Modeling nitrogen dynamics and biomass production in rice paddy fields of cold regions with the ORYZA-N model

This article introduces ORYZA-N as an enhanced soil-water-nitrogen-plant coupling model to simulate agro-hydrological processes and crop growth in rice paddy fields. The model is developed based on the ORYZA2000 (a widely-used rice model with an open-source code) with significant improvements in modeling the nitrogen (N) fate and related processes. Three new modules were designed in ORYZA-N for simulating N transport and transformation processes, including a soil temperature module, a solute module for ammonium nitrogen (NH4-N) and nitrate nitrogen (NO3-N) transport, and a soil nitrogen module for organic carbon and nitrogen turnover. The mechanics of root water uptake and soil water drainage also were improved to better describe the soil water movement and its effects on root N uptake. The ORYZA-N model was then tested and applied in simulating the fate of water and nitrogen and crop growth for Japonica rice in cold regions (JRC) in the Northeast China Plain (NECP), for a two-year field experiment (during 2018 and 2019). Model evaluation shows a good performance was achieved with the ORYZA-N model. The simulated values for pond water depths, N uptake and partitions, and various organ productions were in particularly good agreement with the observed values (R2 > 0.68, NSE > 0.50); meanwhile, the simulated fluctuation trends for NH4-N and NO3-N concen-trations were reasonable as well. The calibrated/obtained nitrogen-related parameters for JRC were obviously different from those for rice varieties in warmer regions, which can efficiently complement the parameter dataset for rice modeling in cold regions. Finally, the N dynamics, balance, and utilization efficiency were interpreted and assessed for the two experimental years, and possible improvement measures for N use for JRC were pro-posed. The total aboveground N content was about 160 kg N ha-1 at harvest, while the N content in the stems, green leaves, dead leaves, and panicles accounted for about 15, 5, 25, and 55% of the total aboveground N content, respectively. Overall, rational simulation proved the enhanced functionality and practicality of ORYZA-N for modeling the N fate and utilization.