Maataloustieteellinen A ikakauskirja Vol. 61: 39—44, 1989 The degradation of linuron in sandy soil HELVI HEINONEN-TANSKI Department of Environmental Engineering, University of Kuopio, FOB 6, SF-70211 Kuopio, Finland Abstract. The degradation of linuron occurs both in aerobic and anaerobic sandy soil with a slight lag when the conditions change from aerobic to anaerobic or from anaerobic to aero- bic. Liming was found to stimulate the degradation rate of linuron so clearly that liming can be recommended for acceleration of linuron degradation as a normal agricultural treatment, particularly in sandy soils. Index words: Liming, degradation acceleration, herbicide, I4C-linuron, flow-through system. liilroduclion Linuron (3,4-dichlorophenyl-l-methyl-1- methoxyurea) has been used in many countries as a pre-emergency or post-emergency herbi- cide in the cultivation carrot and some other plants. The disappearance of linuron from soil oc- curs mainly by microbial cometabolic degra- dation (Glad et al. 1981; Walker & Zimdahl 1981; Stepp et al. 1985; Wang et al. 1985). The mobility of linuron is very low because of its high tendency to adsorb with soil parti- cles and organic compounds as determined by the adsorption constants of Glad et al. (1980) and Walker (1987). Most linuron residues can be found in the soil layer of 0— 6 cm (Walker 1987). Thus Zahnow and Riggleman (1980) did not find linuron in the mud or water of a North American bay de- spite the great amounts of linuron used on the fields near this bay and its tributaries. In one case (Frank et al. 1987), linuron was found in farm well water, but in this case it was as- sumed that linuron was in the water because of spills while mixing and loading the spray equipments near the wells. Linuron forms complex compounds with humic acids (Senesi 1981) as well as some of its potential degradation products (Saxena & Bartha 1983 and Bartha et al. 1983). The microbial degradation of linuron can begin either by dechlorination or by side chain degradation. Stepp et al. (1985) found the reductive dechlorination of para-chlorine to occur in anaerobic pond sediment. The 39 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=b91MzcGDtMkZQRY5.9OnmMFe-YOI-I5K4-YCEyA.CQnVzbDb7iKJNWjqRqY6PJZPPbEIyy8ld_ItxwU9Bfu1H5Bn35m7OeiiD0ppcUpEP3CYQT8rWJXvADKKy7mVCqIekHh5Ag-KSYExUcF8L_9q4tIfjzb4sF_janW0_daDXfu6B4z73n99rdPaw6eQ8azHPiiC66FpEz6PbTnzaotJnjo side chain degradations (demethylation, de- methoxylation and hydrolysis of amide bound) have been known for a long time (Börner, 1965), and at least in aerobic soil they may be more important. When l4C- labelled linuron degraded in soil, the amount of I4C-carbonyl-labelled demethylated and demethoxylated degradation products in soil was only 5 % that of the undegraded 14C-car- bonyl-labelled linuron (Walker 1976), but the formation of 14carbon dioxide was a good indicator of the total degradation of linuron. Thus it can be assumed that hydro- lysis of the amide bound following by decar- boxylation is the most important degradation way of linuron, the main metabolite being 3,4-dichloroaniline. Linuron has been found to degrade in some soils quite rapidly (Klempson-Jones & Hance 1979; Walker 1976 and 1987) or in some other soils very slowly (Glad et al., 1980; Walker & Zimdahl 1981). The degradation rate depends on temperature, soil moisture and pH. The degradation rate was higher at 22°C than at 10°C and the rate was higher when the moisture was neither too high nor too low (Klempson-Jones & Hance 1979). A pH-value above pH 6 seems to be more favourable than lower pH-values (Hance 1979). The very slow degradation of linuron has in some cases damaged the next yields (Eagle 1981; Heinonen Tanski et al. 1986), or the soil residue levels can be critically