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JPPIPA 7(Special Issue) (2021)
Jurnal Penelitian Pendidikan IPA
Journal of Research in Science Education
http://jppipa.unram.ac.id/index.php/jppipa/index
___________
*Email: suwardji@unram.ac.id
Copyright © 2021, Author et al.
This open access article is distributed under a (CC-BY License)
The Fate of Glyphosate in Soil and Water: A Review
Suwardji1*, I Made Sudantha2
1 Department of Soil Science Faculty of Agriculture, University of Mataram, Jalan Pendidikan No 37 Mataram Lombok, Indonesia.
2 Department of Agronomy Faculty of Agriculture, University of Mataram, Jalan Pendidikan No 37 Mataram Lombok, Indonesia.
DOI: 10.29303/jppipa.v7iSpecialIssue.971
Article Info
Received: September 19th, 2021
Revised: December 20th, 2021
Accepted: December 31st, 2021
Abstract: The fate of glyphosate in soil and water is dependent on the properties of
glyphosate and its envoronement. Behaviour of glyphosate in soil, sediment and water is
strongly influenced the way by which it can be adsorbed by soils, sediments, and
suspended material in water. The role of soil organic matter, clay mineral, and amorphous
minerals on the adsorption of glyphosate depends primarily on the nature and properties
of the soil itself and the properties of glyphosate. Environmental factors have some
influence on sorption and degradation of glyphosate. Glyphosate is rapidly inactivated in
soil, is in part due to adsorption. Some soil properties have been identified strongly
influence adsorption of glyphosate, such as clay minerals, composition of cations in
exchangeable site of clay and organic matter, unoccupied phosphate adsorption site, degree
of humification, and soil pH. Adsorption limits the availability of glyposate for microbial
degradation. The sorbed glyphosate is not directly available to microorganisms in soil.
Evidence also suggests that not only a strongly sorbed compound such as paraquat but also
weakly sorbed compounds such as flumetsulam and picloram can persist for long periods
when they are sorbed by soil constituents. This suggests that the interaction between
sorption and biodegradation should be considered in predicting the fate of pesticides in
soils and sediments.
Keywords: The Fate of glyphosate; Glyphosate properties; Adsorption and degradation
behaviour of glyphosate; Sorbed and soluble
Citation:
Suwardji, S., & Sudantha, I M. (2021). The Fate of Glyphosate in Soil and Water: A Review. Jurnal Penelitian
Pendidikan IPA, 7(SpecialIssue), 389–399. https://doi.org/10.29303/jppipa.v7iSpecialIssue.971
Introduction
In order to be able to effectively control the
environmental risk resulting from the application of
pesticides, it is important to understand how pesticides
behave in soils and water. Soil is the ultimate sink for
many pesticides used, while at the same time, soil may
become a source from which pesticide residues can
move into living organisms, ground and surface waters
and the atmosphere (Gerstl and Mingelgrin 1984; Rao
and Hornsby, 2001).
Pesticides can reach soils either by direct
application or indirectly, for example, from inaccurate
spraying technique, runoff, rain and dust. Some
pesticides can also be absorbed by plant through the
leaves and may remain unchanged and become part of
soil organic matter with the death of leaves and
subsequent decay of the plant (Burns 1975; Rampazzo,
2009).
