Degradation of triasulfuron in soil under laboratory conditions

Weed Research, 1992, Volume 32, 167-173

Degradation of triasulfuron in soil under laboratory conditions

F. K. OPPONG AND G. R. SAGAR School ofBiological Sciences, University College of NorthWales, Bangor, Gwynedd LL57 2UW, UK

Received 1 December 1988Revised version accepted 30 September 1991

Summary: Resume: Zusammenfassung

Degradation of triasulfuron in non-autoclavedand autoclaved soil incubated at differenttemperatures and moisture contents wasevaluated in the laboratory using a maize rootgrowth bioassay. Disappearance of triasulfuronwas faster in non-autoclaved than in autoclavedsoil, indicating the importance of micro-organisms in the breakdown process. Degrada-tion of the herbicide was faster at 30°C than at10°C, with half-lives of 11-13 days at 30°C and30-79 days at 10°C. Degradation of the herbi-cide was influenced more by temperature thanby variation in soil moisture. Disappearance ofthe herbicide was rapid in the non-autoclavedsoil at 30°C during the initial 30 days of incuba-tion, but low levels of residues persisted for upto 90 days. A second application of the herbi-cide, to soil in which an initial dose of triasul-furon had degraded, disappeared at the samerate as herbicide added to previously untreatedsoil, indicating that there was no enhancementof degradation with repeated application ofherbicide.

Degradation du triasulfuron dans le sol enconditions de laboratoire

La degradation du triasulfuron dans des sols nonautoclaves et autoclaves, incubus a des tempera-tures et a des teneurs en humidite differentes, a

•Present address: Cocoa Research Institute of Ghana, P.O.Box 8, Akim-Tafo, Ghana.

ete evaluee au laboratoire en utilisant un bioessai sur la croissance d'une racine de mais. Ladisparition du triasulfuron a ete plus rapide ensol non autoclave qu'en sol autoclave, soulig-nant l'importance des microorganismes dans leprocessus de degradation. La degradation del'herbicide a ete plus rapide k 30°C qu'S 10°Cavec des demi-vies respectives de 11-13 jours etde 30-79 jours. La degradation de l'herbicide aete plus influencee par la temperature que parles variations d'humidite du sol. La disparitionde l'herbicide a ete rapide dans le sol non auto-clave a 30°C pendant les 30 premiers joursd'incubation, mais de faibles residus persistaientau dela de 90 jours. Une seconde applicationd'herbicide sur un sol dans lequel une doseinitiate de triasulfuron avait 6te degradee, adisparu de la meme fa§on qu'une dose appli-quee sur un sol non traitd, montrant qu'il n'yavait pas d'augmentation de la degradation k lasuite d'une repetition d'application.

Abbau von Triasulfuron im Boden unterLaborbedingungen

Der Abbau von Triasulfuron in nicht sterili-siertem und sterilisiertem Boden bei verschie-dener Temperatur und Bodenfeuchte wurde miteinem Maiswurzel-Wachstumstest untersucht.Die Menge des Triasulfurons nahm im nicht-sterilisierten Boden schneller ab als im sterili-sierten, was auf mikrobiellen Abbau hinweist.Das Herbizid wurde bei 30 °C mit einerHalbwertszeit von 11 bis 13 Tagen schnellerabgebaut als bei 10 °C mit einer von 30 bis 79Tagen. Der Abbau wurde durch die Temperaturstarker beeinfluBt als durch Anderung derBodenfeuchte. Das Herbizid unterlag in denersten 30 Tagen bei 30 "C im nichtsterilisiertenBoden einem schnellen Abbau, doch geringeRuckstande wurden bis zu 90 Tagen gefunden.Bei einer zweiten Applikation des Herbizids aufBoden, in dem schon eine erste Dosis von

168 F. K. Oppong and G. R. Sagar

Triasulfuron abgebaut worden war, nahm derWirkstoff im selben MaBe wie zuvor ab, so daBbei wiederholter Anwendung nicht mit einemverstarkten Abbau gerechnet werden kann.

