Transformation and binding of 13C and 14C-labelled atrazine in relation to straw decomposition in soil

As a source of organic matter, crop residues affect the behaviour of pesticides in agricultural soils. The fate of [U‐ring‐¹³C] and [U‐ring‐¹⁴C] atrazine (6‐chloro‐N‐ethyl‐N‐isopropyl‐1,3,5‐triazine‐2,4‐diamine) was investigated during laboratory incubation under controlled conditions in a loamy soil amended with wheat straw at two different states of decomposition: no preliminary decomposition or 6 months’ preliminary decomposition. After 3 months, non‐extractable, so‐called ‘bound’, ¹³C‐atrazine residues were recovered in three particle‐size fractions (> 200, 50–200 and < 50 μm), and investigated with solid‐state ¹³C‐NMR spectroscopy. Parallel incubations with [U‐ring‐¹⁴C] atrazine were carried out to quantify the bound residues as well as the extractable and mineralized fractions. The effect of straw residues on atrazine behaviour depended on whether they had been previously decomposed or not. When straw was decomposed for 6 months prior to incubation, atrazine mineralization was enhanced to 50% of the initial ¹⁴C in contrast to 15% of the initial ¹⁴C in soil alone and soil amended with fresh straw. In parallel, atrazine bound residues were formed in greater amount representing up to 20% of the initial ¹⁴C. CP/MAS ¹³C‐NMR on soil size fractions of soil–straw mixtures after incubation with ¹³C‐atrazine showed that bound residues contained mostly triazinic C, corresponding to atrazine or primary metabolites. Non‐humified organic materials recovered in size fractions > 200 and 50–200 μm contained significant amounts of bound residues, especially when straw was added to the soil. CP/MAS ¹³C‐NMR analysis of humic acids obtained from < 50‐μm fractions was difficult due to overlapping of the native carboxyl ¹³C signal with the ¹³C‐atrazine signal.     

Degradation of an Atrazine and Metolachlor Herbicide Mixture in Pesticide-Contaminated Soils from Two Agrochemical Dealerships in Iowa

The fate of atrazine and metolachlor,applied as a mixture, in soil taken from twopesticide-contaminated sites in Iowa (denoted as Alphaor Bravo) were determined in laboratory studies. Atrazine and metolachlor degradation, as well asatrazine mineralization, were greater in soilcollected from Kochia scoparia L. (Schrader)rhizosphere than in soils from unvegetated areas. Theradiolabeled ¹⁴C-carbinol and¹⁴C-morpholinone metabolites were identified in¹⁴C-metolachlor-applied soil 60 d aftertreatment. The half-life for atrazine in Alpha soilwas significantly less in the rhizosphere soil (50 d)than in unvegetated soil (193 d). Quantities ofspecific atrazine degraders were one to two orders ofmagnitude greater in Bravo soils than in Alpha soils. In an experiment with plants present, significantlymore ¹⁴C-atrazine was taken up by K.scoparia (9.9% of the applied ¹⁴C) than by Brassica napus L. Significantly less atrazine wasextractable from soils vegetated with K.scoparia than from soils vegetated with B.napus or unvegetated soils.     

Aufnahme von Atrazin-Metaboliten durch Hafer neun Jahre nach der Herbizid–Applikation

Uptake of atrazine metabolites by oat plants nine years after the herbicide application The uptake of atrazine metabolites by oats under field conditions was investigated 9 years after the herbicide application. In the spring of 1973 the soil was treated with ¹⁴C-ringlabeled atrazine and cultivated with various plant species each year. The soil was seeded to oats in early May of 1982. In August the oat plants were harvested and analysed. 2-Hydroxy-4-amino-6-isopropylamino-s-triazine and 2-hydroxy-4-ethylamino-6-amino-s-triazine were identified in conjugated form in the plant material. Neither free atrazine nor any other phytotoxic 2-chloro-metabolites could be found.     