When pesticides reach the soils, they may
undergo one or more of several processes. These
include evaporation, photochemical degradation,
leaching, runoff, plant removal, adsorption, chemical
and microbial degradation (Rampazzo, 2009; Kanissery,
et al, 2019; Sarkar, et al, 2020). Adsorption and
degradation are two of the most important processes
influencing the residue behaviour of most pesticides in
soils and sediments (Cox, et al. 1993; Hermosin &
Carnejo 1990; Rao and Hornsby 2001). These processes
are not only control the mobility and potential for
Jurnal Penelitian Pendidikan IPA (JPPIPA)
December 2021, Volume 7, Special Issue, 389-399
390
leaching of the pesticide away from the site of
application but also strongly influence the uptake of
pesticide by plant root and soil fauna (Gerstl &
Mingelgrin, 1984; Simonsen, et al, 2008)
Adsorption and degradation of pesticides and
other organic compounds in soils and sediments have
been extensively reviewed in the literature (Calvet,
1980; Hance, 1988; Hassett&Banwart 1989; Koskinen&
Harper 1990; Bollag & Liu, 1990; Cork & Krueger, 1991;
Arunakumara et al. 2013; Sviridov, et al, 2015; Sadegh-
Zadeh, et al, 2017; Kanissery, et al, 2019). This review
will discuss the basic concept of adsorption in relation
to the sorptive properties of soil, will consider factors
which influence adsorption and degradation of
pesticides in soils, how sorption influences degradation
of pesticides, kinetics of biodegradation of sorbed
chemicals, and the behaviour and metabolism of
glyphosate in soil and sediments.
Glyphosate is pre-emergence herbicide and
widely used in agriculture to control wide range of
weeds (Kremer and Means, 2019). Considerable
information is available on the chemistry of glyphosate,
its mode of action, and the effect of this compound on
the plant metabolism (Piccolo & Celano, 1994; Mallik, et
al. 1989; Sadegh-Zadeh, et al, 2017). However, very
little information is available on the ehavior of this
compound in a wide range of soils (Torstensson, 1985;
Ronaldo, et al, 2017)). The rate of decomposition of this
compound in soil has been shown to be strongly
dependent on the sorption characteristic of soil
(Eberbach 1998; Wang, et al, 2016; Zhang, et al, 2015),
yet a simultaneous process of sorption-desorption, and
the influence sorption on the rate of decomposition in
soils is not well understood. Moreover, little is known
regarding the influence of environmental factors
particularly temperature on the sorption and
decomposition of this compound.
A major gap in our understanding of the fate of
pesticide in soil and water is inability to predict the
influence of sorption on biodegradation. This is due to
a lack of a method, which is capable of quantifying the
various strengths of adsorption that exist where
multiple sorption mechanisms exist. A method should
look at how strength of adsorption influences the rate
of herbicide degradation “in situ”. The use of a Non
Steady State Compartmental Analysis (NSSCA)
(Winkler, 1971) using glyphosate degradation data has
been shown to discriminate between the soluble and
sorbed glyphosate “in situ” (Eberbach, 1998). This
technique shows its applicability for explaining the
dependence of herbicide degradation on strength of
adsorption, but as yet has not been used extensively in
herbicide research (Suwardji, 1998).
This review paper is a part of three consecutive
reseach papers will be published in this Journal of
Research in Science Education.
Method
Some scientific publications used in this review
are from books and articles from scientific papers
published in international journals related to the
sorption behavior of glyphosate in soil and water and
its relationship to the process of glyshoste
decomposition behavior. Data from various sources of
information are then analyzed descriptively to discuss
The Fate of Glyphosate in Soil and Water containing:
What is the glyphosate; Interaction beween glyphosate
and soil: (1) The role of clay minerals on adsorption, (2)
The role of organic material on adsorpsion, (3) The role
of pH in adsorption, (4) Movement of glyphosate in soil
and water; Degradation of glyphosate in soil and water;
Conclussion.
Result and Discussion
What Is the Glyphosate
Glyphosate, the active ingredient of the herbicide
Roundup and others is widely used to control a wide
range of perennial and annual weeds. The chemical
structure of this compound is presented in the Figure 1.
Glyphosate has been reported to be rapidly inactivated
when in contact with mineral and organic soils
(Sprankle, et al, 1975b; Suwardji, 1998), and this rapid
inactivation of glyphosate in soil has been suggested to
be a result of rapid adsorption to soil constituents.
