Download Microbial Degradation of Pesticide in Soil

Executive summary PL0526
The major environmental concern arising from the use of pesticides in
agriculture is their potential to leach to surface and ground water and affect
water quality. The susceptibility of a particular pesticide to leaching depends
on complex interactions between the chemical structure of the pesticide and a
variety of soil physical, chemical and microbiological properties, which control
the pesticide sorption and rate of degradation. For several classes of
pesticide, including phenoxyalkanoic acids and carbamate insecticides,
repeated use on the same site over several years can results in development
of enhanced biodegradation, where pesticide degradation occurs extremely
rapidly, as a result of the proliferation of organisms adapted to utilise the
compound as an energy source. Induction of enhanced degradation can
reduce the eficacy of pesticides, creating uncertainty as to doses required for
pest or weed control. Recently, there has been evidence that some pesticide
classes previously though to be degraded slowly in the environment, including
the phenyl-urea herbicides, may also be susceptible to enhanced
degradation.
The purpose of this project was to investigate the microbial mechanisms
underlying enhanced degradation. The experiments used pesticides with
contrasting susceptibility to the induction of enhanced degradation. Key
pesticides used included carbofuran, for which enhanced degradation can be
readily induced, isoproturon, for which 'hot spots' of rapid degradation have
been found within isolated fields, and diuron, which has only very occasionally
been found to degrade rapidly in soil. Studies investigated how the
proliferation of pesticide degrading organisms is affected by the chemical and
biological environment of the soil. Further experiments investigated spatial
variability in biodegradation rates within top-soil and sub-soil. The
characteristics and diversity of pesticide degrading organisms were also
investigated.
Experiments were conducted to examine degradation in the top- and sub-soil
of fields that had received applications of the pesticide at varying times over
the past 10 years. It was found that concentrations as low as 0.1 mg kg-1 soil
could induce rapid degradation of a subsequent application. There was no
relationship between top soil exposure to carbofuran and degradation rates in
the sub-soil. In general, degradation rates in sub-soil were lower than in subsoil. However, there was evidence that some sub-soils had inherently high
abilities to degrade the compound, with greater rates of degradation than in
top-soils. Patterns of degradation in sub-soil samples indicated that there was
a significant spatial variability in the distribution of carbofuran degrading
organisms.
In the case of isoproturon, it was shown that there was substantial variability
in degradation rate within Deep Slade field at Wellesbourne, with the half life
of the compound ranging from 6.5 to 30 days. Microsites at which isoproturon
degraded rapidly had higher pH and larger biomass contents than slow
degrading sites. Using 14C ring labelled isoproturon, it was shown that spatial
variability in degradation rate had implications for the eventual fate of
pesticide residues. Slow degradation promoted the formation of bound
pesticide residues.
In the case of both carbofuran and isoproturon, variation in the rate of
pesticide degradation did not occur as a result of differences in the initial size
of the pesticide degrading population. For both pesticides, the dynamics of
growth and proliferation of the degradative community following pesticiide
application was shown to control degradation kinetics. The extreme variability
in isoproturon degradtion rate was found to arise from fundamental
differences in the mode of degradation. Fast degradation rates were
associated with the rapid proliferation of degrading organisms. However, in
slow degrading sites, there was no evidence for proliferation of degrading
organisms until at least 60 % of the pesticide had been degraded. This
indicates that degradation had occurred by cometabolism, coincidentally to
the general metabolic activities of the soil microbiota. In a number of slow
degrading microsites, proliferation of isoproturon degrading organisms did
subsequently occur. However in several sites, there had been no proliferation
of degrading organisms even at the point of 90 % pesticide degradation.
The characteristics of pesticide degrading communities were investigated.
Bacteria showing abilities to degrade carbofuran were isolated from
geographically distinct soils, from the UK and Greece. Partial sequence
analysis of the 16S rRNA gene of the isolates revealed high diversity within
the carbofuran degrading communities of both sites. While most UK isolates
were Pseudomonas spp., the Greek isolates were predominantly members of
the Flexibacter / Cytophaga / Bacteroides group. However, there was some
overlap of species within the 2 communities, with Chrysobacterium sp.
isolates from the two soils showing identical 16S rRNA gene sequences and
protein profiles. A number of isoproturon degrading bacteria were isolated
from Deep Slade field. Partial sequencing of the 16S rRNA gene of these
isolates showed that they were Sphingomonas sp. In situ changes in soil
microbial community structure during isoproturon degradation were
investigated by denaturing gradient gel electrophoresis of 16S rRNA. It was
found that 2 organisms proliferated during isoproturon degradation, both of
which were closely related Sphingomonas sp.
A further bacterium which was able to degrade diuron was isolated from Deep
Slade field. 16S RNA sequencing showed that the isolate was an Arthrobacter
sp. This organism could degrade a range of further phenyl-urea compounds,
including isoproturon. The genes for phenyl-urea degradation were found to
be located on a 22KD fragment within the plasmid pHRIM620.
The project has highlighted the extreme spatial variability of pesticide
degradation that can occur within a single field, in both sub-soil and top-soil.
This has implications for the dynamics of pesticide leaching. Further work is
needed to investigate whether the variability in carbofuran and isoproturon
degradation rates are typical of other pesticide classes, and the scale over
which the variability occurs.The work has provided tools that can be used to
investigate the localisation, dynamics and survival of pesticide degrading
organisms, which is necessary for an understanding of the mechansims
underlying variability.