Isoproturon Mineralization Research Articles

Fe–TiO 2 composite beads driven hybrid process of photocatalysis and photo-Fenton for the degradation of isoproturon

The concept of hybrid process of photo-Fenton and photocatalysis, particularly in the fixed mode, has been presented in this study for the degradation of the pesticide isoproturon with reduction at the time of treatment. For fixed-bed studies, spherical beads were prepared by combining definite proportions of clay, foundry sand, and fly ash, which were utilized as iron sources for titanium dioxide (TiO2) immobilization. The optimization of various parameters was performed by utilizing the Box–Behnken design model under response surface methodology. The process of degradation followed first-order kinetics under an optimized condition for the integrated degradation of isoproturon with a 700 mg·L-1 dose of H2O2, 42 spherical beads, and 190 mL of solution for a duration of 176 min at pH 3.7. Approximately 80.96% degradation of isoproturon was observed after inducing the optimized conditions. The integrated treatment was also carried out in a solar batch reactor under optimized conditions to expand its application to industries for treating bio-recalcitrant compounds. The mineralization of isoproturon was confirmed through the generation of nitrate, nitrite, and ammonical nitrogen with a definite chemical oxygen demand reduction. The recyclability of the catalyst was confirmed by recycling the spherical beads characterized by scanning electron microscopy–energy dispersive X-Ray analysis. For confirming the dual effect, that is, the presence of TiO2 along with Fe on the bead’s surface, various analyses including UV–diffuse reflectance spectroscopy, scanning electron microscopy–energy-dispersive spectroscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy were carried out. A tentative pathway for isoproturon removal was also predicted based on intermediate analysis through gas chromatography–mass spectroscopy.

Insights into the Function and Horizontal Transfer of Isoproturon Degradation Genes (pdmAB) in a Biobed System.

Biobeds, designed to minimize pesticide point source contamination, rely mainly on biodegradation processes. We studied the interactions of a biobed microbial community with the herbicide isoproturon (IPU) to explore the role of the pdmA gene, encoding the large subunit of an N-demethylase responsible for the initial demethylation of IPU, via quantitative PCR (qPCR) and reverse transcription-PCR (RT-qPCR) and the effect of IPU on the diversity of the total bacterial community and its active fraction through amplicon sequencing of DNA and RNA, respectively. We further investigated the localization and dispersal mechanisms of pdmAB in the biobed packing material by measuring the abundance of the plasmid pSH (harboring pdmAB) of the IPU-degrading Sphingomonas sp. strain SH (previously isolated from the soil used in the biobed) compared with the abundance of the pdmA gene and metagenomic fosmid library screening. pdmA abundance and expression increased concomitantly with IPU mineralization, verifying its major role in IPU transformation in the biobed system. DNA- and RNA-based 16S rRNA gene sequencing analysis showed no effects on bacterial diversity. The pdmAB-harboring plasmid pSH showed a consistently lower abundance than pdmA, suggesting the localization of pdmAB in replicons other than pSH. Metagenomic analysis identified four pdmAB-carrying fosmids. In three of these fosmids, the pdmAB genes were organized in a well-conserved operon carried by sphingomonad plasmids with low synteny with pSH, while the fourth fosmid contained an incomplete pdmAB cassette localized in a genomic fragment of a Rhodanobacter strain. Further analysis suggested a potentially crucial role of IS6 and IS256 in the transposition and activation of the pdmAB operon.IMPORTANCE Our study provides novel insights into the interactions of IPU with the bacterial community of biobed systems, reinforces the assumption of a transposable nature of IPU-degrading genes, and verifies that on-farm biobed systems are hot spots for the evolution of pesticide catabolic traits.

Inoculation of soil with an Isoproturon degrading microbial community reduced the pool of “real non-extractable” Isoproturon residues