Figure 1. Chemical stucture of glyphosate
In a study of behaviour of glyphosate residue in
soil using bioassay plants, Sprankle, et al. (1975a) found
that the application of 56 kg ha-1glyphosate on clay
loam and mucksoil did not significantly reduce the dry
weight of wheat. The dose used in this experiment was
25 times of the proposed dosage normally used to
control weeds. Similar results have been reported by
other authors (Moshier, et al. 1976; Moshier & Penner,
1978a; Klingman & Murray, 1976; Egley & William,
1978; Blowes, et al. 1985). As the roots of plant have
been reported to be unable to absorb glyphosate
(Hance, 1976), low activity of glyphosate in soil in these
studies may be due to the combination of moderate
adsorption and low acropetal uptake of this compound
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December 2021, Volume 7, Special Issue, 389-399
391
when applied to the root (Hance, 1976).
Due its strong adsorption of this compound
when in contact with the soil constituents and rapid
degradation in soil and water, it is generally believed
that this compound is considered to be an
environmentally safe.
Interaction between Glyphosate and Soil
How glyphosate binds to the soil particles is not
well understood. Using bioassay plants, Sprankle et al.
(1975a) found that a correlation existed between plant
injury and the increase in the amount of inorganic
phosphate applied to soils treated with glyphosate.
This result suggested that glyphosate and inorganic
phosphate may share the same adsorption site on soil,
and that phosphate may have stronger affinity to
compete with glyphosate for the adsorption sites.
Consistent with this result, Hance (1976) showed that
glyphosate adsorption has some correlation with the
amount of unoccupied phosphate adsorption site in a
wide range of soils, r2 = 0.72, n=9. This suggested that
glyphosate is likely to be adsorbed by soil constituents
in a similar mechanism to that of orthophosphate but
that orthophosphate is adsorbed preferentially.
Current work has been performed to further
understand of the mechanism of sorption of glyphosate
in soil. In an attempt to understand the mechanism
binding of glyphosate to soil constituents, the
interaction between glyphosate and humic acids and
metal-humic complexes have been intensively
investigated (Miano, et al., 1992; Piccolo, et al., 1992;
Piccolo & Celano, 1994; Piccolo, et al. 1994; Piccolo, et
al. 1995a; Suwardji, 1998). Piccolo, et al., (1994) showed
that adsorption of glyphosate in some European soils
increased with an increase in the amount of amorphous
iron (oxalate extractable). This finding confirmed the
postulated ligand exchangemechanism as proposed by
Sprankle, et al., (1975a) and Torstensson, (1985) for
glyphosate, in which a hydroxyl of the iron hydration
sphere is exchanged by the P-OH group of the
phosphonic moiety of the glyphosate. Moreover, the
adsorption isotherm study of glyphosate on Fe-humic
complex showed the S-type of the Giles (1960)
classification. The S-type adsorption isotherm explains
the relationship of the attraction between the
glyphosate molecules in solution and those already
adsorbed on the substrate and thereby producing an
enhanced affinity at higher concentration (Piccolo, et al.
1992). This result suggests that there are two adsorption
mechanisms operating simultaneously. At low
glyphosate concentration, adsorption occurs through
the exchange of an hydroxyl associated with the iron
hydration sphere by a P-O- group of the phosphonic
moiety of herbicide (Piccolo et al. 1992; Piccolo, et al.
1995b). This type of interaction is considered to be very
strong binding and occurs during the phosphate
fixation on iron hydrous oxides in soil (Parfitt et al.
1975). At high concentrations of herbicide, glyphosate
to be adsorbed on already adsorbedmolecules by
intermolecular hydrogen bonding that may occur
between the electronegative atoms of the herbicide
(Piccolo, et al., 1992; Piccolo, et al., 1995a). Further
investigations of the interactions between glyphosate
and pure humic acid demonstrated that glyphosate
could establish multiple hydrogen bonding (Miano, et
al., 1992). They showed that the interaction of
glyphosate and humic acids (HA) is through the
formation of multiple hydrogen bond as evidenced by
infrared spectra with strong two absorption bands at
1170 and 1090 Cm-1 for the stretching of the P=O and P-
O bonds of the phosphono groups of glyphosate. Picolo
and Celano, (1994) suggested that the multiple
hydrogen bonding may occur between phosphonic
group of glyphosate and complementary oxygen-
containing functional groups of the humic acid such as
ketones and qui-ketones (Piccolo & Celano, 1994).
