Avis A. Evans

Factors affecting degradation rates of five triazole fungicides in two soil types: 1. Laboratory incubations

Triazole fungicides are now widely used commercially and several are known to be persistent in soil. The degradation rates of five such fungicides were measured in laboratory tests with two soils over 720 days, with analysis of soil extracts by high-pressure liquid chromatography. Behaviour in a sandy loam and a clay loam were similar, and incubation of the compounds either singly or in admixture did not influence loss rates except for those of flutriafol which were lower in the latter. Triadimefon was quite rapidly reduced to triadimenol, though traces of the former were always found, indicating a possible redox equilibrium. Flutriafol, epoxiconazole and triadimenol (derived from triadimefon) were very persistent, breakdown following first-order kinetics with half-lives greater than two years at 10 degrees C and 80% field capacity. Propiconazole was moderately persistent, with a half-life of about 200 days under these conditions. Degradation rates increased about 3-fold as the temperature was increased from 5 to 18 degrees C, though decreasing soil moisture to 60% field capacity only slightly slowed degradation. The rate constants obtained are used in a companion paper describing field studies on these two soils to compare laboratory-measured degradation rates with losses in the field following commercial sprays

1-octanol/water partition coefficient (KOW) and pKa for ionisable pesticides measured by a pH-metric method

pK a values for a wide range of commonly used ionisable pesticides, together with the log K ow values of the most lipophilic form of each, have been measured using pH-metric techniques. Examples of acids, bases and multiprotic compounds from the major classes of herbicides, and a number of insecticides and fungicides that contain ionisable groups, are included. The pK a and log K ow values so obtained were generally in good agreement with values taken from the literature that were measured by other methods. The lower limit of log K ow that could be measured by the pH-metric method lay below the -0.97 obtained for amitrole, but the method could not be applied to glyphosate for which shake-flask measurements indicated log K ow below -3. The highest log K ow obtained in this study was 5.12 for pentachlorophenol. The pH-metric technique offers a rapid and convenient method to determine pK a and log K ow for ionisable compounds, especially when utilising an automatic titration system linked to a dedicated computer.

Factors affecting degradation rates of five triazole fungicides in two soil types: 2. Field studies

The criteria for registering pesticides persistent in soil are still a matter of debate. Amongst modern pesticides, several triazole fungicides are very persistent, though no deleterious effects on soil microbial processes have been reported. The behaviour of five such compounds (flutriafol, epoxiconazole, propiconazole, triadimefon and triadimenol) has been examined in two field trials utilising different agronomic treatments. These fungicides were applied in June 1996 at rates of 0.5 kg ha−1, and soil cores were taken to 20 cm depth at intervals over 2.5 years and analysed by extraction and high-pressure liquid chromatography. Triadimefon was quite rapidly reduced to triadimenol. Triadimenol, flutriafol and epoxiconazole were all very persistent with DT50 > 400 days, whilst propiconazole had DT50 c 200 days; behaviour was similar in the Rothamsted clay loam and Woburn sandy loam. Only flutriafol, the most polar and hence weakly sorbed of these fungicides, was appreciably leached, with traces reaching the 15–20 cm deep soil layer. Sprays applied to plots of fallow soil suffered loss of up to 50% of applied compound in the first four weeks, a loss eliminated by shallow incorporation, indicating an early role for surface loss processes such as photolysis and/or volatilisation. A young barley crop intercepted about one-third of the spray, though subsequent rain caused some wash-off. After one to two years, amounts of each compound remaining in the plots were similar for the three agronomic treatments, especially for flutriafol, though with a tendency for the incorporated plots to have the most chemical and the barley plots the least. Computer simulation of behaviour in the field using the model CALF, utilising sorption and degradation measurements made in laboratory incubations with these same soils together with daily climate measurements, overestimated persistence especially for flutriafol, epoxiconazole and triadimenol. This was due both to lack of inclusion of surface loss processes in the model, which caused initial deviations in the plots not receiving cultivation after spraying, and to a longer-term underestimation of breakdown in the field. This latter was especially noticeable for triadimenol, which was not detected after 2.5 years despite predictions of c 50% remaining. Thus field measurements of behaviour are desirable, as simulations based on laboratory measurements can overestimate persistence. © 1999 Society of Chemical Industry

The Effect on Soil Fertility of Repeated Applications of Pesticides over 20 Years

Concern has been expressed that repeated use of pesticides may be leading to accumulation of residues in soil and to damaging effects on the environment. A long-term experiment, known as the Chemical Reference Plots, was started in 1974 on a silty clay loam soil at Rothamsted in which plots received applications of up to five pesticides (aldicarb, benomyl, chlorfenvinphos, glyphosate and chlorotoluron or triadimefon), each plot receiving the same treatment annually for up to 20 years. Spring barley was grown each year, and its yield was taken as an indicator of soil fertility. The glyphosate and triadimefon were applied to the autumn stubble prior to ploughing from growing seasons 1980 and 1982 respectively, chlorotoluron was sprayed pre-emergence (1974 and 1976 only) and the other compounds were incorporated into the soil in spring immediately before sowing (1974-1993 inclusive). No deleterious effects on crop productivity were observed from these pesticide applications, and no differences could be found in microbial processes in soils sampled in April 1992 save for a small increase in the amount of microbial-biomass carbon in plots receiving aldicarb. No pesticide residues could be detected in soil taken in August 1994, 17 months after the last experimental treatment. In laboratory incubations using these same soil samples, the degradation of aldicarb residues was greatly enhanced in plots that had received aldicarb for 20 years, whereas degradation rates of benomyl, chlorfenvinphos and triadimefon residues were not influenced by the treatment history.