroecosystems. Laboratory incubation and field experiments were conducted to determine the effects of surface soil compaction on soil N mineralization in a claypan soil amended with poultry litter (i.e., Turkey excrement mixed with pine shavings as bedding). In a laboratory study, soil from the surface horizon of a Mexico silt loam soil was compacted to four bulk density levels (1.2, 1.4, 1.6 and 1.8 Mg m −3) with and without poultry litter and incubated at 25 °C for 42 days. A field trial planted to corn ( Zea mays L.) was also conducted in 2002 on a Mexico silt loam claypan soil in North Central Missouri. Soil was amended with litter (0 and 19 Mg ha −1) and left uncompacted or uniformly compacted. Soil compaction decreased soil inorganic N by a maximum of 1.8 times in the laboratory study; this effect was also observed at all depths of the field trial. Compacted soil with a litter amendment accumulated NH 4 +-N up to 7.2 times higher than the noncompacted, litter-amended soil until Day 28 of the laboratory incubation and in the beginning of the growing season of the field study. Ammonium accumulation may have been due to decreased soil aeration under compacted conditions. Application of litter increased soil N mineralization throughout the growing season. In the laboratory study, soil inorganic N in unamended soil was negatively correlated with soil bulk density and the proportion of soil micropores, but was positively related with soil total porosity and the proportion of soil macropores. These results indicate that soil compaction, litter application and climate are interrelated in their influences on soil N mineralization in agroecosystems.

Indicator of risk of water contamination by phosphorus from Canadian agricultural land

The indicator of risk of water contamination by phosphorus (IROWC_P) is designed to estimate where the risk of water P contamination by agriculture is high, and how this risk is changing over time based on the five-year period of data Census frequency. Firstly developed for the province of Quebec (2000), this paper presents an improved version of IROWC_P (intended to be released in 2008), which will be extended to all watersheds and Soil Landscape of Canada (SLC) polygons (scale 1:1, 000, 000) with more than 5% of agriculture. There are three objectives: (i) create a soil phosphorus saturation database for dominant and subdominant soil series of SLC polygons--the soil P saturation values are estimated by the ratio of soil test P to soil P sorption capacity; (ii) calculate an annual P balance considering crop residue P, manure P, and inorganic fertilizer P--agricultural and manure management practices will also be considered; and (iii) develop a transport-hydrology component including P transport estimation by runoff mechanisms (water balance factor, topographic index) and soil erosion, and the area connectivity to water (artificial drainage, soil macropores, and surface water bodies).

Soil hydraulic properties near saturation, an improved conductivity model

The hydraulic properties near saturation can change dramatically due to the presence of macropores that are usually difficult to handle in traditional pore size models. The purpose of this study is to establish a data set on hydraulic conductivity near saturation, test the predictive capability of commonly used hydraulic conductivity models and give suggestions for improved models. Water retention and near saturated and saturated hydraulic conductivity were measured for a variety of 81 top and subsoils. The hydraulic conductivity models by van Genuchten [ van Genuchten, 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892–898.] (vGM) and Brooks and Corey, modified by Jarvis [ Jarvis, 1991. MACRO—A Model of Water Movement and Solute Transport in Macroporous Soils. Swedish University of Agricultural Sciences. Department of Soil Sciences. Reports and Dissertations 9.] were optimised to describe the unsaturated hydraulic conductivity in the range measured. Different optimisation procedures were tested. Using the measured saturated hydraulic conductivity in the vGM model tends to overestimate the unsaturated hydraulic conductivity. Optimising a matching factor ( k 0) improved the fit considerably whereas optimising the l-parameter in the vGM model improved the fit only slightly. The vGM was improved with an empirical scaling function to account for the rapid increase in conductivity near saturation. Using the improved models, it was possible to describe both the saturated and the unsaturated hydraulic conductivity better than a previously published model by Jarvis. The pore size boundary of the macropores was found at a capillary pressure of −4 hPa corresponding to a circular pore diameter of 750 μm.

