The effect of pesticide mixtures on the degradation of xenobiotics in soil and aquatic sediment systems

  • Der Effekt von Pestizidmischungen auf den Abbau von Xenobiotika in Boden und Aquatischen Sedimentsystemen

Wijntjes, Christiaan Wilhelmus; Schäffer, Andreas (Thesis advisor); Hollert, Henner (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022


Exploding human population demands for an increase in agricultural production. This leads to enhanced use of fertilizers which promotes pathogenic fungi infestation on crops. Consequently, fungicides are utmost important in modern agricultural pest control. Fungicides and other pesticides, however, can disturb ecologically relevant soil microbial communities. Furthermore, pesticides that are dismissed to the terrestrial environment might infiltrate groundwater or nearby water bodies by leaching, atmospheric drift, runoff, and drainage, or from wastewater treatment plant emissions. Here, pollutants can also interfere with the aquatic ecosystem, whereas such interactions have only few attention in research programs. In the production of agricultural crops, the successive application of distinct pesticides, or (tank) mixtures of them, is a frequent practice. As a result, agrochemicals that enter the soil and aquatic environment, might there co-exist with other pesticides. The impact of such mixtures on various soil parameters has been extensively studied, whereas investigations on the consequences of combined chemicals on the degradation of parent compounds remain deficient. Besides, current international Guidelines for testing of chemicals are designed for the examination and risk assessment of a single chemical compound in their respective test system only. Therefore, the research presented in this dissertation aimed to investigate the potential environmental impact of tank mixtures containing broad spectrum fungicides on the degradation of herbicides in US American and German soils under controlled laboratory conditions. Results showed that biodegradation of the radiolabeled sulfonylurea herbicide iodosulfuron-methyl-sodium, recommended for use together with respective fungicides, in four different soils is decelerated in the presence of the fungicides tebuconazole and prothioconazole, when applied according to good agricultural practice. This effect was observed from the inhibited biotic transformation of the herbicide to its main soil metabolite. Except for one soil, the adverse fungicidal effect was transient and mitigated towards the end of the respective incubation periods. Half-lives (DT50) and DT90 values for the herbicide in the four fungicide-treated soils increased 1.5 to 2.9-fold and 1.3 to 2.2-fold, respectively when compared to untreated control soils. Additionally, potential effects by combined application of the agriculturally employed tank mixture to aquatic sediment systems under controlled laboratory conditions were elaborated. Again, the route and rate of degradation of the radiolabeled herbicide was followed. One set of aquatic sediment samples was incubated in the dark. A second set of samples was controlled at identical conditions, except for being continuously irradiated to promote algal growth. In addition, the algal biomass was monitored on effects of varying treatments in irradiated aquatic sediment, and a potential role of algae in the degradation of iodosulfuron-methyl-sodium was evaluated. These studies showed that the herbicide, which was hydro- and photolytically stable throughout the experiment, metabolized faster in continuously irradiated when compared to dark aquatic sediment, with DT50 values shortened 1.1 to 1.2-fold and DT90 2.8 to 4.5-fold. Beyond the half-life of the herbicide, both fungicides also had a significant adverse effect on its degradation rate with DT90 values prolonged 1.5-fold in the irradiated and 2.5-fold in the dark systems. Furthermore, potential effects of the algae population on the metabolization of the herbicide in irradiated aquatic sediment were observed. Even so, the algal influence was concluded to be indirectly. Besides, a significant adverse effect from tebuconazole and prothioconazole on the fungal biomass in a silt loam soil was confirmed, whereas on the total active soil biomass, a cognate effect was not observed. Hence, as beyond the scope of the presented work, it remains undefined, which specific interactions from the fungicides with the soils and aquatic microflora are leading to the decelerated metabolization of the herbicide iodosulfuron-methyl-sodium, and these processes call for further investigations. The presented findings demand for more environmentally realistic laboratory research on pesticide mixtures in soil and aquatic ecosystems, including their impact on the proliferation of harmful algal and cyanobacterial communities. This is crucial to improve the perception on pesticide and specifically fungicide effects in the natural environment. Furthermore, reduction of agricultural fungicide use by promotion of more sustainable food production in combination with impeding progression of arable land into surrounding areas and minimizing agrochemical leakage to the aquatic environment is highly desirable.