Homogenized Soil Samples Research Articles

Topographic correction of visible near‐infrared reflectance spectra for horizon‐scale soil organic carbon mapping

AbstractUnderstanding soil organic carbon (SOC) response to global change has been hindered by an inability to map SOC at horizon scales relevant to coupled hydrologic and biogeochemical processes. Standard SOC measurements rely on homogenized samples taken from distinct depth intervals. Such sampling prevents an examination of fine‐scale SOC distribution within a soil horizon. Visible near‐infrared hyperspectral imaging (HSI) has been applied to intact monoliths and split cores surfaces to overcome this limitation. However, the roughness of these surfaces can influence HSI spectra by scattering reflected light in different directions posing challenges to fine‐scale SOC mapping. Here, we examine the influence of prescribed surface orientation on reflected spectra, develop a method for correcting topographic effects, and calibrate a partial least squares regression (PLSR) model for SOC prediction. Two empirical models that account for surface slope, aspect, and wavelength and two theoretical models that account for the geometry of the spectrometer were compared using 681 homogenized soil samples from across the United States that were packed into sample wells and presented to the spectrometer at 91 orientations. The empirical approach outperformed the more complex geometric models in correcting spectra taken at non‐flat configurations. Topographically corrected spectra reduced bias and error in SOC predicted by PLSR, particularly at slope angles greater than 30°. Our approach clears the way for investigating the spatial distributions of multiple soil properties on rough intact soil samples.

NATURAL RADIOACTIVITY LEVELS AND RADIOLOGICAL HAZARDS INDICES OF SOIL SAMPLES FROM SELECTED MINING SITES, DANGE-SHUNI, SOKOTO STATE, NIGERIA

Elevation of environmental radioactivity is a growing concern that requires monitoring and measurement via nuclear analytical procedures and have been conducted from Dange-Shuni in this study. The unabated mining activities whose motive is to constantly keep industries running on locally sourced minerals and raw materials often leave the host communities with heavy burden of radioactivity impacts that have be monitored in this study. this should explicitly and clearly state main objectives of the work. Hyper Pure Germanium detector coupled to a Multi-Channel Analyzer (MCA) was used for gamma-ray spectrometry to ascertain the activity concentration results of ,,in 15 packs of homogenized soil samples collected from mining sites, Dange-Shuni LGA, Sokoto State, Nigeria. Mutually exclusive and collectively exhaustiveness approach of sampling was adopted in order to overcome the limitations of large and diverse nature of mining localities terrain. Results obtained from gamma-ray spectroscopy reveals a range of 30.72.9 to 95.67.2, 30.72.9 to 95.67.2 and 499.2051.5 to 971.2421.5 for ,and respectively. The mean values were calculated to be 58.04, 54.5 and 767.62 respectively. The radiological hazards of , , , , , and were obtained for the results of activity concentration ascertained in this study. The results were found in elevations to the world mean values in comparison which is an implication of dangers of environmental radioactivity to the generality of the dwellings. These results are in good agreements with previous studies and hence, suggests for similar investigation to be conducted.

Soil Reclamation Models by Soil Water Infiltration for Refuse Dumps in Opencast Mining Area of Northern China

The water infiltration rules of five different homogeneously or heterogeneously-constructed soil samples were determined to select the best soil construction module for refuse dump reclamation in the opencast mines of the Shanxi-Shaanxi-Inner Mongolia energy circle. Five treatments, including three homogeneous soil samples consisting of sandy soil, Montmorillonite-enriched sandstone, and sand-Montmorillonite-enriched sandstone mixture, together with two heterogeneous soil samples composed of sandy soil + Montmorillonite-enriched sandstone + sandy soil and sandy soil + sandy − Montmorillonite-enriched sandstone mixture (7:3) + sandy soil. Three replicates of each treatment were prepared in the indoor pillars to measure the infiltration process by auto-recording geometry, to investigate the infiltration features of various soil configurations by testing their infiltration rate, cumulative infiltration capacity, wetting front migration, and profile soil content, and to evaluate the infiltration of newly constructed soil in the natural conditions of the research area. The experiment demonstrated that the addition of Montmorillonite-enriched sandstone into sandy soil significantly slowed down soil water infiltration, especially in the heterogeneous soils. Traditional models perfectly simulated the soil water infiltration in the three homogeneous soils in which soil infiltration capacity could be segmentally fitted by Kostiakov model and linear model, and wetting front could be fitted by a power function. Compared with the homogeneous soil samples, heterogeneous soil could reduce the direct surface runoff and deep percolation, and is an idealized structure for soil reconstruction in opencast coal mine dump.