Introduction

Triasulfuron, 1- [2- (2-chloroethoxy) phenylsul-fonyl] -3- (4-methoxy-6-methyl-l, 3, 5-triazin-2-yl)urea, is a sulphonylurea herbicide used tocontrol broad-leaved weeds in wheat andbarley. It is recommended for post-emergenceapplication between the 2- and 3-leaf growthstage of the crop (Amrein & Gerber, 1985).Although applied to the foliage, some of theherbicide will directly or indirectly enter thesoil. Some sulphonylurea herbicides have beenreported to undergo degradation in soil by bothchemical and biological processes, the rates ofloss being controlled by soil temperature andmoisture level, and increasing at lower soil pH(Walker & Brown, 1983; Flom et al. 1986;Smith, 1986). Half-lives ranging from 0-5-2 daysfor thiameturon (Brown et al., 1987) and from4-8 weeks for chlorsulfuron (Palm et al., 1980)have been reported. Previous studies with tria-sulfuron have indicated considerable variationin degradation rates between soils. Iwanzik &Amrein (1988) reported half-lives in 13 soils at20°C and 50% of field capacity ranging from12-38 days, and Martin & Blair (1988) observeda half-life of 11 days at 30°C in an organic soil.Walker & Welch (1989) reported half-lives of33-76 days at 2()°C in a range of mineral topsoils,and of 52-120 days in subsoils from the samesites, suggesting an effect of soil microbialactivity on rates of degradation- The objectivesof the present study were to investigate the roleof micro-organisms in the dissipation oftriasulfuron, and to study the influence of soiltemperature and moisture content on the ratesof loss.

Materials and methods

Experiment 1

Soil from the surface 10 cm of a plot that hadbeen fallow for 3 years at Pen-y-Ffridd FieldStation, University College of North Wales,Bangor, was collected on 21 August 1987, air-dried and passed through a 3-mm sieve. Thephysical and chemical characteristics of the soilare presented in Table 1.

On 28 August 1987, solutions of triasulfuron(2 5 ml) were added to 2-5-kg batches ofautoclaved air-dried soil (autoclaved at apressure of 1 kg cm~^ and 120°C for 4h) or tonon-autoclaved soil to produce a dry soil con-centration of 0-08 mg AI kg~'. The soil wasthoroughly mixed and moistened to either 25%,75% or 100% of field capacity, placed in sealedpolythene bags and incubated in the dark ateither 10 or 30°C. The bags were opened andweighed at weekly intervals and, when neces-sary, water was added to restore the initialmoisture levels. Each treatment had a similaruntreated control, and the experiment wasperformed in duplicate .

Three soil samples {c. 80 g each) wereremoved from each bag after 0, 7, 30,60,90,120and 150 days of incubation, and a bioassaymethod similar to that described by Parker(1964) was used to determine residual concen-trations. For the bioassay, 5 pre-germinatedseeds of maize cv. Anko, with the radicles justbeginning to emerge, were placed in a rowacross the surface of approximately 80 g oftreated or untreated soil in plastic petri dishes (9cm in diameter) which were covered andmounted at an angle of 15° in the dark at 20°Cfor 5 days. At the end of this bioassay incubationperiod, the lengths of the primary roots weremeasured and expressed as percentages of thelengths of the roots of appropriate control plantsgrown in untreated soil. Each of the 42 bioassays

Table 1. Properties of the soil used in the investigation

Origin

Pen-y-Ffridd Experimental Station,University College of North Wales

PH*

5-2

Organic carbont

5-95

Texturai analysis (%)

Sand Silt Clay

85-4 10-5 4-1

•Determined in 1:1 soil:water.tDetermined by the Walkley-Black (1934) method (%).

Degradation of triasulfuron in soil 169

ICX)

90

80

70

60

50

40

30 -

J L _L _L J L J L _L J L20- 3 4 -32 - 3 0 -2-8 -2 6 - 2 4 -2 2 -2 0 -18 -16 -14 -12

Triasulfuron log concentration (mg kg"')

Fig. I. Standard bioassay curve, derived from inhibition of root extension ofmaize by different concentrations of triasulfuron after 5 days of bioassay incuba-tion at 20°C. Vertical bars represent standard errors.

included a reference series of dishes thatcontained maize seedlings exposed to 11 con-centrations of triasulfuron in the range 0-0005-0 08 mg AI kg- ' dry soil.

Experiment 2

A second experiment was performed using theremaining non-autoclaved (25, 75 and 100% offield capacity) and autoclaved (100% of fieldcapacity only) soils that had been incubated for90 days at 30°C in Experiment 1. More than 95%of the initial herbicide dose had disappearedfrom these soils during the initial period ofincubation.

On 4 December 1987, solutions of triasul-furon (1-0 ml) were added to 1-kg batches of thesoils to produce a concentration of c. 0-08 mg AIkg-' on an air-dry basis. The herbicide wasthoroughly mixed into the soil, which was thenplaced in sealed polythene bags that wereincubated in the dark at 30°C. All other experi-mental procedures and conditions were similarto those described for Experiment 1. All treat-ments were performed in duplicate and had un-treated controls. Two samples of approximately80 g treated or untreated soil were removedfrom each bag 0, 3, 7, 15, 30 and 60 days afterincubation had started, and the disappearanceof the herbicide was monitored as described forExperiment 1.

