Non-extractable Residues Research Articles

Hydrogen isotope labeling unravels origin of soil-bound organic contaminant residues in biodegradability testing

Biodegradability testing in soil helps to identify safe synthetic organic chemicals but is still obscured by the formation of soil-bound ‘non-extractable’ residues (NERs). Present-day methodologies using radiocarbon or stable (13C, 15N) isotope labeling cannot easily differentiate soil-bound parent chemicals or transformation products (xenoNERs) from harmless soil-bound biomolecules of microbial degraders (bioNERs). Hypothesizing a minimal retention of hydrogen in biomolecules, we here apply stable hydrogen isotope – deuterium (D) – labeling to unravel the origin of NERs. Soil biodegradation tests with D- and 13C-labeled 2,4-D, glyphosate and sulfamethoxazole reveal consistently lower proportions of applied D than 13C in total NERs and in amino acids, a quantitative biomarker for bioNERs. Soil-bound D thus mostly represents xenoNERs and not bioNERs, enabling an efficient quantification of xenoNERs by just measuring the total bound D. D or tritium (T) labeling could thus improve the value of biodegradability testing results for diverse organic chemicals forming soil-bound residues.

Identifying bound compounds in non-extractable residues of pesticides in soil by 4-pool kinetic analysis

This study explores the feasibility of identifying bound compounds in non-extractable residues (NERs) of pesticides in soil by 4-pool kinetic analysis. The 4-pools refer to parent compound, metabolites, NERs, and CO2 in 14C-labeled pesticide soil degradation studies. We discovered the following two characteristic 4-pool kinetic behaviors of formation of NERs: (1) if parent compound is bound as NERs, the metabolites (m(t) in % applied radioactivity (AR)) kinetically drive the evolution of CO2 only; and (2) if a metabolite (x) in a sequential degradation pathway is bound as NERs, m(t) is split into m1(t) and m2(t) at the metabolite (x) that is bound as NERs, which kinetically drive the formation of NERs and evolution of CO2 respectively. We developed two (i.e., Parent➔NER and metabolite➔NER) 4-pool models to capture the kinetic behaviors respectively. By fitting the models to a set of 4-pool data, we not only determine whether NERs form from parent compound or metabolites but also identify the bound metabolite (x) by resolving the metabolites into M1(t) and M2(t) (i.e., the variables used to simulate m1(t) and m2(t)) and then matching m1(t) (i.e., the sum of bound metabolite (x) and its metabolite precursors) with M1(t). By applying the models to 14C-labeled parent compound-dosed studies and then sequentially to metabolite-dosed studies for 7 pesticides with ≥70 % AR NERs, we identified the bound compounds, which have the moieties known to be responsible for NERs, are bound as NERs instantaneously when dosed into soil, and account for pH dependency of NER formation.

Non-extractable residues of perfluorooctanoic acid (PFOA) in soil

Perand polyfluoroalkyl substances have gained increased attention due to their persistence, ubiquitous presence in the environment, and toxicity. We hypothesised that the formation of non-extractable residues [NER] occurs in soils and contributes to the overall persistence of these priority pollutants, and that NER formation is controlled by temperature. To test these hypotheses, we used 14C-labelled perfluorooctanoic acid [PFOA] as target compound, added it to two arable soils (Cambisol, Luvisol), and incubated them at 10°C and 20°C in the dark. To support potential co-metabolic decomposition, some samples were additionally fed with glucose to enhance microbial activity. The PFOA residues were then sequentially extracted using 0.01 M CaCl2, followed by accelerated solvent extraction (ASE) with methanol or methanol/acetic acid after 0, 1, 3, 9, 30, 62, and 90 days of incubation. In addition, we monitored the release of 14C into the gas phase as well as [14C]-PFOA-NER after dry combustion and liquid scintillation counting. After 90 days, we found that the [14C]-PFOA content declined in the extraction order of CaCl2 ((bio)available fraction) > ASE (residual fraction) > NER > gas fraction), with most rapid changes occurring in the first 9 days of incubation. NER formation was different in the two soils and reached 5-9% of the applied amount in the Cambisol and Luvisol, respectively. Noteworthy the proportion of 14C-PFOA in the (bio)available fraction remained relatively stable over time at 56-62% of the applied amount, indicating the reversible transfer into this fraction from a bi-exponentially declining residual (ASE) pool. These dissipation patterns were neither influenced by temperature nor by the addition of glucose. We conclude that NER exist for PFOA, but that the majority of PFOA remains in (bio)available form, thus maintaining toxicity and mobility in soil for prolonged periods of time.