Fluorescence imaging for mm-scale observation of macropore-matrix mass transfer: Calibration experiments

Macropores are important for gas, water, and heat movement in soils. Still, the transfer of water and solutes through the interface between macropore and matrix like the walls of biopores is less understood. For mass-exchange processes and water storage in soil aggregates, relatively small sizes and volumes limit the analysis of distributed interface properties including the permeability for water or air. Photo-based imaging techniques could perhaps be used to trace the movement of sorbing chemicals at such smaller scale similarly as for larger soil profiles. The objective was to test the suitability of Na-Fluorescein (i.e., fluorescein di sodium salt) for marking small-scaled flow and transport processes through surfaces of biopore walls and coated soil aggregates and for determining water and reactive solute mass exchange.Fluorescence imaging was carried out using fluorescence spectrometer (for aqueous solutions) and a uv/vis photo-based imaging technique for soil and intact biopore and aggregate surfaces of soil samples from the Bt-horizon of a haplic Luvisol. For calibration, a linear relation between fluorescence intensity and initial Na-Fluorescein concentrations could be used for aqueous solutions up to a concentration of 100 mg L−1 Na-Fluorescein. The fluorescence intensities of homogenized soil samples were used to determine the amount of sorbed dye at equilibrium.After calibration, the photo-based imaging technique allows determining mm-scale maps of dye concentrations at intact soil surfaces in both, the dissolved and sorbed fraction of the dye. Uncertainties are caused mainly by effects of fluorescent soil organic carbon that is heterogeneously distributed at biopore wall surfaces. The results suggest that sorption of the dye mostly occurs in the soil region next to the macropore surface. A few high concentration spots at greater distances could be an indication for rapid tracer movement along root channels suggesting occurrence of non-uniform flow and transport even at the scale of soil clods and aggregates.The method seems promising for a small-scale analysis of macropore-matrix exchange of reactive solutes.

Soil zymography as a powerful tool for exploring hotspots and substrate limitation in undisturbed subsoil

It is widely accepted that soil microorganisms are not evenly distributed but are often concentrated in spatially segregated hotspots that are characterized by higher substrate availability compared to the surrounding bulk soil. However, microorganisms outside of hotspots may be in a dormant or inactive state, since they have depleted all available substrates within their vicinity. So far, the knowledge about the spatial distribution and dynamics of microbial activity in subsoil is very scarce, since most available data has been acquired from either homogenized soil samples or as bulk signals from undisturbed soil cores. In this study, we introduced a new incubation approach combining soil zymography and substrate addition on undisturbed soil core surfaces. We mapped three extracellular enzymes (β-glucosidase, chitinase and acid phosphatase) on a subsoil sample from 60 cm depth and analyzed their activity-patterns using different geostatistical and spatial analyses. After initial enzyme mapping, the soil was homogenously sprayed with 14C glucose as model substrate and incubated for 14 days. Soil zymography was suitable for detecting hotspots in undisturbed soil, making up a proportion of 2.4% on average of the total area. Consequently, microbial-driven biogeochemical processes can be expected to be limited to small areas in this subsoil, while the major part of the soil volume is not contributing. Glucose additions considerably increased enzyme activities up to 900% in initial non-hotspots, while the effect was far lower in initial hotspots. These results show that microorganisms in the subsoil outside of hotspots can be activated and release enzymes when substrate is supplied. Thus, dormant or inactive microorganisms outside of hotspots are able to contribute to SOC mineralization when substrate limitation is overcome, thus most likely inducing positive priming effects. Our results clearly demonstrate the benefits of combining enzyme mapping with substrate additions on undisturbed soil to gain new insights about microbial hotspots and C-cycling in subsoils using spatial analyses. In contrast to traditional incubation experiments, this method gives high spatial information about microbial activity, allowing a more differentiated interpretation of incubation results.

