Large Soil Monoliths Research Articles

Flood‐Induced Recharge of Matrix Water in a Vertic Forest Soil

AbstractVertisols shrink and swell with changes in soil moisture, influencing hydraulic properties. Vertisols are often in floodplains, yet the importance of flooding as a source of soil moisture remains poorly understood. We used blue dye and deuterated water as tracers to determine the role of the macropore network in matrix recharge under artificial flood durations of 3 and 31 days in large soil monoliths extracted from a forested soil. Gravimetric soil moisture content increased by 47% in the first three days, then increased only 3.5% from day 3–31. Post‐flood moisture content was greatest in the organic‐rich, top 10 cm and was lower at 10–75 cm where organic matter was less. Deuterium concentration revealed that soil moisture in the top 10 cm was quickly dominated by artificial flood water, but at depth remained <80% floodwater even after 31 days. Pervasive dye staining of ped surfaces in the top 4 cm indicated connectivity to flood waters but staining at depth was less and highly variable. The isotopic composition of soil water at depth continued to shift toward flood water despite no differences in dye staining between days 3 and 31. Results indicate flooding initially but incompletely recharges matrix water via macropores and suggest the importance of flooding as a source of matrix recharge in vertic floodplain soils may depend more on flood frequency than duration. Isotopic composition of matrix water in vertic soils depends on both advective and diffusional processes, with diffusion becoming more dominant as porosity decreases.

Earthworm populations in forestry plantations (Araucaria angustifolia, Pinus elliottii) and Native Atlantic forest in Southern Brazil compared using two sampling methods

Earthworm populations have been little studied in forest systems in South America, and the choice of appropriate sampling methods is an important factor to consider. Hence, the present study evaluated earthworm populations using two methods: handsorting of large soil monoliths (40 × 40 cm × 20 cm depth), and application of formalin (0.5%) to the soil surface. Worms were sampled in fifteen areas at the Embrapa Forestry Research Station, including native Atlantic forest fragments, and native (Araucaria angustifolia) or exotic (Pinus elliottii) tree species plantations. Eight species of earthworms were collected overall, including four native species (Urobenus brasiliensis, Andiorrhinus duseni, Fimoscolex nivae and Glossoscolex embrapaensis) and one exotic (M. schmardae) in low abundance, and three other exotics in higher abundance (Pontoscolex corethrurus, Amynthas gracilis and Amynthas corticis). Total earthworm density ranged from 11.5 to 102 individuals m−2 and biomass from 5.6 to 31.6 g m−2, with higher values being found in P. elliottii plantations. Handsorting was more effective than formalin for sampling earthworms, resulting in higher species richness, overall abundance and biomass (particularly of the endogeic P. corethrurus) in both Pine and Araucaria plantations. Formalin extraction was better for extracting epi-endogeic species (particularly M. schmardae). Eveness and A. corticis densities were higher in native forests, while A. gracilis abundance was higher in Araucaria plantations and P. corethrurus predominated in both plantations, likely due to soil-related differences between the vegetation types. Earthworm population assessments in Brazilian forest systems should prioritize handsorting, while biodiversity inventories may benefit from combining handsorting with formalin extraction.

Nitrate leaching losses from lysimeters simulating winter grazing of fodder beet by dairy cows

ABSTRACTNitrate-N (NO3−-N) leaching losses from dairy cow urine and non-urine areas of simulated winter grazing of fodder beet (Beta vulgaris L.) were quantified using large intact soil monolith lysimeters (50 cm diameter × 70 cm depth), containing Balmoral/Lismore stony silt loam soil. Fresh cow urine was applied to half of the lysimeters in mid- to late-June at 300 kg N ha−1 in 2012 and 250 kg N ha−1 in 2013, following simulated fodder beet grazing. The remaining half did not receive urine to represent non-urine areas of a grazed paddock. Most of the N leached was from the urine-treated lysimeters and 92%–98% was in the form of NO3−-N. Total NO3−-N leached from the urine-treated lysimeters represented an equivalent of 21% (64 kg NO3−-N ha−1) and 32% (84 kg NO3−-N ha−1) of the total urine-N applied in 2012 and 2013, respectively. Nitrate-N leaching losses from lysimeters receiving no urine were between 10 and 11 kg NO3−-N ha−1.