high if the weather is unfavourable (Mundell & Olafsson 1982). Thus, there is a great need for methods to accelerate the degradation of linuron in prac- tical agriculture. This need may be greatest in the Nordic climate after cold and short sum- mers, such as summer 1987, or after very dry or very rainy summers. The cometabolic degradation could be accelerated by addition to soil substrates, which increase the microbial activity of soil. In practice, such compounds could be organic or inorganic fertilizers or lime. Doyle et al. (1978) found that dairy manure and sewage sludge increased the degradation rate of linuron. Liming may generally increase microbial activity in easily acidified Finnish soils. Therefore it was select- ed for this experiment as a possible accelera- tor for the degradation of linuron. Materials and methods Soil: Carrot was cultivated in sandy soil in Laukaa, Central Finland (62° 28' N and 25° 56' E), weeds were controlled annually for seven years by two sprayings of 1.8 kg/ha, and then for four years by one spraying of 1.8 kg/ha linuron (as Afalon). The plots were then limed with dolomitic lime (0, 5 or 10 t/ha) in May before the last seeding and linuron application. Soil samples were taken in autumn five months after the liming, when harvesting the eleventh carrot yield. The or- ganic matter of soil was 2.6 % and mechani- cal analysis gave the following percentages: medium coarse sand 3.0 %, fine sand 35 %, very fine sand 25 %, silt 26 %, and clay 11%. This soil had earlier contained up to 0.4— 0.5 mg/kg linuron one year after the last linu- ron application (Heinonen-Tanski et al. 1986). The other properties of autumn sam- ples are presented in Table 1. Laboratory tests: Soil samples were air- dried at room temperature and added to a flow-through system bottles (Goswami & Koch 1976). I4C-carbonyl linuron (Hungari- an Academy of Sciences, Institute of Isotopes, Budapest) and unlabelled linuron (Hoechst) were applied to 1 mg/kg. The field capacity of the soil was adjusted to 60 °/o with tap water Table 1. The chemical analyses of the soils five months after liming (unlimed control, limed with 5 t/ha and 10 t/ha). Unlimed Limed Limed 5 t/ha 10 t/ha pH K(l 6.26.7 6.7 Conductivity uS lOVcm 0.45 0.54 0.81 Ca mg/l 875 1 175 1 975 K mg/l 105 100 95 Mg mg/l 120 150 210 40 41 and the temperature was set at 15°C. The soils were watered when the field capacity had decreased 30—40 %, which is too low for op- timal microbial activity and linuron degrada- tion. Radioactive carbon dioxide was trapped with ethanolamine and measured with a scin- tillation counter (Lionell et al. 1984). The trapping capacity was tested with NaI4C0 3 and HCI, and it was better than 95 %. The flow-through gas was synthetic air (80 % N 2 and 20 % 02) until the 164th day, then nitro- gen until the 234th day, and air again until the end of the experiment on the 252th day. After the flow-through system experiment, the soil (1.25 g) was extracted three times with water to separate the water-soluble metabo- lites. After the water extraction, linuron and related aromatics in the soil were extracted for 8 hours in a Soxhlet apparatus with acetone. The humins were then separated from the soil by extraction with 0.5 N NaOH solution, first overnight and then twice for 2 hours, and by centrifugation for 30 min at 10 000 rpm. The humins were found in vacuum-dried precipitant. The supernatant was acidified with concentrated HCI to pH 1.0 and cen- trifuged again as above. The HCI-supernatant contains then fulvic acids and HCI-precipitant humic acids. The separation was based on the method described by Hänninen et ai. (1981). The radioactivities of soil extractions were measured by using 1.5 ml of sample solution or water and 10 ml of scintillator