SA Graan September 2018

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What is microbial biomass and why does
it matter?
Microbial biomass is a measure of the total weight of the living com­
ponent (mostly bacteria and fungi) of soil organic matter. It plays a
fundamental role – especially in the flow of carbon and nitrogen from
newly deposited plants or other materials to the mineral forms of
carbon dioxide and ammonium or nitrate ions in the soil.
It serves as an early indicator of changes in total soil organic carbon
(C), because unlike organic biomass, it responds quickly to manage­
ment changes. It is estimated that approximately half the microbial
biomass is located in the top 10 cm of a soil profile in which most of
the nutrient release occurs.
Microbial biomass contributes up to 5% of the total organic carbon
and N in soil that is available to plants once the micro-organisms
die. The soil microbial biomass acts as the transformation agent of
organic matter in soil and is both a source and sink of the nutrients
C, N, P and S contained in the organic matter. It is the centre of the
majority of biological activity in soil.
Microbial respiration is a measure of carbon dioxide (CO2) released
by soil microbes from the soil trough the decomposition of soil or­
ganic matter. It is an important indicator of soil health because it
indicates the level of microbial activity, soil organic matter content
and its decomposition. It can account for 10% to 90% of the CO2
efflux from soils, which has a substantial effect on the atmospheric
CO2 concentration.
The rate of soil respiration is an indicator of the nutrients contained
in organic matter that is being converted into forms that is available
to crops (e.g., phosphate as PO4, nitrate-nitrogen as NO3, and sul­
phate as SO4). Literature states that in the short term, high soil respi­
ration rates are not always desirable because it may be an indication
of unstable systems and loss of soil organic matter due to excessive
tillage or to other factors degrading soil health.
Herbicides and microbial biomass
The application of herbicides can have both positive and negative
effects on different members of the microbial community. It can
be toxic to some microbes resulting in reduced microbial biomass.
Alternatively, herbicides can be a food source supporting microbial
growth. Micro-organisms play a central role in the degradation of
herbicides.
Examples of cases in which the application of herbicides had an ini­
tial negative impact are readily available. Bromacil reduced microbial
biomass significantly for up to eleven months after application3. It
is possible that this reduction in microbial biomass contributes to
the retarded bromacil degradation when this herbicide is repeatedly
applied.
During a laboratory study the soil microbial community structure
shifted after application of imazethapyr, but were able to recover
again after 60 days4. Compared to untreated soil, imazamox and
benfluralin resulted in a 25% and almost 65% decrease in microbial
biomass-C content, respectively. However, the microbial biomass-C
content did return to initial values but at varying times, which de­
pended on the incubation conditions5.
Numerous studies investigated the impact of glyphosate on soil mi­
crobiology. Wardle and Parkinson6,7 reported in two separate publi­
cations published in 1990 that there was a transitory increase in soil
microbial biomass and soil microbial respiration after glyphosate
application.
Two other separate research groups8,9 reported that there was a sig­
nificantly negative impact on microbial community structure and soil
microbial biomass. Some research studies reported no significant
effect at all10,11 . If one only read the first set of articles, the conclusion
drawn would not have been based on all of the research conducted
internationally. On the other hand, if all of these articles have been
read, the reader would have been left confused and unsatisfied be­
cause of the variable and contradictory results presented.
During 2016, a research group from Australia, however, investigated
the impact of glyphosate on soil microbial biomass and respiration
using meta-analysis. They published their findings in the journal Soil
biology and biochemistry12 . Based on their method used, they com­
piled a dataset from peer reviewed literature published up to 2015,
which dealt with studies in which glyphosate was applied to soil
after measuring soil microbial biomass or soil microbial respiration.
Only studies that had suitable controls and replicating statistics
were included. These studies had to adhere to specific inputs made,
similar outputs measured and sound scientific approaches. Con­
sequently, out of the total of 191 scientific articles published on
this topic, only 36 were selected for this paper. Based on the data
generated and meta-analysis conducted, they were able to con­
clude on how soil microbial respiration and soil microbial biomass
would likely react under various conditions in response to glypho­
sate application.
The authors converted kg glyphosate applied per ha to mg active
ingredient (a.i.)/kg soil (note: glyphosate concentration is in gen­
eral expressed as acid equivalent per litre), thereby establishing a
standardised form for application rates used. European regulatory
procedures where then used where the bulk density of dry soil is
assumed to be 1,5 g/cm 3 and the average depth of surface-applied
herbicide penetration into the soil, 50 mm.
Based on the formulations used by the Nguyen group12 , a glypho­
sate product with a 480 g a.i./l formulation applied at 2,2 l/ha is
the equivalent of 1,4 mg a.i./kg soil. During this study various fac­
tors and not single aspects determined the eventual response of
the microbial community observed.
The following observations were made regarding microbial respira­
tion:
Glyphosate concentration: Of all of the factors investigated,
glyphosate concentration applied had the strongest influence
on the soil microbial respiration response. At concentrations
of more than 10 mg/kg but less than 200 mg/kg glyphosate
showed a negative effect on respiration while at concentrations
greater than 200 mg/kg it generally stimulated respiration. Con­
centrations of less than 10 mg/kg had no effect on microbial
respiration.
Days after application: Over the short term (2 - 60 days after ini­
tial exposure), respiration was more likely to increase. However,
after 60 days and onward, respiration declined to levels below
that of control soils not receiving glyphosate.
Soil pH: Glyphosate generally stimulated respiration in soils with
pH <5,5, but in more neutral soils (pH 5,5 - 7,5) glyphosate had a
tendency to show a negative impact on respiration. Limited data
on alkaline soils with pH >7,5 prevented an assessment of the
impacts of glyphosate on soil microbial respiration when applied
on these soils.
Carbon content: Although soil organic carbon content played a
statistically significant role in moderating the impact of glypho­
sate, its effect was not particularly strong. Glyphosate was more
likely to stimulate respiration in soil low in organic carbon than in
soils with a higher organic carbon content.
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September 2018