Results

The results of all 462 reference bioassays werepooled in order to obtain mean values of reduc-tions in root growth, from which an overallbioassay response curve was derived (Fig. 1). Aplot of root length (expressed as a percentage ofthe control value) against log herbicide concen-tration gave a linear relationship with a highlysignificant (P<O-Ol) correlation coefficient of—0-982. The herbicide concentration resultingin 50% root inhibition (GR,,,) was 0 02 mg AIkg-' dry soil, and the lower limit of detectionwas 0-002 mg AI kg-' dry soil. Percentage rootlengths of seedlings grown in soil samples con-taining unknown concentrations were convertedto herbicide concentrations remaining in thesoil, using the response curve.

Experiment 1

Degradation of triasulfuron at 10°C (Fig. 2) wasgenerally slow, but was faster in the non-autoclaved than in the autoclaved soil. In auto-claved soils stored at 10°C with moisturecontents of 25 and 75% of field capacity, therewas virtually no breakdown of the herbicideafter 150 days, whereas in similar soil at fieldcapacity soil moisture, only approximately 50%of the herbicide remained at that time (Fig. 2).For the non-autoclaved soils stored at 10°C,


0 0 8 0 0 8

30 60 90 120Incubation time (days)

150

Fig. 2. Degradation of triasulfuron in non-autoclaved andautoclaved soils with moisture contents of 25, 75 or 100% fieldcapacity at 10°C: (D) = autoclaved (25% moisture);(•) = autoclaved (75% moisture); (o) = autoclaved (100%moisture); (•) = non-autoclaved (25% moisture); (A) = non-autoclaved (75% moisture); (A) = non-autoclaved (100%moisture).

significant linear correlation coefficients(/'<0-05) were obtained from plots of logconcentration of herbicide remaining vs. time,and mean half-lives of 79, 49 and 30 days wereobtained from soils with moisture contents of25, 75 or 100% of field capacity, respectively(Table 2).

In non-autoclaved soil at 30°C, degradationwas rapid at all moisture levels. Half-lives of 12,11 and 13 days were observed for soils withmoisture contents of 25, 75 and 100% of fieldcapacity, respectively, and were not significantlydifferent from each other (Table 2). After 90days of incubation, only trace amounts ofherbicide were detected in these treatments(Fig. 3). Breakdown of the herbicide in theautoclaved soil was also more rapid at 30°C thanat 10°C. Half-lives of 27, 24 and 18 days wererecorded for moisture levels of 25, 75 and 100%of field capacity, respectively (Table 2).

Experiment 2

The patterns of degradation of the second appli-cation of triasulfuron in the non-autoclaved andautoclaved soils were similar to those observedin Experiment 1. Significant Hnear correlationcoefficients (P<0-05) for plots of log concen-tration of residual herbicide against time wereagain recorded for all of the treatments.

30 60 90 120 150Incubation time (days)

Fig. 3. Degradation of triasulfuron in non-autoclaved andautoclaved soils with moisture contents of 25.75 or 100% fieldcapacity at 30°C: (n) = autoclaved (25% moisture);(•) = autoclaved (75% moisture); (o) = autoclaved (100%moisture); (•) = non-autoclaved (25% moisture); (A) = non-autoclaved (75% moisture); (A) = non-autoclaved (100%moisture).

Compared to the data for the first application(Experiment 1), half-lives were slightly reducedin the non-autoclaved and autoclaved soils in-cubated at field capacity soil moisture, but therewere no differences in the other treatments(compare Tables 2 and 3).

Discussion

Triasulfuron degraded more rapidly in non-sterilized than in sterilized soil (Figs 2 and 3), in-dicating the involvement of soil micro-organismsin the breakdown of this herbicide, a conclusionthat is in agreement with that of Iwanzik &Amrein (1988). Although herbicide degradationin autoclaved soils incubated at 10°C was veryslow (Fig. 2), in the same soils incubated at 30°Cit was relatively rapid at all moisture levels (Fig.3). This indicates that decomposition of triasul-furon by non-biological processes is alsopossible, since autoclaving should have drasti-cally reduced the microbial population of thesoil. The pH of the soil used in these experimentswas relatively low (5 2), and there is evidencethat chemical hydrolysis of other sulfonylureaherbicides is also important at low soil pH (Duffye/a/., 1987).

Although the data usually gave statisticallysignificant fits to the first-order rate equation


Table 2. Half-lives and correlation coefficients for first-order triasul-furon degradation in non-autoclaved and autoclaved soils

TemperatureCC)

Non-autoclaved soil101010303030

Autoclaved soil303030

Moisture content(% field capacity)

2575

1002575

100

2575

100

Half-life(days)

79493012It13

272418

Correlationcoefficient (r)

-0-973'-0961 '-0968*-0986*-0941 '-0930*

-0%5*-0992*-0962*

Correlation coefficients followed by an asterisk (*) indicate signi-ficance at/'<0-05.

