Abstract:To elucidate the effects of water-saving irrigation and nitrogen reduction combined with biochar application on the photosynthetic characteristics and water and nitrogen utilization of rice, a combination of field trials test and microzone test was used. 15N tracing technology was applied, with B0N1 (no biochar application+conventional nitrogen application level) as the control. B1N2 (10% nitrogen reduction+12.5t/hm2 biochar), B2N2 (10% nitrogen reduction+25t/hm2 biochar), B1N3 (20% nitrogen reduction+12.5t/hm2 biochar), B2N3 (20% nitrogen reduction+25t/hm2 biochar), B1N4 (30% nitrogen reduction+12.5t/hm2 biochar) and B2N4 (30% nitrogen reduction+25t/hm2 biochar) were set up, the photosynthetic characteristic parameters of rice plant leaves, as well as dry matter accumulation and water consumption processes, were observed, and the relationship between photosynthetic characteristic parameters and WUE, NUE, amount of dry matter, and yield was established. The results showed that under water-saving irrigation, appropriate nitrogen reduction combined with biochar application can increase leaf area index (LAI), chlorophyll content (SPAD), net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr), while reducing stomatal limitation values (Ls). Excessive reduction of nitrogen fertilizer or application of biochar would increase Ls, decrease LAI, SPAD, Pn, Gs, and Tr Compared with B0N1 treatment, B1N2 treatment increased the total accumulation of dry matter in plants by 14.79%, while B2N4 treatment decreased that by 16.02%. The NUE, yield, and WUE of rice treated with B1N2 were significantly higher than those treated with B0N1（P<0.05), with increases of 12.92%, 9.95%, and 1258%, respectively. The NUE, yield, and WUE of rice treated with B2N4 was significantly lower than that of B0N1（P<0.05), with decreases of 22.87%, 18.20%, and 5.66%, respectively;WUE and photosynthetic characteristic parameters (except LAI-tillering stage, SPAD, Tr-grouting stage) were significantly or extremely significantly positively correlated, and were significantly negatively correlated with Ls （P<0.01). NUE, amount of dry matter, yield, and photosynthetic characteristic parameters (except LAI-tillering stage, SPAD-tillering stage, Tr-grouting stage) were significantly or extremely significantly positively correlated, and significantly or extremely significantly negatively correlated with Ls. Overall, B1N2 treatment was the most optimal, which meant reducing the application of 10% nitrogen fertilizer and apply 12.5t/hm2 of biochar under water-saving irrigation was beneficial for improving water and nitrogen utilization efficiency and yield. The research results can provide theoretical basis and technical support for the application of water-saving irrigation with nitrogen reduction and biochar application in cold and black soil paddy fields.