Single-porosity and dual-porosity modeling of water flow and solute transport in subsurface-drained fields using effective field-scale parameters

Water and solute flux from the subsurface drains of macroporous agricultural fields are simulated using two-dimensional single-porosity and dual-porosity models. Field-averaged (i.e. effective) parameters are calibrated from drainage outflow and validated for water flow and the transport of non-reactive solutes applied at discrete locations on the field. Both the single-porosity and the dual-porosity simulations capture the observed trends in the drainage hydrographs, with the dual-porosity model performing slightly better than the single-porosity model. The values of the effective hydrologic parameters, however, were not fully characteristic of macroporous soils. The physical meaning of the effective parameters was further questioned as neither the single-porosity nor the dual-porosity models could simulate the rapid transport of solutes to the subsurface drain. The discrepancies between the simulated and observed solute flux indicate that the actual spatial area contributing to drainage outflow in the field experiment was much greater than the integrated area of the simulated domain over which the effective parameters were calibrated. Supplementary simulations using parameters calibrated from solute flux data (outflow solute concentration multiplied by the outflow water flux) also fail to match both water and solute fluxes. The failure of the simulations is attributable to factors such as non-unique parameters and problems with representing a three-dimensional heterogeneous domain as a two-dimensional homogeneous system, including a misrepresentation of macropore flow paths. This study shows the fallacy of interpreting a hydrograph fit as evidence of the physical meaning of model parameters.

Comportamento de atributos relacionados com a forma da estrutura de Latossolo Vermelho sob sistemas de preparo e plantas de cobertura

Os sistemas de preparo de solo e de culturas têm influência significativa na estrutura do solo e nos fluxos de água e ar. Este trabalho objetivou avaliar os efeitos do manejo do solo sobre as propriedades relacionadas com estrutura de um Latossolo Vermelho cultivado com milho intercalado com plantas de cobertura de verão. Dois experimentos foram realizados em Chapecó (SC), em preparo reduzido (PR), de 1993 a 1998, e preparo convencional (PC), de 1994 a 1998, nos quais foram avaliados três sistemas de cultura: milho + guandu anão, milho + mucuna cinza e milho isolado. Adjacente aos experimentos, foi avaliada uma mata nativa. Em comparação com a mata nativa, os sistemas de preparo modificaram a estrutura do solo, com aumento da densidade e da resistência do solo à penetração e redução da macroporosidade e da porosidade total. Após cinco anos de uso, o PR com milho isolado não recuperou as propriedades relacionadas com a forma da estrutura; entretanto, no sistema milho intercalado com mucuna cinza, notou-se aumento da macroporosidade, porosidade total e condutividade hidráulica saturada, bem como a redução da densidade do solo. Estas melhorias, advindas do uso de plantas de cobertura, não foram observadas no sistema de preparo convencional. O uso de culturas intercalares foi adequado para melhorar a qualidade física desse Latossolo Vermelho argiloso, especialmente pelo maior aporte de resíduos de culturas.

Determining soil quality indicators by factor analysis

Soil quality indicators (SQIs) can be used to evaluate sustainability of land use and soil management practices in agroecosystems. The objective of this study was to identify appropriate SQI from factor analysis (FA) of five treatments: no-till corn ( Zee mays) without manure (NT), no-till corn with manure (NTM), no-till corn–soybean ( Glycine max) rotation (NTR), conventional tillage corn (CT), and meadow (M) in Coshocton, Ohio. Soil properties were grouped into five factors (eigenvalues > 1) for the 0–10 cm depth as: (Factor 1) water transmission, (Factor 2) soil aeration, (Factor 3) soil pore connection 1, (Factor 4) soil texture and (Factor 5) moisture status. Factor 2 was the most dominant, with soil organic carbon (SOC) the most dominant measured soil attribute contributing to this factor. For the 10–20 cm depth, factors identified were: (Factor 6) soil aggregation, (Factor 7) soil pore connection 2, (Factor 8) soil macropore, and (Factor 9) plant production. At 10–20 cm depth, Factor 6 was most dominant with SOC the most dominant measured soil attribute. Management × sample and slope position × sample interactions were significant among some factors for both depths. Overall, SOC was the most dominant measured soil attribute as a SQI for both depths. Other key soil attributes were field water capacity, air-filled porosity, pH and soil bulk density for the 0–10 cm depth, and total N and mean weight diameter of aggregates for the 10–20 depth. Therefore, SOC could play an important role for monitoring soil quality.

