Glyphosate behavior in rice paddy fields across different rotation systems

The intensification of irrigated rice production systems requires a comprehensive assessment of the environmental impact of pesticide use. Here, we investigated the behavior of the herbicide glyphosate (GLY) and its metabolite aminophosphonic acid (AMPA), throughout four rice rotations (continuous rice cropping (R_C), rice-soybean (R-S), rice-short term pasture (R-P_S) and rice-long term pasture R-P_L) reflecting different agricultural intensification scenarios in a long-term experimental (LTE) site. GLY was applied twice at the same amount in all rotations as a chemical fallow and a rice pre-emergent herbicide. Application timing and frequency determined GLY and AMPA levels, but not their decay rates. GLY decay in soil differed between the non-flooded and flooded phases, fitting a first-order exponential decay in all rotations. Floodwater showed no significant differences in decay rates for the evaluated rotations. GLY concentration in soil was determined close to 200 mg kg^-1 whereas AMPA 600 mg kg^-1 ca. after 161 days post-application. During the non-flooded phase, GLY degradation in soil showed a half-life (DT₅₀) of 31.5 days. Differently, GLY and AMPA decay in floodwater followed a first-order kinetic with DT₅₀ values of approximately 5.3 and 7.4 days, respectively. GLY transference from soil to floodwater was evidenced, peaking after 8 days, followed by a significant decay in the first 39 days and negligible decay thereafter. AMPA showed the same pattern with a slower decay rate. Our findings revealed land use intensification scenarios affect background GLY levels in soil, where rice-pasture based rotations (R-P_S and R-P_L) result in lower GLY residues than R_C and R-S cropping systems. These findings hold meaningful implications, aiming at indicators and practices for defining more sustainable productive schemes and water management practices.