During pesticides degradation, biogenic non-extractable residues (“apparent NER”) may not share the same environmental fate and risks with the “real NER” that are bound to soil matrix. It is not clear how microbial community (MC) inoculation for pesticides degradation would influence the NER composition. To investigate degradation efficiency of pesticides Isoproturon (IPU) and NER composition following MC inoculation, clay particles harboring MC that contains the IPU degrading strain, Sphingomonas sp., were inoculated into soil receiving 14C-labeled IPU addition. Mineralization of IPU was greatly enhanced with MC inoculation that averagely 55.9% of the applied 14C-IPU was consumed up into 14CO2 during 46 days soil incubation. Isoproturon degradation was more thorough with MC than that in the control: much less amount of metabolic products (4.6% of applied IPU) and NER (35.4%) formed in MC treatment, while the percentages were respectively 30.3% for metabolites and 49.8% for NER in the control. Composition of NER shifted with MC inoculation, that relatively larger amount of IPU was incorporated into the biogenic “apparent NER” in comparison with “real NER”. Besides its well-recognized role on enhancing mineralization, MC inoculation with clay particles benefits soil pesticides remediation in term of reducing “real NER” formation, which has been previously underestimated.

Enhanced degradation of isoproturon in an agricultural soil by a Sphingomonas sp. strain and a microbial consortium

Isoproturon (IPU) degradation in an agricultural soil inoculated with an isolated IPU-degrader strain (Sphingomonas sp. strain AK1, IS) or a microbial consortium (MC) harboring this strain, with or without carrier material, were investigated in soil microcosm experiments during 46 days. Effect of the carrier material and inoculation size on IPU-degradation efficacy of the inoculants were studied. Mineralization, extractable residues and non-extractable residues of 14C-labeled IPU were analyzed. The low IPU mineralization in untreated soil (7.0%) was enhanced to different extents by inoculation of IS (17.4%–46.0%) or MC (58.9%–67.5%). Concentrations of IPU residues in soils amended with MC (0.002–0.095 μg g dry soil−1) were significantly lower than in soils amended with IS (0.02–0.67 μg g dry soil−1) and approximately 10 times lower than in the uninoculated soil (0.06–0.80 μg g dry soil−1). Less extractable residues and non-extractable residues were detected in soil with higher IPU mineralization. Inoculation size (as indicated by the volume of liquid cultures or by the number of carrier particles) determined the IPU-removal efficacy of IS in soil, but this effect was less pronounced for MC. The low sorption of IPU to soil and the decreasing IPU-mineralizing rates suggested incapability of IS to establish the IPU-mineralizing function in the soil. The thorough removal of IPU and persistent IPU-mineralizing activity of soil inoculated with MC indicated a high persistence of IPU-metabolic trait. Our results showed that microbial consortia might be more efficient than single degrader strains to enhance clean-up of organic chemicals in soil.

Biodegradation of isoproturon in agricultural soils with contrasting pH by exogenous soil microbial communities

Soil pH is a limiting factor for biodegradation of isoproturon (IPU) in the field. IPU dissipation is hampered in soil with low pH. Efficacy of two IPU-mineralizing microbial communities in IPU dissipation was investigated during 189 d in microcosms of three agricultural soils with contrasting pH (Marsdorf, pH 3.8; Neumarkt, pH 5.8 and Dürneck, pH 7.3). The microbial communities enriched from an acidic soil (MC-AS) and a neutral soil (MC-NS), respectively, were established on carrier material, namely expanded clay particles, and introduced to the tested soils at a carrier-soil ratio of 1%. IPU was applied to the soil twice, on day 0 and day 133. The effect of inoculation size, sorption–desorption and pH on biodegradation of IPU were studied. IPU mineralization, extractable residues and non-extractable residues were analyzed with uniformly ring-labeled [14C]-isoproturon. Both microbial communities resulted in significantly enhanced IPU mineralization (52%–60%) and low concentrations of IPU residues in soil Dürneck (pH 7.3). The acquired IPU-mineralizing activity was persistent in the soil for at least 133 d. Initially a 9-fold higher IPU mineralization rate was attained in soil Neumarkt (pH 5.8) by inoculating MC-AS. However, no difference between treated and untreated soils was detected after IPU re-application. Both communities had negligible effect on the fate of IPU in soil Marsdorf (pH 3.8), where biodegradation of IPU was inhibited, with ≈2% IPU mineralized over 189 d. Lowering the carrier-soil ratio to 0.1% sharply reduced the IPU-mineralizing capacity of MC-AS in soil Neumarkt, however, this effect of inoculation size was less pronounced for both microbial communities in soil Dürneck. The low Kd of IPU (1.3–2.0 mL g−1) indicate that IPU bioavailability is not a limiting factor of IPU degradation in the soils. Relationships between pH and maximal IPU mineralization rendered by MC-AS or MC-NS in soil closely approximate those observed in mineral salts medium, suggesting that pH is an important factor influencing biodegradation of IPU by the exogenous microbial communities. We propose that MC-AS, which has a broad pH tolerance for IPU degradation, is a promising candidate for accelerating IPU dissipation in acidic soils. The effect of inoculant density on IPU degradation is microbial community-soil specific. Using degrader microorganisms according to their physiological requirements and properties of the targeted soils may maximize the effectiveness of IPU dissipation.

