ABSTRACT Field microtopography critically regulates water and nitrogen transport pathways, thereby governing the risk of nitrogen pollution in rice fields with significant spatiotemporal heterogeneity. Under an increasing trend of extreme rainfall events, its role in governing nitrogen loss has become even more pronounced. This study aimed to investigate the mechanisms of water flow and nitrogen transport and loss driven by microtopography. A 2‐year field experiment was conducted during the rice growing seasons of 2022 and 2023 in a naturally undulating rice field under conventional flooding irrigation. The HYDRUS‐3D model was calibrated and validated with field data to quantify the processes of nitrogen transformation and transport. The results demonstrated that the model successfully captured the measured surface water depth dynamics across different microtopographic regions, as well as the NH 4 + –N and NO 3 − –N concentrations at different depths. Microtopography significantly influenced nitrogen dynamics, with low‐lying areas exhibiting greater vulnerability to extreme rainfall than elevated areas. Prolonged ponding in depressions led to nitrogen loss primarily via denitrification and runoff. Whereas rapid drainage in elevated areas promoted nitrification, resulting in higher peak concentrations of NO 3 − –N leaching into deeper soil layers. Adjusting fertilization timing based on rice growth stages and extreme rainfall start time in 2023 increased grain yield by approximately 7.5% and mitigated nitrogen loss compared to conventional management in 2022. This study provides a scientific basis for mitigating agricultural non‐point source pollution and improving nitrogen use efficiency in rice fields.
Wu et al. (2026) studied this question.