Results from fluorescence spectra of the HA-glyphosate
sample confirmed the occurrence of an increasing
desegregation of humic molecules, possibly ascribed to
the formation multiple hydrogen bonds by glyphosate
(Miano, et al., 1992).
1. Role of Clay Minerals In Adsorption
Results of many studies of the role of clay
minerals on sorption of glyphosate are ambiguous in
the literature. The type of clay mineral is considered to
have some influence on adsorption of glyphosate. For
example, Miles and Moye, (1988) reported that
glyphosate was much more extensively adsorbed by
kaolinite and bentonite than by montmorillonite. By
contrast, Glass (1987) showed that more glyphosate was
adsorbed by illite than by kaolinite or montmorillonite.
Inconsistent results in the literature of the effect of type
of clay minerals on the adsorption of glyphosate may
partly be due to the different concentration ranges of
solutions used for adsorption study. For example the
concentration of glyphosate solution used for
adsorption study by Glass (1987) is almost 100 times
greater than that used by Miles and Moye, (1988).
Further studies showed that the composition of
cations on cation exchange sites of soil solids has a
strong influence on adsorption of glyphosate. Sprankle
et al., (1975b) investigated the influence of cation
composition on bentonite clay using a wheat bioassay.
They saturated bentonite clay with particular cations
and treated with glyphosate at either 0 or 4.8 L ha-1 of
Roundup (360a.i. g-1 ha-1). Poor wheat growth was
observed on the bentonite clay saturated with Na+,
Ca2+, Mg2+, suggesting lower adsorption of glyphosate
in clays saturated with these cations. As glyphosate is
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December 2021, Volume 7, Special Issue, 389-399
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unlikely to be absorbed through plant roots (Hance
1976), poor wheat growth in the clay saturatedwith
Na+, Ca2+, Mg2+ may be due to inhibition of wheat roots
by the soluble glyphosate and subsequent lower uptake
of nutrients and water. In contrast, greater plant growth
was observed under bentonite clay saturated with the
Zn2+, suggesting greater adsorption of glyphosate. No
differences in the fresh weight of the shoot of wheat
plant was noted on clay saturated with Mn2+ and Al3+
and then treated with glyphosate (Sprankle et al.
1975b). Results from an adsorption studyusing 14C-
glyphosate confirmed the bioassay study (Sprankle et
al. 1975b). In this study, glyphosate adsorption on
bentonite clays saturated with cations decreased as:
Al3+> Fe3+> Mg2+> Zn2+> Mg2+>Zn2+> Mn2+>
Ca2+(Sprankle et al. 1975b) The strong adsorption of
glyphosate to Al3+ and Fe3+ further supported the
concept that phosphate might be implicated in
adsorption (Torstensson, 1985).
It is possible that bentonite saturated with Fe and
Al exchanges sufficient amounts of the Fe and Al into
the solution to enable these two metals to bind
glyphosate by forming a chelate. A chelation of
glyphosate has been postulated by McBride (1994). He
pointed out that glyphosate is capable of forming a
terdenate (three bonds) or tetradenate (four bonds)
chelate with several of the coordination position on the
surface metal ions being occupied by ligand group
(Figure 2).
