Analysis Of Soil Extracts Research Articles

Integrating liquid chromatography-electrochemical detection-surface Enhanced Raman Spectroscopy on microfluidic chip for phenylurea herbicides analysis

The widespread use of phenylurea herbicides (PHs) is a serious threat to human health and environmental safety. These pollutants usually have similar molecular structures but distinct toxicities. Therefore, developing rapid detection methods to analyze PHs quantitatively and qualitatively is highly desirable. This study fabricated a microfluidic glass liquid chromatography (LC) coupled with electrochemical detection (ECD) and Surface-Enhanced Raman spectroscopy (SERS) synchronous system, which demonstrated excellent separation and analysis performance for PHs. The complete separation of three PHs could be achieved using the microfluidic chip LC with a theoretical plate number of 342525 plates m–1. The limit of detection (LOD) of the PHs for optimized electrochemical analysis reached 0.0099–0.1388 mmol/L. More importantly, the detection system integrated with ECD and SERS had high sensitivity and molecular recognition ability, with intra and inter-day precision of less than 6.7630% and 7.5601%, respectively. The recovery of the PHs could reach 102.9861% when this detection system was applied to the analysis of soil extracts in the small potted plant and actual water samples in the environment. It is expected that the microfluidic chip with the LC-ECD-SERS synchronous detection system has significant potential in various environmental, food safety, drug, and biomedical analyses.

Novel Adsorbents for the Determination of Amino Acids in Soil Extracts by Hydrophilic Interaction Liquid Chromatography with Mass Spectrometric Detection

Adsorbents based on various substrates—silica and a copolymer of styrene with divinylbenzene—are developed for the determination of amino acids by hydrophilic interaction liquid chromatography—mass spectrometry. The optimal version of the structure of the functional layer in two series of the obtained stationary phases was chosen, which provides the best hydrophilization for each substrate. Retention mechanisms were studied and the conditions for the mass-spectrometric detection, separation, and determination of 16 amino acids were chosen. The applicability of the obtained adsorbents and a method for determining amino acids for the analysis of soil extracts were estimated.

Formation, classification and identification of non-extractable residues of 14C-labelled ionic compounds in soil

The influence of an ionic functional group on the fate of chemicals in the environment, especially the formation of non-extractable residues (NER), has not been systematically investigated. Using 4-n-dodecylphenol[phenylring-14C(U)], 4-n-dodecylbenzenesulfonicacid[phenylring-14C(U)] sodiumsalt (14C-DS-) and 4-n-dodecylbenzyltrimethylammoniumchloride[phenylring-14C(U)] (14C-DA+) all with a high structural similarity, the formation, classification and identification of NER of negatively (14C-DS-), positively (14C-DA+) and uncharged (14C-DP) chemicals were investigated in a sterilized and non-sterilized soil. After 84 days of incubation in non-sterile soil, 40.6%, 21.7% and 33.5% of the applied radioactivity (AR) of 14C-DP, 14C-DS- and 14C-DA+, respectively, were converted to NER. In contrast, in sterile soil NER formation was markedly lower. The NER were further investigated with respect to sequestered, covalently bound and biogenic residues (i.e. NER types I, II, and III). Silylation of 14C-DP, 14C-DS- and 14C-DA+ derived NER released 3.0–23.2% AR, indicating that these were sequestered, whereas the residual NER (12.9–33.1% AR) was covalently bound to the soil. Analysis of extracts derived by silylation showed that 14C-DP, but neither 14C-DS- nor 14C-DA+, were released by silylation, suggesting that DP might be part of the sequestered NER. Acid hydrolysis of the NER containing soil and subsequent analysis of soil extracts for 14C-aminoacids indicated that 2.5–23.8% AR were biogenic residues. Most DP and DS− derived NER were biogenically or covalently bound, whereas DA+ predominantly forms sequestered NER in soil. From these results we propose that chemicals forming high amounts of NER should be investigated regarding types I-III NER because sequestered parent compounds should be considered in persistence assessments.