Comparative genomic analysis of isoproturon-mineralizing sphingomonads reveals the isoproturon catabolic mechanism.

The worldwide use of the phenylurea herbicide, isoproturon (IPU), has resulted in considerable concern about its environmental fate. Although many microbial metabolites of IPU are known and IPU-mineralizing bacteria have been isolated, the molecular mechanism of IPU catabolism has not been elucidated yet. In this study, complete genes that encode the conserved IPU catabolic pathway were revealed, based on comparative analysis of the genomes of three IPU-mineralizing sphingomonads and subsequent experimental validation. The complete genes included a novel hydrolase gene ddhA, which is responsible for the cleavage of the urea side chain of the IPU demethylated products; a distinct aniline dioxygenase gene cluster adoQTA1A2BR, which has a broad substrate range; and an inducible catechol meta-cleavage pathway gene cluster adoXEGKLIJC. Furthermore, the initial mono-N-demethylation genes pdmAB were further confirmed to be involved in the successive N-demethylation of the IPU mono-N-demethylated product. These IPU-catabolic genes were organized into four transcription units and distributed on three plasmids. They were flanked by multiple mobile genetic elements and highly conserved among IPU-mineralizing sphingomonads. The elucidation of the molecular mechanism of IPU catabolism will enhance our understanding of the microbial mineralization of IPU and provide insights into the evolutionary scenario of the conserved IPU-catabolic pathway.

Impact of spatial distribution of exogenous organic matter on C mineralization and isoproturon fate in soil

Organic matter (OM) is known to affect the behaviour of pesticides in soil (transfer, degradation, retention). In cultivated soils, crops residues and compost incorporation by ploughing results in a heterogeneous OM distribution in soil with the formation of spots with a mm to dm size. This study aimed to compare the impact of OM (straw and compost) addition and its spatial distribution in soil on the total mineralization and the fate of isoproturon (IPU) in cm repacked soil cores. OM was homogeneously or heterogeneously (in small spots or in a larger spot) distributed in the soil cores. 14C IPU was uniformly added at the regular agronomic dose to the soil and OM. We followed total carbon mineralization, 14C IPU mineralization, and extractable and non-extractable 14C residues during a 43-days incubation. We analysed the fate of 14C at the core scale, and characterised what happened separately on soil and spots of OM after their separation. The results showed that i) the addition of exogenous OM stimulated microbial respiration, but the effect was similar regardless of the spatial distribution of OM in soil (13.9%–19.5% of the total organic carbon); ii) IPU degradation was negligible in OM but was significantly stimulated when OM was added to soil (compared to soil incubated alone) up to a factor of 2; iii) the fate of IPU was impacted by the OM spatial distribution in soil locally and at the core scale and degradation and mineralization was maximal when compost was homogeneously distributed in soil; and iv) these effects were different for maize straw and compost. The nature of OM and its spatial distribution that can be impacted by agricultural practices seem to be important factors to be considered to better understand the fate of pesticides in soil and their transfer to superficial or underground water.

Impact of dry-wet and freeze-thaw events on pesticide mineralizing populations and their activity in wetland ecosystems: A microcosm study.

