Non-UV Light Influences the Degradation Rate of Crop Protection Products

Abstract

Crop protection products (CPPs) are subject to strict regulatory evaluation, including laboratory and field trials, prior to approval for commercial use. Laboratory tests lack environmental realism, while field trials are difficult to control. Addition of environmental complexity to laboratory systems is therefore desirable to mimic a field environment more effectively. We investigated the effect of non-UV light on the degradation of eight CPPs (chlorotoluron, prometryn, cinosulfuron, imidacloprid, lufenuron, propiconazole, fludioxonil, and benzovindiflupyr) by addition of non-UV light to standard OECD 307 guidelines. Time taken for 50% degradation of benzovindiflupyr was halved from 373 to 183 days with the inclusion of light. Similarly, time taken for 90% degradation of chlorotoluron decreased from 79 to 35 days under light conditions. Significant reductions in extractable parent compound occurred under light conditions for prometryn (4%), imidacloprid (8%), and fludioxonil (24%) compared to dark controls. However, a significantly slower rate of cinosulfuron (14%) transformation was observed under light compared to dark conditions. Under light conditions, nonextractable residues were significantly higher for seven of the CPPs. Soil biological and chemical analyses suggest that light stimulates phototroph growth, which may directly and/or indirectly impact CPP degradation rates. The results of this study strongly suggest that light is an important parameter affecting CPP degradation, and inclusion of light into regulatory studies may enhance their environmental realism.

Inclusion of seasonal variation in river system microbial communities and phototroph activity increases environmental relevance of laboratory chemical persistence tests

“…Chlorophyll a data clearly demonstrates that there was an increased abundance of phototrophs in the illuminated microcosms, suggesting that phototrophic communities likely played a role in the increased degradation of isopyrazam, as has been described in multiple studies with a range of chemicals ( Roldán et al, 1998 ; Lima et al, 2003 ; Thomas and Hand, 2011 ; Davies et al, 2013a ), including specifically isopyrazam transformation ( Hand and Oliver, 2010 ; Hand and Moreland, 2014 ). These data suggest that phototrophs resident in both the water column and within the sediment contributed to biodegradation.…”

“…This could occur via a number of mechanisms. Phototrophs, particularly cyanobacteria, have been shown to directly metabolise a range of chemicals, while carbon fixation by phototrophs could also stimulate the biomass of heterotrophs, thereby increasing degradation rate ( Davies et al, 2013a ). Furthermore, photosynthesis is associated with the formation of reactive oxygen species ( Foyer, 2018 ), which could indirectly contribute to degradation.…”

“…In the environment, light stimulates growth of phototrophic microorganisms, which use natural sunlight as an energy source ( Overmann and Garcia-Pichel, 2006 ). Phototrophs have been shown to be metabolically capable of chemical biodegradation ( Roldán et al, 1998 ; Lima et al, 2003 ; Davies et al, 2013a ) and several studies have shown that inclusion of non-UV light and the presence of active microbial phototroph communities stimulate CPP biodegradation ( Hand and Oliver, 2010 ; Thomas and Hand, 2012 ). Evidence from experiments using both soil ( Davies et al, 2013a ) and water ( Thomas and Hand, 2012 ) suggests that enhanced biodegradation of chemicals under non-UV light could result from a combination of direct degradation by phototrophs and indirect degradation by heterotrophs, which are stimulated by the presence of phototrophs.…”

Evaluation of the Rhizosphere Contribution to the Environmental Fate of the Herbicide Prometryn

“…Although measures were taken to suppress the colonization of the soil surface by phototrophic organisms, this was not completely successful and some greening on the vessel sides was observed. It is known from previously published studies that some phototrophs are competent to degrade prometryn (Thomas and Hand ) and prometryn degradation is in fact enhanced in colonized (green) soils (Davies et al ). However, further work would be required to fully explain this observation.…”

“…Once the seedlings had emerged and were well established (at 21 d after treatment), a sand layer (~4–10 mm/~50–70 g, general purpose grade sand; Fisher Scientific) was added to cover the soil surface. This was necessary to suppress colonization by phototrophs, which have previously been shown to have competence for prometryn degradation (Thomas and Hand ) and the ability to impact the soil dissipation of prometryn (Davies et al ). To further discourage growth of soil phototrophic communities, the sides and the bottoms of the vessels were covered in aluminum foil up to a height of approximately 1 cm above the soil surface.…”