Figure 2.Chelate formation between lyphosate and iron
(McBridge 1994)
Like clays, cations in association with the surface
of organic matter have a strong influence on the extent
of glyphosate adsorption. When glyphosate was
applied in the organic mattersaturated with Ca2+ or
Na+, a significant reduction of fresh weight of wheat
plants was apparent (Sprankle, et al., 1975b) indicating
of little adsorption of glyphosate. However, the organic
matter saturated with Mn2+, Fe3+ or Al3+ did not
significantly reduce the fresh weight of wheat,
indicating greater adsorption of glyphosate. Moreover,
Hensley, et al., (1978) reported that the increased
percentage of soil organic matter and the addition of
Fe3+ and Al3+ reduced the soil activity of glyphosate,
but inactivation of glyphosate did not well correlate
with the amount of soil organic matter and cation
exchange capacity. This indicates that saturation of
cations on the organic matter was much more
important than both the amount of soil organic mater
and the amount of cation exchange sites available. They
suggested that adsorption of glyphosate onto organic
matter might be enhanced by the formation of Fe3+ and
Al3+ complexes with glyphosate. They also noted that
there was no significant inactivation of glyphosate with
the addition of CaCl2, NaCl, and KCl butthe addition of
FeCl2, FeCl3, and AlCl3 significantly reduced the
activity of glyphosate. Further, a precipitation test
indicated that a red-brown precipitate was formed
when glyphosate and FeCl3were allowed to stand in
solution. They suggested that the adsorption of
glyphosate may partly be due to the formation of metal-
glyphosate complexes, perhaps similar to those
postulated by McBride (1994).
2. The role of organic material on adsorption
The adsorption of glyphosate on soil organic
matter has been shown to be related to the degree of
humification of organic matter. The application of
glyphosate to a muck (organic) soil did not injure
Barnyard grass (Echinochloa crugalli (L) Beauv) and
Italian grass. Sprankle, et al., (1975b) demonstrated that
glyphosate was rapidly inactivated in muck soil.
However, when glyphosate was applied into
sphagnum peat (non-decomposed plant materials),
injury of Barnyardgrass (Echinochloa crugalli (L)
Beauv) and Italian ryegrass (Lilium multiflorium Lam)
was observed, indicating that non-decomposed organic
matter did not render glyphosate unavailable. As
previously mentioned the plant root system is unlikely
to be able to absorb glyphosate, hence the injury of
Barnyardgrass or Italian ryegrass could be due to
substantial amount of glyphosate residue in soil
solution which may inhibit the root system of the
seedling plants in the early establishment. These results
suggest that the fresh plant materials do not render
glyphosate biologically unavailable.
3. The role of pH in Adsorption
The pH of soil systems influences the adsorption
of glyphosate. Glyphosate is zwitterionic, therefore the
charge structure of the molecule is dependent on the
pH of the systems (Sprankle, et al., 1975b) as shown in
the Figure 2. 12. Some studies showed that soil pH had
little effect on adsorption of glyphosate (Sprankle, et al.
1975b; Hance, 1976). However, McConnell and Hossner
(1985) showed a strong negative correlation between
pH and adsorption of glyphosate by montmorillonite
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December 2021, Volume 7, Special Issue, 389-399
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and nontronite minerals. At lower pH, there was a
greater attraction between glyphosate and these clay
minerals. This may have been due to an increase in
positive charge on the end of layer clays at low pH. As
the soil pH increases (> 4.5), glyphosate becomes an
activated anion, resulting in the electrostatic repulsion
between the negatively charged clay mineral and
glyphosate (McConnell & Hossner, 1985). Similar
observations were observed for the adsorption of
glyphosate on kaolinite, hematite and goethite
(McConnell & Hossner, 1985). In this study, the
maximum adsorption of glyphosate by kaolinite was
achieved at pH 4.5, the adsorption decreases as pH
increases or decreases to the value of 4.5. This was
probably due to the zero point of net charge (ZPNC) of
this clay mineral being 3.7. At pH below 3.7 the end
surface of kaolinite was net positively charged while at
above pH 3.7 was neutral to negatively charged
(McConnell & Hossner, 1985).
Figure 3. Proposed dissociation diagram and ionisation
constant for glyphosate (Sprankle et al. 1975).