cocktail (University Pharmacy, Helsinki YA-gel). The total radioactivity of the soil was combusted in a sample oxidizer, trapped and counted as described by Lignell et al. (1984). Results The cumulative evolution of 14C0 2 from I4C-linuron during the incubation is shown in Fig. 1. In limed plots the degradation of linu- ron was clearly accelerated. This acceleration was statistically significantly higher in the two limed plots as compared to the unlimed plots, both in the first sampling (limed 5 t/ha) or af- ter one week (limed 10 t/ha). The degradation rate was practically the same in both soils limed with either 5 or Fig. I. The evolution of ,4 C02 in flow-through systems. A = unlimed, B = limed 5 t/ha and C = limed 10 t/ha. 10 tn/ha, and there was no statistically signifi- cant difference. The initial degradation rate was highest, but after I—21 —2 weeks the degradation rate was more stable. The main rates per day are presented in Table 2. When air was substituted for nitrogen, the degradation rate first de- creased, but it increased again gradually with- out ever reaching the degradation rates before incubation with nitrogen as the flow-through gas. Again, when nitrogen was substituted for air, the degradation rates first decreased and then increased. The distribution of I4 C-activity between C02 and different soil fractions after 252 days’ of incubation is shown in Table 3. Discussion In this experiment, the degradation of linu- ron was much slower than presented by Table 2. The degradation rate per day in unlimed and limed soils (5 t/ha and 10 t/ha) using a flow-through gas air or nitrogen. Flow-through Degradation rate of linuron % / gas phase day of initially added in soils.day of initially added in soils. Unlimed Limed Limed 5 t/ha 10 t/ha Air before N, 0.10 31 days with N, 0.05 Air after N, 0.03 0.17 0.16 0,11 0.10 0.06 0.06 Hance (1979), Maier-Bode and Härtel (1981) and Walker (1987). The half-life wouldbe more than eight months, calculated from the formation of 14C02. At the same time, half or almost half of the radioactivity added as linuron was still found in soil, most of it in acetone extract (and possible partly in water, too) in the form of linuron. The binding of linuron or its metabolites to humus was less than 15 % of the linuron added. The possible binding of 3,4-dichloro- aniline, an important metabolite of linuron known to bind to humus (Bartha et al. 1983) is not included in this figure because this ani- line derivate would not be radioactive. The changes from aerobic to anaerobic and from anaerobic to aerobic initially caused a microbial lag, which might be more important in natural soils with dry and rainy periods. Lime accelerates the degradation of linuron so clearly that liming could be recommended in acid soils, for instance, after spills or ac- cidental overdoses of linuron or if linuron has been used for many years in the same plot, or perhaps after unfavourable growing sea- sons (cold, short, very rainy or very dry), like summer 1987 in Northern Europe. After such growing seasons, it would be worth perform- ing liming earlier, which is a normal and regu- lar operation in Nordic agricultural soils lack- ing calcium buffer. Organic fertilizers, such as manure or Table 3. The distribution percentage of 14C-activity in unlimed and limed soils after 252 days. (Mean ± standard deviation). Fraction Unlimed Limed 5 t/ha Limed 10 t/ha MCO 2 21.012.0 35.1 ±3.3 33.7 ±5.1 In soil 54.7111.1 37.017.4 40.9113.4 Recovery 75.7112.0 72.118.4 76.6114.3 In soil: Water soluble compounds (maybe partly linuron) 9.112.0 4.311.5 6.612.9 Acetone extract (linuron ect.) 20.717.7 20.514.4 23.015.1 Fulvic acids 6.816.1 6.612.3 5.311.5 Humic acids 0.310.1 0.610.3 0.310.1 Humins 7.615.8 5.4 + 3.6 2.511.2 Sum in soil found 