Nitrogen leaching losses from lysimeters containing winter kale: the effects of urinary N rate and DCD application

Nitrogen (N) leached from the plant-soil system is an environmental concern. The objective of this study was to quantify N leaching losses from cow urine deposited during grazing of a winter forage crop (kale), and determine the effect of using the nitrification inhibitor dicyandiamide (DCD) to reduce N leaching losses. Fresh cow urine was applied to the surface of large soil monolith lysimeters in late June 2011 at two urinary N deposition rates; 500 and 700 kg N ha−1 (with and without DCD), and compared with losses from lysimeters receiving no urine. Most of the N leaching loss was in the form of nitrate-N (NO3–-N), with an average of 25% leached as ammonium-N (NH4 +-N). Nitrate-N leaching losses were significantly higher under urine-treated lysimeters (213 and 380 kg NO3–-N ha−1 at the 500 and 700 kg N ha−1 rates, respectively) than those without urine (c. 33 kg NO3–-N ha−1). The application of DCD reduced NO3–-N leaching losses by up to 54%. The results indicate that N leaching losses from winter forage crop systems are high and that the application of DCD can reduce these losses.

Soil‐gas diffusivity in large soil monoliths

SummaryThe variability of gas diffusion in soil is not well known, but is important for assessing greenhouse gas emissions, soil decontamination, oxidation in soil and plant and root respiration. The goal of this study was to assess small‐scale variability of the relative soil‐gas diffusivity (Ds / Do, msoil air) using large intact soil monoliths and to compare Ds / Do calculation methods. Neon (Ne) was maintained constant at the lower boundary of three monoliths of two soils (a sand and an organic soil). Ne concentration was measured at large spatial and temporal frequencies. Calculation methods included the use of average concentration, and average Ds / Do per horizon, per section, or for the entire soil profile. Considering all sections of the monoliths, Ds / Do varied from 3.5 × 10−3 to 1.2 × 10−1 for the Ap horizon and from 4.8 × 10−3 to 8.3 × 10−1 for the Bf horizon in the sand and from 1.0 × 10−3 to 7.9 × 10−3 for the Ohp horizon and from 2.4 × 10−4 to 7.7 × 10−2 for the Of horizon in the organic soil. For the entire soil profile, variations in Ds / Do between monoliths reached 125% in the sand and 56% in the organic soil. The Ds / Do calculation method influenced the apparent variability (CV) of Ds / Do and, to a lesser extent, Ds / Do values of the overall soil profile. Differences in Ds / Do between monoliths could not be explained solely by the variability of total soil porosity and air‐filled porosity. Soil macroporosity (cracks and earthworm burrows) and layering greatly influenced variability of gas movement. Thus, the choice of sampling procedure, calculation method and modelling must be governed by the scale of the processes of interest and soil variability attributes.

Effects of soil warming and drying on methane cycling in a northern peatland mesocosm study

