Large Soil Cores Research Articles

Uranium stability in a large wetland soil core probed by electron acceptors, carbonate amendments and wet-dry cycling in a long-term lysimeter experiment

Understanding the hydro-biogeochemical conditions that impact the mobility of uranium (U) in natural or artificial wetlands is essential for the management of contaminated environments. Field-based research indicates that high organic matter content and saturation of the soil from the water table create favorable conditions for U accumulation. Despite the installation of artificial wetlands for U remediation, the processes that can release U from wetland soils to underlying aquifers are poorly understood. Here we used a large soil core from a montane wetland in a 6 year lysimeter experiment to study the stability of U accumulated to levels of up to 6000 ppm. Amendments with electron acceptors showed that the wetland soil can reduce sulfate and Fe(III) in large amounts without significant release of U into the soil pore water. However, amendment with carbonate (5 mM, pH 7.5) resulted in a large discharge of U. After a six-month period of imposed drought, the re-flooding of the core led to the release of negligible amounts of U into the pore water. This long-term experiment demonstrates that U is strongly bound to organic matter and that its stability is only challenged by carbonate complexation.

Spatially-distributed microbial enzyme activities at intact, coated macropore surfaces in Luvisol Bt-horizons

Soil macropores serve as preferential pathways for water and solute transport as well as for root growth. They are often coated with organic material and known as "hotspots" of nutrient and C turnover. Differences in the SOM composition between macropores and soil matrix as well as between macropore types (biopores, cracks, pinhole fillings) imply potential differences in the microbial community composition and enzymatic activities. The objective of this work was to detect and assess the spatial distribution of enzyme activities related to C turnover, xylanase (XYL) and phenol oxidase (POX), and the composition of microbial communities in structural components of Luvisol Bt-horizons, developed from loess and glacial till. We applied conventional enzyme assays and phospholipid fatty acids (PLFA) analysis to study materials separated from different types of macropores and soil components as well as bulk soil samples. The spatial distribution of enzyme activities on surfaces of large soil core slices (20 cm in diameter) was quantified by soil zymography. Higher XYL activities were detected in separated burrow wall materials, clay-organic coatings, and pinhole fillings from both sites, as compared to the respective soil matrix or bulk soil samples. The XYL activities correlated with bacteria-specific PLFAs. POX activities were solely found increased for earthworm burrow walls from the loess-derived Bt-horizon, but not for burrows from till samples. Zymograms revealed particularly increased XYL activities at rooted earthworm burrows, emphasising evidence for hotspots of enzyme activity and C turnover. The zymography of POX was hampered by methodological restrictions.

Infiltration into frozen soil: From core‐scale dynamics to hillslope‐scale connectivity

AbstractInfiltration into frozen soil is a key hydrological process in cold regions. Although the mechanisms behind point‐scale infiltration into frozen soil are relatively well understood, questions remain about upscaling point‐scale results to estimate hillslope‐scale run‐off generation. Here, we tackle this question by combining laboratory, field, and modelling experiments. Six large (0.30‐m diameter by 0.35‐m deep) soil cores were extracted from an experimental hillslope on the Canadian Prairies. In the laboratory, we measured run‐off and infiltration rates of the cores for two antecedent moisture conditions under snowmelt rates and diurnal freeze–thaw conditions observed on the same hillslope. We combined the infiltration data with spatially variable data from the hillslope, to parameterise a surface run‐off redistribution model. We used the model to determine how spatial patterns of soil water content, snowpack water equivalent (SWE), and snowmelt rates affect the spatial variability of infiltration and hydrological connectivity over frozen soil. Our experiments showed that antecedent moisture conditions of the frozen soil affected infiltration rates by limiting the initial soil storage capacity and infiltration front penetration depth. However, shallow depths of infiltration and refreezing created saturated conditions at the surface for dry and wet antecedent conditions, resulting in similar final infiltration rates (0.3 mm hr−1). On the hillslope‐scale, the spatial variability of snowmelt rates controlled the development of hydrological connectivity during the 2014 spring melt, whereas SWE and antecedent soil moisture were unimportant. Geostatistical analysis showed that this was because SWE variability and antecedent moisture variability occurred at distances shorter than that of topographic variability, whereas melt variability occurred at distances longer than that of topographic variability. The importance of spatial controls will shift for differing locations and winter conditions. Overall, our results suggest that run‐off connectivity is determined by (a) a pre‐fill phase, during which a thin surface soil layer wets up, refreezes, and saturates, before infiltration excess run‐off is generated and (b) a subsequent fill‐and‐spill phase on the surface that drives hillslope‐scale run‐off.

