Abstract

Urea is the most widely used nitrogen fertilizer worldwide, and its application leads to soil acidification, which can potentially change the behavior of agrochemicals such as glyphosate and its main degradation product, aminomethylphosphonic acid (AMPA). This study assessed how urea-induced acidification influences the adsorption of glyphosate and AMPA in an Andisol. Batch equilibrium experiments were conducted to evaluate adsorption kinetics and isotherms with and without urea (200 kg N ha(-1)), as well as under controlled pH conditions (pH 4, 5, and 6). Kinetic data were analyzed using pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models, while adsorption isotherms were described using the Freundlich model. Results showed clear differences in sorption behavior between both compounds. AMPA exhibited higher sorption capacity, faster equilibrium, and minimal effect from urea addition. In contrast, glyphosate adsorption was significantly reduced by urea, showing lower kinetic parameters. Mechanistic analysis indicated that AMPA retention is governed by chemisorption and intraparticle diffusion processes, whereas glyphosate adsorption is more influenced by surface interactions and competition with urea. Overall, urea application may increase glyphosate mobility in Andisols, while AMPA remains strongly retained, highlighting the role of fertilization in herbicide fate.