Separate hydrolysis and fermentation of softwood bark pretreated with 2-naphthol by steam explosion

Background2-Naphthol, a carbocation scavenger, is known to mitigate lignin condensation during the acidic processing of lignocellulosic biomass, which may benefit downstream processing of the resulting materials. Consequently, various raw materials have demonstrated improved enzymatic saccharification yields for substrates pretreated through autohydrolysis and dilute acid hydrolysis in the presence of 2-naphthol. However, 2-naphthol is toxic to ethanol-producing organisms, which may hinder its potential application. Little is known about the implications of 2-naphthol in combination with the pretreatment of softwood bark during continuous steam explosion in an industrially scalable system.ResultsThe 2-naphthol-pretreated softwood bark was examined through spectroscopic techniques and subjected to separate hydrolysis and fermentation along with a reference excluding the scavenger and a detoxified sample washed with ethanol. The extractions of the pretreated materials with water resulted in a lower aromatic content in the extracts and stronger FTIR signals, possibly related to guaiacyl lignin, in the nonextractable residue when 2-naphthol was used during pretreatment. In addition, cyclohexane/acetone (9:1) extraction revealed the presence of pristine 2-naphthol in the extracts and increased aromatic content of the nonextractable residue detectable by NMR for the scavenger-pretreated materials. Whole-slurry enzymatic saccharification at 12% solids loading revealed that elevated saccharification recoveries after 48 h could not be achieved with the help of the scavenger. Glucose concentrations of 16.9 (reference) and 15.8 g/l (2-naphthol) could be obtained after 48 h of hydrolysis. However, increased inhibition during fermentation of the scavenger-pretreated hydrolysate, indicated by yeast cell growth, was slight and could be entirely overcome by the detoxification stage. The ethanol yields from fermentable sugars after 24 h were 0.45 (reference), 0.45 (2-naphthol), and 0.49 g/g (2-naphthol, detoxified).ConclusionThe carbocation scavenger 2-naphthol did not increase the saccharification yield of softwood bark pretreated in an industrially scalable system for continuous steam explosion. On the other hand, it was shown that the scavenger's inhibitory effects on fermenting microorganisms can be overcome by controlling the pretreatment conditions to avoid cross-inhibition or detoxifying the substrates through ethanol washing. This study underlines the need to jointly optimize all the main processing steps.

Strategies and mechanisms for improving the detection accuracy of nonextractable residues of polycyclic aromatic hydrocarbons in soils

The methods that can accurately measure the concentrations of nonextractable residues (NERs) of hydrophobic organic contaminants (HOCs) in soil are still lacked in current studies. In this study, three methods, namely methanolic saponification treatment (MST), silylation treatment (ST), and acid deashing treatment (ADT), were investigated and then combined to extract the NERs of six types of polycyclic aromatic hydrocarbons (PAHs) from nine soil samples. The NER concentrations of PAHs obtained by ST (2.43–521.73 ng g−1) were comparable to or significantly higher than those obtained by MST (1.94–291.54 ng g−1), owing to the properties of soil and target compounds. Additionally, ADT could further release a considerable amount of PAH NERs (0.39–276.99 ng g−1) from the soils that had been treated with ST. The mechanism was that acid solution dissolved mineral components, significantly increasing the pore size of the soil matrices from 9.37–15.57 nm to 17.11–27.51 nm. The average percentage of each PAH obtained by ADT (the ratio of the amount obtained by ADT to the total NER content) exhibited a negative correlation with their ring numbers (R2 = 0.62, p < 0.05), whereas the percentage of targets recovered through ST increased linearly with their log KOW values (R2 = 0.75, p < 0.05). Moreover, there is a positive correlation (R2 = 0.73, p < 0.05) between the NER percentages of phenanthrene (obtained by ST–ADT) and the specific surface areas of soils, and the NER percentages of benzo(g,h,i)perylene is positively correlated to the content of total organic carbon (R2 = 0.62, p < 0.05). These results suggested that the amounts and locations of NERs were influenced by both the physicochemical characteristics of PAHs and soils. These findings provide some basic understandings of the entrapped mechanisms of PAH NERs, helping to establish strategies for improving their detection accuracy.

