Degradation of triasulfuron in soil under laboratory conditions

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  • 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 30C than at10C, with half-lives of 11-13 days at 30C and30-79 days at 10C. 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 30C 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 30C qu'S 10Cavec 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 30C 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 faon 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 at20C and 50% of field capacity ranging from12-38 days, and Martin & Blair (1988) observeda half-life of 11 days at 30C 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 120C 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 30C. 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 20Cfor 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 -12Triasulfuron 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 20C. 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 30C 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 30C. 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

  • 170 F. K. Oppong and G. R. Sagar

    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 10C: (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 co