Glyphosate displacement from New Zealand soils and its effect on non-target organisms : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural Science at Massey University, Palmerston North, New Zealand
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Date
2019
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Massey University
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Abstract
Glyphosate (GlyP) is the most commonly used herbicide worldwide; it is retained in
the soil and is decomposed by soil microorganisms. The main degradation product of
GlyP is Aminomethyl-phosphonic acid (AMPA). Phosphorus and GlyP are antagonistic
anions that compete for the soil’s reaction sites; P accumulation in the soil can
increase GlyP translocation through the environment and increase its bioavailability.
Residual GlyP and AMPA accumulation in the soil has generated concerns about their
potential toxicity to non-target organisms such as crops and soil microorganisms. GlyP
in situ remediation has therefore emerged as an option to reduce the residence time
of the herbicide in soil.
Laboratory experiments were carried out in order to elucidate the effect of the
interaction between soil chemical and physical properties, and phosphorus addition
on GlyP sorption to soil surfaces. The results of the GlyP-AMPA batch adsorption-
desorption experiment demostrated that the Kd and fixation of GlyP and AMPA in the
soil was proportional to the Al-Fe oxy-hydroxides content of the soil, in the following
order Allophanic>Brown>Pallic. In another experiment, phosphorus addition to soil
reduced GlyP adsorption, which demonstrated that phosphate will occupy the same
soil reaction sites as GlyP. These results suggest that due to the stability of the bond
formed between Al oxy-hydroxides and P, Al oxy-hydroxides will fix GlyP; while the
higher reactivity of Fe oxy-hydroxides will facilitate the exchange of phosphate by
GlyP. A column leaching experiment demonstrated that the interaction between the
physical and chemical characteristics of the soil will influence water infiltration and
solubilisation of GlyP. Phosphorus addition to the columns enhanced GlyP’s vertical
displacement through the soil and AMPA detection in the leachate. The Pallic soil with
a poor physical structure had reduced GlyP vertical displacement. In contrast, the free-
drained Brown soil had higher AMPA percolation regardless of the P addition. The
Allophanic soil had the lowest GlyP percolation risks, despite the fact that P addition
increased AMPA detection at the bottom of the column. However, AMPA was
undetected in the Allophanic soil’s leachate. A soil induced respiration (SIR)
experiment demonstrated that GlyP (variable doses) did not affect soil microorganism respiration, while Agave amendments were used as an exogenous source of carbon
and triggered soil respiration (Agave applied had 0.382 mg TC/g soil and control C
applied was 1.25 mg C/g soil). The SIR ratio values observed in the soils were as
follows Allophanic>Pallic>Brown, and were inversely proportional to the total dissolved
carbon concentration in soil extracts. These results demonstrate that the greater Al-
Fe oxy-hydroxide content of the Allophanic soil protected organic matter from
mineralisation enabling greater microbial activity over the GlyP molecule. The P
adsorption-desorption experiment using Agave powder demonstrated that Agave
constituents desorbed phosphorus from soil surfaces, which might help in the
desaturation of P from soil, while increasing its bioavailability.
Glasshouse experiments using Roundup doses and Agave amendment applied to the
soil of white clover potted plants were carried out in order to elucidate the potential for
GlyP degradation in soil and the biochemical responses of white clover plants. The
results demonstrated that Agave amendment attenuated the translocation of GlyP to
white clover shoots for a Roundup dose of 90 kg a.i./ha. The chemical constituents of
Agave, 12 hrs after GlyP application to the soil, enhanced GlyP degradation to AMPA
in soil at the 15 kg GlyP treatment. A similar improved GlyP degradation was observed
during three days of evaluation at the 7.5 kg dose. The biochemical responses of white
clover shoots demonstrated an increase of gallic acid and tartaric acid accumulation
proportional to the increasing Roundup doses. This suggested that Roundup alone,
and in combination with Agave amendments, exerted oxidative stress on the plants.
Alternatively, the herbicide could have affected the EPSPS enzyme disrupting the
carbon cycle. These results demonstrate that the white clover metabolic disruption
caused by the Roundup treatments of 7.5 and 15 kg/ha, expressed through tartaric
acid and gallic acid, was alleviated at the third day of evaluation.
The results of this thesis can support decision-making for the implementation of
strategies which could mitigate glyphosate and AMPA displacement from New
Zealand farmland; as example, it may encourage the prevention of phosphorus
accumulation in the farmland. In addition, these results can encourage the
development of further research related to the potential use of Agave amendments for
glyphosate remediation, and help in the understanding of the effects of the herbicide
on the metabolism of non-target organisms.
Description
Figures 2.1 & 2.2 were removed for copyright reasons but correspond to Zhan et al., 2018 Figs 1 & 3.
Keywords
Glyphosate, Environmental aspects, New Zealand, Soils, Herbicide content, Soils, Leaching, Soil amendments, Agaves, White clover