Der Einfluß des Wassergehaltes auf den Atrazin-Abbau im Boden

The influence of water-content on atrazine degradation in soil In samples of the standard soil 2.2 (loamy sand, 3 % C, pH 7,0) and of a Luvisol (Ap-horizon, loam 1,4 % C, pH 5,2), the degradation of [ethyl-1-¹⁴C]atrazine was investigated in dependence of the soil water content. The experimental conditions were choosen in accordance with the methods proposed by the Biologische Bundesanstalt to study the degradation of pesticides in the soil. The soil water content was varied to simulate the moisture conditions observed in a soil during plant growth. Therefore, besides a steady water content of 20, 40, 60, and 80 % of the maximum water holding capacity of the soils, the soil water contents were fluctuated by 20 to 60 % of the maximum water holding capacity by passing dry air through the soil. At a concentration of 10mg atrazin/kg of soil between 4 and 6 % of the ethyl-1-carbonatom of the atrazine molecule was mineralized to CO₂ within 71 days at a constant soil temperature of 22°C. In the standard soil 2.2 the mineralization in total was reduced to 2/3 compared to the degradation in the Luvisol. With decreasing water content increasing hydroxilated metabolites were formed. About 30–40 % of the applied radioactivity was determined as non-extractable residue in the soil. In general the degradation processes were more enhanced and more intense in the Luvisol as compared to the Standard soil 2.2 which again unterlines that for this type of experiments a fresh soil should be used. In conclusion, the variation of the soil water content did not have a pronounced influence on the mineralization rates of atrazine, but did influence the metabolism and the formation of certain metabolite fractions.     

Interactions between 14C-labelled atrazine and the soil microbial biomass in relation to herbicide degradation

A laboratory incubation experiment was set up to determine the effects of atrazine herbicide on the size and activity of the soil microbial biomass. This experiment was of a factorial design (0, 5, and 50 g g^–1 soil of non-labelled atrazine and 6.6×10³ Bq g^–1 soil of ¹⁴C-labelled atrazine) x (0, 20, and 100 g g^–1 soil of urea-N) x (pasture or arable soil with a previous history of atrazine application). Microbial biomass, measured by substrate-induced respiration and the fumigation-incubation method, basal respiration, incorporation of ¹⁴C into the microbial biomass, degradation of atrazine, and ¹⁴C remaining in soil were monitored over 81 days. The amount of microbial biomass was unaffected by atrazine although atrazine caused a significant enhancement of CO₂ release in the non-fumigated controls. Generally, the amounts of atrazine incorporated into the microbial biomass were negligible, indicating that microbial incorporation of C from atrazine is not an important mechanism of herbicide breakdown. Depending on the type of soil and the rate of atrazine application, 18–65% of atrazine was degraded by the end of the experiment. Although the pasture soil had twice the amount of microbial biomass as the arable soil, and the addition of urea approximately doubled the microbial biomass, this did not significantly enhance the degradation of atrazine. This suggests that degradation of atrazine is largely independent of the size of the microbial biomass and suggests that other factors (e.g., solubility, chemical hydrolysis) regulate atrazine breakdown. A separate experiment conducted to determine total amounts of ¹⁴C-labelled atrazine converted into CO₂ by pasture and arable soils showed that less than 25% of the added ¹⁴C-labelled atrazine was oxidised to ¹⁴CO₂ during a 15-week period. The rate of degradation was significantly greater in the arable soil at 24%, compared to 18% in the pasture soil. This indicates that soil microbes with previous exposure to atrazine can degrade the applied atrazine at a faster rate.     

Lumbricid macrofauna alter atrazine mineralization and sorption in a silt loam soil