Riparian wetlands are proposed to mitigate diffuse pollution of surface water by pesticides in agricultural landscapes. Wetland ecosystems though are highly dynamic environments and seasonal disturbances such as freezing and drying can affect microbial population sizes in the sediment and their functionality including pesticide biodegradation, which has hardly been studied. This study examined the effect of artificially induced dry-wet or freeze-thaw events on the mineralization of the pesticides isoproturon (IPU) and 2-methoxy-4-chlorophenoxy acetic acid (MCPA) in wetland microcosms, either without or with prior enrichment of IPU/MCPA degrading populations. Without prior enrichment, mineralization of IPU and MCPA was significantly reduced after exposure to especially freeze-thaw events, as evidenced by lower mineralization rates and longer lag times compared to non-exposed microcosms. However, herbicide mineralization kinetics correlated poorly with cell numbers of herbicide mineralizers as estimated by a most probable number (MPN) approach and the number of IPU and MCPA mineralizers was unexpectedly higher in freeze-thaw and dry-wet cycle exposed setups compared to the control setups. This suggested that the observed effects of season-bound disturbances were due to other mechanisms than decay of pesticide mineralizers. In addition, in systems in which the growth of pesticide mineralizing bacteria was stimulated by amendment of IPU and MCPA, exposure to a freeze-thaw or dry-wet event only marginally affected the herbicide mineralization kinetics. Our results show that season bound environmental disturbances can affect pesticide mineralization kinetics in wetlands but that this effect can depend on the history of pesticide applications.

Predicting isoproturon long-term mineralization from short-term experiment: Can this be a suitable approach?

A worldwide used pesticide - isoproturon (IPU) - was selected to test whether short-term experiments can be used to predict long-term mineralization of IPU in soil. IPU-mineralization was measured for 39 and 265 days in four different agricultural soils with a low mineralization dynamic. Additionally, in one soil IPU dissipation, formation and dissipation of metabolites, formation of non-extractable residues (NER) and (14)C-microbial biomass from (14)C-IPU were monitored for 39 and 265 days. The data from short-term and long-term experiments were used for model fitting. The long-term dynamics of IPU mineralization were considerably overestimated by the short-term experiments in two soils with neutral pH, while in two other soils with low pH and lower mineralization, the long-term mineralization of IPU could be sufficiently predicted. Additional investigations in one of the soils with neutral pH showed that dissipation of IPU and metabolites could be correctly predicted by the short-term experiment. However, the formation of NER and (14)C-microbial biomass were remarkably overestimated by the short-term experiment. Further, it could be shown that the released NER and (14)C-microbial biomass were the main contributors of (14)CO2 formation at later incubation stages. Taken together, our results indicate that in soils with neutral pH short-term experiments were inadequate to predict the long-term mineralization of IPU.

Potential use of Foundry Sand as Heterogeneous Catalyst in Solar Photo- Fenton Degradation ofHerbicide Isoproturon

This study deals with employing foundry sand (FS) as new low cost iron source in heterogeneousphoto-Fenton process for the degradation of herbicide isoproturon (IPU) in aqueous solution. Thecharacterization of the FS by EDS confirmed presence of iron (23%) required for photo-Fenton andsimultaneously confirmed absence of any heavy metals which may be susceptible to leaching. The photo-Fenton effects of different reaction parameters like H2O2 concentration [H2O2]0, operating pH, initialconcentration of IPU [C0], FS dose, recycling of FS, effect of area/volume ratio were investigated. Resultsshowed that the maximal removal efficiency were achieved when reaction parameters were [H2O2]0= 2.2 mM,pH 3, FS dose=0.5 g/L, [C0] =25 mg/L. Under optimum condition, 97% degradation efficiency of IPU wasachieved within 150 min of reaction. The catalyst recycling test was performed and FS was effectivelyrecycled for 4 times with 20% reduction in degradation efficiency. SEM-EDS analysis of recycled FS confirmedmorphology of FS unchanged. Solar baffled batch reactor (SBBR) with recirculation confirmed 70% degradationof IPU after 6 h. IPU mineralization was confirmed with TOC reduction, NH4+ generation along with formationof intermediates as confirmed through LC-MS.

Effect of pyrochar and hydrochar amendments on the mineralization of the herbicide isoproturon in an agricultural soil

