Elevated temperature and enhanced UV-B radiation alter soil nitrogen supply and allocation within rice plants in high-altitude paddy fields

Elevated temperature and enhanced ultraviolet B (UV-B) radiation are two major global climate change issues of concern today, but their effects on high-elevation rice paddy ecosystems are of limited understanding. A Two-year field experiment was conducted in a paddy field at an altitude of 1600 m in the Yuanyang Terraces, southwest China, to investigate the individual and combined effects of elevated temperature (2℃) and enhanced UV-B radiation (5 kJ m⁻2) on soil nitrogen supply and nitrogen allocation in rice. Elevated temperature inhibited rhizosphere urease activity during the jointing stage, leading to a 1.63–22.53% decrease in inorganic nitrogen concentration. In contrast, enhanced UV-B radiation increased rhizosphere protease activity in 2021, resulting in a 13.35–20.95% rise in NO₃⁻–N concentration. The combined treatment further reduced rhizosphere inorganic nitrogen concentration by 24.75–39.47% during the jointing and booting stages. Enhanced UV-B radiation reduced rice biomass by an average of 19.03%, while elevated temperature increased it by 52.47%, effectively mitigating the growth inhibition caused by UV-B. Both elevated temperature and enhanced UV-B radiation individually promoted nitrogen allocation to roots and stems, increasing their concentration and accumulation. However, the combined treatment weakened this effect. Elevated temperature also promoted nitrogen accumulation in grains, whereas UV-B radiation exhibited an inhibitory effect. The partial least squares path model indicated that elevated temperature primarily influenced nitrogen allocation by promoting rice growth, while UV-B radiation acted through regulating soil inorganic nitrogen concentration. Although elevated temperature did not enhance soil nitrogen availability, it effectively mitigated the inhibitory effects of UV-B radiation on rice growth by optimizing nitrogen allocation among roots, stems, and grains. This finding provides valuable insight into crop response mechanisms under future climate change and is crucial for predicting the stability and productivity of vulnerable agroecosystems.