Soil sterilization affects aging-related sequestration and bioavailability of p,p'-DDE and anthracene to earthworms

Ageing effect on conazole fungicide bioaccumulation in arable soils

Widely used conazole fungicides (CFs) belong to the most frequently detected pesticides in Central European arable soils. However, data on their environmental behaviour and bioavailability to soil organisms are surprisingly scarce. In the present laboratory microcosm study prochloraz, tebuconazole, epoxiconazole and flusilazole were applied to 12 different agricultural soils at background levels. Bioaccumulation to earthworm E. andrei and lettuce L. sativa roots and leaves was evaluated in non-aged (biota exposure after addition of pesticides) and aged (exposure started three months later) systems. In contrast with expectations from ageing effect (decrease of bioavailability), bioaccumulation in E. andrei was both reduced and enhanced after ageing depending on soil properties. The reduction of bioaccumulation correlated positively to the percentage of clay but negatively to soil organic matter. The affinity of compost worm E. andrei towards organic matter where hydrophobic pesticide molecules are sorbed is discussed as a possible explanation. An apparent effect of ageing (reduction of bioavailability) was particularly observed in lettuce roots, where bioaccumulation was significantly reduced in time. However, bioaccumulation in leaves changed ambiguously in aged variants among CFs, possibly as a combined result of bioconcentration, dilution by plant growth and metabolism. This study brings first insights into how the bioaccumulation of conazole fungicides is affected by sequestration in agricultural soils. The results indicate that in complex systems, the ageing is not necessarily connected with decrease of bioaccumulation.

Uptake kinetics of four hydrophobic organic pollutants in the earthworm Eisenia andrei in aged laboratory-contaminated natural soils

Laboratory studies of pollutant uptake kinetics commonly start shortly after experimental soil contamination when it is not clear if the processes between soil and chemicals are equilibrated and stabilized. For instance, when the concentration in soil quickly decreases due to initial biodegradation, bioaccumulation may show a peak-shape accumulation curve instead of conventional first order kinetics with a plateau at the end. The results of such experiments with soil freshly contaminated in the laboratory are then hardly comparable to bioaccumulation observed in soils from historically contaminated sites. Therefore, our study focused on the uptake kinetics of four hydrophobic organic compounds (pyrene, lindane, p,p'-DDT and PCB 153) in two laboratory-contaminated natural soils with different soil properties (e.g. total organic carbon content of 1.6 and 9.3%) aged for 203 days to mimic long-term contamination. For pyrene, the results surprisingly showed peak-shape accumulation curves despite long aging. It seems compound biodegradation might be significant in aged soils when the conditions change (e.g. by distribution to the experimental vessels) and this should be also considered when testing historically contaminated soils. For lindane, longer aging seems to guarantee stability of the soil-compound-earthworm system and the steady state was reached after 5 days of exposure. Furthermore, although concentrations of p,p'-DDT and PCB 153 in earthworms after 11-15-day exposure did not statistically differ, which is a commonly-used indicator that a steady state was reached, they continuously increased until the end of the exposure. Therefore, despite the aging, longer exposure was probably needed to reach the true equilibrium between concentrations in earthworms and soil. In summary, aging does not warranty the conventional first order kinetic curve with the equilibrium at the end of the exposure but may have diverse effects for compounds with different environmental properties and should be taken into account in the bioaccumulation factor calculation and the risk assessment.

Prediction of organic contaminant uptake by plants: Modified partition-limited model based on a sequential ultrasonic extraction procedure

Predicting the translocation of organic contaminants to plants is crucial to ensure the quality of agricultural goods and assess the risk of human exposure through the food web. In this study, the performance of a modified plant uptake model was evaluated considering a number of chemicals, such as polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs) and polybrominated diphenyl ethers (PBDEs), with a range of physicochemical properties; different plant species (Ipomoea aquatica Forsk (swamp morning glory), Chrysanthemum coronarium L. (crown daisy), Zea mays L. (corn), Brassica rapa pekinensis (Chinese cabbage), Cucurbita moschata (pumpkin), Raphanus sativus L. (radish), Spinacia oleracea L. (spinach) and Capsicum annuum L. (pepper)); and different types of soil (paddy soil, laterite soil and black soil). The biases of predictions from a previously used partition-limited model were -76.4% to -99.9% relative to the measured concentrations. An overall transmission factor (α_tf=0.39), calculated from a linear regression of the measured bioavailable fraction (C_bio) and the total concentration in plants, was considered a crucial modification and was included in the modified model. C_bio was found to better represent the chemical content available in soil for root uptake. The results from this study improve the accuracy of predictions for vegetation-uptake assessments by modifying the partition-limited model and then validating the modified model using comparisons between predicted data and measured values. The accuracy of the concentrations of organic contaminants in plants improved: when using the modified model, 89.5% of the predictions were within 40% of the actual value. The average bias was limited to 1.5%-30.5%. The model showed great potential to predict plant uptake using the bioavailable fraction concentration in soil.