44.5111.6 37.416.3 37.716.2 42 sludge which Doyle et al. (1978) found to ac- celerate the degradation of linuron, would also be worth trying. The results presented in Table 3 show the fate of only 75 —80 % of linuron. A leakage in the flow-through system would easily ex- plain this lack but it is not a probable expla- nation because the parallel results were too close to each other. There is always some leakage during the sampling of 14C02 results, which occurred 85 times during this experi- ment, each timetaking approximately 30 secs. In addition to the weighing and changing of gas bottles, watering also caused some leak- age. Watering was done approximately 10 times, each time taking 10—15 minutes. As the entire experiment took 252 days, these er- rors may have some importance. It is also pos- sible that the original application of linuron solution in 100 pi had an error of up to 10— 15 °7o. Acknowledgements. I am grateful for Dr. Eva Eklund for her critical comments. I thank Mr. Paavo Simojoki, M. Sc. (Agr.) of the Central Finland Experimental Station, for all his field works during these years and for the soil samples and Mr. Antti Uusi-Rauva, M. Sc. of the Univer- sity of Helsinki, Instrument Centre, Faculty of Agricul- ture and Forestry, where the 14C02 -evalutions were done. 1 would like to express my appreciation to both of them for their long collaboratorion. Miss Arja Halinen has done the fractionation of linuron metabolites with great accurancy, and Mr. Hannu Eskelinen, M. Sc. has done the computer work. Mr. Kari Puukko, M. Sc. (Agr.) from Hoechst Fennica is thanked for providing linuron. Mrs. Sevastiana Ruusamo M, A. has corrected the English language. References Bartha, R., You,' 1.-S. & Saxena, A. 1983. Humus- bound residues of phenylamide herbicides: Their na- ture, persistence and monitoring. lUPAC Pesticide Chemistry. Ed. J. Miyamoto et al. Pergamon Press, Oxford, pp. 345—350. Börner, H. 1965. Untersuchungen iiber den Abbau von Afalon (N-(3,4-Dichlorphenyl)-N’-methoxy-N’- methylharnstoff) und Aresin (N-(4-Chlorphenyl)- N’-methoxy-N’-methylharnstoff) im Boden. Zeitschr. Pflkranh. Pflsch. 72: 516—531. Doyle, R.C., Kaufman, D.D. & Burt, G.W. 1978. Effect of dairy manure and sewage sludge on l4C- pesticide degradation in soil. J. Agric. Food Chem. 26: 987—989, Eaole, D.J. 1981. Residue problems encountered in England. Proc. EWRS Symp. Theory Pract. Use Soil Applied Herbicides pp. 201—207. Frank, R., Dipley, 8.D., Braun, FI.E., Cleog, 8.5., Johnston, R. & O’Neill, T.J. 1987. Survey of farm wells for pesticide residues. Southern Ontario, Cana- da, 1981 —1982, 1984. Arch. Environ. Contam. Toxicol. 16: I—B. Glad, G., Göransson, 8., Popoff, T., Theander, O. & Torstensson, N.T.L. 1981. Decomposition of linu- ron by fungi isolated from soil. Swed. J. Agric. Res. 11: 127—134. —, Nilsson, H., Popoff, T., Theander, O. & Torstens- son, N.T.L. 1980. Performance of linuron in four Swedish soil types. Swed. J. Agric. Res. 10: 133—137. Goswami, K.P. & Koch, B.L. 1976. A simple apparatus for measuring degradation of 14C-labelled pesticides in soil. Soil Biol. Biochem. 8: 527—528. Hance, R.J. 1979. Effect of pH on the degradation of atrazine, chlorprop, linuron and propyzamide in soil. Pestic. Sci. 10; 83—86. Hänninen, K., Lehto, O. & Mälkönen, P. 1981. Iden- tification of phenolic acids, molecular weight deter- minations of fulvic acids and quantitative measure- ments of phenolic and fulvic acids by HPLC from for- est and moss humus. Proc. Intern. Peat Symposium, Bemidji, Minnesota, pp. 117—155. Heinonen-Tanski, H., Siltanen, H., Kilpi, S., Simojoki, P., Rosenberg, C. & Mäkinen, S. 1986. The effect of the annual use of some pesticides on soil microor- ganisms, pesticide residues in soil and carrot yields. Pestic. Sci. 17: 135—142. Klempson-Jones, G.F. & Hance, R.J. 1979. Kinetics of linuron and metribuzin degradation in soil. Pestic. Sci. 10: 449—454. Lionell, R., Heinonen-Tanski, H. & Uusi-Rauva, A. 1984. Degradation of trichloroacetic acid (TCA) in soil. Acta Agric. Scand. 