How important is soil microbial diversity for pesticide biodegradation function and robust biodegradation kinetics?

Princivalle, J. (2017) How important is soil microbial diversity for pesticide biodegradation function and robust biodegradation kinetics? PhD thesis, University of Reading.

Abstract/Summary

Soil microbial communities are highly diverse and implicated in delivering many ecosystem services including the degradation of pesticides. The aim of this thesis was to study the relationship between microbial species richness and pesticide biodegradation kinetics in order to contribute to the understanding of the causes of variability in pesticide degradation kinetics between test systems used as a component part of the discovery process for new crop protection active ingredients. This thesis was in collaboration with Syngenta who were keen to gain an understanding of the importance of soil microbial diversity for pesticide biodegradation function in order to underpin the design of improved test systems for screening pesticides at an early stage of their development. A series of soils encompassing a microbial diversity gradient using a dilution to extinction approach were created, in which the soil was y-irradiated/lautoclaved and then re-inoculated with a suspension of serial dilutions of fresh soil (la', 10', 106, 10', 101"). Illumina MiSeq sequencing demonstrated gradual decrease of bacterial OTUs of 45, 54, 66, 83 and 88 % in the dilutions compare to the control fresh soil while the y-irradiated control soils showed a reduction of 97% after a soil equilibration period of 20 weeks to allow microbial re-colonisation of the soil. These manipulated soils were used to conduct pesticide degradation studies following the OEC0307 guideline using radio labelled compounds: 2,4-Dichlorophenoxyaceticacid (2,4-0), Terbuthylazine (TBA), Azoxystrobin (AZ) and Bicyclopyrone (BIR). The degradation of 2,4-0 was robust up to 106 dilution, but was disrupted beyond 10'. Conversely, AZ half-life was much prolonged (>388 days) in all diluted soils compared to the 'fresh control' DT50 of 77 days, indicating a high dependency on microbial diversity. TBA was degraded similarly in all soils (DT50 range 75-166 days) suggesting a possible reliance on microbial community structure. BIR degradation was slower than the other compounds with half-life exceeding the duration of the study, however, a reduction in the extrapolated half-life in diluted soils compare to the fresh control was recorded. Analysis of compound degradation kinetics therefore revealed pesticide-specific sensitivity to microbial diversity erosion. The findings of this project contribute to the understanding of the causes of variability in pesticide biodegradation behaviour between the test systems component to the pesticide discovery pipeline. The dilution to extinction system that was evaluated in this thesis could potentially provide a valuable tool to agrochemical R&D through providing a means to identify (and discard) compounds not displaying robust biodegradation kinetics at an early stage in the discovery process.