Stability and transportability of biosolid colloids through undisturbed soil monoliths

This study evaluated the stability of water-suspended biosolid colloids fractionated from municipal and agricultural wastes and their transportability through undisturbed soil monoliths. The aim of the study was to assess potential risks posed by dispersed biosolid colloid particles as carriers of contaminants associated with organic waste amendments applied to soils. All biosolid colloids showed remarkable stability over a wide range of pH conditions, with 50–90% remaining in suspension after 24 h. Lime-stabilized biosolid colloids (LSB) were more stable than poultry manure (PMB) and aerobically digested (ADB) biosolid colloids, with pH and organic matter (OM) content being the dominant factors influencing colloid stability. However, increased colloid stability did not always result in greater transportability. In spite of their higher overall stability, LSB colloids showed low transportability potential through undisturbed soil monoliths averaging < 0.1 C/ C 0, probably due to carbonate dissolution and increased ionic interaction with the soil matrix. The ADB and PMB colloids, with the highest OM content, exhibited increased mobility, particularly through soils with significant macroporosity. Breakthrough curves were mostly irregular, suggesting considerable macropore flow and increased interaction with the soil matrix. Eluent pH and EC fluctuations appeared to mainly affect LSB colloid transport, while the mobility of the remaining biosolid colloids was maintained over a wide range of ionic conditions. The findings of this study suggest that some biosolid colloids derived from soil-applied agricultural or municipal wastes may exhibit considerable mobility through soil macropores with potential to migrate to great distances in subsoil environments. Considering the high surface charge and electronegativity of these colloids, they may be a significant vector for hydrophobic contaminant transport and pollution of groundwater resources.

The fate of cadmium in field soils of the Danubian lowland

The susceptibility of soils to deep penetration of cadmium was assessed by measuring cadmium adsorption on soil particles <0.01 mm, easily mobile in soil macropores, and bypassing ratio. The latter is defined as a ratio of the rate of macropore flow to the rate of total (macropore and matrix) flow. Measurements were made on soils from the Danubian lowland, which is a large (1260 km 2) agriculturally managed area situated in the south-west of Slovakia, with a shallow (0.5–3.8 m deep) underlying aquifer containing about 10 km 3 of freshwater. In this study, the susceptibility of soils to deep penetration of cadmium was assessed on light, loamy-sand soil in Kalinkovo, medium heavy, loamy soil in Macov, and heavy, clay soil in Jurova. It was found that when the interaction between soil and cadmium lasted 1 min, more than 35, 32, and 48% of cadmium was adsorbed on the particles <0.01 mm in soils from Kalinkovo, Macov, and Jurova, respectively. In the case of ponding infiltration, more than 50% of water can flow via topsoil macropores in Kalinkovo, about 70% in Macov, and 96% in Jurova. This value of bypassing ratio can be met during an irrigation/rain with higher intensity then the infiltration rate into the soil matrix of studied soils. As the rains resulting in the macropore flow can occur 24 times on average in south-western Slovakia during the vegetation season, the probability of deep penetration of cadmium is very high, mainly during an initial stage of rainfall. For this reason, some mitigative agricultural practices (e.g. subsurface fertilizer banding or shallow ploughing) should be used in this region to prevent soils from the deep penetration of cadmium.