Improving the Accuracy of Diffusion for Inorganic 15N Analyses of Soil Extracts and Water

ABSTRACTSequential diffusion techniques used to speciate inorganic nitrogen-15 (15N) during soil or water analysis are complicated by incomplete recovery of ammonium (NH4+)-N, introducing error in the subsequent determination of nitrate (NO3–)-N. Based on studies to evaluate different strategies for minimizing cross-contamination error in Mason-jar diffusions, a simple cleaning technique was developed that involves an additional 6-h diffusion using 0.6 M boric acid (H3BO3) at room temperature following the recovery of NH4+-N. This technique was 60–87% effective for reducing cross-contamination of unlabeled NO3–-N by labeled NH4+-N and became more effective for controlling analytical error with decreasing sample volumes, lower NH4+-N enrichment, and larger quantities of NO3–-N. When used with the cleaning technique described, sequential diffusions were far superior for 15N analysis of NO3–-N, as compared to the nonsequential approach that involves an isotope dilution calculation after separate diffusions to determine NH4+-N and total mineral N.

Speciation of Chromium in Alkaline Soil Extracts by an Ion-Pair Reversed Phase HPLC-ICP MS Method.

The aim of this work was to study by a hyphenated HPLC-ICP MS technique the chromium species released during alkaline extraction of various soils collected from a contaminated area of an old tannery. An ultrasound-assisted extraction procedure using 0.1 mol L−1 Na2CO3 solution was developed for the release of chromium species from the soil. The chromium species in the soil extracts were separated on a C8 column using EDTA and TBAH solution as a mobile phase. The use of an ICP-QQQ MS spectrometer in tandem mass configuration (MS/MS) combined with an octopole reaction system (ORS3) pressurized with helium allows one to eliminate spectral interferences during Cr determination in the soil extracts. The detection limit of the procedure was 0.08 µg L−1 for Cr(III) and 0.09 µg L−1 for Cr(VI) species. The trueness of the IP RP HPLC-ICP MS method was proved by an analysis of CRM 041 and CRM 060. The advantage of the proposed method is the analysis of soil extracts without their preliminary neutralization, which limits the losses of Cr(VI) due to the reduction process. The analysed soils mainly contained chromium in immobile forms (94.6–98.5% of the total Cr content). In all alkaline soil extracts mostly the Cr(VI) form was found, but in the extract of organic soils Cr(III) was also present. This arose from the reduction of Cr(VI) species by organic matter (humic acids) and Fe(II). The amount of formed Cr(III) species was dependent on the type of soil (content of organic matter, Mn and Fe) and its moistness. For the first time, the presence of neutral and non-polar chromium fractions in the soil extracts was also demonstrated. It was found that reliable speciation analysis results could be obtained for mineral soils.

Development of a Cobalt Electrode for the Determination of Phosphate in Soil Extracts and Comparison with Standard Methods

ABSTRACTA phosphate-sensitive cobalt electrode was evaluated in detecting orthophosphate ions () in ammonium lactate–acetic acid soil extracts. The dependence of the mixed potential of a cobalt electrode on concentration was investigated through potentiometry. The mechanism of detection is based on the consumption of a surface cobalt (II) oxide layer to form (Co3(PO4)2), which leads to a concentration-dependent shift of the mixed potential. Two reference electrodes were evaluated: Ag/AgCl (3 M) KCl and a platinum (Pt) wire. A linear response was observed using both reference electrodes. However, application of a Pt wire quasireference electrode increased the linear dynamic response range of the detector from 10–103 mg L−1 or 10−4–10−1 M (Ag/AgCl (3 M) KCl) to 0.1–105 mg L−1 or 10−6–101 M. In addition, the response time using the Pt wire was less than 5 min compared to a minimum of 10 min using Ag/AgCl (3 M) KCl. There was close agreement between the response of the phosphate-sensitive cobalt electrode with a standard colorimetric method. As dissolved organic substances can potentially interfere with electrochemical techniques, an investigation into the use of a nonpolar resin for decolorization and removal of organic matter in soil extracts was performed and successfully used. The phosphate-sensitive cobalt electrode was found to be a fast method for the analysis of soil extracts with high sensitivity and selectivity. It has the potential to be developed into a sensor for the in situ measurement of phosphate in various environmental matrices.