Atrazine is a widely used herbicide and is often a contaminant in terrestrial and freshwater ecosystems. It is uncertain, however, how the activity of soil macrofauna affects atrazine fate and transport. Therefore, we investigated whether earthworms enhance atrazine biodegradation by stimulating herbicide degrading soil microflora, or if they increase atrazine persistence by facilitating herbicide sorption. Short (43 d) and medium term (86 d) effects of the earthworms Lumbricus terrestris and Aporrectodea caliginosa on mineralization, distribution, and sorption of U-ring-¹⁴C atrazine and on soil C mineralization was quantified in packed-soil microcosms using silt loam soil. A priming effect (stimulation of soil C mineralization) caused by atrazine supply was shown that likely lowered the earthworm net effect on soil C mineralization in atrazine-treated soil microcosms. Although earthworms significantly increased soil microbial activity, they reduced atrazine mineralization to ¹⁴CO₂-C from15.2 to 11.7% at 86 d. Earthworms facilitated formation of non-extractable atrazine residues within C-rich soil microsites that they created by burrowing and ingesting soil and organic matter. Atrazine sorption was highest in their gut contents and higher in casts than in burrow linings. Also, gut contents exhibited the highest formation of bound atrazine residues (non-extractable atrazine). Earthworms also promoted a deeper and patchier distribution of atrazine in the soil. This contributed to greater leaching losses of atrazine in microcosms amended with earthworms (3%) than in earthworm-free microcosms (0.003%), although these differences were not significant due to high variability in transport from earthworm-amended microcosms. Our results indicated that earthworms, mainly by casting activity, facilitated atrazine sorption, which increased atrazine persistence. As a consequence, this effect overrode any increase in atrazine biodegradation due to stimulation of microbial activity by earthworms. It is concluded that the affect of earthworms of atrazine mineralization is time-dependent, mineralization being slightly enhanced in the short term and subsequently reduced in the medium term.     

Biomineralization of atrazine and analysis of 16S rRNA and catabolic genes of atrazine degraders in a former pesticide mixing site and a machinery washing area

Purpose

The purpose of this study was to determine the first-order rate constants and half-lives of aerobic and anaerobic biomineralization of atrazine in soil samples from an agricultural farm site that had been previously used for mixing pesticide formulations and washing application equipment. Atrazine catabolic genes and atrazine-degrading bacteria in the soil samples were analyzed by molecular methods.

Materials and methods

Biomineralization of atrazine was measured in soil samples with a [U-ring-¹⁴C]-atrazine biometer technique in soil samples. Enrichment cultures growing with atrazine were derived from soil samples and they were analyzed for bacterial diversity by constructing 16S rDNA clone libraries and sequencing. Bacterial isolates were also obtained and they were screened for atrazine catabolic genes.

Results and discussion

The soils contained active atrazine-metabolizing microbial communities and both aerobic and anaerobic biomineralization of [U-ring-¹⁴C]-atrazine to ¹⁴CO₂ was demonstrated. In contrast to aerobic incubations, anaerobic biometers displayed considerable differences in the kinetics of atrazine mineralization between duplicates. Sequence analysis of 16S rDNA clone libraries constructed from the enrichment cultures revealed a preponderance of Variovorax spp. (51 %) and Schlesneria (16 %). Analysis of 16S rRNA gene sequences from pure cultures (n?=?12) isolated from enrichment cultures yielded almost exclusively Arthrobacter spp. (83 %; 10/12 isolates). PCR screening of pure culture isolates for atrazine catabolic genes detected atzB, atzC, trzD, trzN, and possibly atzA. The presence of a complete metabolic pathway was not demonstrated by the amplification of catabolic genes among these isolates.

Conclusions

The soils contained active atrazine-metabolizing microbial communities. The anaerobic biometer data showed variable response of atrazine biomineralization to external electron acceptor conditions. Partial pathways are inevitable in soil microbial communities, with metabolites linking into other catabolic and assimilative pathways of carbon and nitrogen. There was no evidence for the complete set of functional genes of the known pathways of atrazine biomineralization among the isolates.

     

Spatial distribution and characterization of long-term aged 14C-labeled atrazine residues in soil