Maize (Zea mays L.) Plants Alter the Fate and Accumulate Nonextractable Residues of Sulfamethoxazole in Farmland Soil.

The fate of sulfonamide antibiotics in farmlands is crucial for food and ecological safety, yet it remains unclear. We used [phenyl-U-14C]-labeled sulfamethoxazole (14C-SMX) to quantitatively investigate the fate of SMX in a soil-maize system for 60 days, based on a six-pool fate model. Formation of nonextractable residues (NERs) was the predominant fate for SMX in unplanted soil, accompanied by minor mineralization. Notably, maize plants significantly increased SMX dissipation (kinetic constant kd = 0.30 day-1 vs 0.17 day-1), while substantially reducing the NER formation (92% vs 58% of initially applied SMX) and accumulating SMX (40%, mostly bound to roots). Significant NERs (maximal 29-42%) were formed via physicochemical entrapment (determined using silylation), which could partially be released and taken up by maize plants. The NERs consisted of a considerable amount of SMX formed via entrapment (1-8%) and alkali-hydrolyzable covalent bonds (2-12%, possibly amide linkage). Six and 10 transformation products were quantified in soil extracts and NERs, respectively, including products of hydroxyl substitution, deamination, and N-acylation, among which N-lactylated SMX was found for the first time. Our findings reveal the composition and instability of SMX-derived NERs in the soil-plant system and underscore the need to study the long-term impacts of reversible NERs.

Characteristics and Biodegradation of Nonextractable Residues of Polycyclic Aromatic Hydrocarbons in Historically Polluted Soils

Characteristics and Biodegradation of Nonextractable Residues of Polycyclic Aromatic Hydrocarbons in Historically Polluted Soils

Impact of different sterilisation techniques on sorption and NER formation of test chemicals in soil

Standard OECD tests are used to generate data on biodegradation (OECD 307) and sorption (OECD 106) of test chemicals in soil. In such tests, data on abiotic degradation using sterile samples are utilised to investigate any losses due to abiotic processes. The data from sterile samples are also used to interpret results and findings of non-sterile samples, especially in the context of sorption and non-extractable residue (NER) formation. However, to ensure the comparability of the data obtained from sterile and non-sterile experiments, effects of sterilisation on the soil matrix should be minimal. The objective of this study was to investigate the efficiencies of different sterilisation techniques and the impact of the sterilisation on sorption and NER formation in soil. In this study, experiments in accordance with OECD 307 and OECD 106 guidelines were performed with two soils covering wide range of soil characteristics and treated with the three sterilisation techniques autoclaving, gamma(γ)-radiation and adding 1% (w/w) sodium azide. As a test item, 14C-labelled phenanthrene and bromoxynil was used for OECD 307 test, whereas non-labelled phenanthrene and atrazine was used for OECD 106. The sterilisation efficiencies were investigated using traditional viable plate count and molecular approaches (RNA extraction method). The results suggest that none of the tested techniques resulted in completely sterilised soil with autoclaving being the most efficient technique. Adding sodium azide led to most inefficient sterilisation and a significant increase (0.56 units) in soil pH. OECD 307 results showed differences in NER formation of the test chemicals, especially for soil poisoning and γ-radiation, which could be due to inefficient sterilisation and/or change in soil physico-chemical properties. OECD 106 results suggest that none of the sterilisation techniques considerably affected sorption behaviour of the test chemicals. Based on our results, we recommend autoclaving as most suitable sterilisation technique.

