1. J. Princivalle1 | K. Thomas2 | S. Marshall2 | I. Bramke2 | L. Shaw1
Department of Geography and Environmental Science
Introduction
Soil microbial communities are phylogenetically and functionally highly
diverse. And although they are key contributors to pesticide degradation,
little is known about the influence of microbial diversity on pesticide
degradation function – both in the field and in laboratory studies.
The following types of diversity-function relationship can be hypothesised
for soil (Fig. 1).
• Type A (linear), where degradation function is highly sensitive to
diversity loss
• Type B, where high functional redundancy allows soil function to be
maintained despite a decrease in biodiversity
• Type C (idiosyncratic), where degradation function relies on
community structure rather than overall diversity
The aim of this project was to explore the sensitivity of the biodegradation
of four model pesticides to microbial diversity erosion.
Impact of Erosion of Microbial Diversity on Pesticide Degradation
Ecosystemfunction
Type A
Type B
Type C
Biodiversity
Inoculum
10-2
100x 100x 100x100x 100x
Dilution
10-6 10-8 10-1010-4
Fresh
Soil
Species lost in dilution
undiluted 10-2 10-6 10-8 10-10
10-4
γ-
irradiated
Soil
Equilibrated mesocosms of decreasing diversity but comparable biomass
Figure 1:
Hypothetical relationships
between soil microbial
diversity and pesticide
Degradation function
Modified from
Nielsen et al., 2011
N
N
H
N
N
NH
Cl
O
N
O
F
F
F
O
O
O
O
O
O
O
N
O
N
N
Cl
O
Cl
OH
O
Construction of a Microbial Diversity Gradient and Degradation Studies
Figure 2:
Mesocosm Construction
Four successive generations
of mesocosms were
constructed to encompass
a gradient of microbial species
richness using a “dilution to
extinction” approach
(Wertz et al., 2006).
Fresh (undiluted) and
γ-irradiated control soils
were equilibrated and used
alongside these test soils.
Figure 3:
Recovery of biomass assessed by SIR
Following construction, mesocosms (generations A-D) were
constructed, left to equilibrate for 22 weeks and levels of
14C Glucose-induced respiration measured. This surrogate
measure of biomass showed no statistically relevant
differences between any of the soils including the controls
(27.5% F-test probability).
Pesticide degradation studies using radiolabelled compound
were conducted according to the OECD307 guideline.
Vessels were destructively sampled, solvent extracted and
extracts analysed by LSC and HPLC with radiodetection to quantify the remaining 14C and parent compound.
Rate (k) and DT50 of degradation were modelled using simple first order kinetics.
2,4-D Azoxystrobin TerbuthylazineBicyclopyroneResults
Figure 4: Degradation kinetics of 2,4-D, Azoxystrobin, Bicyclopyrone and Terbuthylazine
Fitted curves and data are colour coded according to dilution (legend top right). Graphs (A) show the loss of parent compound over the time course
of the degradation studies for all four compounds. Tables (B) show the corresponding modelled rates (k) and half-lives (DT50) of compound degradation.
The fraction of Terbuthylazine parent remaining at the end of incubation is shown in bar chart (C).
k
DT50
[days]
Fresh 0.227 3
10-2 0.092 7
10-4 0.085 8
10-6 0.052 13
10-8 0.008 87
10-10 0.004 188
γ-irradiated 0.003 238
k
DT50
[days]
Fresh 0.009 77
10-2 0.001 464
10-4 0.001 542
10-6 0.001 649
10-8 0.001 417
10-10 0.001 1191
γ-irradiated <0.001 > 10 years
k
DT50
[days]
Fresh 0.003 238
10-2 0.002 389
10-4 0.002 414
10-6 0.002 284
10-8 0.002 338
10-10 0.001 608
γ-irradiated <0.001 > 10 years
k
DT50
[days]
Fresh 0.01 75
10-2 0.01 77
10-4 0.01 115
10-6 0.01 101
10-8 0.001 166
10-10 0.01 104
γ-irradiated <0.001 > 10 years
B
A
C
Discussion
We have evaluated our methodology for studying the sensitivity of biodegradation kinetics to microbial diversity erosion.
The data shows that the overall biodegradability of a pesticide as well as its dependency relationship with microbial diversity are chemistry specific.
• 2,4-D was quickly degraded in fresh and moderately diluted soils (up to 10-6) indicating functional redundancy (B type relationship). Further erosion of diversity
led to a substantial loss in 2,4-D degradation function.
• Azoxystrobin degradation reacted sensitively to dilution with DT50 and degradation rate changing by an order of magnitude compared to the fresh soil. This
indicates an A-type relationship.
• Bicyclopyrone degradation in soil is slow compared to the other compounds with half-lives exceeding the duration of the study. While this makes comparison of
the kinetics observed in diluted soils difficult, we did observe a reduction of degradation in diluted soils compared to the fresh control.
• Terbuthylazine was degraded similarly in all soils including the fresh soil control (DT50 range 75-166 days), but does not follow the usual trend of higher dilution
leading to longer half-life/slower rate. Instead, the 10-10 soil contained much less remaining parent at the end of the study than in the less diluted 10-8 soil. If this
difference was shown to be robust and significant it would suggest a possible reliance on community structure in addition to species richness and therefore a
C-type relationship.
Work is underway to estimate degrader numbers (MPN approach) and to fully characterise the microbial communities in all soils by next generation sequencing.
Contacts
1 Soil Research Centre,
Department of Geography
and Environmental
Science, University of
Reading, Whiteknights,
RG6 6DW
2 Product Safety, Syngenta,
Jealott's Hill International
Research Centre, Bracknell,
RG42 6EY, United Kingdom
References
Nielsen et. al., European
Journal of Soil Science,
2011, 62, 105–116
Wertz et al., Environ
Microbiol, 2006, 8,
2162-2169
J. Princivalle1, L. Hand2, K. Thomas2, S. Marshall2, L.J. Shaw1 , I. Bramke2