DNA analysis of soil extracts can be used to investigate fine root depth distribution of trees.

Understanding the root distribution of trees by soil coring is time -: consuming as it requires the separation of roots from soil and classification of roots into particular size classes. This labour-intensive process can limit sample throughput and therefore sampling intensity. We investigated the use of quantitative polymerase chain reaction (qPCR) on soil DNA extractions to determine live fine root DNA density (RDD, mg DNA m(-2)) for mango (Mangifera indica) trees. The specificity of the qPCR was tested against DNA extracted from 10 mango cultivars and 14 weed species. All mango cultivars and no weeds were detected. Mango DNA was successfully quantified from control soil spiked with mango roots and weed species. The DNA yield of mango root sections stored in moist soil at 23-28 °C declined after 15 days to low concentrations as roots decayed, indicating that dead root materials in moist soil would not cause false-positive results. To separate large roots from samples, a root separation method for field samples was used to target the root fragments remaining in sieved (minimum 2 mm aperture) soil for RDD comparisons. Using this method we compared the seasonal RDD values of fine roots for five mango rootstock cultivars in a field trial. The mean cultivar DNA yields by depth from root fragments in the sieved soil samples had the strongest relationship (adjusted multiple R(2) = 0.9307, P < 0.001) with the dry matter (g m(-2)) of fine (diameter <0.64 mm) roots removed from the soil by sieving. This method provides a species-specific and rapid means of comparing the distribution and concentration of live fine roots of trees in orchards using soil samples up to 500 g.

Altitudinal distribution of polybrominated diphenyl ethers (PBDEs) in the soil along Central Tibetan Plateau, China

The distribution of PBDEs in the mountains of the Central Tibetan Plateau (CTP) was determined by sampling soil along an elevation transect. The analysis of soil extracts was performed by gas chromatography and high-resolution mass spectrometry, through which 42 congeners were detected. The samples were also characterized with respect to the soil organic carbon (SOC) and mineral contents. The logarithmic concentration for three of the fractions and the ΣPBDEs increased significantly and exponentially with altitude. The slope value of the linear regression between the logarithm of the clay-normalized three fractional concentrations and the altitude is in the following order: light>intermediate<heavy. The coefficient of determination between the logarithm of normalized concentration and the altitude indicates that in the CTP, the clay normalization exhibits a better correlation to altitude than does the SOC normalization. The exponential equation between the concentration and altitude was modified by accounting for the localized soil properties. The exponential equation between the concentration and altitude was modified by accounting for the localized soil properties.

Residues and Dissipation of Fenitrothion in Green Bean (Phaseolus vulgaris) and Soil

The objective of this study were to determine the residue levels of fenitrothion in/on green bean pods, leaves, and soil under the treated plant and to identify the degradative metabolites of fenitrothion in soil under the treated plant. The results showed half-life (t1/2) values of 1.79, 2.00, and 6.08 days for fenitrothion in green bean pods, leaves, and soil, respectively. The results of GC-MS analysis of soil extracts under the treated plant showed that, at zero time, unchanged fenitrothion was found. The proportion of fenitrothion in soil extracts detected decreased with the time. GC-MS analysis of soil extracts shows the presence of compound having the formula of C7H7NO3 which is suggested to be 3-methyl-4-nitrophenol which is the major degrades of fenitrothion in soil. The other transportation products were not founded. According to the maximum residue limits (MRLs), the preharvest intervals (PHIs) of fenitrothion on green bean pods were 13 days after the treatment.

