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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

View larger version (21K): [in a new window]   Fig. 1. Measured degradation of cyanazine under fluctuating temperature or moisture conditions and simulated degradation based on DT50 values (time for 50% decline of the initial pesticide concentration) measured under constant conditions. The term mwhc is maximum water holding capacity.

View larger version (21K): [in a new window]   Fig. 2. Measured degradation of bentazone under fluctuating temperature or moisture conditions and simulated degradation based on DT50 values (time for 50% decline of the initial pesticide concentration) measured under constant conditions (lines). The term mwhc is maximum water holding capacity.

View larger version (20K): [in a new window]   Fig. 3. Degradation of (a) cyanazine and (b) bentazone in <3-mm or 3- to 5-mm aggregates. Solid and dashed lines represent first-order fits to data for <3-mm and 3- to 5-mm aggregates, respectively (Day 0 data excluded from curve fitting). The term mwhc is maximum water holding capacity.

View larger version (28K): [in a new window]   Fig. 4. (a) Sorption coefficients (Kd values) of cyanazine, isoproturon, and chlorotoluron in soil amended with different amounts of lignin. (b) Sorption coefficients normalized to organic carbon content (Koc values).

View larger version (53K): [in a new window]   Fig. 5. (a) Cyanazine, isoproturon, and chlorotoluron extractable with organic solvent from soil amended with different amounts of lignin (as a percentage of applied pesticide = 20 mg kg–1; Day 0). (b) Percentage extractable with aqueous solution from soil amended with different amounts of lignin (as a percentage of total extractable residues; Day 0).

View larger version (16K): [in a new window]   Fig. 6. Residues of cyanazine, isoproturon, and chlorotoluron extractable with organic solvent for soil amended with different amounts of lignin. Data are expressed as % of the initial amount extractable and are the means of three replicates.

View larger version (24K): [in a new window]   Fig. 7. Vertical distribution of cyanazine and bentazone, 9 and 20 d after uniform mixing into the soil of replicate soil columns incubated under static (dashed lines) and flow conditions (solid lines). Individual replicates are shown.