Pesticide Residues in the Environment - Pesticides: Evaluation of Environmental Pollution - page 177

asdeducedfromPCA.Ninety-twopercentofthetotalvariancewasexplainedbytheirst
principal component. In the irst principal plane, it is possible to discriminate between
threegroupsofsoilsamples(Piuttietal.2002).Theirstgroupcaneasilybeseparatedin
thelowerrightsideoftheplane.Itcontainsafewtreatedandamajorityofuntreatedsam-
plesshowingnoorlowaccelerateddegradation.Amongothersoilsamples,Piuttietal.
(2002) have distinguished those that have received diclofop-methyl in the greenhouse
fromthosethathavenot.Forthosetwogroups,theauthorshavereportedthevaluesof
kineticparametersasfunctionsoftheirpositionsalongtheirstcomponentaxisinFigure
6.8band8c(Piuttietal.2002).
6.4.4.4 Phenoxyacetic Herbicides
Theherbicide2,4-dichlorophenoxyaceticacid(2,4-D)isknowntoinhibitmethanotrophic
bacteria.Methaneoxidationwasthereforeusedasaparametertoevaluatetheresidual
2,4-D after bioaugmentation of an agricultural soil by Top et al. (1999). Several strains
harboringcatabolicplasmidsthatcodeforthedegradationof2,4-Dwerecomparedfor
their potentials to alleviate the negative impact of 2,4-D on methane oxidation by soil
microorganisms.AlldataobtainedbyTopetal.(1999)showthatbioaugmentationofa
2,4-D-treated soil with strains with the necessary genetic catabolic information short-
ensthetimeneededforthecompleteremovalof2,4-Danditstoxicitytowardsmetha-
notrophs. The transfer of the plasmid pEMT1k, resulting in increased 2,4-D removal,
conirms the results that dissemination of catabolic genes can be an alternative to the
classicalbioaugmentationapproach,whichreliesontheactivityoftheinoculatedstrain
(Top et al. 1999). Top et al. (1999) have shown that measuring the methane oxidation
in soil can be a very good bioassay to evaluate the success of bioaugmentation if the
pollutant or a possible metabolite inhibits this microbial process. Gonod et al. (2006)
described2,4-Dimpactonbacterialcommunitiesandtheactivityandgeneticpotential
of2,4-Ddegradingcommunitiesinsoil.ThestudyofKumaretal.(2010)carriedoutto
investigatethechronicresponseofthecyanobacteria Anabaena fertilissima to2,4-Dethyl
esteratdifferentconcentrations:15,30,and60ppm.Basedontheinhibitoryeffectand
growth arrest, the release of certain products, such as carbohydrates, proteins, amino
acids, and phenols was also affected at 15, 30, and 60 ppm of 2,4-D. The results of the
experiments on enzyme activity obtained by Kumar et al. (2010) revealed that 2,4-D
inhibitsthesynthesisofnitratereductaseandglutaminesynthetase.Onthebasisofthe
resultsdescribedbyKumaretal.(2010),itcanbestatedthattheapplicationof2,4-Dto
riceieldsshouldbeconsideredbecauseofitstoxicitytotheheterocystousilamentous
cyanobacteria. Adsorption and degradation of four acidic herbicides (2,4-D, dicamba,
metsulfuron-methyl, and lupyrsulfuron-methyl sodium) in soils was described by
Villaverdeetal.(2008).Adsorptionanddegradationoffourpesticidesweremeasuredin
foursoilscharacterizedbysmallorganicmatter(OM)contents(0.3%-1.0%)andvarying
claycontents(3%-66%).Ingeneral,sorptionincreasedintheorder:dicamba<metsul-
furon-methyl<2,4-D<lupyrsulfuron-methylsodium(Villaverdeetal.2008).BothOM
and clay content were found to be important in determining adsorption, but relative
differences in clay content between soils were much larger than those in OM content,
and therefore clay content was the main property determining the extent of herbicide
adsorption for soils of this type. pH was negatively correlated with adsorption for all
pesticides apart from metsulfuron-methyl. The contrasting behavior shown by these
fouracidicherbicidesindicatesthatchemicaldegradationinsoilismoredificulttopre-
dictthanadsorption(Villaverdeetal.2008).