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

## TL;DR

The study shows how land use intensity affects water quality in flooded rice fields in Uruguay, with continuous rice systems increasing phosphorus but not nitrogen levels.

## Contribution

This is the first high-frequency dataset of paddy water quality dynamics in temperate South American rice systems.

## Key findings

- Nutrient concentrations in paddy water followed a biphasic pattern with elevated levels during early flooding.
- Phosphorus concentrations were higher in continuous rice systems compared to rice-pasture rotations.
- Including pasture periods in rotation mitigated phosphorus enrichment and eutrophication risk.

## Abstract

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.

High‐frequency monitoring revealed phase‐specific water quality dynamics in flooded rice.Land use intensification increased P but not N concentrations in paddy water.The early flooding phase was the critical window for nutrient mobilization and mitigation efforts.Dissolved reactive P and organic N dominated nutrient fractions across systems.Including pasture periods in rotation mitigated P enrichment and eutrophication risk.

High‐frequency monitoring revealed phase‐specific water quality dynamics in flooded rice.

Land use intensification increased P but not N concentrations in paddy water.

The early flooding phase was the critical window for nutrient mobilization and mitigation efforts.

Dissolved reactive P and organic N dominated nutrient fractions across systems.

Including pasture periods in rotation mitigated P enrichment and eutrophication risk.

Rice fields are often flooded, which can move nutrients like phosphorus (P) and nitrogen (N) from soil into water, potentially affecting nearby rivers and wetlands. This study examined how different land use intensities influence water quality in rice fields in Uruguay, comparing a traditional rice–pasture rotation with a more intensive continuous rice system used elsewhere as a contrasting case. Over two growing seasons, water in rice paddies was monitored weekly for key nutrients, their chemical forms (speciation), and basic water quality indicators. We found that nutrient concentrations followed a two‐phase pattern: high N and P during the first 7 weeks after flooding, then stabilization. P was higher in the intensive system, while N remained similar across systems, mostly in dissolved organic forms, showing that internal cycling controlled N rather than fertilizer. Maintaining pastures can help protect water quality while sustaining rice production.