Table 3. Half-lives and correlation coefficients for first-order triasul-furon degradation in previously treated non-autoclaved and autoclavedsoils

Temperature(°C)

Non-autoclaved soil303030

Autoclaved soil30

Moisture content(% field capacity)

2575

100

100

Half-life(days)

1211

n

13

Correlationcoefficient (r)

-0994*-0-967*-0 938*

-0%9*

Correlation coefficients followed by an asterisk (*) indicate signi-ficance at F<005.

(Tables 2 and 3), in the case of some of the datathis was only an approximation. In sometreatments, rapid disappearance of triasulfuronwas observed during the first 30 days ofincubation, followed by a somewhat slower rateof dissipation (e.g. in the non-autoclaved soil at30°C; Fig. 3). In this soil with moisture levels at25,75 and 100% of field capacity, 87-5,97-5 and96-3% of the initial concentrations of theherbicide, respectively, had disappeared by theend of the first 30 days of incubation, with 50%degradation occurring after 17-22 days.However, the small residues at this time thenpersisted throughout the following 60 days (Fig.3). A similar pattern of breakdown has beenreported for chlorsulfuron (Thirunarayanan etal., 1985). Duffy et al. (1987) also reported asimilar effect with chlorsulfuron, and suggestedthat the transition to a slower breakdown phaseusually occurred 2-4 weeks after application ofthe herbicide.

Temperature and soil moisture affected therate of degradation of triasulfuron in both

non-autoclaved and autoclaved soils. Ingeneral, breakdown of the herbicide was slowerat 10°C than at 30°C (Figs 2 and 3). Degradationof the herbicide approximated to first-orderreaction kinetics, with much longer half-lives atthe lower (10°C) than at the higher (30°C)temperature (Table 2). The degradation rategenerally increased with increasing soil moisturecontent, but the effects of soil moisture contenton rate of loss were generally smaller than theeffects of temperature.

In Experiment 2, a second application oftriasulfuron to autoclaved and non-autoclavedsoil in which a previous dose of the herbicidehad degraded did not affect the pattern ofbreakdown rates; the pattern of herbicide loss(Fig. 4) was similar to that observed in Experi-ment 1 (Fig. 3). Sixty days after the start ofincubation, the levels of herbicide residues inthe non-autoclaved soils were similar to thoseobserved in Experiment 1 after the same pyeriod(Figs 3 and 4). Brown et al. (1987) also observedrapid microbiai degradation of thiameturon in


15 30 45Incubation time (days)

60

Fig. 4. Degradation of triasulfuron in previously treated non-autoclaved ( ) and autoclaved ( ) soils withmoisture cotitents of 25. 75 or 100% field capacity at 30°C:(o) = autoclaved soil (100% moisture); ( A) = non-autoclavedsoil (100% moisture); ' (A) = non-autoclaved soil (75%moisture); (•) = non-autoclaved soil (25% moisture).

several fresh field soils, and reported thatrepeated applications of the herbicide to thesoils did not result in any enhancement of thedegradation rate. The half-life of the herbicidein the autoclaved soil incubated at field capacitysoil moisture was reduced to 13 days (Table3; calculated after 60 days of incubation),compared to 18 days in Experiment 1 (Table 2;calculated after 90 days of incubation). Thisdifference may be partly due to the calculationof half-lives over different time intervals, but itmay also have resulted from an inability tomaintain sterile conditions throughout both thetreatment and the bioassay periods of incuba-tion.

The rates of loss observed in the presentexperiment are similar to those recorded byIwanzik & Amrein (1988), but somewhat fasterthan those reported by Walker & Welch (1989).Walker & Welch suggested that degradation oftriasulfuron was strongly influenced by soil pH,and they derived an equation relating the first-order rate constant at 20°C and soil moisture atapproximately 75% of field capacity to this soilparameter. Use of this equation and the pH ofthe soil used in the present work gives acalculated half-life of approximately 30 days.The observed half-lives at 75% field capacitywere 49 days at 10°C and 11 days at 30°C. Thuswhen these temperature differences are taken

into account, together with soil pH, the presentresults are consistent with those of Walker &Welch (1989).

The results of this study therefore demon-strate that microbial decomposition is mainlyresponsible for the disappearance of triasul-furon from soils, particularly at lower tempera-tures, and that the rate of breakdown is affectedby soil moisture content as well as by soiltemperature. In warm and moist soils break-down of triasulfuron may be rapid, but in coldsoils herbicide losses are slower, with residuespersisting for lengthy periods.

Acknowledgements

We thank Ciba-Geigy Agrochemicals (UK) forproviding the herbicide. F. K. Oppong is grate-ful to the Commonwealth Scholarship Commis-sion (UK) for providing financial support forthis study.

References

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Degradation of triasulfuron in soil under laboratory conditions