Sorption and dissipation of aged metolachlor residues in eroded and rehabilitated soils

Abstract

The results of this laboratory study support a field dissipation study. Both showed that S-metolachlor has the same retention and dissipation rate throughout an eroded landform, which was not expected owing to the large variability in soil properties, including OC concentrations. Altering soil properties by adding topsoil increased metolachlor sorption and persistence. The method of extraction (conventional versus ASE) affected calculated sorption coefficients and dissipation rates. In all cases, groundwater ubiquity scores (GUSs) categorized metolachlor as having intermediate mobility.

Low Sorption and Fast Dissipation of the Herbicide Saflufenacil in Surface Soils and Subsoils of an Eroded Prairie Landscape

“…At each depth, we observed more saflufenacil sorption to soils from the lower slope than to those from the upper slope (Figure A), again in agreement with trends in soil organic carbon concentration. At this site and another eroded site in the same county, previous assessments have shown more herbicide sorption in the lower slope compared with the upper slope for the weak acids 2,4-D, aminocyclopyrachlor, and picloram, but no or small differences for S -metolachlor and glyphosate . Together with previous observations, these results concur with Farenhorst et al, who noted that soil properties and terrain attributes tend to have a stronger influence on herbicides demonstrating low sorption (including saflufenacil) than on more highly sorbed herbicides.…”

“…In the surface soils, individual K d values ranged from 0.02 to 0.2 L kg –1 , and the mean K d was plotted in Figure . These sorption coefficients are similar to those found for the weak acid herbicide aminocyclopyrachlor (0.04–0.12 L kg –1 ) but much lower than those measured for the acetanilide S -metolachlor (3 L kg –1 ) in soils from the same field site. Our results are consistent with those of Hixson, who reported K d values of 0.06–2.4 L kg –1 for saflufenacil sorption to seven surface soils, including a Chernozem of pH 7.5 from Saskatchewan ( K d = 0.07 L kg –1 ).…”

“…Values of K oc for the lower slope are higher than those for the upper slope (Figure B), suggesting that the organic matter in the depositional area has a stronger affinity for saflufenacil than does organic matter in the eroded upper slope. Metolachlor showed an opposite trend and K oc values for aminocyclopynachlor and picloram were the same in the upper slope and lower slope . Differences in organic matter sorptivity across this landscape appear to be compound-specific, emphasizing the need for compound- and soil-specific information to enable accurate predictions of pesticide fate.…”

“…3 In these landforms, laboratory assessments of sorption and transformation of a variety of herbicides have been shown to be spatially variable, with more sorption and slower degradation observed in the high-organicmatter soils of the lower slope compared to the eroded upper slope. 4−8 However, our recent work in one eroded prairie landform showed no differences due to landscape position in Smetolachlor retention and dissipation in field studies 9 or laboratory studies 10 despite a 50% difference in organic carbon concentration. Landscape position effects on herbicide fate and transport are not universal and require further investigation.…”

Mineralization of S-metolachlor in soil as affected by moisture content, application history, and association with glyphosate

“…The average K d value of S-metolachlor in cornfield soil was 0.74 mL g -1 , whereas 3.22 mL g -1 was observed for the non-cultivated soil (Figure 5). Since S-metolachlor mineralization in soil has been demonstrated to be negatively correlated with sorption (Alletto et al, 2013, Cabrera et al, 2012, likely, mineralization of the herbicide from cornfield soil was confounded by differences in herbicide sorption owing to differences in soil organic matter content. Thus, further research is required to determine the extent of S-metolachlor mineralization that could be solely attributable to enhanced adaptation.…”