A new approach for removing iron interference from soil nitrate analysis

The accurate measurement of soil nitrate ( NO 3 − ) is critical for determining rates of nitrogen (N) cycling and potential N losses from ecosystems. Iron (Fe) can interfere with the colorimetric NO 3 − analysis of soil extracts to cause the appearance of anomalously low NO 3 − concentrations. To resolve the interference, imidazole or NH 4Cl–DTPA has been recommended to replace NH 4Cl–EDTA as the buffer in the analysis. Here we show that phosphate ( PO 4 3 − ) filtration can completely remove Fe interference whereas any of these buffers alone may not. Regardless of which buffer was used, 5.5–55 mg Fe L −1 ferrous iron (Fe 2+) interfered with NO 3 − determination in 0.3 mg N– NO 3 − L −1 2 M KCl solutions. Phosphate filtration removed all detectable Fe 2+ in 2 M KCl anaerobic soil slurry extracts with high Fe 2+ concentrations (25.9 ± 1.7 mg Fe 2+ L −1). With each of the three buffers tested, the measured NO 3 − concentrations in the anaerobic soil slurry extracts were significantly higher with PO 4 3 − filtration compared to without filtration. After filtration, the measured NO 3 − concentrations were similar across all three buffers, suggesting that NO 3 − concentrations were accurately measured in PO 4 3 − filtered soil extracts regardless of the buffer used. The Fe:N ratio of Fe interference with NO 3 − determination depended on Fe concentration, NO 3 − concentration, buffer, and cadmium column age, so that the amount of Fe interference that could occur can be difficult to predict. We suggest comparing measured NO 3 − concentrations for unfiltered and PO 4 3 − filtered soil extracts to determine the potential for Fe interference in colorimetric NO 3 − determination as standard additions may not detect all forms of Fe interference.

Liquid extraction of low molecular mass organic acids and hydroxamate siderophores from boreal forest soil

Low molecular mass organic acids (LMMOAs) and hydroxamate siderophores (HS) are molecules secreted by microbes and have previously been found in soil solution and in cultures. Mycorrhizal fungi are suggested to be involved in the nutrient uptake processes of trees and weathering of minerals. In this study soil samples taken from the O and E horizons of a podzol were extracted with 10 mM potassium phosphate buffer at pH 7.2. Variable parameters included addition of methanol to the extraction buffer and the use of ultrasonication or rotary shaking during extraction. LMMOAs and HS content of the soil extracts were determined. Analysis of soil extracts were carried out by liquid chromatography mass spectrometry (LC–MS) and the extraction results compared to results for soil solution samples obtained by centrifugation of the soils sampled. The extraction yields were significantly increased by addition of methanol to the extraction buffer, especially for the O horizon samples. Rotary shaking of the samples for 90 min gave slightly higher yields than ultrasonication for 15 min but the reduction in extraction time makes ultrasonication an attractive option. Of the HSs determined, ferricrocin was found in all samples. Optimal extraction conditions showed citric acid and isocitric acid to be the most abundant organic acids in the O and E horizons, respectively.

Level and Fate of Etoxazole in Green Bean (Phaseolus vulgaris)

The objective of this study was to estimate the total etoxazole residues balance (residue in pods, leaves and soil under the treated plant) in green bean and for identification of the degradative metabolites of etoxazole in soil under the treated plant. The results showed half life (t(1/2)) values of 3.13, 2.73 and 2.11days for etoxazole in green bean pods, leaves and soil, respectively. According to the maximum residue limits (MRL) the pre harvest intervals (PHI) of etoxazole on green bean pods was 4-days after the treatment. The results of GC-Ms analysis of soil extracts under the treated plant showed that, at zero time unchanged etoxazole was found. The proportion of etoxazole in soil extracts detected decreased with the time. GC-Ms analysis of soil extracts show the presence of compound having the formula of C(14)H(23)NO(2) which was suggested to be 2-amino-2(4-tert-butyl-2-ethoxyphenyl) ethanol. The other founded compound has the formula C(13)H(18)O(3) and suggested to be 4-tert-butyl-2-ethoxybenzoic acid.