The long-term behavior of the herbicide atrazine and its metabolites in the environment is of continued interest in terms of risk assessment and soil quality monitoring. Aqueous desorption, detection, and quantification of atrazine and its metabolites from an agriculturally used soil were performed 22 years after the last atrazine application. A lysimeter soil containing long-term aged atrazine for >20 years was subdivided into 10 and 5 cm layers (at the lysimeter bottom: soil 0-50 and 50-55 cm; fine gravel 55-60 cm depth, implemented for drainage purposes) to identify the qualitative and quantitative differences of aged (14)C-labeled atrazine residues depending on the soil profile and chemico-physical conditions of the individual soil layers. Deionized water was used for nonexhaustive cold water shaking extraction of the soil. With increasing soil depth, the amount of previously applied (14)C activity decreased significantly from 8.8% to 0.7% at 55-60 cm depth whereas the percentage of desorbed (14)C residues in each soil layer increased from 2% to 6% of the total (14)C activity in the sample. The only metabolite detectable by means of LC-MS/MS was 2-hydroxyatrazine while most of the residual (14)C activity was bound to the soil and was not desorbed. The amount of desorbed 2-hydroxyatrazine decreased with increasing soil depth from 21% to 10% of the total desorbed (14)C residue fraction. The amount of (14)C residues in the soil layers correlated well with the carbon content in the soil and in the aqueous soil extracts ( p value = 0.99 and 0.97, respectively), which may provide evidence of the binding behavior of the aged atrazine residues on soil carbon. The lowest coarse layer (55-60 cm) showed increased residual (14)C activity leading to the assumption that most (14)C residues were leached from the soil column over time.     

Effect of long-term liquid pig manure application on atrazine mineralization in a soil cultivated with maize

Soil samples were collected in plots from a field experiment in maize monoculture receiving 0, 60 and 120 m³ ha^-1 liquid pig manure (LPM) for 19 years. Soils were sampled from the 0- to 20-cm layer in August and October 1997 and in June, July and September 1998. Subsurface samples were also evaluated in September 1998. Laboratory soil radiorespirometry was used to evaluate atrazine mineralization using [U-ring-¹⁴C]-atrazine mixed with commercially available product. The effect of atrazine dose (50, 100 and 500 mg atrazine kg^-1 soil) was evaluated on soils sampled in August 1997. For the other sampling dates, the soils were spiked with 50 mg atrazine kg^-1 soil. No LPM dose effect on atrazine mineralization was obtained in the different experiments. Increasing atrazine dose to 500 mg kg^-1 decreased significantly the mineralization rate (R_i) and the maximum of atrazine mineralized (MAX), while the time needed to mineralize 50% of MAX (DT-50%) was not significantly affected. Sampling time had a significant effect on atrazine mineralization. Atrazine mineralization in the soils sampled in June 1998 showed lower R_i and MAX than in the soils sampled at the other dates. Atrazine mineralization in subsurface soils (20–60 cm) was very variable and quite high in some samples. This may be due to atrazine pre-exposure in subsoils resulting from atrazine deep movement by preferential flow.     

Influence of methanol and hexane on soil adsorption of atrazine

The increasing frequency of chemically contaminated groundwater, occurring as a result of improperly managed waste disposal or accidental spills, presents a need for research on the fate of chemical mixtures in the soil. The batch equilibration technique was used to measure adsorption of ¹⁴C ring-labeled atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) for a Palouse silt loam (Pachic Ultic Haploxeroll) and a Pembroke silty clay loam (Typic Paleudalf). The solution phase consisted of mixtures of methanol-water and hexane-water containing up to 33.3 % organic solvent by volume. Aqueous solubility limited atrazine concentrations to 100 μmol L^?1 except for an additional isotherm determined in 33.3 methanol-water at up to 1542 μmol L^?1 The Freundlich adsorption coefficient indicated that the Palouse adsorbed more atrazine than the Pembroke with K values of 4.95 and 0.54, respectively. Both soils showed a significant decrease in K as the percentage methanol increased. Adsorption isotherms from a 33. 3 methanol-water system were of the Freundlich type for atrazine concentrations of 0.25 to 1542 μmol L^?1. In the hexane-water systems, K decreased as the fraction of hexane increased and the Pembroke soil adsorbed less atrazine than the Palouse soil. These results suggest that the introduction of nonaqueous solvents such as methanol and hexane decreased adsorption and increased the potential for atrazine mobility.     

