Bo V. Iversen

Air and water permeability in differently textured soils at two measurement scales

Air permeability can be used to describe the structure of the soil but may also be used to predict saturated hydraulic conductivity. This raises the question of whether the two parameters exhibit the same degree of scale dependency. In this study the scale dependency of water permeability (saturated hydraulic conductivity, Kw) and air permeability (ka, at a matric water potential of −50 cm H2O) was tested at four different sites (three horizons at each site), by using two measurement scales (100 cm3 and 6280 cm3). No clear effect of scale on variability was observed. Air and water permeability displayed higher variabilities for two structured loamy soils compared with two sandy soils. For the more structured soils, the variability between measurements was lower for air compared with water permeability. Both air and water permeabilities were higher at the large scale compared with the small scale, but this scale-dependent difference was less pronounced in sandy soils, suggesting a smaller representative elementary volume. For three of the four soils, a highly correlated relationship between Kw and ka on both small and large soil samples was observed. For the fourth soil, water retention data revealed that the samples were not sufficiently drained at −50 cm H2O to validate a comparison between the two parameters. Predictive Kw (ka) relations for the remaining three soils at the two scales compared favorably with a general Kw (ka) relation proposed by Loll et al. (1999). This study supports the use of a general predictive relation between ka near field capacity (at around −50 to −100 cm H2O) and Kw, but caution should be taken if the soil has a large content of pores that will drain at or close to a matric water potential of −50 cm H2O.

Comparative Mapping of Soil Physical–Chemical and Structural Parameters at Field Scale to Identify Zones of Enhanced Leaching Risk

Preferential flow and particle-facilitated transport through macropores contributes significantly to the transport of strongly sorbing substances such as pesticides and phosphorus. The aim of this study was to perform a field-scale characterization of basic soil physical properties like clay and organic carbon content and investigate whether it was possible to relate these to derived structural parameters such as bulk density and conservative tracer parameters and to actual particle and phosphorus leaching patterns obtained from laboratory leaching experiments. Sixty-five cylindrical soil columns of 20-cm height and 20-cm diameter and bulk soil were sampled from the topsoil in a 15-m × 15-m grid in an agricultural loamy field. Highest clay contents and highest bulk densities were found in the northern part of the field. Leaching experiments with a conservative tracer showed fast 5% tracer arrival times and high tracer recovery percentages from columns sampled from the northern part of the field, and the leached mass of particles and particulate phosphorus was also largest from this area. Strong correlations were obtained between 5% tracer arrival time, tracer recovery, and bulk density, indicating that a few well-aligned and better connected macropores might change the hydraulic conductivity between the macropores and the soil matrix, triggering an onset of preferential flow at lower rain intensities compared with less compacted soil. Overall, a comparison mapping of basic and structural characteristics including soil texture, bulk density, dissolved tracer, particle and phosphorus transport parameters identified the northern one-third of the field as a zone with higher leaching risk. This risk assessment based on parameter mapping from measurements on intact samples was in good agreement with 9 yr of pesticide detections in two horizontal wells and with particle and phosphorus leaching patterns from a distributed, shallow drainage pipe system across the field.

SPATIAL AND TEMPORAL DYNAMICS OF AIR PERMEABILITY IN A CONSTRUCTED FIELD

Soil air permeability in undisturbed soil is closely related to soil structure and heterogeneity and hydraulic properties. Knowledge of air permeability behavior for different types of soils is, therefore, valuable. In this study, we investigated the variability and spatial correlation structure of soil air permeability and soil textural properties in an undisturbed constructed field at Hiroshima University, Hiroshima, Japan. Air permeability was measured on undisturbed soil samples along two 70-m-long transects in the top 10 cm of a field constructed approximately 20 years ago from sandy loam soil collected from a nearby mountain. Air permeability (ka) showed spatial correlation along both transects, with a range of approximately 20 m. Measurements of ka on large (3140 cm3) samples were similar, showing a soil with little small-scale heterogeneity. Local-scale measurements of ka within a 1.6- by 1.6-m grid taken 4 months after the transect sampling suggested that soil structure and pore size distribution had changed significantly over time, as a result of tilling and precipitation, causing an increase in ka. No spatial drift in soil physical parameters within the 1.6- by 1.6-m grid was found. On-site ka measurements after 1- to 2-day rainy periods compared well with laboratory measurements at a soil-water potential of −100 cm H2O, suggesting that natural field capacity occurs at this potential. Measurements of ka as a function of air-filled porosity (&epsis;) at the mid-point of the two transects were compared with predictions by two recently presented ka (&epsis;) models, and good agreement between simulated values and measurements was found.