Carbon (C)-rich, solid products from pyrolysis (pyrochars) and hydrothermal carbonization (HTC, hydrochars) are expected to reduce the bioavailability and bioaccessibility of pesticides as side effect of soil addition. To compare effects of different feedstocks (digestate, miscanthus, woodchips) and production processes (pyrolysis at 750°C, HTC at 200°C and 250°C), 14C-labeled isoproturon (IPU) was applied at 0.75kgha−1 to loamy sand amended either with 0.5% or 5% pyrochars or hydrochars, which was then incubated for 50d. Mineralization of IPU was measured as 14C–CO2 released from soil-char composites. Pore-water and methanol extractable 14C-IPU was quantified as well as non-extractable 14C-residues (NER). Furthermore, C mineralization of pyrochars, hydrochars and feedstocks was studied to assess the relationship between IPU bioaccessibility and char decomposability.In pure soil, 8.1% of applied IPU was mineralized after 50d. This was reduced more strongly in pyrochar treatments (81±6% reduction) than in hydrochar treatments (56±25% reduction). Different feedstocks had no significantly different effect when 5% char was added, but their effect was significant and dependent on the production process in 0.5% amendments. Pesticide binding can occur by surface sorption as well as by diffusion and subsequent occlusion in micropores. The latter can be expected to result in high amounts of NER, as it was observed in the pyrochar treatments. Hydrochars were less stable than pyrochars and contained lower amounts of NER. Thus, in hydrochar amended soils, better accessibility of IPU to microbial degradation may be a result of full char decomposition within decades ensuring controlled pesticide degradation.

An efficient TiO2 coated immobilized system for the degradation studies of herbicide isoproturon: Durability studies

The investigation presents the observations on the use of cement beads for the immobilization of TiO2 for the degradation of herbicide isoproturon. The immobilized system was effective in degrading and mineralizing the herbicide for continuous thirty cycles without losing its durability. Catalyst was characterized by SEM–EDAX for checking the durability of the catalyst. The degradation rate followed first order kinetics as measured by change in absorption intensity in UV range as well as HPLC analysis. Two rounds of TiO2 coating on inert cement beads with average diameter 1.5cm at UV Intensity 25Wm−2 calcined at 400°C were the optimized conditions for the degradation of herbicide isoproturon. More than 90% TOC and COD reduction along with ammonium ions generation (80%) confirmed the mineralization of isoproturon. Fixed bed baffled reactor studies under solar irradiations using the TiO2 immobilized beads confirmed 85% degradation after 6h. LC–MS studies confirmed the intermediates formation and their subsequent degradation using immobilized system.

Photocatalytic degradation of herbicide isoproturon in TiO2Aqueous Suspensions: Study of Reaction Intermediates and Degradation Pathways

Heterogeneous photocatalytic degradation and mineralization of Isoproturon (IPU) was investigated in aqueous solutions containing titanium dioxide. The degradation was monitored by observing the change in absorption intensity in UV range and through HPLC analysis. The degradation rate was found to be strongly dependent on catalyst concentration, initial pH, and substrate concentration. The effect of variation in intensity of UV irradiations, catalyst recycling as well as area/volume is also studied for the practical applications. The degradation rate was observed to follow first‐order kinetics. TiO2loading 0.5 g L−1, pH 5.0,C0= 25 mg L−1were the optimized conditions for obtaining the better degradation rates. Reduction in COD and TOC values along with the generation of ammonium further indicated the mineralization of the IPU. Fixed bed studies on glass sheet and small cement slabs eliminates the drawbacks of slurry mode photocatalysis. An attempt was also made to identify the intermediates (degradation products) through LC‐MS analysis. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 402–409, 2014

Mapping field spatial distribution patterns of isoproturon-mineralizing activity over a three-year winter wheat/rape seed/barley rotation

The temporal and spatial variability of the activity of soil microorganisms able to mineralize the herbicide isoproturon (IPU) pesticide was investigated over a three-year long crop rotation between 2008 and 2010. Isoproturon mineralization was higher in 2008, when winter wheat was treated with this herbicide, than in 2009 and 2010, when rape seed and barley were treated with different herbicides. Under laboratory conditions, we showed that isoproturon mineralization was not promoted by sulfonylurea herbicide applied on barley crop in 2010. IPU mineralization was shown to be highly variable at the field scale in years 2009 and 2010. Principal component analyses and analyses of similarities revealed that soil pH and equivalent humidity, and to a lesser extent soil organic matter content and cation exchange capacity (CEC) were the main drivers of isoproturon-mineralizing activity variance. Using a rather simple model that yields the rate of isoproturon mineralization as a function of soil pH and equivalent humidity, we explained up to 85% of the variance observed. Mapping field-scale distribution of isoproturon mineralization over the three-year survey indicated higher variability in 2009 and in 2010 as compared to 2008, suggesting that isoproturon treatment applied to winter wheat promoted isoproturon mineralization activity and reduced its spatial variability. Field-scale distribution of isoproturon mineralization showed important similarity to the distribution of soil pH, equivalent humidity and to a lesser extent to soil organic matter and cation exchange capacity (CEC) thereby confirming our model.

