High surface area biochar negatively impacts herbicide efficacy

Background and Aims: Amendment of soil by biochar may reduce efficacy of soil-applied herbicides due to sorption. Methods: Bioassays with Green Foxtail (Setaria viridis) tested the influence of two biochars on phytoavailability of S-metolachlor and sulfentrazone under biochar amendment of 0, 13, 26 and 52 Mg ha ^-1. Results: Adsorption of both herbicides was an order of magnitude greater on a high specific surface area (SSA) biochar (EUC-800; SSA 242 m ² g ^-1) than on a low SSA biochar (BC-1; SSA 3.6 m ² g ^-1). Herbicide doses near the lowest recommended label rates controlled the weed at 13 and 26 Mg ha ^-1 of BC-1; sulfentrazone was also effective at 52 Mg BC-1 ha ^-1. These same herbicide doses controlled weed germination and development only at 13 Mg ha ^-1 of EUC-800; at herbicide doses near the highest label rates, weed control was also achieved at 26 Mg EUC-800 ha ^-1, but not at 52 Mg EUC-800 ha ^-1. Conclusions: Increased doses of soil-applied herbicides cannot necessarily offset decreases in herbicide phytoavailability in biochar-amended soils, particularly if the biochar has a high SSA. Considering the long half-life of biochar in soil, pest control needs will be best served by low SSA biochars.