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46
Oktober 2018
W
eeds are a persistent threat to crop productivity
worldwide. In an effort to optimise yield by controlling
these unwanted plants, modern agriculture has become
heavily dependent on the use of herbicides. Non-tar
get organisms are unfortunately equally exposed to these chemi-
cals which have the potential to, unintentionally, also harm them.
According to international literature, this may lead to a reduction
in biodiversity and the provision of ecosystem services that support
food security and on-farm profit
ability. Glyphosate is the world’s
most commonly used herbicide
and has been extensively studied
for its effect on soil microbial com
munities than any other herbicide.
In this second part of our series
we will focus on some of the inter
national findings surrounding the
effects of glyphosate and oth
er herbicides observed on the
beneficial fungal and bacterial
communities. Due to the sheer
magnitude of research findings
available, we will only highlight
some of the findings published
internationally and the various
views surrounding this topic.
Why are fungi and
bacteria important?
The simple answer to this ques
tion would be that fungi and
bacteria are the decompos
ers in the biological world.
They digest the decaying leaves and rotting bodies of
plants and animals, and in so doing return their elements to
the soil. Without them, the earth would basically be littered with
dead organic material, and all life would cease to exist as plants
will not have the required nutrients to grow, which have a knock-on
effect on every living thing.
However, fungi and bacteria degrade plant residues differently
and have different roles in the recycling of nutrients, which is part
ly due to the different habitats they prefer within the soil and the
different types of organic matter they consume. Bacteria within the
rhizosphere (the region of soil in the vicinity of plant roots) are in
volved in biogeochemical processes within the soil such as carbon,
nitrogen, and phosphorus cycling.
Fungi and bacteria differ in their responses to changes in agricul
tural management practices with fungi usually being more sensi
tive to these changes. The fungal-to-bacterial ratio is therefore a
good indicator of environmental changes in the soil. Based on the
function of fungi and bacteria in soil, a reduction of their numbers
may affect soil respiratory activity, soil enzymes and microbial
diversity, as well as rates of carbon and nitrogen turnover.
Herbicides and its effect on fungi and
bacteria in general
As already indicated in Part 1 of our series (
SA Graan/Grain
Sep
tember-issue), there is an overwhelming number of contradictory
findings concerning the impact of herbicides on fungi and bacteria.
The effects of paraquat and gly
fosinate on soil bacteria and fungi
are either positive or negative ef
fects
1 - 4
, or no significant effect
at all
5
.
A similar trend is observed with in
ternational research findings per
taining to glyphosate, with some
studies reporting a negative effect
on fungal growth
6
and reduced
colonisation by beneficial bac
teria
7
, or negligible or minor effects
on microbial community structure
when glyphosate was applied at
recommended field-rate (stated as
50 mg/kg)
8 - 12
.
Other studies state that despite
an initial negative effect directly af
ter application, the rhizobacterial
community managed to recover by
the time the plants reached their fi
nal growth stage
13
.
Researchers speculate that the
reason for inconsistencies in findings can be anything from mi
crobes reacting differently to different chemicals
11
, to differences
in soil characteristics
14
or to different formulations being used
15
, to
name but a few. Some studies, however, refute certain aspects of
such speculations. A research study, published during 2012
13
con
cluded that although the composition of the bacterial communities
differed between the soil types included in their study, the effects
of the herbicides tested remained very similar with regards to the
same bacterial groups.
Comparative studies have, however, indicated that not all herbici
des are equally damaging. An acetochlor and terbuthylazine
product evaluated, demonstrated to be in general much more
aggressive than the glyphosate-based product tested in various
studies
13,16
.
What about herbicides and
biocontrol agents?
Several Pseudomonas species have been employed as effective
biocontrol agents due to their production of a range of antibiotic
compounds, iron-scavenging molecules and plant growth promo
Biodiversity / Ecosystem services
On farm level
Integrated pest control
The effects of herbicides on soil life
Part 2: Beneficial fungi and bacteria
Dr Maryke Craven, Charné Myburgh, Owen Rhode
and
Dr Jeanetta Saayman-du Toit,
ARC-Grain Crops, Potchefstroom
“
Fungi and bacteria
differ in their
responses to changes
in agricultural
management practices
with fungi usually being
more sensitive to
these changes.
“