Combining Fractional Order Derivative and Spectral Variable Selection for Organic Matter Estimation of Homogeneous Soil Samples by VIS–NIR Spectroscopy

Visible and near-infrared (VIS–NIR) spectroscopy has been extensively applied to estimate soil organic matter (SOM) in the laboratory. However, if field/moist VIS–NIR spectra can be directly applied to estimate SOM, then much of the time and labor would be avoided. Spectral derivative plays an important role in eliminating unwanted interference and optimizing the estimation model. Nonetheless, the conventional integer order derivatives (i.e., the first and second derivatives) may neglect some detailed information related to SOM. Besides, the full-spectrum generally contains redundant spectral variables, which would affect the model accuracy. This study aimed to investigate different combinations of fractional order derivative (FOD) and spectral variable selection techniques (i.e., competitive adaptive reweighted sampling (CARS), elastic net (ENET) and genetic algorithm (GA)) to optimize the VIS–NIR spectral model of moist soil. Ninety-one soil samples were collected from Central China, with their SOM contents and reflectance spectra measured. Support vector machine (SVM) was applied to estimate SOM. Results indicated that moist spectra differed greatly from dried ground spectra. With increasing order of derivative, the spectral resolution improved gradually, but the spectral strength decreased simultaneously. FOD could provide a better tool to counterbalance the contradiction between spectral resolution and spectral strength. In full-spectrum SVM models, the most accurate estimation was achieved by SVM model based on 1.5-order derivative spectra, with validation R2 = 0.79 and ratio of the performance to deviation (RPD) = 2.20. Of all models studied (different combinations of FOD and variable selection techniques), the highest validation model accuracy for SOM was achieved when applying 1.5 derivative spectra and GA method (validation R2 = 0.88 and RPD = 2.89). Among the three variable selection techniques, overall, the GA method yielded the optimal predictability. However, due to its long computation time, one alternative was to use CARS method. The results of this study confirm that a suitable combination of FOD and variable selection can effectively improve the model performance of SOM in moist soil.

Assessment the effect of homogenized soil on soil hydraulic properties and soil water transport

Soil hydraulic properties play a crucial role in simulating water flow and contaminant transport. Soil hydraulic properties are commonly measured using homogenized soil samples. However, soil structure has a significant effect on the soil ability to retain and to conduct water, particularly in aggregated soils. In order to determine the effect of soil homogenization on soil hydraulic properties and soil water transport, undisturbed soil samples were carefully collected. Five different soil structures were identified: Angular-blocky, Crumble, Angular-blocky (different soil texture), Granular, and subangular-blocky. The soil hydraulic properties were determined for undisturbed and homogenized soil samples for each soil structure. The soil hydraulic properties were used to model soil water transport using HYDRUS-1D.The homogenized soil samples showed a significant increase in wide pores (wCP) and a decrease in narrow pores (nCP). The wCP increased by 95.6, 141.2, 391.6, 3.9, 261.3%, and nCP decreased by 69.5, 10.5, 33.8, 72.7, and 39.3% for homogenized soil samples compared to undisturbed soil samples. The soil water retention curves exhibited a significant decrease in water holding capacity for homogenized soil samples compared with the undisturbed soil samples. The homogenized soil samples showed also a decrease in soil hydraulic conductivity. The simulated results showed that water movement and distribution were affected by soil homogenizing. Moreover, soil homogenizing affected soil hydraulic properties and soil water transport. However, field studies are being needed to find the effect of these differences on water, chemical, and pollutant transport under several scenarios.

