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TECHNICAL REPORTS

Bioremediation and Biodegradation

Evaluation of Simplifying Assumptions on Pesticide Degradation in Soil

^a Cranfield Centre for EcoChemistry, Cranfield University, Silsoe, Bedford, MK45 4DT, UK
^b Environment Department, University of York, Heslington, York, YO10 5DD, UK, and Central Science Laboratory, Sand Hutton, York, YO41 1LZ, UK
^c Warwick HRI, University of Warwick, Wellesbourne, Warwick, CV35 9EF, UK

^* Corresponding author (s.beulke{at}csl.gov.uk)

Received for publication November 26, 2004. There is evidence that degradation of pesticides in simple laboratory systems may differ from that in the field, but it is not clear which of the simplifications inherent in laboratory studies present serious shortcomings. Laboratory experiments evaluated several simplifying assumptions for a clay loam soil and contrasting pesticides. Degradation of cyanazine [2-(4-chloro-6-ethylamino-1,3,5-triazin-2-ylamino)-2-methylpropiononitrile] and bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] at fluctuating temperature and moisture was predicted reasonably well based on parameters derived from degradation under constant conditions. There was a tendency for slower degradation of cyanazine and bentazone in soil aggregates of 3 to 5 mm in diameter (DT50 at 15°C and 40% maximum water holding capacity of 25.1 and 58.2 d, where DT50 is the time for 50% decline of the initial pesticide concentration) than in soil sieved to <3 mm (DT50 of 19.1 and 37.6 d), but the differences were not significant for most datasets. Degradation of cyanazine, isoproturon [3-(4-isopropylphenyl)-1,1-dimethylurea], and chlorotoluron [3-(3-chloro-p-tolyl)-1,1-dimethylurea] was measured in soil amended with different amounts of lignin. The effect of lignin on degradation was small despite considerable differences in sorption. The DT50 values of cyanazine, isoproturon, and chlorotoluron were 16.2, 18.6, and 33.0 d, respectively, in soil without lignin and 19.0, 23.4, and 34.6 d, respectively, in soil amended with 2% lignin. Degradation of bentazone and cyanazine in repacked soil columns was similar under static and flow conditions with 50.1 and 47.2% of applied bentazone and 74.7 and 73.6% of applied cyanazine, respectively, degraded within 20 d of application. Thus, the assumptions underpinning laboratory to field extrapolation tested here were considered to hold for our experimental system. Additional work is required before general conclusions can be drawn.

Abbreviations: DT50, time for 50% decline of the initial pesticide concentration • K_d, sorption coefficient • K_oc, sorption coefficient normalized to organic carbon content • mwhc, maximum water holding capacity • Q10, factor by which degradation increases when temperature increases by 10°C

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