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Soil water potential and fertilizer nutrients are the two main factors affecting rice growth and development. A soil‐grown experiment was conducted using mid‐season japonica rice cultivar Xudao 3 with three different nitrogen (N) fertilization rates: (i) none, (ii) moderate nitrogen (240 kg ha–1), and (iii) high nitrogen (360 kg ha–1) and three irrigation regimes: (i) submerged irrigation (0 kPa), (ii) alternate wetting and moderate drying (–20 kPa), and (iii) alternate wetting and severe drying (–40 kPa). In the same N level, root length, root dry weight, root activity (represented by root‐bleeding sap), net photosynthetic rates at main growth stages, and ATPase activity during the grain‐filling stage are higher under the treatment of alternate wetting and moderate drying, when compared with those of submerged irrigation. Meanwhile, non‐structural carbohydrate (NSC) remobilization and shoot dry accumulation are also increased from heading to maturity, which contribute to higher grain yield and higher nitrogen‐use efficiency. In the same irrigation regime, root‐bleeding sap, NSC remobilization, and ATPase are increased significantly under the moderate‐N treatment compared with those of the high‐N treatment, which contribute to higher nitrogen‐use efficiency. However, no significant difference exists in grain yield and crop growth rate. These results indicate that the coordinated relationship between the root and shoot, especially increased root length, enhanced root‐bleeding sap, increased NSC remobilization, enhanced shoot dry accumulation, and ATPase activity during the grain‐filling stage, can promote rice yield and nitrogen‐use efficiency by using the appropriate nitrogen application and alternate wetting and moderate drying. Core Ideas Root and shoot traits were significantly different in response to irrigation regime and nitrogen level treatment. Alternated wetting/moderate drying interacted with moderate N to coordinate the development of root and shoot. Deeper roots increased root‐bleeding sap, NSC remobilization from stems to grain, and activity of grain sink. Greater root length (via root‐shoot relationships) contribute to increased yield and enhanced N‐use efficiency. Alternated wetting/drying irrigation with moderate N promotes grain filling and increased yield and N‐use.

Appropriate irrigation methods and N management can effectively increase the yield and N efficiency of rice (Oryza sativa L.). A soil‐grown experiment was conducted in 2018 and 2019 using Liangeng 7 with different N forms (NH3–N, NH4+ and NO3− mixed 50:50 [50:50], and NO3–N), as well as three irrigation regimes (submerged irrigation [0 kPa], alternate wetting and moderate drying [−20 kPa], and alternate wetting and severe drying [−40 kPa]). Results showed that yield and N agronomic efficiency were maximized at the alternative wetting and moderate dying treatment with NH4NO3 mixed among all treatments. Nitrogen forms in different irrigation regimes had different effects on root characteristic and leaf photosynthesis rate. In submerged irrigation, NH3–N treatment improved root morphology, increased the activities of NH3 assimilation enzymes in roots, and promoted root oxidation and leaf photosynthesis rate at different growth stages compared with other N forms. By contrast, moderate drying interacted with 50:50 and significantly enhanced root length, average root diameter, root volume, root tips, and root oxidation activity and promoted C and N metabolism in root and photosynthesis rate of leaves compared with other N forms under alternate wetting and drying irrigation. Multiple regression analysis showed that root length, the glutamate synthase (GOGAT) activity of roots, and the photosynthetic rate of leaves closely related with high rice yield and efficient N utilization. All these results suggested that improved root morphology and physiology activity, enhanced C and N metabolism, and increased leaf photosynthesis rate through the appropriate regulation of irrigation‐regime interaction with N forms can help to increase the grain yield and N agronomic efficiency of rice.

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The interaction betweennitrogen management and irrigation regimes plays important roles in regulating the rice (Oryza sativa L.) source, sink characteristic, and grain yield. However, if and how nitrogen forms could synergistically interact with irrigation methods in rice remain unclear. Herein, a soil-grown experiment was conducted with three nitrogen forms, namely, ammonium nitrogen (AN), ammonium, nitrate mixed (ANM), and nitrate nitrogen (NN), as well as three irrigation regimes, namely, submerged irrigation (0 kPa), alternate wetting and moderate drying (−20 kPa), and alternate wetting and severe drying (−40 kPa). Results demonstrated that grain yield was the highest in the ammonium nitrate and alternate wetting and moderate drying irrigation. Nitrate nitrogen application decreased grain yield in each irrigation regime. The alternate wetting and moderate drying and ammonium nitrate treatment had higher chlorophyll content, leaf nitrogen content, leaf area index (LAI), photosynthesis rate, and chlorophyll fluorescence, superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and sucrose-synthase activity and decreased leaf malondialdehyd (MDA) content than the other treatments at mid-tillering, panicle initiation, heading, and maturity stages. By contrast, AN treatment exhibited very small difference between AN and ANM treatment in submerged irrigation. Besides, grain yield was positively correlated with chlorophyll content, leaf nitrogen content, LAI, photosynthesis rate, antioxidant enzymes, and sucrose-synthase activity at main growth stages. Therefore, the adoption of ANM and alternate wetting and moderate drying can synergistically increase grain yield by promoting the leaf-source and grain-sink activity of rice. Results of the present investigation provided new ideas for increasing grain yield by nitrogen forms management in alternate wetting and drying irrigation.