Factors Affecting Mehlich III Soil Test Methodology for Extractable P

ABSTRACTAgronomic and environmental testing laboratories in Texas and elsewhere have adopted Mehlich III (M3) as their official soil test phosphorus (P) methodology. However, M3-P data could be skewed due to non-homogenous soil samples or failure to follow standard protocol which could influence recommendations or restrictions. Twelve agricultural soils with a wide range of properties were collected from across Texas. Exhaustive efforts via multiple methods were made to prepare homogeneous representative soil samples. The standard M3 protocol selected was a 2 g weighed soil sample placed in a 148 ml disposable plastic cup, using a 1:10 soil:M3 solution ratio, shaken on a 200 rpm orbital shaker with a 2.5 cm throw for exactly 5 min, and filtered through Whatman No. 2 filter paper. The standard protocol was compared with nine different protocol variations with variables including soil weighing versus scooping, scooping repeatability of different technicians, soil sample weight, shaking type, speed and time, different filter papers, and varying soil:extractant ratios. Extent of soil pulverization on M3-P results was also evaluated. Tests were performed in four replications for all protocols to assess effects on M3-extractable soil P. Percent recovery of soil during grinding had no effect on M3-extractable P. Little difference in M3-extractable P was observed between scooping and weighing of 2 g soil samples. Shaker type had no effect on extractable P in soils with low clay contents, however, increasing shaking speed and using an orbital shaker resulted in higher extractable P, especially in clayey soils. Both Whatman No. 1 and 2 filter papers were found suitable for M3-P analyses. Different soil:extractant ratios resulted in a highly significant influence on the amount of M3-P extracted. However, when ratios were maintained between 1:9 and 1:11, few differences in extractable P were observed. Using sample weights below 3 g did not significantly alter precision or accuracy of results. However, technician variation in scooping of 2 or 5 g soil samples resulted in significant differences in M3-P.

Critical Analysis of the van Bemmelen Conversion Factor used to Convert Soil Organic Matter Data to Soil Organic Carbon Data: Comparative Analyses in a UK Loamy Sand Soil

Converting soil organic matter (SOM) data to soil organic carbon (SOC) data usually uses the van Bemmelen factor of 0.58 (or in reverse its reciprocal of 1.724) as a universal conversion factor. The accuracy of this conversion factor has been questioned. Under the Kyoto Protocol (1997) dry combustion is recommended to provide reproducible analyses to measure soil carbon stocks. However, dry combustion equipment is expensive and entails high maintenance. For rapid and inexpensive measurements, loss-on-ignition (LOI) is often used. A total of 278 loamy sand topsoil (0-5 cm depth) samples were taken during three soil sampling sessions (9 January 2007, 22 January 2009 and 10 October 2011) from runoff plots, splash erosion plots and grassed/cultivated plots on the Hilton Experimental Site, Shropshire, UK. A total of 124 soil samples were collected from both runoff and splash plots in both 2007 and 2009 (Bhattacharyya et al., 2011a). Some 22 of the collected samples in 2011 were from grassland (Ah horizon) and eight from cultivated soils (Ap horizon). Homogenized soil samples were split and SOM was determined on oven-dried samples by LOI and total SOC was determined by dry combustion. A conversion factor of 0.845 was used to obtain SOC from total soil C, following Rawlins et al. (2011). Results showed strong associations (R² = 0.70, P 0.001, n = 278) between SOM and SOC data. For all data, SOM to SOC conversion factors varied between 0.36-0.98, with a mean value of 0.66 (SD = 0.105). The mean values of the conversion factor were 0.64, 0.69 and 0.56, respectively, for the samples collected in 2007, 2009 and 2011. Results indicate the van Bemmelen factor (0.58) is a reasonable predictor, but both temporal and spatial variations occur around it within a specific soil type. Thus, caution should be exercised in SOM/SOC data conversions using the van Bemmelen factor.

3D-electrical resistivity tomography monitoring of salt transport in homogeneous and layered soil samples

Monitoring transport of dissolved substances in soil deposits is particularly relevant where safety is concerned, as in the case of geo-environmental barriers. Geophysical methods are very appealing, since they cover a wide domain, localising possible preferential flow paths and providing reliable links between geophysical quantities and hydrological variables. This paper describes a 3D laboratory application of electrical resistivity tomography (ERT) used to monitor solute transport processes. Dissolution and transport tests on both homogeneous and heterogeneous samples were conducted in an instrumented oedometer cell. ERT was used to create maps of electrical conductivity of the monitored domain at different time intervals and to estimate concentration variations within the interstitial fluid. Comparisons with finite element simulations of the transport processes were performed to check the consistency of the results. Tests confirmed that the technique can monitor salt transport, infer the hydro-chemical behaviour of heterogeneous geomaterials and evaluate the performances of clay barriers.

Determination of Small‐Scale Spatial Heterogeneity of Water Repellency in Sandy Soils

The degree of water repellency in soils is commonly characterized by the contact angle (CA). For its determination, disturbed and homogenized soil samples are used. Thus, information about the small‐scale spatial heterogeneity of soil water repellency is lost. We modified the sampling technique of the sessile drop method to allow the determination of the CA on particles located at their original positions, reflecting the natural wetting properties of the site. The proposed preparation technique further allows the determination of small‐scale variability of water repellency (millimeter to centimeter scales) along transects with a length of approximately 1 m or more. Hence, the small‐scale spatial distribution of water repellency within a soil profile or transect can be determined and its impact on physical, biological, or chemical processes in soil can be evaluated.