What are the effects of soil treatment procedures (sterilization by γ-irradiation and solvent-assisted spiking) on DDE bioaccumulation by earthworms?

Ionizing γ-irradiation and solvent-assisted spiking are frequently applied to eliminate microbial activity and to induce hydrophobic organic compounds (HOCs) into soil, respectively, when studying the accumulation of chemicals in terrestrial organisms. However, the side-effects that may arise from these treatments on soil-HOC interaction and, subsequently, the kinetics and extents of bioaccumulation are not thoroughly understood. To this end, the accumulation of 1,1-dichloro-2,2-bis(p-chlorophenyl)etylene (p,p'-DDE) by Eisenia andrei was studied in sterilized or unsterilized and freshly spiked (FS) or historically contaminated (HC) soils in parallel with an analysis of aliphatic and hydrophilic soil organic matter (SOM) moieties using mid-infrared diffuse reflectance spectroscopy (DRIFT-S). Irradiation did not impart significant changes on spectral SOM descriptors. In contrast, earthworm inhabitation increased the relative presence of aliphatic moieties to a greater extent than hydrophilic ones, reaching or exceeding pre-treatment levels. Overall, effects on SOM chemistry can be ranked as earthworms > spiking > irradiation. Corresponding changes at the bioaccumulation level were observed for the FS soil (i.e., a 27% reduction in bioaccumulation upon sterilization) but not for the HC soil. This implies that in contrast to the interactions between aged p,p'-DDE and sterilized HC soil, the interactions established between freshly added p,p'-DDE and sterilized FS soil were altered by γ-irradiation-induced secondary effects alone or in combination with earthworm inhabitation. Thus, although the soil treatment processes studied here should not drastically impact compound bioaccumulation, they should be considered in mechanistic studies where the qualitative and quantitative aspects of compound-soil (organic matter)-earthworm interactions are at the centre of attention.

Influence of soil γ-irradiation and spiking on sorption of p,p'-DDE and soil organic matter chemistry

The fate of organic chemicals and their metabolites in soils is often investigated in model matrices having undergone various pre-treatment steps that may qualitatively or quantitatively interfere with the results. Presently, effects associated with soil sterilization by γ-irradiation and soil spiking using an organic solvent were studied in one freshly spiked soil (sterilization prior to contamination) and its field-contaminated (sterilization after contamination) counterpart for the model organic compound 1,1-Dichloro-2,2-bis(4-chlorophenyl)ethene (p,p'-DDE). Changes in the sorption and potential bioavailability of spiked and native p,p'-DDE were measured by supercritical fluid extraction (SFE), XAD-assisted extraction (XAD), and solid-phase microextraction (SPME) and linked to qualitative changes in soil organic matter (SOM) chemistry measured by diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy. Reduced sorption of p,p´-DDE detected with XAD and SPME was associated more clearly with spiking than with sterilization, but SFE showed a negligible impact. Spiking resulted in an increase of the DRIFT-derived hydrophobicity index, but irradiation did not. Spectral peak height ratio descriptors indicated increasing hydrophobicity and hydrophilicity in pristine soil following sterilization, and a greater reduction of hydrophobic over hydrophilic groups as a consequence of spiking. In parallel, reduced sorption of p,p´-DDE upon spiking was observed. Based on the present samples, γ-irradiation appears to alter soil sorptive properties to a lesser extent when compared to common laboratory processes such as spiking with organic solvents.