Boreal peatlands contain a large portion of the Earth's terrestrial organic carbon and may be particularly vulnerable to changes in climate. Temperatures in boreal regions are predicted to increase during the twenty‐first century which may accelerate changes in soil microbial processes and plant community dynamics. In particular, climate‐driven changes in plant community composition might affect the pathways and rates of methanogenesis, the plant‐mediated emission of methane, and the scavenging of methane by methanotrophic bacteria. Climate change may also affect methane cycling through changes in pore water chemistry. To date, these feedbacks have not been incorporated into the carbon cycling components of climate models. We investigated the effects of soil warming and water table manipulations on methane cycling in a field mesocosm experiment in northern Minnesota, USA. Large intact soil monoliths removed from a bog and fen received infrared warming treatments crossed with water table treatments for 6 years. In years 5 and 6, concentrations, fluxes, and isotopic compositions of methane were measured along with aboveground and belowground net primary productivity and pore water concentrations of acetate, sulfate, ammonium, nitrate, and dissolved organic carbon. Water table level was the dominant control over methane flux in the fen mesocosms, likely through its effect on methane oxidation rates. However, pore water chemistry and plant productivity were important secondary factors in explaining methane flux in the fen mesocosms, and these factors appeared to be the predominant controls over methane flux in the bog mesocosms. The water table and IR treatments had large effects on pore water chemistry and plant productivity, so the indirect effects of climate change appear to be just as important as the direct effects of changing temperature and water table level in controlling future methane fluxes from northern peatlands. Pore water sulfate, ammonium, nitrate, and acetate had a relatively consistent negative relationship with methane emissions, pore water DOC had a positive relationship with methane emissions, and BNPP had mixed effects. The bog mesocosms had much higher methane emissions and pore water methane concentrations than the fen mesocosms, despite a much lower average water table level and peat that is a poor substrate for methanogenesis. We suggest that the relatively high methane fluxes in the bog mesocosms can be explained through their low concentrations of inhibitory pore water compounds, high concentrations of DOC, and high plant productivity. Stable isotopic data from pore water support acetate fermentation as the principal pathway of methanogenesis in bog mesocosms (meanδ13CH4= −41.0‰, meanδD‐CH4= −190‰). Fen mesocosms had lower pore water concentrations and emissions of methane than bog mesocosms, despite much higher methane production potentials in fen peat. The methane from the fen mesocosms was isotopically heavy (meanδ13CH4= −28.9‰, meanδD‐CH4= −140‰), suggesting a strong oxidative sink. This is likely related to the dominance of graminoid vegetation and the associated oxygen transport into the rhizosphere. Our results illustrate the need for a more robust understanding of the multiple feedbacks between climate forcing and plant and microbial feedbacks in the response of northern peatlands to climate change.

Flow Regime Effects on Reactive and Non-reactive Solute Transport

The lower boundary maintained throughout leaching experiments in column and lysimeter studies influences the transport behavior of solutes. In this study we wanted to determine the effects of different flux regimes and soil organic matter treatments on reactive (pesticide) and non-reactive (bromide) solute movement in lysimeters. Four large soil monoliths, collected from the same experimental site but differing in the organic carbon content by an added manure layer, were subjected to two consecutive leaching tests. Unsaturated steady-state flow with a unit hydraulic gradient was established in Test 1 by applying a constant negative pressure at the base of the lysimeter while during Test 2 atmospheric pressure was maintained (zero-tension). Although less herbicide (isoproturon) was leached from the soil with added manure, the flux regime, as controlled by the lower boundary, dominated the adsorption and the degradation coefficients. The lower moisture content of Test 1 enhanced isoproturon binding, whilst chemical degradation was more effective under the increased but non-uniform soil water content of Test 2. This study demonstrates how solute leaching studies using zero-tension lysimeters do not represent unsaturated in-field processes; the physicochemical processes occurring in unsaturated field soils are only adequately reflected in leaching tests when a constant negative soil water pressure head is maintained throughout the soil sample.

Advances In Out Door Lysimeter Techniques

Different methods exist for measuring soil water and solute fluxes in and below the root zone and have been critically reviewed. Besides indirect methods (e.g. water balance, tensiometer, time domain reflectometry – TDR, frequency domain reflectometry – FDR, environmental tracer) direct methods (e.g. drainage-type lysimeter, water fluxmeter) have a long tradition and have been successfully used in seepage research. A large weighable out door lysimeter is the best method for obtaining reliable data about seepage water quantity and quality, but it involves significant investment and additional expenses for maintenance. To tackle this problem new methods for the vertical collection of large volume soil monoliths (up to 6 m3) as well as for the horizontal collection (up to 6 m3) have been developed. For the placement of the lysimeter a container lysimeter unit was constructed, which is cheaper than a conventional steel or concrete cellar. Furthermore, the technical design of the newly developed lysimeter types as a weighable gravitation lysimeter, a weighable groundwater lysimeter and a lateral flow lysimeter are presented.

