How do pesticides impact soil microbial structure and functioning?

Doctoral thesis English OPEN
Howell, Christopher Carl
  • Subject: S1

Pesticides are used worldwide and exhibit a plethora of different modes of action against a wide\ud spectrum of organisms. Therefore, before they can be marketed they have to be tested against certain\ud standardised regulations. These include the Food and Environment Protection Act 1985 (FEPA), the\ud Control of Pesticides Regulations 1986 (COPR), and more recently the European Council Directive\ud 1991 91/414 and the Plant Protection Products Regulations 1991 (PPPR). However, the current tests\ud used to determine pesticide impacts on microorganisms as detailed by the OECD focus on only broadscale\ud analytical methods that may mask more subtle effects that may still be ecologically significant.\ud Therefore, this project aimed to determine the effects of a widely-used model pesticide, azoxystrobin\ud on both target and non-target microbial communities across different trophic levels.\ud The techniques used to perform this included broad- (soil microbial biomass and soil dehydrogenase\ud activity) and fine-scale (T-RFLP, cloning/sequencing, and qPCR) analytical methods. The results of\ud these analyses showed that the application of azoxystrobin had a significant, concentration-dependent\ud impact on soil dehydrogenase activity whilst biomass was unaffected. The molecular analyses showed\ud that azoxystrobin significantly impacted fungal community structure, diversity and gene copy number.\ud Additionally, pesticide application significantly altered nematode community structure and general\ud eukaryotic diversity. Soil and liquid culture enrichments showed that azoxystrobin degradation can be\ud enhanced following repeated applications and enabled the isolation of two degrader organisms with\ud sequence homologies to a Cupriavidus sp. and a Rhodanobacter sp. Further work showed that\ud sequential enrichments with azoxystrobin also conferred cross-enhanced degradative abilities for three\ud other strobilurin fungicides: pyraclostrobin, kresoxim methyl and trifloxystrobin. The work performed\ud in this thesis served to illustrate how the current OECD test procedures may benefit from the\ud incorporation of finer-scale molecular methods into its tests, as well as how difficult the task can be to\ud produce compounds that persist in the environment long enough to perform their required function,\ud but do not have significant deleterious impacts on non-target organisms when present.
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