Isoproturon mineralization in an agricultural soil

The impact of soil moisture content and temperature on isoproturon (3-(4-isopropylphenyl)-1,1-dimethyl-urea [IPU]) mineralization activity was assessed on an agricultural soil regularly exposed to this herbicide. Mineralization of 14C-IPU was monitored on soil microcosms incubated at different temperatures (10°C, 20°C, 28°C) and soil moisture contents (9%, 12%, 15, 18%, 21%, 24%). An increase in temperature and/or soil moisture significantly enhanced the maximum rate and percentage of IPU mineralization while it decreased the lag time before mineralization. The maximum rate and percentage of IPU mineralization respectively ranged from 0.18% day−1 and 9% for the lowest temperature and soil moisture content pair (10°C–9%) to 1.51% day−1 and 27.1% for the highest pair (28°C–24%). Statistics revealed a cross interaction of temperature and soil moisture content on the maximum rate of IPU mineralization. The optimum conditions for IPU mineralization, estimated from the double Gaussian model, were 25.8°C and 24% soil moisture content. The influence of fluctuations in soil moisture content on IPU-mineralization was investigated by subjecting the soil microcosms to drought stress. When IPU was added at the end of the drought stress, it had no statistical effect on IPU mineralization. However, when it was added before the drought stress, two mineralization phases were observed: (1) one corresponding to the drought stress for which mineralization was low and (2) another one observed after restoration of soil moisture content characterized by higher mineralization rate. It can be concluded that climatic fluctuations affect the activity of IPU mineralizing microbial community, and may lead to an increase in IPU persistence.

Impact of soil water regime on degradation and plant uptake behaviour of the herbicide isoproturon in different soil types

The environmental fate of the worldwide used herbicide isoproturon was studied in four different, undisturbed lysimeters in the temperate zone of Middle Europe. To exclude climatic effects due to location, soils were collected at different regions in southern Germany and analyzed at a lysimeter station under identical environmental conditions. 14C-isoproturon mineralization varied between 2.59% and 57.95% in the different soils. Barley plants grown on these lysimeters accumulated 14C-pesticide residues from soil in partially high amounts and emitted 14CO 2 in an extent between 2.01% and 13.65% of the applied 14C-pesticide. Plant uptake and 14CO 2 emissions from plants were inversely linked to the mineralization of the pesticide in the various soils: High isoproturon mineralization in soil resulted in low plant uptake whereas low isoproturon mineralization in soil resulted in high uptake of isoproturon residues in crop plants and high 14CO 2 emission from plant surfaces. The soil water regime was identified as an essential factor that regulates degradation and plant uptake of isoproturon whereby the intensity of the impact of this factor is strongly dependent on the soil type.

Mineralization of isoproturon, mecoprop and acetochlor in a deep unsaturated limestone and sandy aquifer

Isoproturon ( N, N-dimethyl- N′-[4-(1-methylethyl)phenyl]urea), mecoprop (MCPP) (2-(4-chloro-2-methylphenoxy)propanoic acid) and acetochlor (2-chloro-N-(2-ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide) are agricultural pesticides that may leach through the vadose zone down to groundwater. Sediment samples were collected from intact sediment cores from 0 to 59 m below surface, including soil, unsaturated limestone and aquifer sand. In the unsaturated limestone, the initial pesticide concentrations (0.5–100 μg kg −1) did not systematically affect the proportion of mineralized pesticides or the kinetics. However, in the aquifer, mecoprop and to some degree isoproturon mineralization was found to increase with increasing initial concentration (0.5–100 μg L −1 equivalent to 1–220 μg kg −1) demonstrating the importance of using environmentally relevant concentrations when predicting pesticide fate. The mineralization of isoproturon, mecoprop and acetochlor was studied in 40 samples at low concentrations (1–3 μg L −1) and specific pesticide-mineralizing bacteria were enumerated using 14C-MPN. Presence of the mineralizers documented a degradation potential of the pesticides within the catchment. The number of mineralizers varied from <0.18 to >16 000 g −1 and was not found to correlate with depth. Mecoprop, isoproturon and acetochlor were substantially mineralized in the soils (19–44% after 8 months incubation at 1 μg kg −1), in sub-surface unsaturated limestone samples (≤2% for acetochlor, ≤21% for isoproturon and ≤31% for mecoprop) and in aquifer samples (4–28% for mecoprop, ≤4.7% for isoproturon and ≤5.6% for acetochlor). The finding of isoproturon and acetochlor mineralization in deep aquifers is novel and important for the evaluation of the fate of these pesticides, as even low mineralization rates can be important in aquifers exhibiting long residence times.