Both glyphosate and kaolinite lose their
exchangeable proton as pH is raised. As a result both
glyphosate and kaolinite become anionic, a decrease in
the strength of adsorption occurs due to increasing
repulsion between glyphosate and clay system. Similar
patterns were observed for hematite and goethite. This
evidence indicates that the pH of soil systems might
influence the adsorption of glyphosate by two ways: (i)
determine the charge structure of glyphosate, and (ii)
influence the surface charge of pH dependent clays
(McConnell & Hossner, 1985).
More recently, Nicholls and Evans, (1991)
showed that the maximum sorption of phosphate was
similar in strength to that of glyphosate (Figure 4) and
occurred at similar values of pH. However when pH
increased from pH 8 to 10, adsorption of glyphosate did
not decrease as much as that of phosphate. At pH
values above 8.5, the phosphate started to increase in
such a way notobserved for glyphosate. While there
may be some similarities in the binding mechanism
between glyphosate and phosphate, the current
evidence suggests that this mechanism is not the only
binding mechanism operable and that other binding
mechanisms of glyphosate are likely to exist.
Figure 4. The influence of pH on the sorption of glyphosate
(a) and inorganic phosphate (b). The R and W are silty clay
and sandy loam soils respectively. Closed symbols are for soil
where pH was adjusted by adding HCl or Ca (OH) solution
(Nicholls & Evans 1991).
4. Movement of Glyphosate In Soil And Water
Glyphosate is considered to be fairly immobile in
soil. Using soil thin-layer plates, Sprankle, et al. (1975b)
found that mobility of glyphosate is very slow with RF
values from 0.04-0.2. The mobility of glyphosate in the
thin soil layer increased with increasing soil pH and
level of phosphate in soil. The addition of phosphate is
likely to compete with glyphosate for some adsorption
sites, and leave more glyphosate in the soil solution.
However runoff studies confirmed that glyphosate is
relatively immobile and leaching is unlikely to occur in
the field (Edwards, etal., 1980).
A recent study suggests that glyphosate may
leach to deep layers in the soil profile. Piccolo and
Cellano, (1994) investigated the binding interaction
between glyphosate and water-soluble humic
substances, and found that these substances were
capable of rendering glyphosate becomes “biologically
inactive” thatwater soluble. This humic substances
finding may influence suggest the partitioning
glyphosate at the soil-water interface and sufficient
amount of glyphosate may bedistributed on the surface
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December 2021, Volume 7, Special Issue, 389-399
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of soluble organic matter. And as the water-soluble
humic substances are regarded as an important carrier
of organic contaminants (Spark 1998), the adsorption of
glyphosate on the surface of water-soluble humic
substance may serve as a mechanism for enhanced
mobility (Piccolo & Cellano, 1994). The significance of
dissolved humic acids inenhancingthe mobility of
glyphosate in agriculture. However, as the amount of
the dissolved humic substances in most soils is
considered to be very low, it is likely to have only a
minor role in affecting the movement of glyphosate in
soil. However in agricultural practices in which sewage
sludge is applied, leaching of glyphosate may be more
prominent.
Degradation of Glyphosate in Soil and Water
Rapid biodegradation has been suggested to be
the main route of glyphosate disappearance in soil and
sediments (Bronstad & Friestad, 1985; Sprankle, et al.,
1975b; Torstensson, 1985; Rueppel, et al., 1977). Other
processes such as chemical and photo decomposition
have been shown to be oflittle importance regarding the
decay of glyphosate in soil (Nomura & Hilton, 1977;
Rueppel, et al., 1977). However, Bronstad and Friestad,
(1985) pointed out that photolysis may be an important
process in influencing the disappearance of glyphosate
in the aquatic system.
Results of many studies have shown that while
soil microflora are responsible for decomposition of
glyphosate in soil (Al Rajab et al, 2019; Sprankle, et al.