Soil macropores and compaction control the leaching potential of Escherichia coli O157:H7

The influence of soil structure in controlling leaching of Escherichia coli O157:H7 through soil was investigated under controlled conditions using both intact and repacked soil cores. Leaching rates of E. coli O157:H7 decreased with increasing dry bulk density and were significantly increased by the presence of earthworm (Lumbricus terrestris) burrows in repacked cores. For intact cores, the percentage of E. coli O157:H7 that leached through replicate cores within 72 h varied from 0.01% to 24%. In contrast, the dry bulk densities of intact cores varied only slightly and were not significantly correlated with leaching. Differences in the numbers of E. coli O157:H7 cells in the leachates were not related to variability in the flow volumes, which were relatively constant as a result of the experimental design, but were strongly correlated with the variations in concentrations of E. coli O157:H7 in the leachates. Relatively small variations in the internal structure of soil cores can therefore significantly affect the pathway that cells can take through soil. Factors such as compaction and the occurrence of pores providing preferential flow are prime determinants in the degree of leaching of E. coli O157:H7 through soil.

Dual-domain solute transfer and transport processes: evaluation in batch and transport experiments

Soil macropore networks establish a dual-domain transport scenario in which water and solutes are preferentially channeled through soil macropores while slowly diffusing into and out of the bulk soil matrix. The influence of macropore networks on intra-ped solute diffusion and preferential transport in a soil typical of subsurface-drained croplands in the Midwestern United States was studied in batch- and column-scale experiments. In the batch diffusion studies with soil aggregates, the estimated diffusion radius (length) of the soil aggregates corresponded to the half-spacing of the aggregate fissures, suggesting that the intra-ped fissures reduced the diffusion impedance and preferentially allowed solutes to diffuse into the soil matrix. In the column-scale solute transport experiments, the average diffusion radius (estimated from HYDRUS-2D simulations and a first-order diffusive transfer term) was nearly double that of the batch-scale study. This increase may be attributed to a loss of pore continuity and a compounding of the small diffusion impedance through macropores at the larger scale. The column-scale solute transport experiments also suggest that two preferential networks exist in the soil. At and near soil saturation, a primary network of large macropores (possibly root channels and earthworm burrows) dominate advective transport, causing a high degree of physical and sorption nonequilibrium and simultaneous breakthrough of a nonreactive (bromide) and a reactive (alachlor) solute. As the saturation level decreases, the primary network drains, while transport through smaller macropores (possibly intra-ped features) continues, resulting in a reduced degree of nonequilibrium and separation in the breakthrough curves of bromide and alachlor.

Chloride and atrazine transport through saturated soil columns

A possible contamination of water resources by the application of pesticides is a problem confronting many irrigated areas in arid and semi-arid areas. The best management practices have to be adopted to minimize pesticide transport and leaching under irrigated conditions. Atrazine dissipation in loam and sandy loam soils has been tested in the laboratory using disturbed soil columns under saturated flooding conditions. All the experiments were performed in replicates. The chloride transport was also studied to test its behavior as an inert tracer in both the soils. Atrazine and chloride breakthrough curves were analyzed with the parameter optimization program CXTFIT to determine transport parameters including pore-water velocity (v), retardation coefficient (R), hydrodynamic dispersion coefficient (D), and pulse duration (t o ). The pore-water velocity and pulse duration of the solute were estimated from the experimental conditions and kept constant during the optimization procedure. The results indicated that the R of chloride was not significantly different from 1, indicating that chloride is an inert tracer for the types of soil tested in this study. The average R of atrazine was 4.56 and 3.15 for sandy loam and loam soils, respectively. Results also showed that the hydrodynamic dispersion coefficient was much higher in the case of sandy loam soil compared to the loam soil for the two solutes, thus indicating non-equilibrium transport conditions. In the case of chloride, D increased from 0.4 for the loam soil to 16.2 cm2/min for the sandy loam soil. Similar results were observed in the case of atrazine in which D for the sandy loam soil was 60% higher than that for the loam soil. More atrazine leaching is expected under field conditions due to the presence of soil cracks and macropores.