Mitigation of dimethazone residues in soil and runoff water from agricultural field

Dimethazone, also known as clomazone [2-[(2-chlorophenyl) methyl]- 4,4-dimethyl-3-isoxaolidinone] is a pre-emergent nonionic herbicide commonly used in agriculture. A field study was conducted on a silty-loam soil of 10 % slope to monitor off-site movement and persistence of dimethazone in soil under three management practices. Eighteen plots of 22 × 3.7 m each were separated using stainless steel metal borders and the soil in six plots was mixed with municipal sewage sludge (MSS) and yard waste (YW) compost (MSS+YW) at 15 t acre−1 on dry weight basis, six plots were mixed with MSS at 15 t acre−1, and six unamended plots (NM) were used for comparison purposes. The objectives of this investigation were to: (i) monitor the dissipation and half-life (T1/2) of dimethazone in soil under three management practices; (ii) determine the concentration of dimethazone residues in runoff and infiltration water following natural rainfall events; and (iii) assess the impact of soil amendments on the transport of NO3,NH4, and P into surface and subsurface water. Gas Chromatography/Mass Spectrometery (GC/MS) analyses of soil extracts indicated the presence of ion fragments at m/z 125 and 204 that can be used for identification of dimethazone residues. Intitial deposits of dimethazone varied from 1.3 μg g−1 dry native soil to 3.2 and 11.8 μg g−1 dry soil in MSS and MSS+YW amended soil, respectively. Decline of dimethazone residues in the top 15 cm native soil and soil incorporated with amendments revealed half-life (T1/2) values of 18.8, 25.1, and 43.0 days in MSS+YW, MSS, and NM treatments, respectively. Addition of MSS+YW mix and MSS alone to native soil increased water infiltration, lowering surface runoff water volume and dimethazone residues in runoff following natural rainfall events.

Analytical Possibilities of Total Reflection X-ray Spectrometry (TXRF) for Trace Selenium Determination in Soils

Selenium content of soils is an important issue due to the narrow range between the nutritious requirement and toxic effects upon Se exposure. However, its determination is challenging due to low concentrations within complex matrices that hamper the analysis in most spectroscopic techniques. In this study, we explored the possibilities of several analytical approaches combined with total reflection X-ray (TXRF) spectrometry for soil Se determinations. The direct analysis of a solid suspension using 20 mg of fine ground material (<50 μm) has a relatively high Se limit of detection (LOD) of 1 mg/kg (worldwide Se average in soils = 0.4 mg/kg) and is therefore only suitable for seleniferous soils. Several fast and simple analytical strategies were developed to decrease matrix effects and improve the LOD for Se determination in soil digests. On one hand, the application of a liquid-liquid extraction procedure using ethyl ether and the introduction of a Cr absorbent in the instrument configuration were carried out to avoid the associated problems on TXRF analysis of soil extracts due to the high Fe concentrations (∼700 mg/L). On the other hand, a dispersive liquid-liquid microextraction procedure (DLLME) before the TXRF analysis of the soil digest was also developed. The effects of various experimental parameters such as sample volume, effect of major elements present in the soil matrix (Fe), and Se concentration in the sample were investigated. The LOD using this analytical methodology (0.05 mg/kg of Se) was comparable to or lower than those obtained in previous works using other popular spectrometric techniques such as GFAAS, ICPMS, and AFS. The calculated Se concentration for JSAC-0411 ([Se] = 1.32 ± 0.27 mg/kg) using the combination of DLLME and TXRF ([Se] = 1.40 ± 0.23 mg/kg) was in agreement with the certified value.

Elemental Analysis of Leaching Solution from Soils in the Mountain District of Shikoku with a Handy-type Liquid Electrode Plasma Atomic Emission Spectrometer

The elemental analysis of a 1M hydrochloric acid leaching solution from soil samples in the mountain district of Shikoku was carried out by a portable liquid electrode plasma (LEP) atomic emission spectrometer. The contents of Na, K, Ca, Mg, Mn, and Fe were determined, and compared with those by flame photometry, flame atomic absorption spectrometry, ICP-MS and ICP-AES. The 600 V of the applied voltage was found to be suitable for the present sample solution at the use of a sample holder made of resin. The precision of the emission intensities for iterative measurements was improved by a normalization procedure with an emission line from the atomic hydrogen, and the relative standard deviations were ca. 20%. Results of quantitative analysis of soil-extracts by LEP atomic emission spectrometry were discussed based on geological data around the sampling area.