1975b; Torstensson & Aamisepp, 1977; Rueppel, et al.
1977, & Torstensson, 1985), the dominant process for
decomposition is co-metabolism. Hence
microorganisms do not use this compound as a carbon
(C) or energy source (Sprankle, et al., 1975b;
Torstensson & Aamisepp, 1977; Nomura & Hilton,
1977), but that degradation is likely to be due to soil
enzymatic activity and no adaptation to growth on the
herbicide or energy gain by organism has been
observed (Torstensson & Aamisepp, 1977; Torstensson,
1985). To date no soil microorganisms have been
isolated from the field soil, which utilize glyphosate as
carbon source (Carlisle & Trevors, 1988). However,
some Pseudomonas spp. isolated from activated sludge
have been reported to be able to utilise glyphosate as a
P source (Moorman, 1993).
Robertson and Alexander (1994) investigated the
relationship between the occurrence of accelerated
pesticide biodegradation and the susceptibility of the
pesticide to growth-linked degradation and
cometabolism. They found a slight increase in
glyphosate metabolism in the second application but
was not significantly different (P < 0.05). The initial
amount of the population of glyphosate metabolising
micro-organisms of 3.9 x 10-4 mL-1 rose shortly after the
first addition of glyphosate, but the second addition of
glyphosate was not accompanied by a marked increase
in cell number. As there is no significant increase in the
rate of metabolism and the number of cells observed in
the second addition, it is evident that microbes are
unlikely to be able to utilise glyphosate as an energy
source.
Many decomposition studies have used 14C-
glyphosate to evaluate the rate of decomposition of
glyphosate in soil (Torstensson, 1985). This method is
attractive because measuring the evolution of 14CO2 is
simple. However, the major disadvantage of this
method is that the primary substance (glyphosate) is
not analysed for (Torstensson, 1985). The use of the
14CO2-evolution rate as a measure of the rate of
glyphosate degradation is only correct if 14CO2 is
released as 14C-glyphosate degrades (Torstensson 1985).
Eberbach (1998) showed that the loss of triethylamine
extractants glyphosate (250C) occurred at the same rate,
as did the evolution of 14CO2. Other work by Eberbach
1998 also indicated that AMPA was only a transitory
immediate of glyphosate metabolism. These findings
indicate that the rate of 14CO2 evolution closely reflects
the rate of 14C-glyphosate decomposition in soil. The
degradation of glyphosate has been reported to be very
rapid in the first few days slowing down with time to
the steady state rate of decomposition (Eberbach, 1998;
Nomura & Hilton, 1977; Torstensson, 1985). Rapid
decomposition of glyphosate in the first few days has
been thought to be due to rapid metabolism of soluble
(un-bound) glyphosate. While the slow degradation in
the later stage was ascribed to the decomposition of
glyphosate from the most slowly available (the bound
fraction) (Eberbach, 1998; Nomura & Hilton, 1978;
Torstensson, 1985).
A number of factors affect the degradation of
glyphosate in soil. The addition of phosphate enhanced
the degradation of glyphosate in some, but not all soils
(Sprankle, et al., 1975b; Hensley, et al., 1978). The
reason for this is not clear but this may in part be due to
differences in the extent ofun-occupied phosphate
adsorption sites as, it has been found to correlate highly
with the adsorptionof glyphosate (Hance, 1976). The
addition of cations such as Fe and Al inhibited
glyphosate degradation (Moshier & Penner 1978b). This
is particularly due to a change in the adsorption site,
and hence the availability of glyphosate in soil. More
recently, Picollo, et al. (1995a) found that the complexes
Fe-humic substances were capable of adsorbing
glyphosate in soil and may reduce its availability for
decomposition. Soil pH has also been reported to have
little effect on the degradation of glyphosate (Moshier
& Penner, 1978b).