Laboratory versus field soil aging: Impact on DDE bioavailability and sorption

Solid-phase microextraction (SPME), XAD, and the sequential supercritical fluid extraction (SFE) were used to assess the influence of aging of p,p'-DDE in a laboratory contaminated soil for up to 730 days. The end points determined were the freely dissolved concentration (C_free) using SPME, the potentially bioaccessible fraction (F_XAD, %) and the distribution of p,p'-DDE among fast, moderate, and slow desorbing soil sites determined by three sequentially stronger SFE conditions. C_free and F_XAD decreased during the first 35 days of aging by up to 40%. After this, no significant changes were observed up to the end of the aging experiment. The relative percentage of fast desorbing sites tended to exponentially decrease with aging, while the percentage of moderate and slow desorbing sites increased over time. These changes were most apparent within the first 90 days of aging, after which the relative distribution of p,p'-DDE among desorbing sites remained relatively constant. Significant correlations between SFE and XAD results demonstrated that the XAD method preferentially desorbed p,p'-DDE from fast and moderate desorbing sites and is capable of extracting the bioaccessible fraction. The distribution among desorbing sites, C_free and F_XAD values determined after different periods of laboratory aging were then compared to those measured for a field-contaminated soil where p,p'-DDE had resided for more than 40 years. C_free, F_XAD and SFE profiles measured for the field-aged p,p'-DDE were similar to those observed for p,p'-DDE aged in laboratory for between 35 and 90 days. These results suggest that aging in the laboratory must be carried out for periods of months if it is to approximate field aging.

Bioavailability of five hydrophobic organic compounds to earthworms from sterile and non-sterile artificial soils

Bioaccumulation factors (BAFs) of organic pollutants to soil biota, often required by risk assessment, are mostly obtained in non-sterile laboratory-contaminated artificial soils. However, microbial degradation has been indicated by many authors to influence the fate of hydrophobic organic compounds (HOCs) in soils. A question arises if the microbial community of peat which is used for artificial soil preparation affects the measured values of BAFs. In this study the effect of soil microorganisms on bioavailability of HOCs was studied and a portion of each soil was sterilized by gamma irradiation. Results indicated that the sterilization process significantly affected the fate of polycyclic aromatic hydrocarbons (PAHs; phenanthrene and pyrene) and increased bioavailability of these compounds to earthworms with BAFs several times higher in the sterile soils compared to their non-sterile variants. This suggests that sterilization of soils can be used as the "worst-case scenario" for laboratory tests of toxicity or bioaccumulation of biodegradable HOCs such as PAHs. It represents a situation of limited microbial degradation resulting in higher bioavailable fractions to other organisms (e.g. invertebrates). This may be the case in soils where microbial communities face stresses caused by contamination or land management. The bioavailability of chlorinated HOCs (lindane, 4,4'-DDT and PCB 153) was not affected by sterilization, as their BAFs were similar in the sterile and non-sterile soils during the experiment.

Dissipation of pterosin B in acid soils - Tracking the fate of the bracken fern carcinogen ptaquiloside

Bracken ferns (Pteridium spp.) are well-known for their carcinogenic properties, which are ascribed to the content of ptaquiloside and ptaquiloside-like substances. Ptaquiloside leach from the ferns and may cause contamination of drinking water. Pterosin B is formed by hydrolysis of ptaquiloside. In soil, Pterosin B is adsorbed more strongly and it is expected to have a slower turnover than ptaquiloside. We thus hypothesized that pterosin B may serve as an indicator for any past presence of ptaquiloside. Pterosin B degradation was studied in acid forest soils from bracken-covered and bracken-free areas. Soil samples were incubated with pterosin B at 3 and 8 μg g^-1 for 10 days, whereas sterile (autoclaved) samples were incubated for 23 days. Pterosin B showed unexpected fast degradation in soils with full degradation in topsoils in 2-5 days. Pterosin B dissipation followed the sum of two-first order reactions. The initial fast reaction with half-lives of 0.7-3.5 h contributed 11-59% of the total pterosin B degradation, while the slow reaction was 20-100 times slower than the fast reaction. Total dissipation half-lives were shorter for loamy sand (4 h) than for sandy loam soils (28 h). No degradation of pterosin B took place under sterile conditions assuming observed dissipation during the first 3 h could be attributed to irreversible sorption. Our results demonstrate that pterosin B is microbially degraded and that pterosin B is as unstable as ptaquiloside and hence cannot be used as an indicator for former presence of ptaquiloside in soil.

