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September 2018
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Focus
Integrated pest control
The effects of herbicides on soil life
Grain SA/Sasol photo competition
– Amanda van Blerk 2012
This study further showed that the effects that glyphosate applica
tion had on microbial biomass were, similar to that on respiration,
dependent on various variables such as pH, glyphosate concentra
tion, organic carbon content and time after application
12
.
Soil microbial biomass was more likely to increase with short-term
exposure (<100 d), low soil pH (<5,5) and high glyphosate concen
tration (>200 mg/kg), whereas the relative microbial biomass was
more likely to decrease in the longer term (>100 d), at neutral pH and
at lower glyphosate concentrations.
Soil organic matter had a positive influence on the soil microbial bio
mass response to glyphosate exposure. The application of glypho
sate to high organic carbon soils was more likely to favour higher
soil microbial compounds. Mid-range concentrations of glyphosate
(10 mg/kg - 100 mg/kg) were associated with significantly lower soil
microbial biomass. No effect was again observed at <10 mg/kg.
The Nguyen research group concluded that at field applied rates of
glyphosate (<10 mg/kg), there was no significant effect on soil mi
crobial respiration and soil microbial biomass, but that soil micro
bial biomass was significantly lower at glyphosate concentrations of
10 mg/kg to 100 mg/kg.
Their study ultimately concluded that management and environmen
tal factors regulated the soil microbial response. Research that re
ports on either the toxicity or safety of glyphosate to soil microbial
respiration and soil microbial biomass should, therefore, stipulate
details of the conditions under which glyphosate were tested and
applied. Before drawing conclusions in this regard, a reader should
evaluate the approach followed to obtain the results reported in an
article as well as the conditions under which the findings were made.
What is the effect of repeated, long term glyphosate use? A study
published by a USA based research group investigated the effects
of glyphosate on soil microbial communities maintained in soil from
pine plantations. Glyphosate was repeatedly applied for nine to
13 years in these plantations in order to control vegetation
13
.
This study concluded that repeated application of glyphosate had
minimal effect on seasonal microbial characteristics despite sub
stantial changes in vegetation composition and growth. This situa
tion does, however, not necessarily reflect what is happening under
standard agricultural practices and additional research is required
to evaluate the effect of long-term use of a specific product such as
glyphosate on microbial life.
In Part 2 of this series we will focus on research findings published
on the impact of glyphosate and other herbicides on fungi and
bacteria.
Readers are welcome to contact the authors at ARC-
Grain Crops with any enquires they might have at
018 299 6100.
References
1
Atwood, D and Paisley-Jones, C. 2017.
Pesticides industry sales and usage 2008
- 2012 market estimates
. US Environmental Protection Agency Washington, DC.
2
Benbrook, C. 2016.
Trends in glyphosate herbicide use in the United States
and globally
. Environmental Sciences 28:3
https://doi.org/10.1186/s12302-016-
0070-0.
3
Sanders, P, Wardle, D and Rahman, A. 1996.
Persistence of bromacil in soils with
different management histories
. Proc. 49th N.Z. Plant Protection Conference
1996:207 - 211.
4
Zhang, C, Xu, J, Liu, X, Dong, F, Kong, Z, Sheng Y and Zheng, Y. 2010.
Impact of
imazethapyr on the microbial community structure in agricultural soils
. Chemos
phere 81:800 - 806.
5
Vischetti, C, Casucci, C and Perucci, P. 2002.
Relationship between changes of
soil microbial biomass content and imazamox and benfluralin degradation
. Biol
ogy and Fertility of Soils 35:13 - 17.
6
Wardle, DA and Parkinson, D. 1990a.
Effects of three herbicides on soil microbial
biomass and activity
. Plant and Soil 122:21 - 28.
7
Wardle, DA and Parkinson, D. 1990b.
Influence of the herbicide glyphosate on soil
microbial community structure
. Plant and Soil 122:29 - 37.
8
Andrea, MMD, Peres, TB and Luchini, LC. 2003.
Influence of repeated applica-
tions of glyphosate on its persistence and soil bioactivity
. Pesquisa Agropecuaria
Brasileira 38:1 329 - 1 335.
9
Lancaster, SH, Hollister, EB, Senseman, SA and Gentry, TJ. 2009.
Effects of re-
peated glyphosate applications on soil microbial community composition and the
mineralization of glyphosate
. Pest Management Science 59 - 64.
10
Zabaloy, MC, Gomez, E, Garland, JL and Gomez, MA. 2012.
Assessment of micro-
bial community function and structure in soil microcosms exposed to glyphosate
.
Applied Soil Ecology 61:333 - 339.
11
Rosenbaum, KK, Miller, GL, Kremer, RJ and Bradley, KW. 2014.
Interactions
between glyphosate, fusarium infection of common waterhemp (
Amaranthus
rudis
), and soil microbial abundance and diversity in soil collections from Mis-
souri
. Weed Science 62:71 - 82.
12
Nguyen, DB, Rose, MT, Rose, TJ, Morris, SG and Van Zwieten, L. 2016.
Impact of
glyphosate on soil microbial biomass and respiration: A meta-analysis
. Soil Biol
ogy and Biochemistry 93:50 - 57.
13
Busse, MD, Ratcliff, AW, Shestak, CJ and Power, RF. 2001.
Glyphosate toxicity
and the effect of long-term vegetation control on soul microbial communities
.
Soil Biology and Biochemistry 33:1 777 - 1 789.