Impact of Biochar Amendment On Pesticide Efficacy: Bioassays and Sorption Studies.

Sorption of many organic chemicals on biochars greatly exceeds their sorption on soil organic matter, and frequently, desorption kinetics are hindered. A soil amendment with such sorption characteristics may reduce pesticide leaching and decrease its degradation, or, alternatively, cause pesticide inactivation meaning that greater amounts of pesticide would be needed to obtain the same level of pest protection. To help elucidate some of these processes, we examined the impact of biochar on sorption, volatilization and efficacy of a common soil fumigant, 1,3-Dichloropropene (DCP) used against nematodes in fresh and incubated systems. The biochar had a low specific surface area (SSA; 3.0 m²/g). In addition, we examined the influence of two biochars having significantly different specific surface areas (SSA; 3.0 m²/g and 242 m²/g) on sorption and efficacy of two widely used herbicides (Metolachlor and Sulfentrazone) against a common weed, Green Foxtail. With low SSA biochar, adequate weed and nematode protection could be obtained at 13 and 26 t biochar/ha. However, pest control with high SSA biochar was inadequate at common biochar application rates. Aging did not significantly affect sorption to the biochar, such that pest protection is not expected to be substantially improved under aged conditions. Biochars produced at higher temperatures generally have substantially higher sorption affinities and capacities than those produced at lower temperatures. From our bioassays, it is apparent that the potential detrimental impact of biochar amendment on pest control must be taken into account when contemplating the use of biochar in field soils, particularly as the half-life of biochar in the soil is estimated to be 100s to thousands of years. For now it is prudent to suggest that pest control requirements would be best served by biochars bearing relatively low SSAs.