Changes in sorption and bioavailability of herbicides in soil amended with fresh and aged biochar

Knowledge of long-term pesticide behavior in biochar (BC) amended soil is still contradictory. In this work, we compared the sorption of three highly persistent and ionizable pesticides [two anionic (imazamox and picloram) and one weak base (terbuthylazine)] on both fresh and field-aged BC as well as on a soil amended with these biochars. The aging process was performed by burying the biochar at 10 cm in a silt loam soil [Upper Midwest USA (Wisconsin)] for six months. Field aged BC removed the three pesticides from solution to a higher extent (> 85%) than the fresh BC (< 16%). This removal was attributed to water-soluble components on the biochar. Laboratory incubation experiments demonstrated that dissipation in soil and biochar mixtures were both a function of pesticide and biochar type. The amounts of soil extractable (i.e, bioavailable) pesticide were inversely related to the extent of sorption. Similar dissipation and sorption of imazamox was observed in unamended and BC-amended soils. Terbuthylazine dissipated similarly in all treatments, but sorption increased with incubation time in the aged BC-amended soil. Conversely, picloram dissipated to a higher extent in aged BC-amended soil when compared to unamended or fresh BC-amended soil. This work demonstrates temporal variability of biochar sorption capacities due to soil exposure, which alters the efficacy and bioavailability of soil applied pesticides.