Sodium Chloride Enhances Nitrogen Use Efficiency but Reduces Yield Benefits Under Elevated CO₂ in Upland Rice

Abstract

Climate-change-driven elevation of atmospheric CO2 (e[CO2]) disrupts rice physiology by impairing nitrogen use efficiency (NUE) and leaf carbon balance. This study investigated how sodium chloride (NaCl) amendment modulates these processes in upland rice (Oryza sativa L. cv. CMG 2085) under current (400 μmol mol−1) and elevated (700 μmol mol−1) CO2 concentrations. Using a randomized block design with factorial treatments (CO2 × NaCl), we analyzed leaf nutrients, gas exchange, chlorophyll fluorescence, and yield parameters. Our findings revealed that 3 mmol L−1 NaCl under ambient CO2 (1) reduced photorespiration by half, (2) increased grain yield, and (3) enhanced leaf area despite lower leaf N content, indicating improved NUE. Conversely, under e[CO2], NaCl supplementation decreased rice yield by 15%, demonstrating CO2-dependent reversal of sodium benefits. Photosynthetic modeling showed higher Vcmax and J values at ambient CO2, while e[CO2] increased J/Vcmax, suggesting altered nitrogen allocation to photosynthetic reactions. These results demonstrate that applying low-dose NaCl (3 mmol L−1) can optimize carbon and nitrogen economy under current CO2 concentrations, although its efficacy diminishes under e[CO2]. These findings support climate-resilient cultivation strategies for upland rice in tropical and subtropical regions where mild salinity can be used to enhance nitrogen use efficiency and yield under present-day atmospheric conditions.

Keywords

• sodium fertilization
• climate change
• <i>Oryza sativa</i> L.
• photorespiration
• photosynthetic responses
• climate-smart agriculture