The Impact of Disturbed Soil Structure on the Degradation of 2 Fungicides Under Constant and Variable Moisture.

Degradation of agrochemicals in soil is frequently faster under field conditions than in laboratory studies. Field studies are carried out on relatively undisturbed soil, whereas laboratory studies typically use sieved soil, which can have a significant impact on the physical and microbial nature of the soil and may contribute to differences in degradation between laboratory and field studies. A laboratory study was therefore conducted to determine the importance of soil structure and variable soil moisture on the degradation of 2 fungicides (azoxystrobin and paclobutrazol) that show significant differences between laboratory and field degradation rates in regulatory studies. Degradation rates were measured in undisturbed cores of a sandy clay loam soil (under constant or variable moisture contents) and in sieved soil. For azoxystrobin, degradation rates under all conditions were similar (median degradation time [DegT50] 34-37 d). However, for paclobutrazol, degradation was significantly faster in undisturbed cores (DegT50 255 d in sieved soil and 63 d in undisturbed cores). Varying the moisture content did not further enhance degradation of either fungicide. Further examination into the impact of soil structure on paclobutrazol degradation, comparing undisturbed and sieved/repacked cores, revealed that the impact of sieving could not be mitigated by repacking the soil to a realistic bulk density. Examination of fungal and bacterial community structure using automated ribosomal spacer analysis showed significant initial differences between sieved/repacked and intact soil cores, although such differences were reduced at the end of the study (70 d). The present study demonstrates that disruption of soil structure significantly impacts microbial community structure, and for some compounds this may explain the differences between laboratory and field degradation rates. Environ Toxicol Chem 2021;40:2715-2725. © 2021 SETAC.

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

Plant protection products (PPPs) undergo rigorous regulatory assessment to ensure that they do not pose unacceptable risks to the environment. Elucidation of their fate and behavior in soil is an integral part of this environmental risk assessment. The active substance degradation in soil of PPPs is first assessed in laboratory studies (typically following Organisation for Economic Co-operation and Development [OECD] test guideline 307). Conditions in guideline laboratory studies are far removed from those occurring under agricultural use, and the contribution of crop roots has currently not been assessed. We integrated viable plant root systems, representative of 3 different crop types, into the OECD test guideline 307 design to assess their impact on the dissipation of the herbicide prometryn. Significantly faster decline of parent residue and higher formation of nonextractable residues were observed in all 3 planted systems. This led to a reduction in the time required for 50% of the compound to dissipate (DT50) of approximately one-half in the presence of rye grass and hot pepper and of approximately one-third in the presence of red clover. These findings imply that plants and their associated root networks can have a significant influence on PPP dissipation. Based on these data, greater environmental realism could be added to the standardized laboratory study design by the inclusion of plant root systems into higher tier studies, which, in turn, could serve to improve the environmental risk assessment process. Environ Toxicol Chem 2020;39:450-457. © 2019 SETAC.

Quantifying soil surface photolysis under conditions simulating water movement in the field: a new laboratory test design.

Soil surface photolysis can be a significant dissipation pathway for agrochemicals under field conditions, although it is assumed that such degradation ceases once the agrochemical is transported away from the surface following rainfall or irrigation and subsequent drainage of soil porewater. However, as both downward and upward water movements occur under field conditions, relatively mobile compounds may return to the surface, prolonging exposure to ultraviolet light and increasing the potential for degradation by photolysis. To test this hypothesis, a novel experimental system was used to quantify the contribution of photolysis to the overall dissipation of a new herbicide, bicyclopyrone, under conditions that mimicked field studies more closely than the standard laboratory test guidance. Soil cores were taken from 3 US field study sites, and the surfaces were treated with [(14) C]-bicyclopyrone. The radioactivity was redistributed throughout the cores using a simulated rainfall event, following which the cores were incubated under a xenon-arc lamp with continuous provision of moisture from below and a wind simulator to induce evaporation. After only 2 d, most of the test compound had returned to the soil surface. Significantly more degradation was observed in the irradiated samples than in a parallel dark control sample. Degradation rates were very similar to those observed in both the thin layer photolysis study and the field dissipation studies and significantly faster than in the soil metabolism studies conducted in the dark. Thus, for highly soluble, mobile agrochemicals, such as bicyclopyrone, photolysis is not terminated permanently by rainfall or irrigation but can resume following transport to the surface in evaporating water.

Light structures phototroph, bacterial and fungal communities at the soil surface.