Nonextractable Residues, an Analytical Iceberg, Contribute Significantly to the Pool of Halogenated Organic Chemicals in Soils across Various Regions

Nonextractable Residues, an Analytical Iceberg, Contribute Significantly to the Pool of Halogenated Organic Chemicals in Soils across Various Regions

Fate of sulfamethoxazole in wetland sediment under controlled redox conditions

Redox condition is an important controlling factor for contaminant removal in constructed wetlands; however, the redox-sensitivity of antibiotic removal in wetland sediments under controlled conditions with specific electron acceptors remains unclear. Here, using a 14C radioactive tracer, we explored fate of sulfamethoxazole (SMX) in a wetland sediment slurry under oxic, nitrate-reducing, iron-reducing, and methanogenic conditions. In the sterile treatment, unlike the comparable SMX dissipation from the water phase under four redox conditions, non-extractable residues (NERs) of SMX was highest formed in the sediment under oxic condition, mainly in sequestered and ester/amide-linked forms. Microorganisms markedly promoted SMX transformation in the slurry. The dissipation rate of SMX and its transformation products (TPs) followed the order: oxic ≈ iron-reducing > methanogenic >> nitrate-reducing conditions, being consistent with the dynamics of microbial community in the sediment, where microbial diversity was greater and networks connectivity linking dominant bacteria to SMX transformation were more complex under oxic and iron-reducing conditions. Kinetic modeling indicated that the transformation trend of SMX and its TPs into the endpoint pool NERs depended on the redox conditions. Addition of wetland plant exudates and sediment dissolved organic matter at environmental concentrations affected neither the abiotic nor the biotic transformation of SMX. Overall, the iron-reducing condition was proven the most favorable and eco-friendly for SMX transformation, as it resulted in a high rate of SMX dissipation from water without an increase in toxicity and subsequent formation of significant stable NERs in sediment. Our study comprehensively revealed the abiotic and biotic transformation processes of SMX under controlled redox conditions and demonstrated iron-reducing condition allowing optimal removal of SMX in constructed wetlands.

Fate of dimethylsilanediol (DMSD) in soil-plant systems

Dimethylsilanediol (DMSD) is the degradation product of methylsiloxane polymers and oligomers such as volatile cyclic methylsiloxanes (cVMS). To better understand the environmental fate of this key degradation product, we conducted a three-part study on the movement of DMSD in soil. The objective of this third and final study was to determine the fate of DMSD in soil-plant systems under constant irrigation. Soil columns were constructed using two soils with the upper 20 cm layers spiked with 14C-labeled DMSD. Corn seedlings were transplanted into the soil columns and placed in a field plot underneath a transparent cover that prevented rainwater from reaching the soil columns while allowing soil water to be volatilized freely. The soil-plant columns were regularly irrigated with known amounts of DMSD-free plant growth solution to sustain the plant growth. At pre-determined time intervals (15–67 days), the plant and soil columns were sectioned and the distribution of 14Corganosilicon species in the soil profile and plant parts was determined using a combination of Liquid Scintillation Counting and High-Performance Liquid Chromatography-Flow Scintillation Analysis, while soil water loss was determined gravimetrically. It was found that the majority (>92 %) of DMSD initially spiked into the soil was removed from the soil-plant systems. Although DMSD was transported from the soil to the plant, it was subsequently volatilized from the plant via transpiration, with only a small fraction (∼5%) remaining at the conclusion of the experiments. In addition, little non-extractable DMSD was found in the top layer of soil in the soil-plant systems, suggesting that the air-drying of soil is a necessary pre-condition for the formation of such non-extractable silanol residue on topsoil.

Nitrogen-fertilizer addition to an agricultural soil enhances biogenic non-extractable residue formation from 2-13C,15N-glyphosate