Comparison of calibration curves prepared by soil compaction and ball milling methods for direct quantification of the potato cyst nematode Globodera rostochiensis in soil

A direct quantification method consisting of soil compaction and real-time PCR was applied to quantify the potato cyst nematode (PCN), Globodera rostochiensis, in soil. We also compared the efficacies of pre-treatment methods for disrupting nematode bodies and obtaining homogeneous soil samples between soil compaction and ball milling. PCN was detected at a density of five second-stage juveniles (J2)/20 g of soil and there were highly significant correlations (r2 > 0.8697) between the cycle threshold (Ct) values and the number of J2 inoculated in all the soils, irrespective of the pre-treatment methods. Calibration curves showing the relationship between Ct values and nematode density were almost identical for both pre-treatment methods in two soils, while the Ct values were consistently lower for the ball milling method in one soil, suggesting that DNA extraction efficiency might be better in the ball milling method. The calibration curves obtained with the ball milling method were nearly identical in three soils, but were different with the compaction method. These results suggest that the ball milling method is more suitable for direct quantification of nematodes than the compaction method.

Methane oxidation kinetics differ in European beech and Norway spruce soils

SummaryConiferous forest soils often consume less of the greenhouse gas methane (CH4) than deciduous forest soils. The reasons for this phenomenon have not been resolved. It might be caused by differences in the diffusive flux of CH4 through the organic layer, pH or different concentrations of potentially inhibitory compounds. Soil samples were investigated from three adjacent European beech (Fagus sylvatica) and Norway spruce (Picea abies) stands in Germany. Maximal CH4 oxidation velocities (Vmax(app)) and Michaelis Menten constants (KM(app)), retrieved from intact soil cores at constant CH4 concentrations, temperature and matric potential, were twice as great in beech as in spruce soils. Also atmospheric CH4 oxidation rates measured in homogenized soil samples displayed the same trend. Greatest atmospheric CH4 oxidation rates were detected in the Oa horizon or in the upper 5 cm of the mineral soil. In contrast to the beech soils, the Oa horizon of the spruce soils consumed no CH4. A differential effect due to divergent diffusive flux through the litter layer was not found. pH and ammonium concentration were similar in samples from both forest soil types. Ethylene accumulation in all soils was negligible under oxic conditions. These collective results suggest that the different atmospheric CH4 uptake by beech and spruce soils is caused by different CH4 oxidizing capacities of methanotrophic communities in the Oa horizon and top mineral soil.

Shrinkage behaviour of transiently‐ and constantly‐loaded soils and its consequences for soil moisture release

SummarySoils under loaded conditions may have different shrinkage behaviour from that of load‐free soils. In this study, we applied two kinds of mechanical stress (σ) on repacked homogeneous soil samples: transient and constant stresses, simulating the traffic load during tillage and the overburden pressure, respectively. Three transient stresses were applied on the soil surface with 150, 400 and 1400 kPa, while the constant stresses ranged from 1.8, 3.8, 5.5, to 7.3 kPa. We hypothesized that the two stresses play different roles in soil shrinkage behaviour as depicted by void ratio (e) and moisture ratio (ϑ), as compared with load‐free soil. Thus, our aim was to build up the relationship between e, ϑ and σ. For a swelling soil, total pores can be divided into rigid and non‐rigid components according to their swelling and shrinkage capacity relative to soil moisture. The non‐rigid pores compacted by the transient stress can be regained in the subsequent wetting at load‐free conditions, whereas the compacted rigid pores do not recover. The reduction in rigid pores does not alter the soil pore shrinkage capacity. The shrinkage curves of transiently‐loaded soils are therefore parallel to each other with an identical coefficient of linear extensibility (COLE) and the same shrinkage slope, although their structural shrinkage phase narrows with an increase of stress. However, the constant stress compresses non‐rigid pores readily through suppressing their swelling capacity during wetting as well as compacting rigid pores. If the change of rigid pores is negligible, the shrinkage curves of constantly‐loaded soils converge at the zero shrinkage or the dry‐end point with the load‐free soil shrinkage. If the reductions of rigid and non‐rigid pores are both considered, the soil shrinkage combines the part of parallel shrinkage derived from the reduced rigid pores and the intersected shrinkage resulted from the altered non‐rigid pores. On the basis of different shrinkage behaviours resulting from the two mechanical stresses, we propose numerical formulae to illustrate a series of curves for the e‐ϑ‐σ relationship. The different changes in rigid and non‐rigid pores cause soil water release differently.

