Mechanistic insights into glyphosate adsorption and desorption in vineyard soils: Role of organic matter and iron and aluminum oxides.

Glyphosate is the most widely used herbicide worldwide and is extensively applied in vineyard systems, where its environmental behavior is largely governed by soil adsorption-desorption processes. To investigate the mechanisms controlling glyphosate retention in vineyard soils, adsorption and desorption experiments were conducted on 18 vineyard soils from the northwestern Iberian Peninsula with contrasting physicochemical properties. Organic matter removal and phosphate pre-saturation experiments were additionally performed on three representative soils to elucidate sorption mechanisms. Glyphosate showed strong adsorption across all soils, with adsorption percentages ranging from 69.6% to 92.8%. Freundlich adsorption coefficients (KF(ads)) ranged from 103 to 287 Ln mg1-n kg-1. Desorption was consistently low (1.2-11.5%), indicating strong retention and pronounced hysteresis, with KF(des)/KF(ads) ratios of 1.20-4.06. Adsorption decreased with increasing soil pH and sand content, whereas it increased with clay content, organic carbon, and extractable Fe and Al. However, removal of organic matter increased adsorption and reduced desorption, while phosphate pre-saturation produced the opposite effect. These results indicate that organic matter indirectly influences glyphosate behavior by partially blocking reactive mineral surfaces, whereas Fe and Al (hydr)oxides control sorption through ligand exchange mechanisms. A multiple linear regression model including pyrophosphate-extractable Al (Alp), pH, and clay content explained 74.5% of the variability in glyphosate retention after desorption. Overall, glyphosate behavior in vineyard soils is primarily governed by the mineral fraction rather than organic matter, providing mechanistic insight into its environmental mobility.