Effects of long-term inorganic and organic fertilizations on the soil micro and macro structures of rice paddies

The soil structure of paddy soil is very dynamic from the aggregate to the pedon scale because of intensive anthropogenic management strategies. In this study, we tested the hypothesis that long-term inorganic and organic fertilizations can affect soil structure at different scales. Microstructure assessed by soil aggregates (3–5mm in diameter) and macrostructure assessed by small soil cores (CoreS) (5cm in diameter, 5cm in height) and large soil cores (CoreL) (10cm in diameter, 10cm in height) were sampled from three long-term fertilization treatments, including no fertilizer (CK), application of inorganic fertilizer (NPK), and a combination of inorganic fertilizer and organic manure (NPKOM), established in 1982. They were scanned at two scales with two types of micro-computed tomography (micro-CT) and quantified using image analysis. Results showed that relative to CK treatment, long-term NPKOM fertilization increased soil organic C (SOC) by 28% and available water content (AWC) by 20%, but decreased soil bulk density by 0.2gcm−3 whereas NPK showed no difference. Soils under CK and NPK treatments exhibited an identical dense structure at both aggregate and core scales in which pores were mainly cracks resulting from shrink/swell processes, and showed no significant difference in porosity and size distribution of the CT-identified pores (>3.7μm). Compared with the CK treatment, the soil in the NPKOM treatment had greater intra- and inter-aggregate pores, and increased porosity by 58.3%, 144.9%, and 65.9% at aggregate, CoreS, and CoreL scales, respectively. These were attributed to the biopores formed from decayed roots, stubble, and organic manures as a result of increased yields and direct amendment of organic manure. Overall, this study demonstrates that organic fertilization can improve the physical qualities of paddy soils across different scales but inorganic fertilization in isolation does not.

Impact of temperature and moisture on heterotrophic soil respiration along a moist tropical forest gradient in Australia

Tropical forests represent the largest store of terrestrial carbon (C) and are potentially vulnerable to climatic variations and human impact. However, the combined influence of temperature and precipitation on aboveground and belowground C cycling in tropical ecosystems is not well understood. To simulate the impact of climate (temperature and rainfall) on soil C heterotrophic respiration rates of moist tropical forests, we translocated soil cores among three elevations (100, 700 and 1540 m a.s.l.) representing a range in mean annual temperature of 10.9°C and in rainfall of 6840 mm. Initial soil C stocks in the top 30 cm along the gradient increased linearly with elevation from 6.13 kg C m–2 at 100 m a.s.l. to 10.66 kg C m–2 at 1540 m a.s.l. Respiration rates of translocated soil cores were measured every 3 weeks for 1 year and were fitted to different model functions taking into account soil temperature, soil moisture, mean annual temperature and total annual rainfall. Measured data could be best fitted to the model equation based on temperature alone. Furthermore, Akaike’s information criteria revealed that model functions taking into account the temperature range of the entire translocation gradient led to better estimates of respiration rates than functions solely based on the site-specific temperature range. Soil cores from the highest elevation revealed the largest temperature sensitivity (Q10 = 2.63), whereas these values decreased with decreasing elevation (Q10 = 2.00 at 100 m a.s.l.) or soil C stocks. We therefore conclude that increased temperatures will have the greatest impact on soil C stocks at higher elevations, and that best projections for future soil respiration rates of moist tropical forest soils can be achieved based on temperature alone and large soil cores exposed to temperatures above site-specific temperature regimes.