Interaction of earthworm burrows and cracks in a clayey, subsurface-drained, soil

Installation of subsurface tile lines in poorly drained soils is a beneficial management practice that enhances crop productivity. In some instances, however, they can contribute to offsite losses of agricultural chemicals and sediment in drain flow. Movement of these materials through soil macropores (earthworm burrows and cracks) has been shown to contribute to this phenomenon. In order to determine if there was any interaction between these two types of macropores and subsurface drains we investigated water movement in a sandy clay field in southwest Finland that had 1 m-deep tile drains installed in the 1950s. Previous studies at this site suggested that cracks were important in terms of water movement and that Lumbricus terrestris L. populations were greater, and their burrows deeper, above the drains than in the area between drains. Mean infiltration rate for soil above the drains was twice that of the mid-drain position and the infiltration rates were positively correlated to L. terrestris numbers and biomass. Infiltration rates in individual L. terrestris burrows, measured with the plow layer removed to reduce the influence of cracks, ranged from 6 to 1043 ml min −1 (average 358 ml min −1) and did not appear to be related to the position of the burrows relative to the buried tile. Consistently higher infiltration rates (average 1080 ml min −1) were noted when measurements were made with the plow layer intact. Dye poured into the cracks adjacent to these burrows indicated water movement to the base of the plow layer, which acted as a hydraulic barrier, followed by lateral movement until open earthworm burrows were encountered. Water movement to the depth of the tile was exclusively in L. terrestris burrows with 106 dyed burrows m −2 observed 10 cm above the tile. These observations indicate that entry of water into this soil is probably dominated by cracks when it is dry enough for their formation, yet rapid movement of materials entrained by this flow to the subsurface drains depends on connection of the cracks to earthworm burrows.

Herbicide leaching as affected by macropore flow and within-storm rainfall intensity variation: a RZWQM simulation.

Within-event variability in rainfall intensity may affect pesticide leaching rates in soil, but most laboratory studies of pesticide leaching use a rainfall simulator operating at constant rainfall intensity, or cover the soil with ponded water. This is especially true in experiments where macropores are present--macroporous soils present experimental complexities enough without the added complexity of variable rainfall intensity. One way to get around this difficulty is to use a suitable pesticide transport model, calibrate it to describe accurately a fixed-intensity experiment, and then explore the affects of within-event rainfall intensity variation on pesticide leaching through macropores. We used the Root Zone Water Quality Model (RZWQM) to investigate the effect of variable rainfall intensity on alachlor and atrazine transport through macropores. Data were used from an experiment in which atrazine and alachlor were surface-applied to 30 x 30 x 30 cm undisturbed blocks of two macroporous silt loam soils from glacial till regions. One hour later the blocks were subjected to 30-mm simulated rain with constant intensity for 0.5 h. Percolate was collected and analyzed from 64 square cells at the base of the blocks. RZWQM was calibrated to describe accurately the atrazine and alachlor leaching data, and then a median Mid-west variable-intensity storm, in which the initial intensity was high, was simulated. The variable-intensity storm more than quadrupled alachlor losses and almost doubled atrazine losses in one soil over the constant-intensity storm of the same total depth. Also rainfall intensity may affect percolate-producing macroporosity and consequently pesticide transport through macropores. For example, under variable rainfall intensity RZWQM predicted the alachlor concentration to be 2.7 microg ml(-1) with an effective macroporosity of 2.2 E(-4) cm(3) cm(-3) and 1.4 microg ml(-1) with an effective macroporosity of 4.6 E(-4) cm(3) cm(-3). Percolate-producing macroporosity and herbicide leaching under different rainfall intensity patterns, however, are not well understood. Clearly, further investigation of rainfall intensity variation on pesticide leaching through macropores is needed.

