Effects of nitrogen fertilizer reduction and oxygen-enhancing irrigation on the key enzyme activities of nitrogen metabolism and nitrogen utilization in rice

A very low utilization rate of nitrogen fertilizer has posed a great challenge to agriculture production in China in recent years. There is an almost half loss rate of nitrogen fertilizer applied to the soil, leading to the ever-increasing surplus of nitrogen in agricultural nitrogen balance. Once the nitrogen content in soil has not yet reached a steady state, a lot of nitrogen loss can occur, as the nitrogen content in soil increases. The dissolved oxygen concentration in soil can reduce the loss of nitrogen runoff for the high yield. The nitrogen use efficiency of rice can depend mainly on the nitrogen forms, microbial community structure, as well as nitrogen uptake and transformation in rhizosphere. This study aims to clarify the effect of reduced nitrogen fertilizer application and aerobic irrigation on the key growth period of rice, including the nitrogen metabolism-related enzyme activities, plant nitrogen content, nitrogen accumulation, yield, and nitrogen use efficiency. A series of experiments were carried out in the network room of the experimental base at China National Rice Research Institute. The test materials were Zhonghan 221 (upland rice), Zhongzheyou 8 (rice), and IR45765-3B (deep rice). Two irrigation modes were set, including Conventional Flood Irrigation (WL), and Micro-Nano Bubble Water Oxygenation Irrigation (MBWI). Two nitrogen levels were set as the conventional (195.0 kg/hm2) and reduced nitrogen fertilizer application (157.5 kg/hm2). The results showed that: 1) The activities of key enzymes of nitrogen metabolism in rice leaves decreased in the reduced nitrogen fertilizer application, compared with the conventional dosage. The MBWI significantly improved the nitrate reduction of rice leaves, Enzyme, glutamine synthase, glutamate synthase, and alanine aminotransferase activity. 2) The MBWI and increased nitrogen fertilizer dosage greatly contributed to the nitrogen accumulation in different organs of rice, whereas, the reduction of nitrogen fertilizer application under MBWI was better than that under WL. The nitrogen fertilizer utilization rate in the season increased by 15.6%, 36.2%, and 21.5% (P<0.05), respectively, for three rice varieties. 3) The grain yield of different rice varieties significantly increased under the oxygen-increasing irrigation with MBWI and increasing nitrogen fertilizer application rate. By contrast, the grain yield increased by 3.5% (P<0.05) via the reduced nitrogen fertilizer application under medium-dry 221 irrigation, compared with conventional nitrogen application under WL. However, there was no significant difference between the reducing and routine nitrogen application under WL (P>0.05). 4) A correlation analysis showed that the activities of key enzymes of nitrogen metabolism in rice leaves were mostly positively correlated with the leaf nitrogen content and nitrogen accumulation in the same period (P<0.05) or extremely significantly (P<0.01). Consequently, the oxygenated irrigation under MBWI can significantly improve the activities of nitrogen metabolism-related enzymes in different rice varieties, thereby significantly improving the nitrogen accumulation of rice plants for the high efficiency of nitrogen fertilizer use in the current season, as well as the high grain yield of rice. The MBWI under the condition of fertilization can greatly contribute to maintaining the high absorption of nitrogen fertilizer and utilization efficiency, where the glutamine synthase activity can be used to predict the nitrogen accumulation in rice at various stages. The research results provide theoretical and technical support for reducing nitrogen application in rice and improving nitrogen use efficiency in rice.