A review on effects and regulation of paddy alternate wetting and drying on rice nitrogen use efficiency

Abstract: Since Green Revolution in 1960s, rice yield has increased dramatically worldwide due to extensive application of chemical fertilizer, particularly nitrogen fertilizer. In fact, farmers always obtain high rice yield by excessive application of nitrogen fertilizer. This raises environmental problems and causes low nitrogen use efficiency (NUE) in rice. Improvement of rice NUE is one of the most efficient ways to resolve those problems, and it has been one of the most concerned research subjects in the field of rice science. Rice production consumes about 80% of agricultural irrigation water in Asia, which is the largest rice planting area in the world. However, seasonal water deficit caused by climate change is becoming a major threat in rice production in many areas. Paddy alternate wetting and drying (AWD) is an important water management technology in counteraction of water deficit and improvement of rice water use efficiency (WUE). It has been accepted as an efficient water regime in main rice planting countries. It is also found that AWD is an effective approach to improve the NUE. Nevertheless, rice NUE is influenced by many factors under AWD irrigation condition, including rice variety, ecological environment, nitrogen fertilizer management, and soil drying intensity. Drying and re-watering cycle in AWD affects biochemical and physical processes such as nitrification, denitrification, mineralization, percolation, and leaching in soil by changing soil water and air equilibrium, which in turn affect the availability of nitrogen nutrition. However, the processes are complex, involving the interactions of soil pH and Eh values, and microbe community and quantity. Proper AWD (e.g. alternate wetting and moderate drying, WMD) facilitates the growth of rice root, improves its ultrastructure and activity, and promotes nitrogen absorption, assimilation, and transference in rice. As a result, NUE of rice is increased. Nitrate reductase (NR), glutamine synthetase (GS), and glutamate: oxo-glutarate aminotransferase (GOGAT) are the most important enzymes in nitrogen assimilation. Their activities are positively correlated with NUE and improved under appropriate AWD condition. WMD has no adverse effects on rice photosynthesis that is the main source of dry matter for rice yield. In addition, it accelerates the transference of dry matter from vegetative organs to seeds developing, and increases harvest index (HI) of rice. Therefore, both rice yield and NUE are enhanced. AWD leads to the change of phytohormone in rice, which might participate in the regulation of rice NUE. Cytokinin is an essential phytohormone in the regulation of nitrogen metabolism. Many studies observed the increase of cytokinin content in both rice roots and leaves under WMD condition. Cytokinin content was positively correlated with net photosynthetic rate. However, the regulative mechanism of cytokinin on photosynthesis and NUE has remained to be elucidated. In this paper, we reviewed the main factors that impact rice NUE under AWD conditions, with emphasis on the influence mechanisms of AWD on rice NUE in the aspects of rhizosphere nitrogen nutrition and environment, root morphology and function, nitrogen assimilation and re-transference, carbon assimilation and allocation, and regulation of phytohormone. Finally, we proposed some suggestions for further research in the field of the relationship between AWD and NUE.