Glyphosate and nitrogen (N) or (P) phosphorus fertilizers are often applied in combination to agricultural fields. The additional P or N supply to microorganisms might drive glyphosate degradation towards sarcosine/glycine or aminomethylphosphonic acid (AMPA), and consequently determine the speciation of non-extractable residues (NERs): harmless biogenic NERs (bioNERs) or potentially hazardous xenobiotic NERs (xenoNERs). We therefore investigated the effect of P or N-fertilizers on microbial degradation of glyphosate and bioNER formation in an agricultural soil. Four different treatments were incubated at 20 °C for 75 days as follows; I: no fertilizer (2-13C,15N-glyphosate only, control), II: P-fertilizer (superphosphate + 2-13C,15N-glyphosate, effect of P-supply), III: N-fertilizer (ammonium nitrate + 2-13C,15N-glyphosate, effect of N-supply) and IV: 15N-fertilizer (15N-ammonium nitrate + 2-13C-glyphosate, differentiation between microbial assimilations of 15N: 15N-fertilizer versus 15N-glyphosate). We quantified 13C or 15N in mineralization, extractable residues, NERs and in amino acids (AAs). At the end, mineralization (36–41 % of the 13C), extractable 2-13C,15N-glyphosate/2-13C-glyphosate (0.42–0.49 %) &15N-AMPA (1.2 %), and 13C/15N-NERs (40–43 % of the 13C, 40–50 % of the 15N) were comparable among treatments. Contrastingly, the 15N-NERs from 15N-fertlizer amounted to only 6.6 % of the 15N. Notably, N-fertilizer promoted an incorporation of 13C/15N from 2-13C,15N-glyphosate into AAs and thus the formation of 13C/15N-bioNERs. The 13C/15N-AAs were as follows: 16–21 % (N-fertilizer) > 11–13 % (control) > 7.2–7.3 % (P-fertilizer) of the initially added isotope. 2-13C,15N-glyphosate was degraded via the sarcosine/glycine and AMPA simultaneously in all treatments, regardless of the treatment type. The percentage share of bioNERs within the NERs in the N-fertilized soil was highest (13C: 80–82 %, 15N: 100 %) compared to 53 % (13C &15N, control) and to only 30 % (13C &15N, P-fertilizer). We thus concluded simultaneous N & glyphosate addition to soils could be beneficial for the environment due to the enhanced bioNER formation, while P & glyphosate application disadvantageous since it promoted xenoNER formation.

Composition, Release, and Transformation of Earthworm Tissue-Bound Residues of Tetrabromobisphenol A in Soil.

Earthworms accumulate organic pollutants to form earthworm tissue-bound residues (EBRs); however, the composition and fate of EBRs in soil remain largely unknown. Here, we investigated the fate of tetrabromobisphenol A (TBBPA)-derived EBRs in soil for 250 days using a 14C-radioactive isotope tracer and the geophagous earthworm Metaphire guillelmi. The EBRs of TBBPA in soil were rapidly transformed into nonextractable residues (NERs), mainly in the form of sequestered and ester-linked residues. After 250 days of incubation, 4.9% of the initially applied EBRs were mineralized and 69.3% were released to extractable residues containing TBBPA and its transformation products (TPs, generated mainly via debromination, O-methylation, and skeletal cleavage). Soil microbial activity and autolytic enzymes of earthworms jointly contributed to the release process. In their full-life period, the earthworms overall retained 24.1% TBBPA and its TPs in soil and thus prolonged the persistence of these pollutants. Our study explored, for the first time, the composition and fate of organic pollutant-derived EBRs in soil and indicated that the decomposition of earthworms may release pollutants and cause potential environmental risks of concern, which should be included in both environmental risk assessment and soil remediation using earthworms.

The Interaction of Pesticides with Humin Fractions and Their Potential Impact on Non-Extractable Residue Formation.

The constant influx of pesticides into soils is a key environmental issue in terms of their potential retention in the soil, thus reducing their negative impact on the environment. Soil organic matter (SOM) is an important factor influencing the environmental fate of these substances. Therefore, the aim of this research was to assess the chemical behavior of pesticides (flufenacet, pendimethalin, α-cypermethrin, metazachlor, acetamiprid) toward stable soil humin fractions (HNs) as a main factor affecting the formation of non-extractable residues of agrochemicals in soil. This research was conducted as a batch experiment according to OECD Guideline 106. For this purpose, HNs were isolated from eight soils with different physicochemical properties (clay content = 16-47%, pHKCl = 5.6-7.7, TOC = 13.3-49.7 g·kg-1, TN = 1.06-2.90 g·kg-1, TOC/TN = 11.4-13.7) to reflect the various processes of their formation. The extraction was carried out through the sequential separation of humic acids with 0.1 M NaOH, and then the digestion of the remaining mineral fraction with 10% HF/HCl. The pesticide concentrations were detected using GC-MS/MS. The pesticides were characterized based on the different sorption rates to HNs, according to the overall trend: metazachlor (95% of absorbed compound) > acetamiprid (94% of absorbed compound) > cypermethrin (63% of partitioning compound) > flufenacet (39% of partitioning compound) > pendimethalin (28% of partitioning compound). Cypermethrin and metazachlor exhibited the highest saturation dynamic, while the other agrochemicals were much more slowly attracted by the HNs. The obtained sorption kinetic data were congruous to the pseudo-first-order and pseudo-second-order models related to the surface adsorption and interparticle diffusion isotherm. The conducted research showed that the processes of pesticide sorption, apart from physicochemical phenomena, are also affected by the properties of the pollutants themselves (polarity, KOC) and the soil properties (SOM content, clay content, and pHKCl).

