Deactivation of the Biological Activity of Paraquat in the Soil Environment:  a Review of Long-Term Environmental Fate

Abstract

During the many years of paraquat usage, wide ranges of investigations of its environmental impact have been conducted. Much of this information has been published, but key, long-term field studies have not previously been presented and assessed. The purpose of this review is to bring together and appraise this information. Due to the nature of paraquat residues in soils, the major part (some 99.99%) of a paraquat application that reaches the soil within the typical Good Agricultural Practice (GAP) is strongly adsorbed to soils of a wide variety of textures. This is in equilibrium with an extremely low concentration in soil solution. However, the paraquat in soil solution is intrinsically biodegradable, being rapidly and completely mineralized by soil microorganisms. The deactivation of the biological activity of paraquat in soils, due to sorption, has been investigated thoroughly and systematically. It is recognized that the determination of total soil residues by severe extraction procedures provides no insight into the amount of paraquat biologically available in soil. Consequently, the key assay developed for this purpose, namely, the strong adsorption capacity-wheat bioassay (SAC-WB) method, has proved to be valuable for determination of the adsorption capacity relevant to paraquat for any particular soil. This method has been validated in the field with a series of long-term (>10 years) trials in different regions of the world. These trials have also shown that, following repeated applications of very high levels of paraquat in the field, residues not only reach a plateau but also subsequently decline. This demonstrates that the known biodegradation of paraquat in soil pore water plays an important role in field dissipation. The biological effects of paraquat in the field have been assessed under unrealistically high treatment regimes. These trials have demonstrated that the continued use of paraquat under GAP conditions will have no detrimental effects on either crops or soil-dwelling flora and fauna. Any such effects can occur only under extreme use conditions (above the SAC-WB), which do not arise in normal agricultural practice.

Effects of Paraquat-induced oxidative stress on antioxidants and Chlorophyll Fluorescence in stay-green wheat (Triticum aestivum L.) flag leaves

“…Paraquat is a non-selective and fast-acting herbicide that affects all green tissues. It is also very strongly absorbed into the soil and thereby biologically deactivated (Roberts et al 2002).…”

Paraquat Induced Toxicity in Adrenals in Albino Mice

“…It is easily accessible to farmers in developing countries [2]. Chemically paraquat is 1, 1'-dimethyl-4, 4'-bipyridinium dichloride [3]. It is available in liquid form with pH range of 6.5 to 7.5 and is stable in acidic/ neutral solutions.…”

Systematic Analysis of Superoxide‐Dependent Signaling in Plant Cells: Usefulness and Specificity of Methyl Viologen Application

“…Only in the middle of the twentieth century was it discovered that methyl viologen acts as a nonspecific contact herbicide [71] and it was subsequently used in agriculture as rapid desiccant of green plant tissue [72]. Its ability to adsorb to soil particles and organic soil matter [73,74] limits the bioavailability for microorganisms and plants [75], and fosters a rapid biological deactivation. Additionally to this advantage of methyl viologen as an herbicide, its limited mobility leads to the avoidance of ground water contamination [76].…”

“…Additionally to this advantage of methyl viologen as an herbicide, its limited mobility leads to the avoidance of ground water contamination [76]. Furthermore degradation of the unbound fraction to less-toxic compounds is realized by external factors such as ultraviolet light [77], sunlight [78], and a variety of soil microorganisms [79], leading to chemical or biological catabolism, respectively (see Figure 9.1) [73,78]. However, methyl viologen is also a highly toxic compound if ingested or absorbed by humans and animals, with various cases of poisoning and casualties being reported [80,81].…”