Plant availability and phytotoxicity of soil bound residues of herbicide ZJ0273, a novel acetolactate synthase potential inhibitor

The plant availability and phytotoxicity of soil bound residues (BR) of herbicide ZJ0273, a novel acetolactate synthase (ALS) potential inhibitor, to rice ( Oryza sativa L.) and corn ( Zea mays L.) was investigated in three different soils including a Fluvio-marine yellow loamy soil ( S 1), a Red clayey soil ( S 2), and a Coastal saline soil ( S 3), using 14C-labeling tracer and bioassay techniques. When soils were amended with BR at 0.6, 1.2 and 1.8 nmol g −1, dose-dependent and significant inhibition was observed for rice seedlings within 14 d after treatment, but no significant inhibition occurred to corn seedlings in the same treatment. Radioactive analysis of soil extracts following sequential extractions showed that the 14C labeled residues of ZJ0273 were released from the amended soil BR upon planting. For example, when amended with 1.8 nmol g −1, about 68.3%, 57.0%, and 61.1%, respectively, of the added BR were released in S 1, S 2, and S 3 planted with rice seedlings, whereas 38.9%, 32.7% and 32.6% became available for uptake in the corresponding soils planted with corn seedlings. The released compounds were identified as ZJ0273 and its degradation products M1 and M2, with M2 as the primary component. Bioassay on rice showed that concentration for 50% inhibition (IC 50) of ZJ0273, M1, and M2 were 33.16, 1.93 and 0.49 μM, respectively. Therefore, BR formed after application of ZJ0273 may become available for plant uptake during rice cultivation and lead to phytotoxic effects, and the phytotoxicity is mainly caused by the release of the biologically active metabolite M2. This knowledge is valuable for designing crop rotation practices so that crop injury and yield losses due to carry-over herbicide phytotoxicity may be avoided.

Analysis of nerve agent degradation products in high‐conductivity matrices by transient ITP preconcentration and CZE separation coupled to ESI‐MS

Preconcentration of nerve agent degradation products (alkyl methylphosphonic acids) contained in high-conductivity matrices was performed using transient ITP to enhance sensitivity of CE-ESI-MS. The separation conditions of the five studied alkyl methylphosphonic acids in CE-MS were first optimized. The presence of methanol in the separation medium was required to obtain a good separation of the analytes under counter-EOF conditions. Preconcentration by ITP was induced by the BGE acting as leading electrolyte (LE) while the terminating electrolyte (TE) was loaded before the sample because of the counter-EOF conditions. Different leading ions (formate or acetate) and LE concentrations were tested. The best results for the analysis of soil extracts fortified with the analytes were obtained with an LE composed of 30 mM CH(3)COONH(4) adjusted to pH 8.8 with ammonium hydroxide in (35:65 v/v) MeOH/H(2)O mixture. The TE consisted of 200 mM glycine adjusted to pH 10.0 with ammonium hydroxide in the same solvent mixture. The loading length of the TE zone was optimized. The initial pH of the TE, which determined the initial mobility of the terminating ion, appeared to markedly influence the resolution and the sensitivity. This transient ITP-CZE-MS method was then adapted for the analysis of rat urine samples fortified with the analytes, which required the use of a more concentrated LE (50 mM). LODs between 4 and 70 ng/mL in soil extract, and between 5 and 75 ng/mL in rat urine were reached from extracted ion electropherograms.

