Desorption Kinetics of Tricyclazole and Isoprothiolane in Biochar-Amended Paddy Soils and Implications for Pesticide Bioavailability

Assessing pesticide bioavailability in paddy soils is critical for understanding environmental fate and potential food safety implications. This study investigated the desorption and immobilization of isoprothiolane and tricyclazole in a clay- and organic-rich Mekong Delta paddy soil, comparing Tenax and XAD-4 as extraction agents and assessing the effect of biochar amendment, using a two-compartment desorption model. Although both compounds fall below the ISO 16751:2020 criterion for Tenax (log Kow > 3), Tenax and XAD-4 did not differ significantly for the moderately hydrophobic isoprothiolane, whereas for the more polar tricyclazole, Tenax yielded a significantly lower fast-desorbing fraction, despite efficient sorption in matrix-free tests. Both pesticides exhibited high fast-desorbing fractions despite the soil’s high clay and TOC content, consistent with their predominantly neutral form at the soil’s low pH; nevertheless, a considerable slowly desorbing fraction of tricyclazole indicates interactions beyond hydrophobic partitioning. Biochar amendment had compound-specific, opposing effects: it significantly reduced the fast-desorbing fraction of tricyclazole at the applied, above-field concentration, whereas it increased that of isoprothiolane without significantly affecting its remaining fraction in soil. Residues of both fungicides were detected in regional rice grain, but could not be directly linked to the laboratory-derived parameters.