Tillage effects on topsoil structural quality assessed using X-ray CT, soil cores and visual soil evaluation

Soil structure plays a key role in the ability of soil to fulfil essential functions and services in relation to, e.g., root growth, gas and water transport and organic matter turnover. The objective of this paper was: (1) To quantify tillage effects on soil structural quality in the entire topsoil layer (0–20cm) using X-ray CT, visual evaluation and traditional core methods; and (2) To correlate pore network characteristics from X-ray CT imaging with the results from the visual evaluation and the core method. Samples were taken in May 2009 from a long-term rotation and tillage field experiment on a Danish sandy loam. The tillage treatments were direct drilling (D) and ploughing (P). For X-ray CT scanning, we sampled large soil cores (Ø=20cm, height=20cm) from the top layer. Small 100cm3 samples were taken from the 4–8 and 12–16cm layers for water content and bulk density measurements. Visual soil structure evaluation was carried out in the field at the same time as sampling. CT images (0.39×0.39×0.6mm3 voxels) were produced using a medical X-ray CT scanner. The visual assessment showed a good structural quality in the top 5–8cm for both treatments (Sq<2). A poorer soil structure was observed in lower part or the topsoil where a firm structure (Sq=2.9) was observed for D and relatively friable structure (Sq=2.2) for P. Lower bulk density was found for P than for D in the 4–8cm layer (1.34 and 1.52gcm−3, respectively), whereas relatively high bulk density values were observed for both treatments in the 12–16cm layers (1.50 and 1.56gcm−3, respectively). The X-ray CT image analysis showed that the P soil had more networks, branches and junctions but a lower degree of anisotropy and shorter average branch length than the D soil. The image data also confirmed a clear stratification of the 0–20cm topsoil layer for both tillage treatments. The stratification of the direct drilled soil was in accordance with our expectations whereas it was surprising for the ploughed soil. The dense lower topsoil layer for the ploughed soil was probably caused by compaction during secondary tillage and natural consolidation, and aggravated by a poor structural stability due to a low organic matter content. The visual soil evaluation scores were negatively correlated to soil porosity and number of pore networks estimated from X-ray CT imaging and positively correlated to the macropore characteristics of branch length and pore thickness.

Larval sampling and instar determination in field populations of northern and western corn rootworm (Coleoptera: Chrysomelidae).

Abundance and head capsule width were measured for northern (Diabrotica barberi Smith & Lawrence) and western corn rootworm (D. virgifera virgifera LeConte) larvae recovered primarily from maize root systems but also from large soil cores each centered around a root system. Larvae for measurement derived from field populations under infestation and rotation regimes that allowed most specimens to be assigned to species. A frequency distribution of head capsule widths indicated three separate peaks for western corn rootworm, presumably representing frequency of the three larval instars, with no larvae measuring 280 or 420 microm in the valleys between peaks. Multiple normal curves fit to similar but partially overlapping peaks generated by northern corn rootworm suggested that division of first to second and second to third instar can best be made for this species at 267 and 406 microm, respectively (270 and 410 when measurements are made to the nearest 20 microm). These results implied that instar of individuals from mixed northern and western corn rootworm populations can be accurately judged from head capsule width without having to determine species. The relative abundance of western corn rootworm instars was similar in root systems removed from the center of 19-cm diameter x 19-cm deep soil cores and in soil cores from which the root systems were removed. Furthermore, the number of larvae from root systems correlated significantly with that from the surrounding soil. These results indicated that the former and much more convenient sampling unit can be used to estimate population developmental stage and possibly density, at least early in the season when these tests were done and young larvae predominated.

Larval Sampling and Instar Determination in Field Populations of Northern and Western Corn Rootworm (Coleoptera: Chrysomelidae)

Abundance and head capsule width were measured for northern (Diabrotica barberi Smith & Lawrence) and western corn rootworm (D. virgifera virgifera LeConte) larvae recovered primarily from maize root systems but also from large soil cores each centered around a root system. Larvae for measurement derived from field populations under infestation and rotation regimes that allowed most specimens to be assigned to species. A frequency distribution of head capsule widths indicated three separate peaks for western corn rootworm, presumably representing frequency of the three larval instars, with no larvae measuring 280 or 420 μm in the valleys between peaks. Multiple normal curves fit to similar but partially overlapping peaks generated by northern corn rootworm suggested that division of first to second and second to third instar can best be made for this species at 267 and 406 μm, respectively (270 and 410 when measurements are made to the nearest 20 μm). These results implied that instar of individuals from mixed northern and western corn rootworm populations can be accurately judged from head capsule width without having to determine species. The relative abundance of western corn rootworm instars was similar in root systems removed from the center of 19-cm diameter × 19-cm deep soil cores and in soil cores from which the root systems were removed. Furthermore, the number of larvae from root systems correlated significantly with that from the surrounding soil. These results indicated that the former and much more convenient sampling unit can be used to estimate population developmental stage and possibly density, at least early in the season when these tests were done and young larvae predominated.