Modelling environmental impacts of deposition of excreted nitrogen by grazing dairy cows

The soil nitrogen (N) and carbon dynamics model SOILN (which has interactive links to a grass growth model), and the dual-porosity contaminant transport model MACRO, have been used to study environmental pollution arising from grazing dairy cows. The models had been calibrated and tested in previous studies related to livestock agriculture. Information about N contents and other characteristics of urine and faeces excreted by dairy cows was assembled from literature sources. Watercourse pollution by nitrate and ammonium was the main environmental impact considered. Denitrified nitrogen losses were also estimated as an indicator of nitrous oxide pollution of air. Higher levels of nitrate pollution in tile drains (which feed into watercourses) were shown to arise under grazing compared to fields receiving slurry and cut for silage. Much of this raised nitrogenous pollution arises late in the grazing season. High levels of nitrate pollution could be attributed to various factors, including the fact that cows tend to congregate in certain areas of a field at a localised stocking rate much higher than the overall stocking rate, and due to deposition of N at times when grass cannot utilise it as a plant nutrient. The fact that urine and faeces patches are concentrated over a small proportion of the field area did not give an increase in overall loss when this was considered along with field areas receiving no excretions. Rapid transport through soil macropores of ammonium from urine led to high pollution loads during grazing on wet soil. In contrast to leaching, simulated N losses by denitrification were at a low level, and appeared to show little variation with factors which had a large effect on leaching losses. Overall, the forms of pollution most damaging to the environment due to spatially non-uniform excretion by grazing animals, appeared to be leached ammonium from urine transported by macropore flow, and leached nitrate exacerbated both due to cows congregating and due to deposition at times of low plant N uptake.

Inferring flow types from dye patterns in macroporous soils

In this study, we exploit dye patterns to identify flow types in macroporous soils as a step forward from using dye patterns simply as qualitative pictures that illustrate preferential flow. Dye tracer experiments at different irrigation rates and initial soil moisture conditions were carried out on three hillslope sites. Several vertical and horizontal soil sections were prepared after each experiment to analyze the patterns of the dye tracer Brilliant Blue FCF. Photographs of the soil sections were processed by image analysis to discriminate between stained and unstained areas and to classify stained areas into three classes of dye concentration. The images of the vertical sections were first analyzed according to conventional approaches (e.g., dye coverage). Then, a new approach was developed using the extent and distribution of the stained objects to classify flow into five types, two of these flow types occur only in the soil matrix, and three of them are related to the degrees of water flow between macropores and soil matrix (interaction). From horizontal sections, the macropore distribution was classified and, in combination with the dye pattern, the interaction quantified based on statistical description of the spatial relationship between macropores and stained areas. Categorized flow type profiles showed a logical sequence in each of our experiments, and we think the concepts will have broad application in soil science and infiltration research to compare the infiltration regimes of soils.

Preferential spread of the pathogenic fungus Rhizoctonia solani through structured soil

Most studies on soil fungi have been carried out with little explicit characterisation of soil structure within which fungi spread and biotic interactions occur. In this paper we use a combination of epidemiological ( colonisation efficiency) and soil bio-physical ( thin sectioning) techniques to investigate the role of macropores in soil on the spread of a fungal colony. Macropores, in the form of gaps orientated in various directions, were artificially introduced in replicated samples of sand and a sandy loam. The pathogenic fungus Rhizoctonia solani AG4 was introduced on the surface (encountering a gap whilst the colony expands over the surface) or within soil (encountering a gap whilst the colony spreads through the bulk soil). Depending on the orientation, location and width, gaps were demonstrated to act as preferential pathways (increasing the colonisation efficiency of R. solani), or as a partial barrier (reducing the colonisation efficiency). Within bulk soil, R. solani preferentially followed larger pores, enabling the fungus to by-pass more densely areas. Study of soil thin sections revealed that hyphal densities were greater in gaps than in the surrounding bulk soil. We use the results to discuss how macropore structure in soil can either enhance or reduce the parasitic spread and saprotrophic invasion of soil by fungi.