Design and Operability of a Large Weighable Fen Lysimeter

In principle, conventional lysimeters are suitable for the investigation of vertical water and solute fluxes. Lateral fluxes in water-saturated fen sites are characterized by heterogeneities and abnormities due to anisotropic layering. But due to lack of adequate monitoring techniques, these fluxes have been insufficiently analyzed. The newly developed large weighable fen lysimeter (LWFL) overcomes the limitations of conventional lysimetry and enables the measurement of vertical and horizontal transport processes in undisturbed large volume soil monoliths. The LWFL has a volume of 6 m3 (4 m length, 1 m width and 1.5 m depth) and was tested by filling the lysimeter with an undisturbed fen monolith. A special extraction procedure for the horizontal sliding of the lysimeter vessel through the natural fen was developed. In front of the vessel a converted cutting tool assisted in carving the soil monolith out of the peat, both vertically and horizontally. Inlet and outlet of the LWFL was constructed to allow the adjustment of a wide range of hydraulic gradients to depict natural occurring lateral transport processes. The LWFL including the measurement techniques was tested successfully for 3 years. On the basis of these tests, we conclude that complex physical and biogeochemical research problems involving lateral flows can be tackled now with multiphase observations and measurements at high spatial and temporal resolution, transdisciplinary data evaluation and numerical modelling approaches.

Leaching of isothiocyanates through intact soil following simulated biofumigation

Biofumigation can be used as an alternative to conventional soil fumigation to control soil-borne pests. With biofumigation, plant tissue with a natural content of glucosinolates (cruciferous plants) is damaged and incorporated into the topsoil. When the plant tissue is damaged, the glucosinolates come into contact with the endogenous enzyme myrosinase, which catalyse the hydrolysis of glucosinolates into various products depending on the reaction conditions. Isothiocyanates are among the potential products formed from these reactions. We investigated if the isothiocyanates from rape plant material were leached through the soil to drain depth when a heavy rainstorm followed the biofumigation. We applied isothiocyanates from rape plant material (1,480 μmol m−2) to four large (0.6 m diameter, 1.0 m long) intact soil monoliths from a loamy and a sandy soil and conducted a leaching experiment under semi-field conditions. The soil monoliths were irrigated with 70–90 mm (10 mm h−1) and the concentrations of three isothiocyanates (3-butenyl, 4-pentenyl and 2-phenethyl) were monitored in the leachate. Between 0 and 14.8 mmol isothiocyanates were leached for each mol of isothiocyanates applied during application of 70–90 mm irrigation. The distribution coefficient estimated from leached concentrations was 0.04–1.19 for 3-butenyl, 0.04–1.15 for 4-pentenyl isothiocyanate and 0.037–0.97 for 2-phenethyl isothiocyanate. The concentration of total isothiocyanates in the leachate was in the same order of magnitude as the LD50 of isothiocyanates for sensitive aquatic organisms.

Collecting large soil monoliths

Large soil monoliths (> 50 kg) are required to study and monitor soil energy and water budget, gas and liquid fluxes, and contaminant transport. This paper reviews the types of studies requiring large soil monoliths, design and material for the casing, and collection methods for vertically forced cylindrical and for block (orthorhombic) monoliths. Methods of extraction are described along with recommendations. Advantages and disadvantages of each method are discussed. Vertically pushed cylindrical monoliths are easier to sample, but compared with orthorhombic monoliths that are not pushed into the soil, the pushing process has the disadvantage of modifying certain soil physical properties. Key words: Large soil samples, intact soil cores, acrylic boxes, soil sampling