Construction and Performance of Large Soil Core Lysimeters

A method was developed to obtain large undisturbed soil cores and instrument those cores to collect vadose zone leachate data under agricultural field conditions. Twenty undisturbed soil core lysimeters were constructed at an irrigated‐field site in southeastern North Dakota. Construction of the undisturbed lysimeters consisted of utilizing a steel cutting bit to collect large (0.61‐m diam. by 1.68‐m deep) soil cores in sections of polyvinyl chloride (PVC) pipe, installing a leachate collection system, installing time domain reflectometry (TDR) soil moisture probes, and then finally placing them below grade so normal farming practices could continue. The described method of core collection is an improvement over other methods in that the cutting bit is reusable, the resulting core is larger and deeper than other methods, no heavy static weights are required, and it is not necessary to expose a free‐standing soil column. Leachate quality and quantity data is reported for 1990 through 1995 and compared favorably with larger, reconstructed profile lysimeters on the same site. Overall, the lysimeters functioned as intended for the first 6 yr of operation; however, two developed leaks and rodents damaged another. Possible sidewall flow was observed and may be avoidable in future designs.

Quantifying the Influence of Intra‐Aggregate Concentration Gradients on Solute Transport

The physical nonequilibrium of solute concentration resulting from preferential flow of soil water has often led to models where the soil is partitioned into two regions: preferential flow paths, where solute transport occurs mainly by advection, and the remaining region, where significant solute transport occurs through diffusive exchange with the flow paths. These two‐region models commonly ignore concentration gradients within the regions. Our objective was to develop a simple model to assess the influence of concentration gradients on solute transport and to compare model results with experiments conducted on structured materials. The model calculates the distribution of solutes in a single spherical aggregate surrounded by preferential flow paths and subjected to alternating boundary conditions representing either an exchange of solutes between the two regions (a wet period) or no exchange but redistribution of solutes within the aggregate (a dry period). The key parameter in the model is the aggregate radius, which defines the diffusive time scales. We conducted intermittent leaching experiments on a column of packed porous spheres and on a large (300 mm long by 216 mm diameter) undisturbed field soil core to test the validity of the model and its application to field soils. Alternating wet and dry periods enhanced leaching by up to 20% for this soil, which was consistent with the model's prediction, given a fitted equivalent aggregate radius of 1.8 cm. If similar results are obtained for other soils, use of alternating wet and dry periods could improve management of solutes, for example in salinity control and in soil remediation.

An investigation of localised soil heterogeneities on solute transport using a multisegement percolation system

A multisegment percolation system (MSPS) consisting of 25 individual collection wells was constructed to study the effects of localised soil heterogeneities on the transport of solutes in the vadose zone. In particular, this paper discusses the transport of water and nutrients (NO3 _, Cl−, PO4 3‐) through structurally stable, free‐draining agricultural soil from Victoria, Australia. A solution of nutrients was irrigated onto the surface of a large undisturbed soil core over a 12‐h period. This was followed by a continuous irrigation of distilled water at a rate which did not cause ponding for a further 18 days. During this time, the volume of leachate and the concentration of nutrients in the leachate of each well were measured. Very significant variation in drainage patterns across a small spatial scale was observed. Leaching of nitrate‐nitrogen and chloride from the core occurred two days after initial application. However, less than 1% of the total applied phosphate‐phosphorus leached from the soil during the 18‐day experiment, indicating strong adsorption. Our experiments indicate considerable heterogeneity in water flow patterns and solute leaching on a small spatial scale. These results have significant ramifications for modelling solute transport and predicting nutrient loadings on a larger scale.

