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tors. A recent study found negative consequences of glyphosate
on this genus
17
with the effects observed being both species and
strain-dependent.
When
P. protegens
, a bacteria used as a biocontrol agent for
cereal crops, and
P. fluorescens
, used as a fungus biocontrol for
fruit trees, were exposed to varying glyphosate concentrations, no
negative effects were observed. In two species of
P. putida
, used
as soil fungus control for maize and other crops, the bacteria had
notably stunted growth.
A study published during 2008 report similar effects regarding
the toxic effect of glyphosate and 2,4-D on the water hyacinth bio
control agent,
Alternaria alternata
18
. Although neither of the two
herbicides actually killed the fungus, both inhibited its growth.
Glyphosate though, delayed mycelial growth yet stimulated spo
rulation while 2,4-D inhibited both growth and sporulation.
Glyphosate at low concentration did not affect the virulence of
A. alternata
, while fungi grown on 2,4-D amended plates lost their
virulence. Another study
19
discourages the joint application of
P. lagenophorae
(a biocontrol agent for the annual weed
Senecio
vulgaris
) with 2,4-D, glyphosate and linuron to control
S. vulgaris
due the detrimental effect that these herbicides have on the bio
control agent.
Rollan and his research group
20
were, however, not able to find any
detrimental effect of herbicides such as glyphosate, imazamox or
imazethapyr on
Trichoderma harzianum
and again underlined the
critique against herbicide studies which focuses on soil-free media,
rather than adding the herbicide directly to soil.
As herbicide application could potentially have an effect on the
efficiency of the biocontrol agents, producers should discuss this
matter with the relevant suppliers before applying them. Considera
tion could be given to different application times for herbicides and
biocontrol agents.
A study of note…
A study by an USA based research group
21
, which published
their results during 2014, investigated the effect of long term glyp
hosate use on the microbial community. The community evaluation
included gram positive bacteria, gram negative bacteria, fungi,
actinomycetes, arbuscular mycorrhizal fungi and eucaryotes.
Two soils were compared, one with no-glyphosate use history as
opposed to one with a history of more than ten years of glypho
sate tolerant cropping practices. Glyphosate treated
and untreated residues were respectively also
added to these soils and the microbial activity moni
tored for 30 days.
With this study Ney and his colleagues dem
onstrated that the initial response of the
two soil types differed during plant
decomposition, but that the effect
diminished over time. The residue type used, in addition, did not
have an effect on the end result.
They concluded that the microbial community had adjusted over
time to such an extent that the soil with a glyphosate history had
a community primed for glyphosate residues, whereas the non-
glyphosate soil was starting to change, or adjust in response to the
decomposition of glyphosate residues. Their data suggested that
the microbial communities in the soil with a history of long-term
glyphosate exposure had adapted to glyphosate tolerant residues
and thus would show fewer microbial responses during decomposi
tion than that of a soil with no glyphosate use history.
Their study also answered the question whether plant residues
from a glyphosate tolerant cultivar would impact on soil health,
in that they concluded that it is the exposure to glyphosate and
not the genetically tolerant plant material itself that causes microbial
responses.
Food for thought
It is internationally accepted that the behaviour of pesticides in
soil will depend on a number of factors. The chemical and physical
properties of the pesticide itself as well as environmental factors
such as temperature, precipitation, wind, sunlight, soil properties
such as moisture, organic matter, pH, available nutrients, interac
tions between solid, liquid and gaseous phases of the soil are all
part and parcel to establishing the persistence, availability and tox
icity of pesticides
22
.
Based on their review of international research findings, two au
thors (Lock and Zablotowicz)
23
which contributed to the book titled
Managing soil quality: Challenges in modern agriculture
, conclu
ded that microbial and faunal populations generally demonstrated
tolerance of pesticides, with only minor short lived disturbances
being exhibited when recommended rates are used, whilst major
biotic processes such as enzyme activity, respiration and carbon
and nitrogen transformations are generally minimally impacted.
There is, however, the other side of the coin that links in with the
Ney research group findings
21
, and which must be considered. In a
recent study published in February 2018, an Australian-based
research group
24
which focused on the effects of glyphosate,
glyphosinate, paraquat and paraquat-diquat on soil microbial and
bacterial diversity, were unable to provide evidence of any nega
tive effect on the various parameters measured due to a once-off
application by the various herbicides.
They concluded that the lack of observed reaction
of microbial communities might, amongst
others, be due to the fact that years of
herbicide application on the soils used
in the study might have resulted
in only herbicide tolerant popu
lations of microbes being
present in the soil.