Experimental study of water flow and sulphate transport at monolith scale

In this study, sulphur transport processes and effect of flow rate on sulphate breakthrough curves (BTCs) were studied on six undisturbed large soil monoliths (each having roughly a volume of 0.5 m 3), collected from two different agricultural soils being loamy and sandy loam. In the laboratory, each monolith was equipped with different measuring devices to monitor soil water content, bulk soil electrical conductivity (ECa), soil temperature, pressure head, outlet flux, and pH. Four unsaturated steady state experiments were carried out on each monolith using two different imposed fluxes (referred to as low and high). First a chloride breakthrough experiment was performed to identify the basic transport processes followed by sulphate breakthrough experiment for the identification of the important sulphate transport processes. Water and solute (chloride and sulphate) mass balances were made to evaluate effective sulphate transformations at the scale of the monolith. The relative water mass balance errors ranged between −4.0% and 5.0%. The chloride mass balances were almost as good as those obtained for water, whereas the sulphate mass balance revealed that sulphate was subjected to adsorption and immobilization during the transport. The high flux sulphate experiments resulted in relatively large mass balance discrepancies compared to the low flux sulphate experiments. The sulphate breakthrough curves (BTCs) were somewhat retarded in the loamy monoliths whereas both chloride and sulphate BTCs were significantly affected by preferential flow in the sandy loam monoliths. Standard batch experiments showed that the adsorption isotherm was linear and immobilization occurred in both soils studied whereas net mineralization was essentially low. The effect of flow rate on the BTCs and influence of water content on immobilization process was not apparent.

Using TDR and Inverse Modeling to Characterize Solute Transport in a Layered Agricultural Volcanic Soil

Volcanic soils exhibit particular physical‐chemical properties (i.e., strong and stable natural aggregation and high content of variable‐charge minerals) that may influence solute transport. To determine if such techniques like TDR and inverse modeling are useful for analyzing solute transport in volcanic soils, we studied the governing transport processes by means of a miscible displacement experiment of Br− in a large undisturbed soil monolith. Bromide resident concentrations at several depths were monitored successfully with TDR technology, while parameters for the convective–dispersive (CDE) and mobile–immobile (MIM) transport models were estimated by inverse modeling. For the relatively high soil moisture conditions, typical of high frequency‐irrigation systems that we considered, Br− was found to move slowly by convection–dispersion. Simulations with the CDE and MIM transport models yielded very similar results. Although Br− is generally assumed to behave as a tracer, we found that this anion in our experiment was subject to adsorption at the bottom part of the monolith. This may be explained by the variable‐charge nature of the minerals (Fe and Al oxihydroxides) present in this volcanic soil, which exhibited anion exchange when the pH of the soil solution decreased below the zero point of charge.

Potential effects of warming and drying on peatland plant community composition

AbstractBoreal peatlands may be particularly vulnerable to climate change, because temperature regimes that currently constrain biological activity in these regions are predicted to increase substantially within the next century. Changes in peatland plant community composition in response to climate change may alter nutrient availability, energy budgets, trace gas fluxes, and carbon storage. We investigated plant community response to warming and drying in a field mesocosm experiment in northern Minnesota, USA. Large intact soil monoliths removed from a bog and a fen received three infrared warming treatments crossed with three water‐table treatments (n = 3) for five years. Foliar cover of each species was estimated annually.In the bog, increases in soil temperature and decreases in water‐table elevation increased cover of shrubs by 50% and decreased cover of graminoids by 50%. The response of shrubs to warming was distinctly species‐specific, and ranged from increases (for Andromeda glaucophylla) to decreases (for Kalmia polifolia). In the fens, changes in plant cover were driven primarily by changes in water‐table elevation, and responses were species‐ and lifeform‐specific: increases in water‐table elevation increased cover of graminoids – in particular Carex lasiocarpa and Carex livida– as well as mosses. In contrast, decreases in water‐table elevation increased cover of shrubs, in particular A. glaucophylla and Chamaedaphne calyculata. The differential and sometimes opposite response of species and lifeforms to the treatments suggest that the structure and function of both bog and fen plant communities will change – in different directions or at different magnitudes – in response to warming and/or changes in water‐table elevation that may accompany regional or global climate change.

Influence of Soil Colloids on the Migration of Atrazine and Zinc Through Large Soil Monoliths