Variation of MCPA, metribuzine, methyltriazine-amine and glyphosate degradation, sorption, mineralization and leaching in different soil horizons

Pesticide mineralization and sorption were determined in 75 soil samples from 15 individually drilled holes through the vadose zone along a 28km long transect of the Danish outwash plain. Mineralization of the phenoxyacetic acid herbicide MCPA was high both in topsoils and in most subsoils, while metribuzine and methyltriazine-amine was always low. Organic matter and soil pH was shown to be responsible for sorption of MCPA and metribuzine in the topsoils. The sorption of methyltriazine-amine in topsoil was positively correlated with clay and negatively correlated with the pH of the soil. Sorption of glyphosate was tested also high in the subsoils. One-dimensional MACRO modeling of the concentration of MCPA, metribuzine and methyltriazine-amine at 2m depth calculated that the average concentration of MCPA and methyltriazine-amine in the groundwater was below the administrative limit of 0.1mug/l in all tested profiles while metribuzine always exceeded the 0.1mug/l threshold value.

Field-scale variation in microbial activity and soil properties in relation to mineralization and sorption of pesticides in a sandy soil.

Pesticides applied to agricultural soils are subject to environmental concerns because leaching to groundwater reservoirs and aquatic habitats may occur. Knowledge of field variation of pesticide-related parameters is required to evaluate the vulnerability of pesticide leaching. The mineralization and sorption of the pesticides glyphosate and metribuzin and the pesticide degradation product triazinamin in a field were measured and compared with the field-scale variation of geochemical and microbiological parameters. We focused on the soil parameters clay and organic carbon (C) content and on soil respiratory and enzymatic processes and microbial biomass. These parameters were measured in soil samples taken at two depths (Ap and Bs horizon) in 51 sampling points from a 4-ha agricultural fine sandy soil field. The results indicated that the spatial variation of the soil parameters, and in particular the content of organic C, had a major influence on the variability of the microbial parameters and on sorption and pesticide mineralization in the soil. For glyphosate, with a co-metabolic pathway for degradation, the mineralization was increased in soils with high microbial activity. The spatial variability, expressed as the CV, was about five times higher in the Bs horizon than in the Ap horizon, and the local-scale variation within 100 m(2) areas were two to three times lower than the field-scale variation within the entire field of about 4 ha.

Biochar effects on soil aggregate properties under no-till maize.

Abstract Soil aggregates are useful indicators of soil structure and stability, and the impact on physical and mechanical aggregate properties is critical for the sustainable use of organic amendments in agricultural soil. In this work, we evaluated the short-term soil quality effects of applying biochar (0–10 kg m−2), in combination with swine manure (2.1 and 4.2 kg m−2), to a no-till maize (Zea mays L.) cropping system on a sandy loam soil in Denmark. Topsoil (0–20 cm) aggregates were analyzed for clay dispersibility, aggregate stability, tensile strength (TS), and specific rupture energy (SRE) using end-over-end shaking, a Yoder-type wet-sieving method, and an unconfined compression test in soil samples collected 7 and 19 months after final biochar application. The highest rates of biochar and swine manure application resulted in the highest aggregate stability and lowest clay dispersibility. Applying both amendments systematically increased TS and SRE for large aggregates (4–8 and 8–16 mm) but not for small aggregates (1–2 and 2–4 mm). Increased biochar application also decreased the friability index of soil aggregates. Based on X-ray visualization, it was found that aggregates containing larger amounts of biochar particles had higher TS and SRE probably because of bonding effects. Based on the improved soil aggregate properties, we suggest that biochar can be effective for increasing and sustaining overall soil quality, for example, related to minimizing the soil erosion potential.