Spatial and temporal heterogeneity of soil microorganisms and isoproturon degrading activity in a tilled soil amended with urban waste composts

Organic wastes addition may impact the pesticide behaviour in soils. Furthermore the incorporation of crop residues or organic amendments by ploughing can induce a heterogeneous spatial distribution of the added organic matter in the tilled layer. Spatial and temporal heterogeneity of soil microorganisms and isoproturon biodegradation was investigated at the decimetric scale in relation to the spatial distribution of organic matter originating from urban waste composts from 1 to 19 months after their incorporation through ploughing. We compared one control plot without compost addition and two plots receiving either municipal solid waste (MSW) compost or green waste co-composted with sewage sludge (SGW) every two years. According to morphological description, the ploughed layer was divided into four zones: the interfurrows, containing important quantities of fresh organic matter, the plough pan and two types of clods, the Δ clods (with no visible structural porosity) and the Γ clods (with visible macropores). Total organic carbon (C) and microbial and fungal biomasses were measured on homogeneous samples taken from each zone. Total C and isoproturon mineralization were monitored for two months at 2 sampling dates. At the end of incubation, samples were extracted to evaluate the nature and availability of isoproturon residues. The results showed that interfurrows constituted a special local environment with the highest level of microbial biomass and the highest isoproturon mineralizing capacities and respiration levels. The presence of compost in the interfurrows stimulated isoproturon biodegradation relative to controls. This effect was more pronounced for the MSW compost than for the SGW compost. Heterogeneity may persist several months after compost or stubble hiding depending on the nature of added organic matter.

Laboratory Degradation Studies of 14C-Atrazine and -Isoproturon in Soil from Sugarcane Cultivated Fields Under Kenyan Tropical Conditions

A study to compare the degradation rates of atrazine (6-chloro-N(2)-ethyl-N(4)-isopropyl-1,3,5-triazine-2,4-diammine) and isoproturon [3-(4-isopropylphenyl)-1,1-dimethylurea] in soils from sugarcane fields with different practices of herbicides application was carried out. (14)C-atrazine was poorly mineralized to (14)CO(2) (1.10% +/- 0.22%) after 139 days of incubation in soil without previous exposure to atrazine. In the same soil also with no previous isoproturon exposure isoproturon was mineralized to (14)CO(2) by 7.70% +/- 0.94%. Atrazine mineralization after 98 days was 13.4% +/- 0.30% in soil which discontinued the use of atrazine in 1997 while it was 89.9% +/- 1.23% in soil in which atrazine is currently being used. The isoproturon mineralization values were 7.24% +/- 0.85% and 22.97% +/- 0.96% in soil which discontinued atrazine and soil currently using atrazine, respectively.

In situ mass distribution quotient (iMDQ) – A new factor to compare bioavailability of chemicals in soils?

Aim of this work was the development of a new non-biological factor to determine microbial in situ bioavailability of chemicals in soils. Pesticide residues were extracted from ten highly different agricultural soils that had been incubated with the 14C-herbicide isoproturon (IPU) under comparable soil conditions (water tension – 15 kPa; soil density 1.3 g cm −3). Two different pesticide extraction approaches were compared: (i) 14C-pesticide residues were measured in the pore water (PW) which was extracted from soil by centrifugation; (ii) 14C-pesticide residues were extracted from soil samples with an excess of water (EEW). We introduce the pesticide’s in situ mass distribution quotient (iMDQ) as a measure for pesticide bioavailability, which is calculated as a quotient of adsorbed and dissolved chemical amounts for both approaches (iMDQ PW, iMDQ EEW). Pesticide mineralization in soils served as a reference for real microbial availability. A highly significant correlation between iMDQ PW and mineralization showed that PW extraction is adequate to assess IPU bioavailability. In contrast, no correlation exists between IPU mineralization and its extractability from soil with EEW. Therefore, it can be concluded that soil equilibration at comparable conditions and subsequent PW extraction is vital for a isoproturon bioavailability ranking of soils.