The impact of groundwater level on soil seed bank survival

AbstractSeed longevity of plant species is an important topic in restoration management, and little is known about the effects of environmental conditions on seed survival and longevity under natural conditions. Therefore, the effect of groundwater level on the survival of seeds in the soil seed bank of a natural grassland community was investigated. Large soil cores, mesocosms, were sampled from a grassland site and transferred to two basins under a glass roof. The mesocosms were subjected to different groundwater-level treatments (high and low, respectively 5 and 30 cm below the soil surface) for nearly three years. After that period the soil seed bank of the mesocosms was sampled. In total 15 789 seeds of 38 taxa emerged from the experiment. Significant differences between the number of viable seeds that emerged in the two treatments were found for several species. More seeds ofGlyceria fluitans, Cardamine pratensisandMyosotis palustrisgerminated in the high water-level treatment, whereas fewer seeds ofJuncusspp.,Cerastium fontanumandStellaria mediawere found in this treatment than in the low water-level treatment. The experiment showed that the anoxic conditions prevailing in the high water-level treatment were beneficial to the survival of seeds of species of wet grassland communities. Species of dry grasslands, although represented by only two species, survived better under aerobic conditions.

Apparatus for Constant‐Head Solute Transport and Soil Water Characteristic Determination

AbstractWe developed a system to impose, maintain, and monitor constant matric head (h) and water content (θ) within large soil cores during saturated or unsaturated flow. Critical elements include: (i) delivery of water and chemicals to the column inlet using a modified disk permeameter; (ii) paired time domain reflectometry (TDR) and transducer tensiometer sensors; and (iii) solenoid valves at the outlet for uninterrupted negative pressure and simultaneous collection of large effluent volumes. Use of TDR and tensiometry provides detailed water retention data and allows hydraulic conditions within the soil to be continuously monitored during single or multiple experiments using the same soil column. Our experiences have emphasized the need to measure soil column h rather than infer its status based on applied pressure.

Field Extraction of Large Intact Soil Cores for Leaching Studies in the Laboratory

Various samplers have been developed for taking intact soil cores. Very often, the sampler was used once or twice to take only a few soil cores then it became either obsolete or a burden for maintenance and storage. An economic portable hand-operated soil core sampler was developed to obtain large soil cores with a diameter of 19 cm and a length of 120 cm. The quality of large soil cores was evaluated and compared with that obtained from short cores from each soil horizon. In addition, the large intact column was used to evaluate atrazine transport under the saturated condition. Results showed that the hydraulic conductivity of the large cores were of the same magnitude as that of short cores, except for the A horizon; the hydraulic conductivity of the large cores was about 10 times greater than the short cores. Even though the sampling procedure is labor intensive, the soil sampler has the flexibility to collect different size soil cores and can be constructed at a very low cost (less than $200 including labor). Lastly, the sampler is maintenance free and can be stored easily in a limited space.

A modified hydropneumo‐elutriation apparatus for quantitative root separation from large soil core samples

Abstract An apparatus was designed to separate roots automatically from large soil samples (>20 kg). The apparatus (1.50 m height) was constructed of polyvinyl chloride pipes (diameter = 100 mm). The roots were separated from soil mineral particles at the base of the apparatus by a high energetic hydrovortex, and then transfered to a 200‐μm sieve by a flow of water and small air bubbles. The inlet flow of both water and air were ajusted to obtain roots similar to those separated by hand (control). The statistical analysis indicated that 7 out of 42 tested pairs of water and air flows gave results not significantly different from the control. The inlet water flow is fixed to 3 L/min and the air inlet to 100 units for routine analysis. The washing cycle is completed in 5 minutes for 200 g of soil, but for larger soil samples, it could be extended over several hours after evacuating the mineral particules from the base of the apparatus.