Glufosinate and ammonium sulfate inhibit atrazine degradation in adapted soils

The co-application of glufosinate with nitrogen fertilizers may alter atrazine cometabolism, thereby extending the herbicide’s residual weed control in adapted soils. The objective of this study was to assess the effects of glufosinate, ammonium sulfate, and the combination of glufosinate and ammonium sulfate on atrazine mineralization in a Dundee silt loam exhibiting enhanced atrazine degradation. Application of glufosinate at rates of 10 to 40 mg kg⁻¹ soil extended the lag phase 1 to 2 days and reduced the maximum degradation rate by 15% to 30%. However, cumulative atrazine mineralization averaged 85% 21 days after treatment and was independent of treatment. Maximum daily rates of atrazine mineralization were reduced from 41% to 55% by application of 1 to 8 g kg⁻¹ of ammonium sulfate. Similarly, cumulative atrazine mineralization was inversely correlated with ammonium sulfate rates ranging from 1.0 to 8 g kg⁻¹ soil. Under the conditions of this laboratory study, atrazine degradation was relatively insensitive to exogenous mineral nitrogen, in that 8 g (NH₄)₂SO₄ per kilogram soil repressed but did not completely inhibit atrazine mineralization. Moreover, an additive effect on reducing atrazine mineralization was observed when glufosinate was co-applied with ammonium sulfate. In addition, ammonium fertilization alters the partitioning of ¹⁴C-atrazine metabolite accumulation and nonextractable residues, indicating that ammonium represses cleavage of the triazine ring. Consequently, results indicate that the co-application of glufosinate with N may increase atrazine persistence under field conditions thereby extending atrazine residual weed control in adapted soils.     

How increasing availabilities of carbon and nitrogen affect atrazine behaviour in soils

 The effect of increasing amounts of glucose and mineral N on the behaviour of atrazine was studied in two soils. One had been exposed to atrazine under field conditions (adapted soil), the other had not (non-adapted soil), resulting, respectively, in an accelerated degradation of atrazine in the adapted soil and in a slow degradation of the herbicide in the non-adapted soil. The dissipation of ¹⁴C-atrazine via degradation and formation of non-extractable "bound" residues was followed during laboratory incubations in soils supplemented or not with increasing amounts of glucose and mineral N. In both soils, glucose added at rates of up to 16 g C kg^–1 soil did not modify atrazine mineralization but increased the formation of bound residues; this was probably due to the retention of atrazine by the growing microbial biomass. Atrazine dealkylation was enhanced when a large amount of glucose was added. In both soils, the addition of the largest dose of mineral N (2.5 g N kg^–1 soil) decreased atrazine mineralization. The simultaneous addition of glucose and mineral N enhanced their effects. When the largest doses of mineral N and glucose were added, atrazine mineralization stopped in both soils, and the proportion of bound residues increased. Glucose and mineral N additions influenced atrazine mineralization to a greater extent in the adapted soil than in the non-adapted one, as revealed by ANOVA, although glucose addition had a greater effect than N. The competition for space and nutrients between atrazine-degrading microorganisms and the total heterotrophic microflora probably contributed to the decrease in atrazine mineralization. Received: 9 June 1998     

Catechol and chlorocatechols in soil: Degradation and extractability

Catechol and chlorocatechols occur as intermediary metabolites during the degradation of naturally-occurring and synthetic aromatic compounds. Their degradation in soil was assessed under laboratory conditions using ¹⁴C-tracing techniques. Degradation of all compounds to CO₂ was rapid during the first 2 weeks (5–10% week^?1, but gradually decreased to below 1% week^?1 after 3 months. After 6 months. 44% of 4,5-dichlorocatechol, 38% of 4-chloro- and tetrachlorocatechols, and 30% of catechol were degraded to CO₂. In comparison, chlorophenols were degraded at similar rates, and chloroanilines were degraded more slowly. A mixed extradant of citric acid-ascorbic acid-acetone (1:1:2) was found to be most effective in extracting the catcchols from variously-treated soil samples. Recovery of added ¹⁴C from freshly fortified soils ranged from 74% for catechol to 98%, for tetrachlorocatechol. After equilibration of ¹⁴C-chemical with soil for 5–20 days, the extractability decreased to 38% for catechol, but remained over 86% for tetrachlorocatechol. Sterilization of soil before ¹⁴C addition had little effect on ¹⁴C extractability. After incubation of treated soil for 5 months, only 20–35% of residual ¹⁴C could be extracted. More than half of the nonextractable ¹⁴C-residues from incubated soil could be further removed by Na-pyrophosphate extraction.     