34: 3—B. Maihr-Bode, H. & Härtel, K. 1981. Linuron and monolinuron. Residue Rev. 77: 1—152. Mundell, D. & Olafsson, S. 1982. Residue of linuron in soils and potatoes in Iceland. Isl. Landbun. J. Agric. Res. Icel. 14: 3—17. Saxena, A. & Bartha, R. 1983. Modelling of the cova- lent attachment of chloroaniline residues to quinoidal sites of soil humus. Bull. Environ. Contam. Toxicol. 30: 485—491. Senesi, N. 1981. Free radicals in electron donor-accep- 43 tor reactions between a soil humic acid and photoly- sis inhibitor herbicides. Z. Pflanzenernähr. Bodenk. 144; 580—586. Stepp, T.D., Camper, N.D. & Paynter, M.J.B. 1985. Anaerobic microbial degradation of selected 3,4-di- halogenated aromatic compounds. Pestic. Biochem. Physiol. 23: 256—260. Walker, A. 1976. Simulation of herbicide persistence in soil. II Simazine and linuron in long-term experiments. Pestic. Sci. 7: 50—58. 1987. Evaluation of a simulation model for predic- tion of herbicide movement and persistence in soil. Weed Res. 27: 143—152. & Zimdahl, R.L. 1981. Simulation of the persistence of atrazine, linuron and metolachlor in soil at differ- ent sites in the USA. Weed Res. 21; 255—265. Wang, Y.-S., Madsen, E.L. & Alexander, M. 1985. Mic- robial degradationby mineralization or cometabolism determined by chemical concentration and environ- ment. J. Agric. Food Chem. 33: 495—499. Zahnow, E.W. & Riocleman, J.D, 1980. Search for linu- ron residues in tributaries of the Chesapeake Bay. J. Agric. Food Chem. 28: 974—978. Ms received February 5, 1988 SELOSTUS Linuronin hajoaminen hietamaassa Helvi Heinonen-Tanski Kuopion yliopisto. Teknisen ympäristöhygienian laitos, Pl 6, 70211 Kuopio Linuronia on käytetty meillä lähinnä porkkanan vilje- lyssä herbisidinä joko ennen tai jälkeen taimettumisen. Karuissa maissa sekä epäedullisissa sääoloissa (liian kui- vaa, liian sateista tai liian lyhyt tai liian kylmä kasvukausi) linuron ei ehdi hajota riittävän täydellisesti ja jäämävaa- ra maan pintakerroksissa on olemassa. Jäämäriski lisään- tyy myös, jos samaa maata käsitellään toistuvasti tai linuronia käytetään ylisuurina annoksina. Pahimmillaan linuronjäämät ovat alentaneet seuraavan samassa lohkos- sa viljeltävän kasvin kasvua ja satoa, mistä on olemassa kirjallisuudessa esimerkkejä. Tässä työssä on tutkittu keinoja nopeuttaa linuronin hajoamista maassa. Linuron on lisätty 14C-leimattuna suomalaiseen karkeaan ja melko happamaan hietamaa- han. Osa maista oli kalkittu (5 tn tai 10 tn/ha) edellisenä keväänä, viisi kuukautta ennen maanäytteiden ottoa. Maan alkuperäinen pH oli pH 6,2 jakalkituksen jälkeen se oli pH 6,7 kummassakin kalkitussa maassa. Seurat- tiin 14C02:n vapautumista ls°C:ssa 252 päivän ajan. Kalkitus lisäsi erittäin selvästi linuronin hajoamista. Tässä kokeessa ei voitu havaita eroja eri kalkkimäärien välillä. Linuronin hajoaminen jatkui sekä aerobisissa että anaerobisissa olosuhteissa; tosin kun olosuhteita muutet- tiin aerobista anaerobisiksi ja päinvastoin, hajoamisno- peus aina aluksi hidastui, millä seikalla on varmasti mer- kitystä kesinä, joinarunsassateiset jakuivat jaksot vuo- rottelevat. Tämänkokeen perusteella linuronin puoliintumisaika olisi tutkitussa maassa noin 8 kk, mikä on selvästi pidempi kuin kirjallisuudessa tavallisesti esitetyt 2—4 kk. Kokeen perusteella voidaan olettaa, että maan normaali ylläpitokalkitus nopeuttaa merkittävästi linuronin hajoa- mista jatätä kalkitusta kannattaisi käyttää hyväksi jopa aikaistettuna, jos on syytä epäillä, että maahan on jää- nyt liian korkeita linuronjäämiä. 44