Macropores and Macropore Transport: Relating Basic Soil Properties to Macropore Density and Soil Hydraulic Properties

Abstract Preferential transport of water through soil macropores is a governing process in the facilitated transport of strongly sorbing compounds. The aim of this study was to investigate the relationships between macropore density and the hydraulic conductivity of the soil and to test the sampling representativeness of soil columns for the measurement of saturated hydraulic conductivity. Macropore density was determined in three horizons in four typical Danish soil types (third year of pasture), and saturated hydraulic conductivity and near-saturated hydraulic conductivity were measured in the laboratory on undisturbed soil columns (6,280 cm3) in the same three horizons. A strong relationship between macropore density and the mean particle diameter was found in the B and C horizons. A poor relationship was found between macropore density and the hydraulic conductivity at a matric potential of −10 hPa, whereas a stronger relationship was found at saturation. Results of the test of the number of soil columns needed for a representative distribution of macropores for the measurement of saturated hydraulic conductivity indicated that approximately six soil columns were enough. Our work suggests that integrating knowledge of the abundance of macropores in relation to soil type and land use will increase the performance of pedotransfer functions in predicting saturated hydraulic conductivity.

Integrating lysimeter drainage and eddy covariance flux measurements in a groundwater recharge model

Abstract Field-scale water balance is difficult to characterize because controls exerted by soils and vegetation are mostly inferred from local-scale measurements with relatively small support volumes. Eddy covariance flux and lysimeters have been used to infer and evaluate field-scale water balances because they have larger footprint areas than local soil moisture measurements. This study quantifies heterogeneity of soil deep drainage (D) in four 12.5-m2 repacked lysimeters, compares evapotranspiration from eddy covariance (ETEC) and mass balance residuals of lysimeters (ETwbLys), and models D to estimate groundwater recharge. Variation in measured D was attributed to redirection of snowmelt infiltration and differences in lysimeter hydraulic properties caused by surface soil treatment. During the growing seasons of 2010, 2011 and 2012, ETwbLys (278, 289 and 269 mm, respectively) was in good agreement with ETEC (298, 301 and 335 mm). Annual recharge estimated from modelled D was 486, 624 and 613 mm for three calendar years 2010, 2011 and 2012, respectively. In summary, lysimeter D and ETEC can be integrated to estimate and model groundwater recharge. Editor D. Koutsoyiannis

HYDROGEOLOGICAL RELATIONSHIPS OF SANDY DEPOSITS: MODELING OF TWO-DIMENSIONAL UNSATURATED WATER AND PESTICIDE TRANSPORT

Prediction of the movement of water and solutes in the vadose zone requires information on the distribution of spatial trends and heterogeneities in porous media. The present study describes different lithofacies origination mainly from glaciofluvial deposits. Among different lithofacies, hydrological relationships were investigated. By means of a two-dimensional hydrological model it was evaluated how the flow of water and leaching of metribuzin (4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one) was affected. Two selected large outcrop sections consisting of glacial outwash deposits were used in the modeling study. Eleven different lithofacies were distinguished and described in terms of texture distribution, sorting, bedding style, and external boundaries based on excavated soil profiles from 27 locations representing seven predominantly sandy landforms in Denmark. Undisturbed soil columns were sampled from each of the lithofacies and brought to the laboratory to be analyzed. With respect to their soil hydraulic properties, the different lithofacies formed four different hydrofacies having relatively homogeneous, hydrogeological properties. Two large outcrop sections from one of the locations (a gravel pit) located near the terminal moraine of the former Weichsel glacier were used for the HYDRUS-2D modeling. Modeling results revealed that the spatial distribution of sedimentary bodies affected water flow and the leaching of metribuzin.

Relating Water and Air Flow Characteristics in Coarse Granular Materials

This paper investigates the connection between the velocity (V)–pressure drop (ΔP) relationships for air and water flow in coarse porous filter media with the aim of linking the V–ΔP relationships for air and water. Investigations were carried out using a common biofilter medium, Leca® consisting of rounded porous particles of 2- to 18-mm diameter. V–∆P relations for water flow were measured for 14 different Leca® particle size fractions and compared with measurements of V–∆P relations for air flow in 36 different Leca® particle size fractions (including the 14 used for water flow). The measurements showed a strong relationship between the two types of relationships, and further that this relationship could be described using a single constant. An approach for predicting the water flow V–∆P relationship from the corresponding air flow relationship across different particle size fractions from the same material is suggested, tested, and found valid for Re numbers higher than that which is considered in the presently available studies of air/water V–∆P relationships.