Molecular Investigations of the Soil, Rhizosphere and Transgenic Glufosinate-Resistant Rape and Maize Plants in Combination with Herbicide (Basta®) Application under Field Conditions

A field study was conducted during 1994 to 1998 on the Experimental Farm Roggenstein, near Fürstenfeldbruck, Bavaria, Germany to determine the effect of transgenic glufosinate-resistant rape in combination with the herbicide Basta [glufosinate-ammonium, phosphinothricin, ammonium (2RS)-2-amino-4-(methylphosphinato) butyric acid] application on soil microorganisms and the behaviour of the synthetic transgenic DNA in response to normal agricultural practice. No influence of Basta on microbial biomass could be detected. The phospholipid fatty acid analysis of soil extracts showed no difference between Basta application and mechanical weed control, whereas conventional herbicide application revealed a different pattern. Basta application resulted in a changed population of weeds with a selective effect for Viola arvensis. During senescence, transgenic rape DNA was degraded similar to endogenous control DNA. After ploughing the chopped plant material in the soil, transgenic as well as endogenous control DNA sequences could be detected for up to 4 weeks for rape and up to 7 months for maize, whereas PCR analysis of composted transgenic maize revealed the presence of the transgene over a period of 22 months.

Cold-Trapping of Persistent Organic Pollutants in the Mountain Soils of Western Sichuan, China

In the Chinese province of Sichuan steep mountains rise from the very densely populated and intensely cultivated Chengdu basin more than 4000 elevational meters to the Tibetan Plateau. This steep physical gradient is exceptionally well suited to investigate the transport of persistent pesticides and other organic contaminants from low to high elevations. In spring and autumn 2006, 25 soil samples were taken at five elevations ranging from 2636 to 4479 m along the East-facing slope of Balang Mountain in Wolong Nature Reserve. Analysis of soil extracts was done by gas chromatography-high resolution mass spectrometry. Whereas hexachlorobenzene (HCB), hexachlorocyclohexanes (HCH), and dichlorodibenzotrichloroethane and its degradation products (DDTs) were present at levels of a few ng/g, only two light PCB congeners were detected at levels below 1 ng/g in soil. Soil concentration for all analytes increased significantly and exponentially with altitude. The rate of concentration increase, expressed quantitatively through the slope of the linear regression between the logarithm of the concentrations and altitude, increases along the sequence HCB < PCB < HCH < or = DDT. This trend is consistent with, and therefore lends additional observational support to, a mountain cold-trapping mechanism based on the temperature dependence of precipitation scavenging.

Iron interference in the quantification of nitrate in soil extracts and its effect on hypothesized abiotic immobilization of nitrate

Human alteration of the nitrogen cycle has stimulated research on nitrogen cycling in many aquatic and terrestrial ecosystems, where analyses of nitrate (NO3 −) by standard laboratory methods are common. A recent study by Colman et al. (Biogeochemistry 84:161–169, 2007) identified a potential analytical interference of soluble iron (Fe) with NO3 − quantification by standard flow-injection analysis of soil extracts, and suggested that this interference may have led Dail et al. (Biogeochemistry 54:131–146, 2001) to make an erroneous assessment of abiotic nitrate immobilization in prior 15N pool dilution studies of Harvard Forest soils. In this paper, we reproduce the Fe interference problem systematically and show that it is likely related to dissolved, complexed-Fe interfering with the colorimetric analysis of NO2 −. We also show how standard additions of NO3 − and NO2 − to soil extracts at native dissolved Fe concentrations reveal when the Fe interference problem occurs, and permit the assessment of its significance for past, present, and future analyses. We demonstrate low soluble Fe concentrations and good recovery of standard additions of NO3 − and NO2 − in extracts of sterilized Harvard Forest soils. Hence, we maintain that rapid NO3 − immobilization occurred in sterilized samples of the Harvard Forest O horizon in the study by Dail et al. (2001). Furthermore, additional evidence is accumulating in the literature for rapid disappearance of NO3 − added to soils, suggesting that our observations were not the result of an isolated analytical artifact. The conditions for NO3 − reduction are likely to be highly dependent on microsite properties, both in situ and in the laboratory. The so-called “ferrous wheel hypothesis” (Davidson et al., Glob Chang Biol 9:228–236, 2003) remains an unproven, viable explanation for published observations.