Degradation and Leaching of Simazine in Arctic Sandy Soils

Abstract

Degradation and leaching of ¹⁴C-labelled simazine in coarse sandy soils at 15 + 1°C were investigated using radiometric and mass-spectrometric methods. During 6 months incubation approx. 4–7% of the applied ¹⁴C-simazine was evolved as ¹⁴CO₂. 4–9% of the simazine still remained in the soil. Addition of hen manure or acidification by addition of peat did not clearly influence the rate of degradation of simazine, whereas mechanical treatment significantly increased its degradation. In a nitrogen atmosphere the rate of degradation of simazine was reduced.

9–15% of the simazine or its radioactive metabolites leached through a 33 cm sandy moraine soil column (diameter 6 cm) in ca. 1770 mm of precipitation over a 4 month period, and 2% was leached from a fine sand soil under the same conditions.     

DNA damage and effects on antioxidative enzymes in earthworm (Eisenia foetida) induced by atrazine

To evaluate atrazine (2-chloro-4-ethylamino-6-isopropylamino-1, 3, 5-triazine) ecotoxicology in soil, the effect of atrazine on the activity of antioxidative enzymes (superoxide dismutase, SOD; catalase, CAT; and guaiacol peroxidase, POD) and DNA damage induced by atrazine were investigated in earthworms. Atrazine was added to artificial soil at rates of 0, 2.5, 5 and 10 mg per kg of soil. Earthworm tissues exposed to each treatment were collected on the 7th, 14th, 21st, and 28th day of the treatment. Compared to the controls, the CAT activity was stimulated at 2.5 mg kg⁻¹ treatment except on the 14th day, and inhibited at 5, 10 mg kg⁻¹ atrazine except 5 mg kg⁻¹ on the 28th day and 10 mg kg⁻¹ on the 21st day; the overall SOD activity was inhibited, while the POD activities were stimulated by all atrazine concentrations in 28 days. The olive tail moments of single-cell gel electrophoresis of coelomocytes, as an indication of DNA damage, were increased after treatment with different doses of atrazine on the 7th, 14th, 21st, and 28th day, and significant differences were found compared to the controls. In conclusion, atrazine induces oxidative stress and DNA damage on earthworms, and the adverse effects may be the important mechanisms of its toxicity to earthworms.     

Influx and concentration of triazine pesticides in the Amaterska cave system,Moravian Karst,Czech Republic

Purpose

The aim of this study was to detect three triazine pesticides and their metabolites in the drip water and the sediment of the Amaterska cave system. Diversity of the bacterial community in the sediment was also assessed, and the potential role of bacteria in degradation of these pesticides was evaluated.

Materials and methods

Triazines and their metabolites were analyzed in the soil, drip water, and sediment of the Amaterska cave system area in seven sampling sites (S1–S7) based on the above ground cover that included forest, permanent grassland, and agriculture cropland. The bacterial community in the cave sediments (S1–S6) was also analyzed using the Illumina sequencing of the V3 and V4 regions of 16S rDNA.

Results and discussion

Triazines were present in the soil and drip water in all sites below grassland and agricultural land but not under the forest area. Only atrazine metabolites were detected in the surface soil. In contrast, atrazine was detected in all cave sediments regardless of above ground cover, and this is likely due to the occasional alluvial influx. The overall prevalence of bacteria potentially capable of atrazine degradation in the cave sediment ranged from 13.4 to 64.0% of the entire bacterial community. The concentrations of atrazine in the cave sediment were 16 to 70 times higher than in those in drip water.

Conclusions

High concentrations of atrazine in the cave sediment indicate a slow degradation rate of triazines in the cave likely due to low temperatures and absence of photolysis. The main source of atrazine in the Amaterska cave system is likely not drip water but the alluvial influx. Bacteria potentially capable of triazine degradation in the cave sediment were detected; however, their role in this process remains to be investigated.

