Project
title / Microbial degradation of pesticides in soil
/ DEFRA
project code / PLO550

Department for Environment, Food and Rural Affairs CSG 15

Research and Development

Final Project Report

(Not to be used for LINK projects)

Two hard copies of this form should be returned to:
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Project title / Microbial degradation of pesticides in soil
DEFRA project code / PLO550
Contractor organisation and location / Warwick HRI
Wellesbourne
Warwick
CV35 9EF
Total DEFRA project costs / £ 310,635
Project start date / 01/07/01 / Project end date / 31/07/01
Executive summary (maximum 2 sides A4)
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CSG 15 (Rev. 6/02) 2

Project
title / Microbial degradation of pesticides in soil
/ DEFRA
project code / PLO550

A large proportion of any pesticide application reaches the soil where it interacts with organic and mineral constituents and undergoes biological and chemical transformations. Microbial degradation is the primary route for loss, and is therefore the key process affecting the dynamics of pesticide residues in the environment, including their persistence in soil, and their susceptibility to leaching. This project aimed to determine factors controlling microbial degradation of pesticides at the field scale, and in particular, spatial variability of biodegradation rates and its implications. The work addressed the DEFRA Pesticides Safety Directorate policy objectives of 'supporting a system of pesticide approval and monitoring which protects the consumer and the environment' and of 'establishing suitable means to assess and model pesticide behaviour in the environment'. The project had 3 interlinked components; 1. To determine the degree to which spatial variability of pesticide biodegradation rate occurs for a number of key classes of pesticide with contrasting persistence in soil. 2. To characterise patterns of pesticide biodegradation in sub-soil, to give a 3-dimensional characterisation of pesticide degradation rates. The key soil properties governing horizontal and vertical variability of degradation rates in soil were also elucidated. 3. To determine the impact of persistent pesticides on the structure and functioning of soil microbial communities.

1. Spatial variability in the degradation of persistent pesticides in soil

Previous studies have demonstrated considerable spatial variability in the degradation rate of pesticides in soil, with implications for patterns of pesticide leaching from soil. However, previous studies have focussed on compounds with high to moderate mobilities in soil and which are degraded relatively quickly, with time to 50 % decay (DT50) of less than 3 months. Much less is known about spatial variation in the degradation rate of relatively immobile persistent pesticides, where environmental concerns arise from the potential of compounds to accumulate over time, or to exert non-target effects on soil communities. We determined time to 50 % decay of the compounds azoxystrobin, tebuconazole, diflufenican and isoproturon in soil taken from 40 sites within a field on sandy-loam at Wellesbourne, Warwickshire and a field on clay-loam at Kirton, Lincolnshire. It was shown that degradation rates varied widely at the field scale. The variability in DT50 was lower at the Kirton site, in which variation in key soil chemical and biological properties was lower. Additionally, variability increased as pesticide DT50 increased. For diflufenican and tebuconazole, there were isolated locations at both sites at which there was very slow degradation, and at which the pesticides could potentially accumulate with repeated year on year use. Azoxystrobin degradation was strongly correlated to soil pH. However, diflufenican and tebuconazole persistence was not correlated with any of the soil chemical or biological parameters measured.

The relationship between pH and degradation rate of isoproturon was found to vary in adjoining fields at Wellesbourne with similar management histories. In Deep Slade field, degradation was strongly linked to pH. Experiments with isoproturon degrading Sphingomonas spp. isolated from Deep Slade field indicated that this pH relationship was the result of direct effects of pH on the growth and activities of specific strains of the soil bacterium Sphingomonas spp. However, no such relationship between DT50 and pH existed in Long Close field. It was shown that although degradation in Long Close field also involved Sphingomonas spp. the specific strains involved were different. Differences in the specific communities of Sphingomonas spp. involved in isoproturon degradation in the field could account for the differences in the relationship between pH and degradation.

