Can Simple Soil Parameters Explain Field-Scale Variations in Glyphosate-, Bromoxyniloctanoate-, Diflufenican-, and Bentazone Mineralization?

The large spatial heterogeneity in soilphysico-chemical and microbial parameters challengesour ability to predict and model pesticide leaching fromagricultural land. Microbial mineralization of pesticidesis an important process with respect to pesticide leachingsince mineralization is the major process for the completedegradation of pesticides without generation ofmetabolites. The aim of our study was to determinefield-scale variation in the potential for mineralizationof the herbicides glyphosate, bromoxyniloctanoate,diflufenican, and bentazone and to investigate whetherthis variation can be predicted by variations in basic soilparameters. Sixty-five soil samples were sampled froman agricultural, loamy field in Silstrup, Denmark, from a60×165mrectangular grid. The mineralization potentialof the four pesticides was determined using a 96-wellmicroplate 14C-radiorespirometric method. Initial mineralizationrates were determined using first-order kineticsfor glyphosate and bromoxyniloctanoate and zeroorderkinetics for diflufenican and bentazone. Themineralization rates of the four pesticides varied betweenthe different pesticides and the different soil samples,but we could not establish correlations between thepesticide mineralization rates and the measured soilparameters. Only the glyphosate mineralization ratesshowed slightly increasing mineralization potentials towardsthe northern area of the field, with increasing clayand decreasing OC contents. The mineralization potentialsfor glyphosate and bentazone were compared with9-years leaching data from two horizontal wells 3.5 mbelow the field. The field-scale leaching patterns, however,could not be explained by the pesticide mineralizationdata. Instead, field-scale pesticide leaching mayhave been governed by soil structure and preferentialflow events.