Fermented biochar has a markedly different effect on fate of pesticides in soil than compost, straw, and a mixed biochar-product

Current knowledge about how biochars affect the fate of pesticides in soil is based on studies that used pure biochars. After finding that an additional biological post-pyrolysis treatment, such as co-composting or lactic fermentation, is required for biochars for superior performance in temperate arable soils, a knowledge gap formed of how such further processed biochar products would affect the fate of pesticides in soil. This study compared the effects of a novel fermented biochar alone or mixed with biogas residues on the fate of two pesticides, 4-chloro-2-methylphenoxyacetic acid (MCPA) and metalaxyl-M, in a temperate arable soil to the traditional organic amendments wheat straw and compost. The fate of 14C-labeled MCPA was markedly affected in different ways. Fermented biochar effectively reduced the water-extractability and mineralization due to adsorption that was comparable to adsorption strengths reported for pure biochars. However, this effect was weak for the biochar mixed with biogas residues. Straw reduced water-extractable amounts due to increased biodegradation and formation of likely biogenic non-extractable residues of MCPA. In contrast, compost decelerated mineralization and increased the water solubility of the MCPA residues due to released dissolved organic matter. The amendments' effects were minor regarding 14C-metalaxyl-M, except for the fermented biochar which again reduced water-extractability and delayed degradation due to adsorption. Thus, the effects of the organic amendments differed for the two pesticide compounds with only the fermented biochar's effect being similar for both. However, this effect was no longer present in the mixed product containing 20% biochar. Our findings clearly show that biologically treated biochar-containing products can affect the fate of pesticides in soil very differently, also when compared to traditional organic amendments. Such impacts and their desirable and undesirable ecotoxicological implications need to be considered before the large-scale application of biochars to temperate arable soils.

Fate of bisphenol A (BPA) in a flooded soil-rice system

In this study, 14C-tracers were used to investigate the fate of BPA in flooded soil with or without rice plants during a complete growing period. In flooded soil, the dissipation of BPA (half-life 14.8 d) was accompanied by its mineralization (8.4% of the initially applied radioactivity) and the formation of non-extractable residues (NERs) in amounts (79.5%) similar to that formed under oxic conditions. The growth of rice significantly accelerated the dissipation of BPA in flooded soil, resulting in a reduction in both the half-life (5.6 d) and the amount of NERs (35.8%). Two non-polar metabolites were detected both in unplanted and in rice-planted soil. At rice harvest, 57.1% of the radioactivity had accumulated in rice plants, mainly as NERs (54.2%) rather than as extractable radioactivity (2.7%), and mainly in roots (34.5 ± 1.4%), stems (9.4 ± 1.1%), and leaves (8.8 ± 0.6%), with trace amounts in seeds (3.6 ± 0.3%) and seed shells (0.7 ± 0.05%). Our study thus demonstrates that the oxic-anoxic interface stimulates the dissipation of BPA in flooded soil. The link between the releasing of NERs in flooded soil and the uptake of BPA metabolites by rice should be considered in environmental risk assessments of agroecosystems.

Low-temperature thermally enhanced bioremediation of polycyclic aromatic hydrocarbon-contaminated soil: Effects on fate, toxicity and bacterial communities

Remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil using thermal desorption technology typically requires very high temperatures, necessitating coupled microbial treatment for energy and cost reduction. This study investigated the fate and toxicity of PAHs as well as the responses of microbial communities following thermal treatment within a low temperature range. The optimal temperature for PAH mineralization was 20–28 °C, within the growth range of most mesophilic microorganisms. By contrast, 50 °C treatment almost completely inhibited PAH mineralization but resulted in the greatest detoxification effect particularly for cardiotoxicity and nephrotoxicity. A potential increase in toxicity was observed at 28 °C. Co-metabolism and non-extractable residue formation may play an interdependent role in thermally enhanced bioremediation. Moreover, alterations in bacterial communities were strongly associated with PAH mineralization and zebrafish toxicity, revealing that soil microorganisms play a direct role in PAH mineralization and served as ecological receptors reflecting changes in toxicity. Network analysis revealed that Firmicutes formed specific ecological communities at high temperature, whereas Acidobacteria and Proteobacteria act as primary PAH degraders at moderate temperature. These findings will enable better integration of strategies for thermal and microbial treatments in soil remediation.

