Simulation of Herbicide Persistence in Soil

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Long-term persistence of simazine and linuron in soilthe soil (2.5 kg) by the method previously described3 to give a final concentration of 4.0 pglg drysoil. Separate 25 g amounts of the treated soil were weighed into wide-mouthed 100 ml conicalflasks and the appropriate amount of water was added to each to give the required soil moistureccntent. Six samples were prepared for incubation at both 4 and 121 soll moisture and temperaturesof 5, 10. 15,20,25 and 30'C. Further samples were incubated at 25'C with soil moisture contentsof 8 and 16)1. The initial weights of all the flasks were recorded and they were returned to theseoriginal weights by adding water at 7 day intervals.Duplicate flasks from each treatment were removed for analysis on three occasions during thesubsequent 56 days. The analytical method used for simazine has been described previously.3lotsrndsrda106romralfentsfordatatestides.andwereh Organizarfthe same963 to I 968nted for thewas appliedat approxisapplied totence at thesprayed onh year. The: simulation:ropl rndOxford soil2,2. Linur on treatmentsA rnethanol solution containing50 pg IaC]-linuron/ml and 750 pg unlabelled linuronimlwas pipettedover the surface of a sample of the air-dry soil (2.5 kg) to give a final concentration of 4.0 pglinuron/g air-dry soil. The solvent was allowed to evaporate and sufficient water was added to thesoil to give a flnal soil moisture content of approximately 4'%. Mixing was achieved by passingthe treated soil several times through a 2 mm mesh sieve. Treatments were prepared in 100 mlconical flasks for incubation under the same conditions as those used for simazine (section 2.1).Duplicate samples were removed for analysis at intervals during the subsequent 56 days. Theherbicide was extracted by shaking with 50 ml redistilled methanol on a wrist-action shaker forI h. The extracts were filtered and duplicate 2 ml subsamples were transferred to counting vialswith 5 ml dioxan-based scintillator.a Activities were determined using a Tracerlab Corumatic 200liquid scintillation counter. Quench corrections were determined using a sample channels ratiocalibration curve.The extracts remaining after preparation of samples for counting were concentrated to about5 ml and stored in glass vials at 4'C. Several of these extracts were examined by the thin-layerchromatographic method described by Katzs to separate linuron from its possible breakdownproducts. A thin streak of extract was applied to freshly prepared 250 pm layers of silica gel G on5 x 20 cm glass plates which had been activated for 30 min at I10"C. The plates were developed forl0 cnr above the baseline in chloroform-methanol-pyridine (100:5: I by volume).After development the plates were dried and the peaks of activity determined by scraping successive0.5 cnr bands from the plates, dispersing in methanol on 5 cnr aluminium planchets and countingon a Tracerlab, low-background, end-window Geiger counter.A further sample (100 g) of the original linuron-treated soil was weighed into a 250 ml conicalflask, and l0ml water was added to give a moisture content of about 141. Carbon
52A.Woo4% Soit moislu/ec:3l?'h Soil moisturejFlgure 2. Effr'; O, 20"; l, 25".:'rT'.T l,:f::,'!:t,r"+:erature40Trme {doys)on simazine degradation at 4 and r2lsoitmoisture. A,5.; a, 10"; o, 15.;OU20cTable l' The effects of temperature and soil moisture content on simazine and linuron degradationLinuronSoil moisturecontent(%)Simazine+812l68l2loI0683209125I{alf-life (days) at temperature (oC)l0 l5 20 25 30;o,384 67 5745 36:50Il=63I5343I293414080show that the effects of soil moisture content on the rate of degradation aretemperaturessimilar atfortheanydiflerentone herbicide. with tinuron a reduction from 12\75)(ofsoil moisturefieldcapacitv)tolT.soil(aboutmoisture (25/,otfieldcapacity;increaseothehalflifebyabout30"1 and a similar change with simazine increasld the half-life by about g0f(.similar to Thesethose reported increases areby Usoroh and Hance,? and walker3 forlinuron and simazine respectively.925360344028162329l6toc?5t?.3Figure 3. Effe,l, 25'; n, 30'
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Simulation of Herbicide Persistence in Soil

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