2. Spatial variability in degradation rates of pesticides down the soil profile

Degradation rates of pesticides are generally assumed to decrease with soil depth, as the size of the microbial community declines. However, some recent studies have indicated that this is not always true, and that in some cases rates of biodegradation in sub-soil can be higher than in corresponding top-soil. The reasons for such patterns of degradation are unclear. However, changes in pesticide availability down the soil profile, linked to a decline in amounts of organic matter could be a key controlling factor of such degradation patterns. In addition, the movement of degradative bacteria through the soil profile to sites with lower microbial communities and hence less competition, could also contribute. Degradation of pesticides in soils is evidently three dimensional, with highly complex relationships between bio-availability and bio-degradability in sub-soils as well as top-soils. Knowledge of the extent of this 3-dimensional variation, the scale at which it occurs, and the factors that control it, is essential to any risk assessment of environmental exposure, and is particularly important within the context of probabilistic modelling. Studies focussed on mecoprop, isoproturon and bentazone, which are among the most commonly found contaminants of surface freshwater and groundwater in the UK. Degradation of the compounds was studied in soil samples taken from 0-10 to 70-80 cm depth. It was shown that for all compounds, degradation rate declined with soil depth, but became more variable. Bentazone was degraded by cometabolism, without the proliferation of organisms able to use it as an energy source. For this compound, decline in DT50 down the soil profile was strongly related to the vertical decline in microbial biomass. In contrast, at all top- and most sub-soil sites mecoprop and isoproturon were degraded by growth linked metabolism, which involved the proliferation of organisms. Communities capable of growth-linked mecoprop and isoproturon metabolism were present in soil even down to at least 0.8 m depth. For these compounds variability in DT50 was the result of variation in the lag phase prior to a period of rapid degradation. DT50 for mecoprop and isoproturon down the soil profile was not strongly related to soil properties.

3. The effect of persistent fungicides on the structure and functioning of soil microbial communities

There has been much research on the impact of pesticides on the size and functioning of soil microbial communities. However, this work has been completed using relatively crude means of measuring the size and activities of soil microbial populations. We used molecular profiling methods to investigate the impact of the fungicides chlorothalonil, azoxystrobin and tebuconazole on soil microbial communities. The pesticides were applied to two soil types at realistic field rates. Chlorothalonil reduced dehydrogenase, which reflects microbial activity, by up to 45 % over a 3 month period, and reduced the rate of mineralization of subsequent additions of isoproturon and bentazone by up to 36 %. Azoxystrobin had no effect on dehydrogenase, but inhibited mineralization of a subsequent addition of isoproturon by up to 11 %. Tebuconazole had no effect on either dehydrogenase or degradation of subsequent pesticides. After 3 months soil microbial community structure in pesticide treated and untreated soils was compared. None of the pesticides affected the structure of the soil bacterial community. However, all the pesticides were capable of inhibiting members of the eukaryote community, with the evidence suggesting that those organisms inhibited belonged to the protozoa.

Currently models dealing with the environmental fate of pesticides take no account of spatial variability, and the data set derived from this project could prove valuable for developing approaches to incorporate variability into degradation fate modelling. Further work is needed to determine the extent to which the extreme spatial variability in degradation rate of some pesticides occurs in real farming situations, and its implications for soil quality. The mechanisms controlling spatial variability in degradation rate remain largely unknown. In part this is the result of uncertainty in the characteristics of those organisms contributing to pesticide cometabolism in soil. Further, the relationships between the diversity of degradative genes and growth-linked metabolism are unclear. New approaches to examine these aspects could provide new ways of predicting and managing pesticide fate. Work in this project has demonstrated that pesticides can exert non-target effects on soil microbes, but that impacts on the structure, activity and functioning of communities may vary. Further research is needed to examine these effects in order to determine implications for the pesticide registration process and issues to do with soil quality. Examining the specific communities impacted, the duration of the effects, and the impacts on the resilience of the soil community and its activities to further stresses should be key components of this work.

CSG 15 (Rev. 6/02) 2

Project
title / Microbial degradation of pesticides in soil
/ DEFRA
project code / PLO550
Scientific report (maximum 20 sides A4)
To tab in this section press the tab key and the Control key together
Press the DOWN arrow once to move to the next question.