The upper few millimeters of soil harbour photosynthetic microbial communities that are structurally distinct from those of underlying bulk soil due to the presence of light. Previous studies in arid zones have demonstrated functional importance of these communities in reducing soil erosion, and enhancing carbon and nitrogen fixation. Despite being widely distributed, comparative understanding of the biodiversity of the soil surface and underlying soil is lacking, particularly in temperate zones. We investigated the establishment of soil surface communities on pasture soil in microcosms exposed to light or dark conditions, focusing on changes in phototroph, bacterial and fungal communities at the soil surface (0-3 mm) and bulk soil (3-12 mm) using ribosomal marker gene analyses. Microbial community structure changed with time and structurally similar phototrophic communities were found at the soil surface and in bulk soil in the light exposed microcosms suggesting that light can influence phototroph community structure even in the underlying bulk soil. 454 pyrosequencing showed a significant selection for diazotrophic cyanobacteria such as Nostoc punctiforme and Anabaena spp., in addition to the green alga Scenedesmus obliquus. The soil surface also harboured distinct heterotrophic bacterial and fungal communities in the presence of light, in particular, the selection for the phylum Firmicutes. However, these light driven changes in bacterial community structure did not extend to the underlying soil suggesting a discrete zone of influence, analogous to the rhizosphere.

Variation in chlorotoluron photodegradation rates as a result of seasonal changes in the composition of natural waters.

BACKGROUND: It is important to understand the degradation of organic molecules in surface waters to ensure that risk assessments, intended to prevent adverse effects on human health and the environment, are robust. One important degradation mechanism in surface waters is photodegradation. This process is generally studied in laboratory test systems, and the significance of the results is then extrapolated to the field. The aim of this work was to assess how fluctuations in the composition of surface water influence the photodegradation rate of chlorotoluron. RESULTS: Photodegradation DT(50) values in the lake (mean = 26.0 days) and pond (mean = 26.0 days) were significantly slower than in the river (mean = 6.8 days) and stream (mean = 7.3 days) samples. The DT(50) values in the pond and lake samples were similar to the direct photolysis value (mean = 28.6 days). Photodegradation was significantly faster in the stream and river samples, suggesting that indirect photolysis was significant in those waters. Principal component analysis indicated a strong inverse correlation between nitrate concentration and degradation rate. CONCLUSIONS: Nitrate concentration had a strong influence on the rate of photodegradation, with increasing nitrate concentrations sharply reducing the DT(50) . However, this effect was restricted to a narrow concentration range and levelled off quite quickly, such that further increases in the nitrate concentration had no significant effect on the rate of degradation. Extrapolating photodegradation rates of chlorotoluron from the laboratory to the field should be relatively straightforward, provided the nitrate concentrations in the waters are known.

The behavior of isopyrazam in aquatic ecosystems: implementation of a tiered investigation.

Degradation of a new fungicide, isopyrazam, was slow in water-sediment systems maintained in the dark, with degradation half-life (DegT50) values in the total system (water column and sediment) of greater than one year, and only moderately fast in a photolysis study in buffered pure water (DegT50 > 60 d). This indicated that microbial degradation and direct photolysis are not significant loss mechanisms for this compound. Under more realistic conditions, a number of other processes of natural attenuation occur, such as metabolism by aquatic plants, microalgae, and periphyton and indirect photolysis. A photolysis study in sterile natural water, and water-sediment studies incorporating aquatic macrophytes and microalgae under fluorescent light, were therefore conducted to investigate the contribution of these processes to the fate of isopyrazam. Degradation rates were at least one order of magnitude faster in these higher-tier laboratory studies, indicating that all of these processes may have a role to play in complex natural ecosystems. The fate in an outdoor system, designed to mimic conditions in edge-of-field drainage ditches, also was investigated to provide an integrated picture of the contribution of all the different potential loss mechanisms to the overall fate of isopyrazam. The total system DegT50 in the study was similar to that observed in the higher-tier laboratory studies. Furthermore, the pattern of degradation formation allowed for the contribution of the different degradation processes at work in the microcosm study to be contextualized. The implementation of this tiered approach to investigating the aquatic fate of crop protection products provides a comprehensive explanation of the behavior of isopyrazam and clearly demonstrates that it will not persist in the aquatic environment under natural conditions.

The role of indirect photolysis in limiting the persistence of crop protection products in surface waters.

The photodegradation of six crop protection products (CPPs) was studied in 16 natural waters collected from across the midwest of the United States under simulated sunlight to determine the significance of indirect photolysis. The rate of degradation of five of the CPPs was faster in irradiated natural waters than in buffer systems, with the effect particularly significant with the relatively photostable compounds propiconazole and prometryn. Degradation rates were correlated with the concentration of one or more photosensitizers, or ratios thereof, by means of a Pearson's correlation and linear regression analysis. It was found that the photodegradation of chlorotoluron, pinoxaden, propiconazole and prometryn were linked to the concentration of nitrate, pointing to a significant role of hydroxyl radical ((.)OH) as a reactive intermediate. Increased concentrations of dissolved organic carbon (DOC) and bicarbonate relative to nitrate were found to decrease the rate of degradation of these compounds, consistent with a quenching role. Chlorothalonil appeared to be rapidly degraded by means of the carbonate radical ((.)CO(3)(-)), whereas the photodegradation of emamectin was particularly complex. Overall, indirect photolysis significantly enhanced the rate of CPP degradation and fate models based on these experiments appear to offer more realism than those that only take into account direct photolysis.