     

Abiotic soil factors influencing the deleterious effect of atrazine on ungerminated conidia of Cochliobolus sativus

To determine why viability of conidia of Cochliobolus sativus declines in some soils treated with atrazine and not in others, the influence of soil organic matter, texture and pH on the lethal effect of atrazine was examined. Viability of conidia on Boyer sandy loam (SL) (−1 kPa matric potential) containing 25μg atrazine g⁻¹ was 7% after 3 weeks, as compared with 99% in the control. Decreasing the organic carbon of Boyer SL from 0.73 to 0.02% by H₂O₂ digestion, or to 0.04% by NaOH extraction, nullified the lethal effect of atrazine. The addition of 4mg humic acid g⁻¹ to NaOH-extracted Boyer SL containing atrazine partially restored the lethal effect. Increasing the pH of Boyer SL from 5.2 to 7.5 nullified the lethal effect of atrazine. Viability of conidia on Spinks SL (pH 6.6) containing atrazine remained at 99% after 3 weeks. The addition of 4mg humic acid g⁻¹ from Boyer SL to atrarine-treated Spinks SL reduced viability to 86%. Viability of conidia in atrazine-treated acidified Spinks SL (pH 5.4) was 65%. The response of conidia to atrazine in soils supplemented with 4% bentonite clay, or in separated sand or silt and clay fractions of soils was not affected except when the soil pH was altered. Thus, a low pH and the presence of humic acid increased the toxicity of atrazine to conidia of C. sativus.     

CO2 efflux by rapid decomposition of low molecular organic substances in soils

Decomposition rates of the [2-¹⁴C]-glucose and [2-¹⁴C]-glycine in four different soils of the long-term field trial of Moscow were investigated in a 3-months laboratory experiment in which ¹⁴CO₂ respiration was measured. A model with three decomposition components and two distribution parameters was developed and validated with the data of the experiment. The decay rate constants of free [2-¹⁴C]-glucose (4–32 day^-1) were slower than those of [2-¹⁴C]-glycine (16–44 day^-1). The calculated use efficiency for microbial biosynthesis of the second carbon atom was 47% for glucose and 31% for glycine. The potential half-life of labelled carbon in the microbial soil biomass ranged from 0.6 to 4.4 days, depending on the soil type and the initial amount of added substrate. The calculated total utilisation of carbon by the soil biomass from glycine was about 2–5 times lower than that of glucose.The modelled ¹⁴C incorporation into the microbial soil biomass reached its maximum on the first day of the incubation experiment and did not exceed 22% of the ¹⁴C input. Both of the investigated substances decomposed most rapidly in the soil samples from sites that have not being fertilised with organic or mineral fertilisers during an 81-years period.     

Distribution of assimilated carbon in the system <Emphasis Type="Italic">Phragmites australis</Emphasis>-waterlogged peat soil after carbon-14 pulse labelling

 Short-term (3–6 days) and long-term (27 days) laboratory experiments were carried out to determine the distribution of assimilated C in the system Phragmites australis (common reed)-waterlogged fen soil after ¹⁴C pulse labelling. The investigated system of fen plants and anaerobic organic soil showed different patterns of assimilated ¹⁴C distribution when compared to systems with cultivated plants and aerobic mineral soil. Between 90% and 95% of the ¹⁴C in the system was found in the reed plants. A maximum of 2% of the assimilated plant ¹⁴C was released from the fen soil as CO₂ and about 5–9% remained in the soil. The ¹⁴C remaining in the waterlogged fen soil of the reed plant had the same amount as that of a cultivated plant in mineral soil, despite lower ¹⁴C-release (i.e. rhizodeposition and root respiration) from reed roots. Assuming that root respiration of fen plants is low, this indicates that microbial C turnover in waterlogged fen soil is much slower than in mineral soil. The estimated quantity of the assimilated C remaining in the soil was of an ecologically relevant order of magnitude. Received: 8 July 1999     

土壤和水稻中正+六烷的结合残留

利用＾１４Ｃ－正十六烷示踪技术,水稻盆栽和溶剂连续化学提取方法。研究了正十六烷在土壤水稻系统中的某些行为。结果表明,正十六烷容易进入水稻并累积于籽实,对人类健康产生不良影响。实验还探讨了土壤和水稻体内＾１４Ｃ－正十六烷的存在和代谢转化过程。