The role of soil colloids and their potential to co-transport agrochemicals in subsurface soil environments was evaluated in aleaching experiment utilizing large soil monoliths. The monolithswere created by hydraulically driving steel pipe sections (50 cm diameter × 50 cm length) into Maury silt loam (fine, mixed, mesic Typic Paleudalf) and Loradale silt loam (fine, silty, mixed mesic Typic Argiudoll) soils. Water dispersible colloids fractionated from the Bt horizons of the above soilswere spiked with 3 mg L-1 atrazine (2-chloro-4-ethylamino-6-isopropylamine-s-triazine) and 10 mg L-1 zinc (Zn), and after a twenty-four hour equilibration were applied into the monoliths at eight hour intervals using 500 mL pulse applications. Solutions containing atrazine and Znwithout added colloids were applied to separate monoliths from each soil to represent control treatments. Colloid, atrazine, andZn recoveries in the eluent varied greatly with respect to soiltype. Colloid recovery in the Loradale monoliths averaged 65.1 ± 26.5%, with maxima approaching the input level, while in the Maury monoliths the average recovery was low (5.7 ± 6.2%) and never exceeded 25% of the input level. Atrazine eluted from the two monoliths averaged 40.3 ± 12.5% (Loradale) and 29.0 ± 20.0% (Maury), with considerable enhancement in the presence of colloids, especially in the Loradale soil. In contrast, the elution of Zn averaged 3.0 ±3.2%, in the Loradale monoliths and rarely exceeded control concentrations in the Maury monoliths, suggesting a stronger retardation of Zn over atrazine within the soil matrix, especially when colloid transport was deterred. Settling-rateexperiments at varying pH and electrical conductivity (EC) valuessuggested that the transport of Maury colloids may have been hindered due to flocculation within the monoliths, while the Loradale colloids remained stable throughout the leaching experiment. Although the presence of colloids enhanced atrazineelution in all monoliths, the actual amount of atrazine transported bound to either colloid type was minimal, suggestingmainly physical exclusion transport processes. In contrast, stronger chemisorption of Zn to colloid surfaces than the soil matrix appeared to enhance the transport of Zn by both colloids.

Nitrate Leaching as Influenced by Cover Crops in Large Soil Monoliths

Unacceptable levels of NO3 leaching to ground water and drainage systems can occur under corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotations. Cover crops have the potential to reduce NO3 leaching, but this process has not been well documented. Lysimeters utilizing large soil monoliths are an excellent approach for studying NO3 leaching because inputs can be controlled and outputs accurately measured. The objective of this study was to see if fall cover crops could reduce NO3 leaching from large soil monoliths. We used three (1 by 1 by 1.5 m deep) monoliths in each of two controlled climate chambers with oat (Avena sativa L.) or rye (Secale cereale L.) fall cover crop interplanted into soybean in mid‐August. The study was continued for two cover crop cycles in each chamber. In Chamber 1, drainage was significantly reduced due to oat or rye cover crops for the fall through summer of Years 1 and 2 (first cover crop cycle), and NO3 loss was reduced for most of the same time period. In Chamber 2, NO3 loss was reduced for the spring‐summer season of the second year (first cover crop cycle). Although drainage was less under cover crops for Chamber 1, the soil water content was not consistently lower because of replenishment by watering. The soil monoliths were useful for showing that oat and rye cover crops in a corn–soybean rotation can reduce NO3 leaching from lysimeters and suggest that the same trend would be true in the field.

Nitrate Leaching as Influenced by Cover Crops in Large Soil Monoliths

Unacceptable levels of NO3 leaching to ground water and drainage systems can occur under corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotations. Cover crops have the potential to reduce NO3 leaching, but this process has not been well documented. Lysimeters utilizing large soil monoliths are an excellent approach for studying NO3 leaching because inputs can be controlled and outputs accurately measured. The objective of this study was to see if fall cover crops could reduce NO3 leaching from large soil monoliths. We used three (1 by 1 by 1.5 m deep) monoliths in each of two controlled climate chambers with oat (Avena sativa L.) or rye (Secale cereale L.) fall cover crop interplanted into soybean in mid-August. The study was continued for two cover crop cycles in each chamber. In Chamber 1, drainage was significantly reduced due to oat or rye cover crops for the fall through summer of Years 1 and 2 (first cover crop cycle), and NO3 loss was reduced for most of the same time period. In Chamber 2, NO3 loss was reduced for the spring-summer season of the second year (first cover crop cycle). Although drainage was less under cover crops for Chamber 1, the soil water content was not consistently lower because of replenishment by watering. The soil monoliths were useful for showing that oat and rye cover crops in a corn–soybean rotation can reduce NO3 leaching from lysimeters and suggest that the same trend would be true in the field.