CSG 15 (Rev. 6/02) 2

Project
title / Microbial degradation of pesticides in soil
/ DEFRA
project code / PLO550

1. INTRODUCTION

A large proportion of any pesticide application reaches the soil where it interacts with organic and mineral constituents and undergoes biological and chemical transformations. Microbial degradation is the primary route for loss, and is therefore the key process affecting the dynamics of pesticide residues in the environment, including their persistence in soil, and their susceptibility to leaching (Aislabie and Lloyd Jones, 1995).

Work in our previous project on microbial degradation (PL0526) concentrated on the microbial relationships involved in ‘enhanced’ or ‘accelerated’ biodegradation. This is the phenomenon by which pesticides are degraded more rapidly following repeated application at the same site, because of adaptation within the soil microbial community to use the pesticide as an energy source. A key finding was the observation of extreme spatial variation in degrading potential of the phenyl-urea herbicides within a field or group of fields, even when the soil appeared uniform in terms of its major physical and chemical characteristics (Cullington and Walker, 1999; Walker et al., 2001). The variability was found to result from differences in the relative contribution of growth-linked and cometabolic degradation at soil microsites (Bending et al., 2001).

Studies which have investigated spatial variation in pesticide degradation have largely focussed on compounds with high to moderate mobilities in soil and which are degraded relatively quickly, with time to 50 % decay (DT50) of up to 3 months (Price et al., 2002; Walker et al., 2001; Parkin and Shelton, 1992; Muller et al., 2003). Few studies have considered the extent of spatial variability in the degradation rate of persistent pesticides with DT50 of up to 6 months or more. A number of such compounds with DT50 over 6 months are widely used in the UK, including the triazol fungicides (Bromilow et al., 1999a,b) and the herbicide diflufenican (Conte et al., 1998; Rouchaud et al., 1994).

Degradation rates of pesticides are generally assumed to decrease with soil depth, as the size of the microbial community declines. However, work in PL0526 with the carbamate insecticide carbofuran (Karpouzas et al., 2001), and parallel work with atrazine in Australia (Di et al., 2001) has indicated that this is not always true, and that in some cases rates of biodegradation in sub-soil can be higher than in corresponding top-soil. The reasons for such patterns of degradation are unclear. However, changes in pesticide availability down the soil profile, linked to a decline in amounts of organic matter could be a key controlling factor of such degradation patterns. In addition, the movement of degradative bacteria through the soil profile to sites with lower microbial communities and hence less competition, could also contribute. Degradation of pesticides in soils is evidently three dimensional, with highly complex relationships between bio-availability and bio-degradability in sub-soils as well as top-soils. Knowledge of the extent of this 3-dimensional variation, the scale at which it occurs, and the factors that control it, is essential to any risk assessment of environmental exposure, and is particularly important within the context of probabilistic modelling.

There has been much research on the impact of pesticides on the size and functioning of soil microbial communities. However, this work has been completed using relatively crude means of measuring the size and activities of soil microbial populations (e.g. Elmholt, 1992; Anhalt et al., 2000; Bjornlund et al., 2000). Measurement of biomass has been widely used in such studies. However, while this method is effective at providing estimates of living organisms in the soil, it provides no information about the structure of communities inhabiting the soil. Similarly studies of the functioning of soil communities in pesticide amended soils have focussed on N mineralization from added organic matter, and metabolism of soil organic matter. These are processes performed by all organisms inhabiting the soil, and for which there is considerable functional redundancy. This means that multiple organisms are available to take over these functions should particular members of the soil community be negatively affected by a treatment. The impact of pesticides on processes for which there is less functional redundancy, such as pesticide metabolism, is unclear. Over the past 10 years molecular methods have become available for profiling complex soil communities. These provide new opportunities to determine the extent to which pesticides affect the structure and functioning of soil communities.

This project used a number of field sites to investigate the degree to which spatial variability of pesticide biodegradation rate occurs for a number of key classes of pesticide with contrasting persistence in soil. Additionally, at selected sites, patterns of pesticide biodegradation were determined in sub-soil, to give a 3-dimensional characterisation of pesticide degradation rates. The key soil properties governing horizontal and vertical variability of degradation rate in top-soil were elucidated. Finally, we initiated studies to determine the impact of persistent pesticides on the structure and functioning of soil microbial communities.