Mass fractal dimension of soil macropores using computed tomography: from the box‐counting to the cube‐counting algorithm

SummaryTransport phenomena in porous media depend strongly on three‐dimensional pore structures. Macropore networks enable water and solute to move preferentially through the vadose zone. A complete representation of their geometry is important for understanding soil behaviour such as preferential flow. Once we know the geometrical, topological and scaling attributes of preferential flow paths, we can begin computer simulations of water movement in the soil.The box‐counting method is used in three dimensions (i.e. cube‐counting algorithm) to characterize the mass fractal dimension of macropore networks using X‐ray computed tomography (CT) matrices. We developed an algorithm to investigate the mass fractal dimension in three dimensions and to see how it compares with the co‐dimensions obtained using the box‐counting technique in two dimensions. For that purpose, macropore networks in four large undisturbed soil columns (850 mm × 77 mm diameter) were quantified and visualized, in both two and three dimensions, using X‐ray CT. We observed an increasing trend between the fractal dimension and macroporosity for the four columns. Moreover, similar natural logarithm functions were obtained for the four cores by a least squares fit through plots of mass fractal dimension against macroporosity.

Modelling phosphorus leaching to watercourses from extended autumn grazing by cattle

AbstractA modelling approach was undertaken to investigate the effect of grazing animals on phosphorus pollution of water draining from grazed fields. Following a similar study in relation to slurry spreading in winter, the dual‐porosity hydrological and contaminant transport model, MACRO, was calibrated to represent transport of phosphorus from faeces through the soil to field drains. Rapid flows, through water‐filled macropores in wet soil, of phosphorus sorbed onto colloidal particles in the faeces of cattle appeared to be the dominant transport mechanism. The outputs of the model supported experimental evidence that levels of phosphorus pollution of water draining from grazed fields can rise substantially if grazing is extended into late autumn, particularly if grazing is extended until soil has wetted up to around the field capacity water content. The critical housing date, to avoid phosphorus losses rising to an unacceptable level, varied considerably between years. The outputs of the model suggest that phosphorus pollution does not occur during grazing under dry conditions where soil macropores do not contain water, so that losses would remain low if animals are housed before soil substantially wets up in the autumn.

Predicting preferential water flow in soils by traveling-dispersive waves

Rapid preferential drainage or by-pass flow of water and pollutants occurs in soil macropores such as burrows and channels formed by earthworm activity in soils. We show that preferential flow through these non-capillary pores can be described by a traveling-dispersive wave. This wave is the solution of a non-linear convective–dispersive equation (KDW model) that depends on three transport parameters: two are related to a convective celerity and the other one is a dispersive coefficient. We show that the flux–mobile water content relation is hysteretic and that it can be described by a non-linear function of the mobile water content and its first time derivative. By combining the latter relation with the continuity equation we derive the KDW model. This model can be viewed as a second-order correction of the purely convective kinematic wave model. The dispersive term incorporates the large-scale effects of dissipative forces without resolving the small-scale conservation equations in detail. We further present numerical solutions for the signaling problem and a direct method for estimating model parameters. The model is validated with data obtained from laboratory infiltration experiments on soil columns. The experiments were carried out in repacked soil columns inoculated with Allolobophora chlorotica earthworms. Varying rainfall intensities were applied at the top surface of the columns with a rainfall simulator. Both the mean of mobile water content within the columns and the drainage hydrograph at the bottom were recorded in time. The parameters of the model were estimated from the experimental flux–mobile water content relation. A very good agreement was found between model prediction and experimental data.

Lateral Water Diffusion in an Artificial Macroporous System: Modeling and Experimental Evidence

In two‐domain schematizations of macroporous soils or fractured rock systems, lateral mass exchange between macropores and the soil matrix is generally modeled as an apparent first‐order process. With respect to lateral diffusion, the system is thus characterized by a single parameter, the transfer rate coefficient, which is difficult to estimate a priori. We conducted water infiltration experiments in a laboratory column with an artificial macropore. The novel design of the experimental setup allowed us to discriminate between matrix flow and macropore flow, from which we could estimate the water exchange flux between the two domains. Most of the parameters in a dual‐permeability model could be determined independently of the experimental data. In particular, a theoretical expression for the transfer rate coefficient was derived by assuming lateral water and solute diffusion to be similar processes. Numerical analysis of the water exchange process revealed that the transfer coefficient depended also on the macropore conductivity. When this dependency was taken into account, the model reproduced the experimental data reasonably well.