The relationship between concentrations of plutonium and americium in soil interstitial waters and their uptake by plants

Large soil cores (23.5 cm diameter), containing elevated concentrations of plutonium and americium derived from fuel reprocessing effluents (typically 2000–3000 but up to 10000 Bq kg −1 239,240Pu and 3000–5000 but up to 15 000 Bq kg −1 241Am), were collected from three zones of a salt marsh in west Cumbria. The standing vegetation was removed, the cores placed in a greenhouse and the vegetation allowed to regrow. After 2 months, the regrowth was harvested and the amount of plutonium and americium taken up determined. These data, combined with data from earlier, in-situ sampling of soil pore waters and from analysis of soil cores, allow estimation of soil-plant transfers. If these are expressed simply as the ratio of the activity concentrations in the plant tissues to those in the soil, the values lie in the range 10 −5−10 −6, comparable to those found by many other workers. However, the ratio of concentrations in plant tissues to those in the soil solution is much higher; 40–110 l kg −1 for 239,240Pu and 210–640 l kg −1 for 241Am. There appears to be a consistent relationship between soil-plant transfer and the in-situ solid-solution distribution coefficient (Rd).

Large Soil Core Sampler

Large minimally disturbed soil samples for tomographic scanning analysis are difficult to collect. A soil core sampler capable of extracting 150 mm diameter x 600 mm length (6 in. x 2 ft) cylindrical cores was built and field tested. The design features were determined based on an extensive review of literature. The sampler consisted of a rotating exterior tube fitted with auger flights and a nonrotating interior tube, both of which advanced downward, collecting the sample within a PVC tube. The unit was equipped with a hydraulic motor to rotate the external auger. The entire assembly was designed to be mounted on a Giddings-type unit. A hand-operated cutting device cut the sample at the base and was developed for lifting the sample to the surface following sampler insertion. In soil conditions ranging from harrowed clay loams to firm sandy sods with pebbles, the sampler changed bulk density values by less than 4%, with cores up to 300 mm (1 foot) in length.

Soil Coring Machine for Microplots and Large Soil Cores1

AbstractA self‐contained, hydraulically operated soil coring machine was constructed that can push large open‐ended cylinders or boxes into the soil to establish microplots or obtain large soil cores. This machine reduces the time required to create microplots by inserting open‐ended tubes into the soil profile with minimum disturbance to surrounding areas. The equipment is mounted on two wheels and can be towed across the plot area by any small vehicle. The machine has been successfully used to establish undisturbed microplots up to 760 mm in diameter by pushing cylinders up to 1 m deep into the soil. This machine can also remove soil cores of 250 mm in diameter and 900 mm deep from the soil profile.

Measuring air-filled porosity changes in an irrigated swelling clay soil

An in-situ method for measuring air filled porosity (�a) of soils using a neutron meter was developed and evaluated for a layered, swelling clay soil. Bulk density-volumetric moisture content relationships of soil layers were used to calculate the regressions of volumetric moisture content on �a. These regressions were combined with calibration equations of the neutron meter to obtain relationships between �a and neutron counts. The �a values predicted from measured neutron count ratios show good agreement with �a values calculated from the measured bulk densities and moisture contents in a transitional red-brown earth. The method provides a rapid and non-destructive measurement of �a. Soil aeration in a transitional red-brown earth under two irrigation treatments was assessed using neutron counts to monitor the changes in �a profiles. Large soil cores which were flooded had lower profile �a values than did cores which were not flooded. However, differences were small, owing apparently to the very low rates of infiltration and redistribution. The potential use of this method to continuously monitor changes in �a profiles allows frequent calculation of aeration stress indices, which may be used to predict crop responses and yield losses due to poor aeration.

Moisture measurement in a swelling clay soil using neutron moisture meters

Neutron meters were calibrated for moisture determination in a layered swelling soil, showing changes in both swelling characteristics and texture with depth. The bulk density-moisture content relationship of each layer determined from small soil cores agreed with those predicted from in-situ swelling measurements taken during a previous study. The precision of calibration of two neutron meters was markedly improved by correcting for textural and bulk density changes with depth. The regressions of total moisture content on corrected neutron ratios were similar for the two moisture meters and comparable to those obtained for similar soils in previous studies, provided that the same calibration techniques were used. For moisture determination in the field, separate calibration equations of volumetric moisture content against neutron ratios were required to account for the individual bulk density-moisture content relationship of each soil layer. The calibration equation for the surface layer had a markedly lower calibration coefficient and a much higher intercept constant than for the subsoil. Measured changes in soil moisture following irrigation using the above calibration equations of the neutron moisture meter agreed closely with values predicted by water balance within a large soil core in a lysimeter.