Nutrient characteristics of aggregates and rhizosphere of a degraded upland soil: Implication for soil fertility evaluation

The influence of aggregates and rhizosphere on nutrient availability of degraded tropical soils is not yet well understood. The study evaluated differences in the nutrient characteristics between rhizosphere and non-rhizosphere soils, aggregated and homogenized soil samples, and inner and outer portions of aggregates. Soil samples were collected from a degraded upland soil in Pinabacdao, Samar, Philippines, and analyzed for selected soil properties. Results revealed that rhizosphere soil had higher organic matter (OM), available P contents, and exchangeable Ca and Mg contents than the non-rhizosphere soil but with no considerable variation in soil pH. Results also showed no differences in the chemical properties between the outer and inner portions of aggregates and that the aggregated samples slightly differed from the homogenized soil samples in terms of some chemical characteristics. Findings suggest that the standard practice of homogenizing soil sample for analysis may have limitations for fertility evaluation of degraded soils.

The mechanical behavior of structured and homogenized soil under repeated loading

AbstractSoil deformation is increasingly important in crop production since nowadays weights of agricultural machines exceed the bearing capacity of most soils. Often this is counteracted by distributing the weight over more axles leading to an increase in wheeling frequency. Machine passages during one year can, depending on the crop and equipment used, range between two and five times for the majority of the field and up to twenty times and more for a wheeling track. These add up to hundreds of loading events for a crop‐rotation period. In this study, we investigated the effect of multiple loading with the same load in a cyclic‐compression test on soil‐pore‐volume change. The tests were conducted on homogenized soil samples with varying texture and undisturbed soil samples from a field experimental site comparing conventional and conservation‐tillage systems. Of particular interest was the question whether there is significant plastic soil deformation for soil stresses that remained sufficiently below the precompression stress, which is commonly neglected. Our results show that especially for cohesive soils, the assumption of fully elasticity in the recompression range may not be justified since those soils show distinct cyclic‐creep behavior. We found that deformation under cyclic loading follows a logarithmic law. We used the slope of the logarithmic fit of void‐ratio changes vs. loading cycles as a parameter to characterize the sensitivity of soils to cyclic compression. The results suggest that for characterizing the mechanical stability of soils that show cyclic creep, we have (with respect to long‐term deformation effects) to consider both precompression stress and cyclic compressibility.

How to Prepare a Good Plant Sample for Internal Reference Material

To evaluate their daily work in analytical measurements, all soil analysis laboratories must have their own low‐cost reference material to substitute high‐cost certified reference materials. This standard can be used to substitute routine use of high‐cost certified reference material (CRM). This standard can be used to substitute routine use of CRM or conventionalized reference materials in case the former are not available. This article presents a simple procedure for preparing a standard for plant material that has high quality and homogeneity for internal use in laboratories. This procedure can also be used to prepare homogeneous soil samples. As a preliminary analytical indicator of homogeneity, water‐soluble potassium and total zinc contents were used. The Levene test was selected as the statistical test of homogeneity of the material. The process is iterative and concludes when the value of F of the results obtained from the analysis of variance of the residuals is less than the critical value of F for a given selected probability. It is recommended that a p=0.01 be used. The homogenized sample was distributed to participants in an intercomparison exercise to validate the results of the laboratory preparation.