Processes of colloid mobilization and transport in macroporous soil monoliths

Transport of pesticides, PAH and other hydrophobic or surface-complexing contaminants in soils may be enhanced by colloid-facilitated transport. A prerequisite for colloid-facilitated transport is the release and transport of colloids. The mechanisms for colloid mobilization and transport in a macroporous Alfisol have been evaluated by measuring the amount and type of colloids leached in two large soil monoliths during long duration simulated rain events. The soil was irrigated with water having a chemical composition close to natural rainwater and at intensities as expected under natural conditions. The results showed that the colloids were primarily mobilized and transported in the macropores and that the source of colloids was not exhausted for extended rainfall duration. The first flush of water mobilized loosely bound colloids that had a high organic content relative to the bulk soil. After the initial release, the high ionic strength in the percolating water limited the mobilization. For prolonged leaching, the diffusion of colloids from the macropore walls appeared to rate-limit the mobilization process. During the late leaching phase, the rate of colloid mobilization was positively correlated with flow velocity.

137Cs and 90Sr root uptake prediction under close-to-real controlled conditions

Radionuclide soil-to-crop transfer was analysed in large undisturbed soil monoliths installed in lysimeters under controlled climatic conditions. The soils were representative of large agricultural areas in the European Union: Loamy-sand Orthic Podzol from Belgium, Loamysand Fluvisol from France, Loamy Calcic Luvisol from Spain, Silty-loam Orthic Luvisol from Germany, and Sandy-loam Eutric Fluvisol from the UK. On the lysimeters, winter barley was sown for two consecutive years on artificially contaminated soils simulating a post-accidental situation, 1 and 2 years after radionuclide deposition. The amount (Bq m -2) of 137Cs and 90Sr that accumulated in the crop was higher in the loamy-sand soils, regardless of the total radionuclides (MBq m -2) or available fraction content in the soil. The Orthic Podzol produced the highest 137Cs crop accumulation, and the Loamy-sand Fluvisol the highest 90Sr accumulation. Concentration ratios (CR) showed similar results, but with higher 90Sr transfer for the Podzol than for the Loamy-sand Fluvisol. The activity concentration of radionuclides in the plants was negatively correlated with crop yield, indicating that high crop productivity produced a general dilution effect in radionuclide concentration activity in plants. The relative radionuclide crop accumulation expressed in Bq m -2 was predicted from the soil availability parameters, that is the available 137Cs fraction divided by the distribution coefficient ( K D ) and by the K concentration in soil solution, and the available 90Sr divided by the cationic exchange capacity (CEC). These predictions could not be obtained when using the CR, as this does not fully account for the crop growing conditions which influence radionuclide uptake by the crop.

A comparison of near-saturated hydraulic properties measured in small cores and large monoliths in a clay soil

The effect of measurement scale on hydraulic properties close to saturation was investigated in a clay soil. Results from measurements on undisturbed “standard” small soil core samples were compared with results from three large intact soil monolith samples which were assumed to reflect natural soil hydraulic behaviour. An intermediate sample size, which was used to characterize soil water retention relations (θ(ψ)) in each layer in the intact monoliths, was obtained by cutting the intact monoliths in three layers. The small cores were subsequently sampled from the cut monoliths so that all measurements were made on the same soil material. Measurements of θ(ψ) and saturated hydraulic conductivity ( K s ) were compared, as well as the exponential relationships between K s and macroporosity ( ϕ ma ), the latter derived from the θ(ψ) data for small cores and from specific yields for intact monoliths. Between soil water pressure heads of −60 cm and −15 cm the small core data and the cut monolith data showed similar θ(ψ) relations, whereas they diverged closer to saturation. When comparing K s and ϕ ma , small core data were correlated with the intact monolith data although the small cores had smaller values. Thus, the “standard” small cores can satisfactorily predict the investigated soil hydraulic properties for a natural soil at pressure heads equal or less than −15 cm, but they may be in error, especially in soils with macropores, in the pressure head range −15 cm to saturation, i.e. in the largest macropores.