Simulation of Herbicide Persistence in Soil

A. WalkerIrng-term persistcnce of simazinc and linuron in soil55d soil moistureie of linuron in( soil moistures between half-3. The straightuation relatingclusion is further supported by the results from collection of ILlc]-carbon dioxide which are shownin Figure 5. The amount of linuron remaining in the soil (calculated by subtraction of the cumulativeloss of [1aC]-carbon dioxide from the radioactivity present initially, expressed as linuron), is plottedon a log scale against time of incubation. Also shown is the value for extractable activity, againexpressed as linuron, on termination of the experiment at 70 days. The data show a close correspor.ldencewith first-order kinetics giving a half-life of about 30 days. There was good agreement(l)nilar data withres with sinrilarres is a furtherilar at differentt coFs50= :_..erature can be(2)ivati< .tergy.at 4 and 12\plotted against:n. C, 4i{ SoilIn energy forrnts of radio-3r chromato-Cays showed:nt herbicide.raClJinuronn. This con-6(d 4t fbTime {doys)Figure 5, Loss of linuron from soil calculated fromcumulative amounts of liCOz collected ( o) and amountof linuron extracted from the soil after 70 days (O).between the calculated amount remaining after 70 days and that determined. Thin-layer chromatographicanalysis ol the methanol extract at the end of the experiment again showed that over 957"of the extractable radioactivity was in the form of linuron. These data are therefore consistent withthose of Bcirner,lo which showed that loss of linuron from soil is closely related with loss of theerbonyl group and not associated with buildup of the demethylated or demethoxylate derivatives.3.2. Simulation of persistence in the fieldTh,) nleteorological records of rainfall (mm/day), evaporation from an open water surface (mm/day)and l0cm soi[ temperature ("C) for the period January 1964to December 1968 were combined inthe computer programz with the respective constants relating half-lives of simazine or linuron withsoil temperature and soil moisture content. The weather data used were those recorded at theNational Vegetable Research Station since evaporation measurements were not available from theWeed Research Organization. The two stations are approximately 35 miles apart, and the averageweather records are very similar. There would have been differences, particularly in the rainfallpattern, on a day-to-day basis, but such differences were probably of little importance when thesimulation program was run for long periods. The disappearance curves obtained from use of thesitnuiation program are compared with the published results of Fryer and Kirklandl flor the croppedsimazine plots in Figtrre 6 and for the uncropped plots in Figure 7. Similar results for the croppedlinuron plots are shown in Table2,and for the uncropped plots in Table 3. In previous experimentswith the simulation model, the determined degradation curves have been drawn following correctionof measured residue levels for initial recoveries2,3'e but in the present study this was not done.The results from the model, shown in Figures 6 and 7 and Tables 2 and 3 were calculated using theactual amounts repeatedly applied, in order to determine whether residue build-up would be predictedand to give the model a strict test against field results.The results with simazine are encouraging: the simulated disappearance curves are of similarshape to those observed and the predicted residues are often very similar to those determined. Withthe uncropped plots, the predicted residue at the end of the fifth year is 0.58 kg/ha compared withthe determined residue of 0.63 kg/ha. Fryer and Kirklandl conclude that a residue varying from0'56 to 1.89 kg/ha was present in the soil at the time of the next application of simazine to theseand the model predicts a minimum residue of 0.49 kg/ha following spring application and a