Metolachlor Sorption and Degradation in Soil Amended with Fresh and Aged Biochars

Abstract

Addition of organic amendments such as biochar to soils can influence pesticide sorption-desorption processes, and in turn, pesticide availability and biodegradation. Availability is affected by both the physical and chemical properties of soils and pesticides, as well as soil-pesticide contact time, or aging. Changes in sorption/availability of metolachlor with aging in soil amended with three macademia nut shells biochars aged zero (BCmac-fr), 1 year (BCmac-1yr) and 2 years (BCmac-2yr) and two wood biochars aged zero (BCwood-fr) and 5 years (BCwood-5yr) was determined. Sorption coefficient (Kd) values increased with incubation time to a greater extent in amended soil as compared to unamended soils; Kd increased by 1.2X for the unamended soil, 2.0X for BCwood-fr, 1.4X for BCwood-5yr, 2.4X for BCmac-fr, 2.5X for BCmac-1yr, and 1.9X for BCmac-4yr. This increase was the result of a 15% decrease in the metolachlor extractable with CaCl2 solution with incubation time in soil as compared to a 50% decrease in amended soil. Differences could possibly be due to diffusion to less accessible or stronger binding sites with time, a faster rate of degradation (in solution and on labile sites) than desorption, or a combination of the two in the amended soils. These data show that transport models would over-predict depth of movement of metolachlor in soil if effects of aging or biochar amendments are not considered.