Chemical measures of bioavailability/bioaccessibility of PAHs in soil: fundamentals to application

Risk assessment and remediation of contaminated land is inherently dependent on the contaminants present and their availability for interaction with soil biota. An ever-growing body of evidence suggests that current regulatory procedures over-estimate the 'true' fraction available to biota. Thus, a procedure that predicts the 'bioavailable fraction' would be useful for predicting 'actual' exposure limits and provide a more relevant basis for risk assessment. The aim of this paper is to address several important questions: "How should bioavailability be defined?" "What factors affect bioavailability measurement?" "To what extent have existing protocols measured bioavailability?" "What is actually measured by chemical techniques purported to determine bioavailability?" We offer two definitions (namely 'bioavailability' and 'bioaccessibility') and review commonly employed chemical extraction techniques to measure putative bioavailability. Relative advantages and disadvantages of the techniques are highlighted to elucidate underlying factors for the wide range of conclusions observed in the literature. Although the concept of bioavailability is implicit to contaminated land risk assessment and remediation, explicit reference to and use of adjustment factors is rare amongst regulatory bodies and remediators. Use of chemical determinants for bioavailability, applicable within current legislation and due consideration to inherent variability, are proposed and barriers to their implementation discussed.

Fate and bioavailability of ¹⁴C-pyrene and ¹⁴C-lindane in sterile natural and artificial soils and the influence of aging

Soil organic matter is used to extrapolate the toxicity and bioavailability of organic pollutants between different soils. However, it has been shown that other factors such as microbial activity are crucial. The aim of this study was to investigate if sterilization can reduce differences in the fate and bioavailability of organic pollutants between different soils. Three natural soils with increasing total organic carbon (TOC) content were collected and three artificial soils were prepared to obtain similar TOCs. Soils were sterilized and spiked with (14)C-pyrene and (14)C-lindane. Total (14)C radioactivity, HPCD extractability, and bioaccumulation in Eisenia fetida were measured over 56 days. When compared to non-sterile soils, differences between the natural and artificial soils and the influence of soil-contaminant contact time were generally reduced in the sterile soils (especially with middle TOC). The results indicate the possibility of using sterile soils as "the worst case scenario" in soil ecotoxicity studies.

The effects of aging time on the fraction distribution and bioavailability of PAH

Understanding the effects of aging time on the fraction distribution and bioavailability of PAH, such as phenanthrene (PHE) and pyrene (PYR), has considerable benefits for risk assessment, food security and remediation strategies for contaminated soil. The results of the present study show that the proportion of the desorbed PHE decreased from ca. 82% at day 0 to ca. 65% at day 150. In addition, non-desorbed PHE increased from ca. 18% at day 0 to ca. 31% at day 150, whereas the changes of desorbed and non-desorbed PYR showed no significant trend during this aging period. The proportion of desorbed PYR was lower than that of PHE, whereas the opposite occurred with the non-desorbed fraction. After 150 d of aging, the proportion of bound residues (PHE and PYR) increased significantly with the cultivating time from ca. 0.2% to ca. 4.7% and ca. 0.1% to ca. 1.2% for PHE and PYR, respectively. In addition, the bioavailability of PAH (PHE and PYR) to earthworms was also assessed over 0-150 d. The results showed that the uptake rate and bioconcentration factor (BCF) of pollutants by earthworms displayed the following biphasic character: a rapid decrease over the first 15 d followed by a slow decrease over the next 135 d. Moreover, the earthworm uptake rate of PHE was greater than that of PYR throughout the incubation period, indicating that PHE has a higher bioavailability than PYR. In addition, the positive correlation between the uptake rate of earthworms and PAH extractability suggested that a three-step extraction is a reliable approach to predict PHE bioavailability in soil. However, a limit was observed for PYR.

A comparison of POPs bioaccumulation in Eisenia fetida in natural and artificial soils and the effects of aging

The close relationship between soil organic matter and the bioavailability of POPs in soils suggests the possibility of using it for the extrapolation between different soils. The aim of this study was to prove that TOC content is not a single factor affecting the bioavailability of POPs and that TOC based extrapolation might be incorrect, especially when comparing natural and artificial soils. Three natural soils with increasing TOC and three artificial soils with TOC comparable to these natural soils were spiked with phenanthrene, pyrene, lindane, p,p'-DDT, and PCB 153 and studied after 0, 14, 28, and 56 days. At each sampling point, total soil concentration and bioaccumulation in earthworms Eisenia fetida were measured. The results showed different behavior and bioavailability of POPs in natural and artificial soils and apparent effects of aging on these differences. Hence, direct TOC based extrapolation between various soils seems to be limited.