Soil Properties Control Glyphosate Sorption in Soils Amended with Birch Wood Biochar

Despite a contemporary interest in biochar application to agricultural fields to improve soil quality and long-term carbon sequestration, a number of potential side effects of biochar incorporation in field soils remain poorly understood, e.g., in relation to interactions with agrochemicals such as pesticides. In a field-based study at two experimental sites in Denmark (sandy loam soils at Risoe and Kalundborg), we investigated the influence of birch wood biochar with respect to application rate, aging (7–19 months), and physicochemical soil properties on the sorption coefficient, Kd (L kg−1), of the herbicide glyphosate. We measured Kd in equilibrium batch sorption experiments with triplicate soil samples from 20 field plots that received biochar at different application rates (0 to 100 Mg ha−1). The results showed that pure biochar had a lower glyphosate Kd value as compared to soils. Yet, at the Kalundborg soils, the application of biochar enhanced the sorption of glyphosate when tested after 7–19 months of soil–biochar interaction. The relative enhancement effect on glyphosate sorption diminished with increasing biochar application rate, presumably due to increased mineral–biochar interactions. In the Risoe soils, potential biochar effects on glyphosate sorption were affected by a distinct gradient in soil pH (7.4 to 8.3) and electrical conductivity (0.40–0.90 mS cm−1) resulting from a natural CaCO3 gradient. Thus, glyphosate Kd showed strong linear correlation with pH and EC. In conclusion, the results show that biochar, despite initially being a poor sorbent for glyphosate, can increase glyphosate sorption in soil. However, the effect of biochar on glyphosate sorption is depends on prevailing soil physicochemical properties.