Simulation of Herbicide Persistence in Soil

A. Walker Wry-term Persistence ofA feature of the results in Table I is the greater effect of both soil temperature and soil moisturecontent on the rate of degradation of simazine compared with linuron. The half-life of linuron insoil at 30"C andl2l soil moisture is almost twice that of simazine, whereas at4lsoil moistureand 5"C, the half-life of simazine is almost twice that of linuron. The relationships between halflivesand soil moisture content are shown for both simazine and linuron in Figure 3. The straightlines drawn through the data at 25"C show an approximate fit to the empirical equation relatingthe half-life (H) to soil moisture content (M):H:AM B (l)in which A and B are constants. This equation has previously been shown to fit similar data withpropyzamidee, napropamide,2 simazine3 and prometryne.3 The fact that straight lines with similarslope to that at 25"C can be drawn through the two points at the other temperatures is a furtherillustration that the relative effects of moisture content on degradation rates are similar at differenttemperatures.The relationship between the rates of degradation of both herbicides and temperature can beexpressed in terms of the Arrhenius equation:' Ht lF'l l\be'n,:#i(n-Z I tztin which Ht and 11: are the half-lives at temperatures Ir and Tz and -lE is the activation energy.The relationships between the half-lives of simazine and linuron and temperature at 4 and 12i(soil moisture are shown in Figure 4, in which the half-life on a logarithmic scale is plotted against;= oro.oo35| /Absolute temperolure ( K -')Figure 4, Test for the fit of the Arrhenius equation to data for simazine and linuron degradation. O,4%Soilmoisture ; O, l2',r" soil moisture.the inverse of the absolute temperature. From the slopes of the lines, the activation energy forlinuron degradation is calculated as 30.2 kJ/mol and for simazine as 57.4kJ/mol.The data for linuron degradation in Figure 2 and Table I were based on the amounts of radioactivityextracted from the soil expressed as concentrations of linuron. The thin-layer chromatogramsof the extracts from the samples incubated at 25"C and all the extracts at 56 days showedthat over 957" of the activity on the plates was located at the Rr,-value of the parent herbicide(between 0.90 and 0.95), therefore demonstrating that in experiments with [carbonyl-1aC]-linuronchanges in extractability of radioactivity give a valid measure of linuron degradation. This con-clusion is further suPlin Figure 5. The amotloss of [raC]-carbon don a log scale againsexpressed as linuron,spondence with first-'srooQa\t z,)between the calculategraphic analysis of ttof the extractable raCthose of Bcirner,lo wcarbonyl group and n3.2. Simulation of peThe meteorological rrand l0 cm soil temp(the computer prograrsoil temperature ancNational Vegetable IWeed Research Orgaweather records arepattern, on a day-tosimulation program'simulation program €simazine plots in Figlinuron plots are sho,with the simulation nof measured residueThe results from theactual amounts repeidicted and to give thThe results with sshape to those obserrthe uncropped plots,the determined resid0.56 to l.g9 kg/ha wPlots, and the model