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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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.
Figures 2.1 & 2.2 were removed for copyright reasons but correspond to Zhan et al., 2018 Figs 1 & 3.
Glyphosate, Environmental aspects, New Zealand, Soils, Herbicide content, Soils, Leaching, Soil amendments, Agaves, White clover