Improved Assessment of Soil Nonextractable Residues of the Pyrethroid Insecticide Cyphenothrin.

The metabolic fate of pyrethroid insecticide cyphenothrin (1) [(RS)-α-cyano-3-phenoxybenzyl (1RS)-cis-trans-2,2-dimethyl-3-(2-methylprop-1-enyl)cyclopropanecarboxylate] in soils was investigated using 14C-labeled (1R)-cis/trans isomers at the cyclopropane ring. Both isomers degraded with half-lives of 19.0-47.4 days, and 48.9-56.0% and 27.5-38.7% of the applied radioactivity (AR) were mineralized to CO2 and incorporated into nonextractable residues (NER), respectively, after 120 days at 20 °C. NER analyses revealed 37.5-42.2% (cis-1) and 44.9-54.1% (trans-1) of each residue at 30/120 days were comprised of 14C-amino acids (AAs) as microbial products. Assuming that 50% of microbial biomass is AAs, it was estimated that 11.3-22.9%AR (cis-1, 75.0-84.4% of NER) and 13.9-30.4%AR (trans-1, 89.8-108.2% of NER) were nonhazardous biogenic NER (bio-NER), while type I/II xenobiotic NER (xeno-NER) characterized by silylation was insignificant at 0.9-1.0%/2.8-3.3%AR (cis-1). Detailed 14C-AA quantitation indicated a high relevance of the tricarboxylic acid cycle and pyruvate pathway during bio-NER formation, offering new insights into the microbial assimilation of the chrysanthemic moiety.

Divergence in the distribution of di(2-ethylhexyl) phthalate (DEHP) in two soils.

Understanding the distribution of di(2-ethylhexyl) phthalate (DEHP) is necessary for future risk evaluation of DEHP in agricultural soils. This study used 14C-labeled DEHP to examine its volatilization, mineralization, extractable residues, and non-extractable residues (NERs) incubated in Chinese typical red and black soil with/without Brassica chinensis L. Results showed that after incubated for 60 days, 46.3% and 95.4% of DEHP were mineralized or transformed into NERs in red and black soil, respectively. The distribution of DEHP in humic substances as NER descended in order: humin > fulvic acids > humic acids. DEHP in black soil was more bioavailable, with 6.8% of initial applied radioactivity left as extractable residues at the end of incubation when compared with red soil (54.5%). Planting restrained the mineralization of DEHP by 18.5% and promoted the extractable residues of DEHP by 1.5% for black soil, but no such restrain was observed in red soil. These findings provide valuable information for understanding the distribution of DEHP in different soils and develop the understanding for the risk assessments of PAEs in typical soils.

Soil drying legacy does not affect phenanthrene fate in soil but modifies bacterial community response

Alteration of the structure of soil microbial communities following the elimination of hydrophobic organic pollutants (e.g., polycyclic aromatic hydrocarbons, PAHs) is generally assessed using DNA-based techniques, and soil is often required to dry prior to pollutant addition, to facilitate a better mix when establishing microcosms. However, the drying practice may have a legacy effect on soil microbial community structure, which would in turn influence the biodegradation process. Here, we used 14C-labeled phenanthrene to examine the potential side effects of precedent short-term drought events. The results indicate that the drying practice had legacy effects on soil microbial community structure, illustrated by irreversible shifts in the communities. The legacy effects had no significant impact on phenanthrene mineralization and non-extractable residue formation. However, they altered the response of bacterial communities to PAH degradation, leading to a decrease in the abundance of potential PAH degradation genes plausibly attributed to moderately abundant taxa. Based on a comparison of the varied effects of different drying intensity levels, an accurate description of microbial responses to phenanthrene degradation strongly relies on the establishment of stable microbial communities before PAH amendment. Concurrent alterations in the communities resulting from environmental perturbation could greatly mask minor alterations from the degradation of recalcitrant hydrophobic PAH. In practice, to minimize the legacy effects, a soil equilibration step with a reduced drying intensity is indispensable.