The half-life of glyphosate in soil is quite
variable, ranging from few days to several years
Jurnal Penelitian Pendidikan IPA (JPPIPA)
December 2021, Volume 7, Special Issue, 389-399
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(Eberbach 1998; Nomura & Hilton 1977; Torstensson
1985). In a study using four Victorian soils, Eberbach
(1998) showed that the half-life of glyphosate in the
soluble phase was very short; in the order of one week.
However the estimate half-life of glyphosate in the
sorbed phase varied considerably; in the 4 soils
investigated half-lives ranged from 7 months to 6 years.
Eberbach (1998) suggested that this be attributed to the
influence of sorption-desorption characteristic of soil on
the availability of this compound for microbial
cometabolism. A considerable difference in the rate of
glyphosate decomposition between soils reported in the
literature may be due to differences in the strength of
adsorption that render glyphosate un-available for
decomposition.
Conclussion
The behaviour of pesticide particularly
glyphosate in soil and sediment have been discussed.
The nature and properties of a pesticide strongly
influence the way by which it can be adsorbed by soils
and sediments. The role of soil organic matter, clay
mineral, and amorphous minerals on the adsorption of
pesticide depends primarily on the nature and
properties of the soil itself and the properties of
pesticide. Environmental factors have some influence
on sorption and degradation of pesticide.
Adsorption limits the availability of pesticide for
microbial degradation. The sorbed pesticide is not
directly available to microorganisms in soil. Evidence
suggests that not only a strongly sorbed compound
such as paraquat but also weakly sorbed compounds
such as flumetsulam and picloram can persist for long
periods when they are sorbed by soil constituents. This
suggests that the interaction between sorption and
biodegradation should be considered in predicting the
fate of pesticides in soils and sediments.
Glyphosate is rapidly inactivated in soil, is in
part due to adsorption. However adsorption of
glyphosate may not occur in some sandy soils with low
adsorption capacity. Some soil properties have been
identified that strongly influence adsorption of
glyphosate, such clay minerals, composition of cations
in exchangeable site of clay and organic matter,
unoccupied phosphate adsorption site, degree of
humification, and pH. Even though there is a similarity
in the mechanism of binding between glyphosate and
phosphate to soil solids, current evidence suggests that
this mechanism is not the only binding mechanism
operable, and that other binding mechanisms of
glyphosate are likely to exist. The influence of
phosphate on adsorption and decomposition of
glyphosate in soil is not fully understood. The addition
of phosphate increased the rate of glyphosate
degradation in some soils but did not have such an
effect in all soils. As phosphate is the common fertilizer
applied in agriculture practice and it is applied in high
amount in horticulture system, it is important to gain
understanding the competition effect of phosphate on
the adsorption behaviour of glyphosate that may occur
in soil. Understanding the basic mechanism of sorption
and desorption is required to avoid situation which
may trigger the release of bond residues of glyphosate
in field situations. In addition, information is lacking in
the literature on the influence of environmental factors
particularly temperature, concentration and repeated
applications on adsorption and decomposition of
glyphosate in soil.
Degradation of glyphosate is mainly through
biodegradation and the dominant process is
cometabolism. Rapid initial degradation followed by
slower rate of degradation in the further stage suggests
that adsorption influences its availability for microbial
degradation. Little information is available on the
influence of adsorption characteristic of wide ranges of
soils on the rate of glyphosate degradation. The use of
non steady state compartmental analysis provides a
good tool to evaluate the dependence of glyphosate
degradation on sorption characteristic of soil (Suwardji,
2000), and this technique has shown its applicability to
evaluate the simultaneous processes of sorption-
desorption and degradation of this compound to allow
for a clear understanding of their interrelationship.
Repeated applications of glyphosate will become
common practice in the near future with development
of glyphosate tolerant crops. This may influence
partitioning of glyphosate into the soluble and sorbed
phases and sorption-desorption behaviour of this
compound in soil. To date, little, if any information is
available in the literature on the influence of repeated
applications of glyphosate on sorption dynamic and
decomposition of this compound in soil.
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