Pesticide Residues in the Environment - Pesticides: Evaluation of Environmental Pollution - page 178

6.4.4.5 Organophosphorus Herbicides

Glyphosate,N-(phosphonomethyl)glycine,isaherbicidefrequentlyusedforcontrolling
perennialweedsthroughapplicationafterharvest.Theeffectoffreeze-thawactivityon
theavailabilityofglyphosateinsoil,andconsequentlyitsmineralizationbysoilmicroor-
ganisms,wasstudiedbyStenrødetal.(2005).Resultsobtainedbytheseauthorsshowing
lowconcentrationsofglyphosateinsoilwaterareconsistentwithearlierindingsofhigh
K d valuesforglyphosateinthesesoilsbutdonotgiveanyconclusiveevidenceastohow
thebioavailabilityofglyphosateinthesoilsaffectsthebiodegradationpatternsobserved.
ThedifferentglyphosatedegradationratesbetweensoilsobservedbyStenrødetal.(2005)
suggestthatbioavailabilityisinvolvedandthelengthoftimeoffreezingwasanimpor-
tantfactorgoverningtotalglyphosatemineralization.Amorósetal.(2007)describedthe
assessmentoftoxicityofaglyphosate-basedformulationusingbacterialsystemsinlake
waterandshowedthatwhenthepHofthesampleincreases,theEC 50 (mg/l)signiicantly
increases,showingthedirecteffectofthepHonthetoxicityvaluesobtained.Desorption
andtime-dependentsorptionofiveherbicides(abroad-spectrumherbicide,glyphosate,
and four commonly used selective herbicides, triluralin, metazachlor, metamitron, and
sulcotrione) in three soils were described by Mamy and Barriuso (2007). Desorption of
glyphosateanditsmetaboliteaminomethylphosphonicacid(AMPA)wasgreaterinsome
soils(highpHandphosphatecontents,littlecopperandamorphousironcontents),compa-
rablewiththatoftriluralinandsulcotrione(MamyandBarriuso2007).

6.4.4.6 Chloroacetanilide Herbicides

Sietal.(2009)describedadsorption,desorption,anddissipationofmetolachlorinsurface
and subsurface soils. Adsorption of metolachlor was greater in the high-organic-matter
surfacesoilthaninsubsoils.Loweradsorptiondistributioncoeficient(K ads )valueswith
increasing depth indicated less adsorption at lower depths and greater leaching poten-
tialofmetolachlorafterpassagethroughthesurfacehorizon.Desorptionofmetolachlor
showed hysteresis, indicated by the higher adsorption slope (1/n ads ) compared with the
desorptionslope(1/n des ).Soilsthatadsorbedmoremetolachloralsodesorbedlessmetola-
chlor.Theirst-orderdissipationratewashighestatthe0-50cmdepth(0.140/week)and
lowestatthe350-425cmdepth(0.005/week)(Sietal.2009).TheresultsobtainedbySietal.
(2009)indicatethatdegradationoftheherbicidewassigniicantlycorrelatedwithmicro-
bialactivityinsoils.Millsetal.(2001)describedthequantiicationofthechloroacetanilide
herbicide acetochlor degradation in the unsaturated zone using two novel in situ ield
techniques. Two different in situ ield techniques and traditional laboratory-incubation
studieshaveindicatedthatacetochlorisrapidlydegradedandismineralizedinsubsoils
toadepthofapproximately5meters(Millsetal.2001).

6.4.4.7 Nitrile Herbicides

BiodegradationofthenitrileherbicidebromoxynilwasdescribedbyRosenbrocketal.
(2004). The mineralization and formation of metabolites and nonextractable residues
of the herbicide [ 14 C]bromoxynil octanoate ([ 14 C]3,5-dibromo-4-octanoylbenzonitrile)
and ([ 14 C]3,5-dibromo-4-

hydroxybenzonitrile)wereinvestigatedinsoil.Themineralizationof[ 14 C]bromoxynil
and [ 14 C]bromoxynil octanoate in soil within 60 days amounted up to 42% and 49%,
respectively (Rosenbrock et al. 2004). After the experiment, 52% of the originally

the corresponding agent substance [ 14 C]bromoxynil