Contribution of nitrification and denitrification to the emission of N2O in a freeze‐thaw event in an agricultural soil

AbstractThe amounts of N2O released in freeze‐thaw events depend on site and freezing conditions and contribute considerably to the annual N2O emissions. However, quantitative information on the N transformation rates in freeze‐thaw events is scarce. Our objectives were (1) to quantify gross nitrification in a Luvisol during a freeze‐thaw event, (2) to analyze the dynamics of the emissions of N2O and N2, (3) to quantify the contribution of nitrification and denitrification to the emission of N2O, and (4) to determine whether the length of freezing and of thawing affects the C availability for the denitrification. 15NO$ _3^- $ was added to undisturbed soil columns, and the columns were subjected to 7 d of freezing and 5 d of thawing. N2O emissions were determined in 3 h intervals, and the concentrations of 15N2O and 15N2 were determined at different times during thawing. During the 12 d experiment, 5.67 mg NO$ _3^- $‐N (kg soil)–1 was produced, and 2.67 mg NO$ _3^- $‐N (kg soil)–1 was lost. By assuming as a first approximation that production and loss occurred exclusively during thawing, the average nitrate‐production rate, denitrification rate, and immobilization rate were 1.13, 0.05, and 0.48 mg NO$ _3^- $‐N (kg soil)–1 d–1, respectively. Immediately after the beginning of the thawing, denitrification contributed by 83% to the N2O production. The ratios of 15N2 to 15N2O during thawing were narrow and ranged from 1.5 to 0.6. For objective (4), homogenized soil samples were incubated under anaerobic conditions after different periods of freezing and thawing. The different periods did not affect the amounts of N2 and N2O produced in the incubation experiments. Further, addition of labile substrates gave either increases in the amounts of N2O and N2 produced or no changes which suggested that changes in nutrient availability due to freezing and thawing are only small.

Use of diffusive gradients in thin films (DGT) in undisturbed field soils.

The technique of diffusive gradients in thin films (DGT) has been shown to be a promising tool to assess metal bioavailability in soils under laboratory conditions. In this study we used DGT to investigate the resupply kinetics of Cu and Zn under in-situ conditions in a polluted lysimeter soil and compared the results with laboratory measurements using undisturbed soil cores at defined water contents as well as homogenized soil samples. Results differed considerably between these treatments, although the same soil material was used in all experiments. A small pool of rapidly available Zn was found in the field but not in the homogenized soil. Soil solution pH and dissolved metal concentrations also varied significantly between the soil treatments. In addition, we compared the DGT-measured effective concentration with the uptake of Cu and Zn into the shoots of Lolium perenne (Ryegrass) under the same three types of conditions, i.e., field, soil cores, and homogenized soil. A close relationship was found which was not linear but could be described by a saturation-type behavior. L. perenne is a metal excluder plant, and thus, metal accumulation is limited by translocation of metals from roots to shoots. DGT predicted plant metal uptake much better than the soil solution concentration or pH. The results of this study suggest that DGT may be successfully used under field conditions to study the kinetics of metal resupply. Plant metal concentrations were not well predicted in all cases by the effective concentration CE under field conditions. Some plants took up considerably more metals than estimated by CE. Variations in metal uptake independent of their bioavailability can be caused by local variations in microsite conditions, e.g. light, temperature, water, and nutrients. To some degree, such indetermination has to be expected as an inherent feature of the system and the concept of bioavailability.

Contents and Leachability of Heavy Metals (Pb, Cu, Sb, Zn, As) in Soil at the Pantex Firing Range, Amarillo, Texas

ABSTRACT Pantex firing range soil samples were analyzed for Pb, Cu, Sb, Zn, and As. One hundred ninety-seven samples were collected from the firing range and vicinity area. There was a lack of knowledge about the distribution of Pb in the firing range, so a random sampling with proportional allocation was chosen. Concentration levels of Pb and Cu in the firing range were found to be in the range of 11-4675 and 13-359 mg/kg, respectively. Concentration levels of Sb were found to be in the range of 1-517 mg/kg. However, the Zn and As concentration levels were close to average soil background levels. The Sn concentration level was expected to be higher in the Pantex firing range soil samples. However, it was found to be below the neutron activation analysis (NAA) detection limit of 75 mg/kg. Enrichment factor analysis showed that Pb and Sb were highly enriched in the firing range with average magnitudes of 55 and 90, respectively. Cu was enriched ~6 times more than the usual soil concentration levels. Tox-icity characteristic leaching procedure (TCLP) was carried out on size-fractionated homogeneous soil samples. The concentration levels of Pb in leachates were found to be ~12 times higher than the U.S. Environmental Protection Agency (EPA) regulatory concentration level of 5 mg/L. Sequential extraction (SE) was also performed to characterize Pb and other trace elements into five different fractions. The highest Pb fraction was found with organic matter in the soil.