Rate of bentazone transformation in four layers of a humic sandy soil profile with fluctuating water table

Abstract

The rate of transformation of a pesticide as a function of the depth in the soil is needed as an input into computations on the risk of residues leaching to groundwater. The herbicide bentazone was incubated at 15 degrees C in soil materials derived from four layers at depths of up to 2.5 m in a humic sandy soil profile with a fluctuating water table (0.8 to 1.4 m), while simulating the redox conditions existing in the field. Gamma-irradiation experiments indicated that bentazone is mainly transformed by microbial activity in the soil. The rate constant for transformation was highest in the humic sandy top layer; it decreased with depth in the sandy vadose subsoil. However, material from the top of the phreatic aquifer had a higher rate constant than that from the layers just above. The presence of fossil organic material in the fluviatile water-saturated sediment probably stimulated microbial activity and bentazone transformation. The changes in the transformation rate constant with depth showed the same trend as those in some soil factors, viz organic carbon content, water-extractable phosphorus and microbial density as measured by fluorescence counts. However, the (low) concentration of dissolved organic carbon (DOC) in the top of the aquifer did not fit the trend. The rate constant for bentazone transformation in the layers was higher at lower initial contents of the herbicide.

Water and herbicide transient flow transport in field-dried topsoils during controlled infiltration

“…Disc infiltrometer experiments and linked soil sampling occurred a few days after herbicide spraying: 17 March 2003 for the silt loam soil, 18 March 2003 for the silty clay soil and 20 March 2003 for the sandy loam soil, corresponding to a delay from herbicide spraying of 3, 4 and 10 days, respectively. Similar results of very slow biodegadation of either isoproturon or bentazone in various soils and subsurface environments have also been reported in the literature (e.g., Romero et al, 1996;Johnson et al, 1998;Sorensen and Aamand, 2001;Leistra et al, 2001). Analyses of soil structure and soil water have also been already described and discussed in part I (Bartoli et al, 2006).…”

Time effect on bentazone sorption and degradation in soil

“…Bentazone mineralisation during incubation experiments was slow. Low rates of bentazone degradation for topsoils and subsoils have been reported in several studies 12, 13. More mineralisation was observed in the clay and loamy soils than in the sandy soil (Table 2).…”

Study of the spatial variation of the biodegradation rate of the herbicide bentazone with soil depth using contrasting incubation methods

“…Similarly, DT 50 values for bentazone degradation in topsoils range from 2 to 15 weeks in previous studies (Huber and Otto, 1994), and degradation rate has been shown to decline with depth (Leistra et al 2001). In the current study the proportional decline in degradation rate was greater than that of biomass or dehydrogenase, so that the ratio of biomass or dehydrogenase to DT50 increased significantly (p<0.001) with soil depth, in the case of biomass, ranging from 0.85 at 0- (2003) and .…”

“…Contact time and soil type have a significant effect on bentazone availability (Boivin et al, 2004). It has been observed that the rate constant for bentazone dissipation was highest in the topsoil and decreased with depth (Leistra et al, 2001). …”