Paddy ponding water quality responses to land use intensity in flooded rice systems in Uruguay

Understanding water quality responses in flooded rice (Oryza sativa L.) systems is essential to reconcile high productivity with environmental protection. This study provides the first high-frequency dataset of paddy water quality dynamics in temperate South American rice systems, conducted in the Laguna Merín Basin (Uruguay-Brazil). Through detailed field-scale monitoring under real management and objective fertilization criteria, we evaluated how land use intensity influences physicochemical variables and nutrient speciation in paddy ponding water under two contrasting systems: rice-pasture rotation (RP) and continuous rice (CR). Over two seasons, a biphasic temporal pattern was observed, with elevated phosphorus (P) and nitrogen (N) concentrations during early flooding (weeks 1-7) followed by stabilization. Despite both systems following best management practices, the more intensive CR consistently showed higher total and dissolved reactive P, while N concentrations were similar between systems and largely dominated by dissolved organic forms, indicating that N dynamics were governed by internal cycling rather than fertilizer inputs. Overall, land use intensity affected water quality in a nutrient-specific manner: P was more responsive to management and enhanced mobilization under flooded conditions, whereas N appeared buffered by biogeochemical processes. The lower P enrichment observed in the RP rotation suggests that management practices associated with this system may help moderate nutrient concentrations in paddy water. This study provides field-based evidence that differences in land use intensity influence nutrient behavior in flooded rice systems, contributing insights to improve water quality protection and sustainability in temperate rice agroecosystems.