Nitrogen (N) transformation in paddy rice field: Its effect on N uptake and relation to improved N management

Nitrogen (N) is pivotal to crop yield, and the application of N fertilizer in crop production systems is a crucial aspect of modern crop management practices and one of the determining factors to increase crop yield and thereby keeping pace with human population increase. However, most of the N fertilizers (>60%) added to rice fields is not taken up by rice plants, but lost to the environment in forms of ammonia, nitrate, and nitrous oxide. The 'reactive N' causes serious environmental problems and detrimental impacts on human health. Furthermore, the N cycling in a cropping system is complicated, and solutions must be made based on an in-depth understanding of the transformation of N in soil and the biochemical processes of N in rice plants. Here, we described multiple transformations and oxidation/reduction processes of N in the paddy rice field, and then summarized the basic biological processes in N acquisition, transportation, assimilation, and metabolism in rice. We discussed various local N signals that regulate root systems. In the end, we suggested the solutions that can synchronize the supply of available N in the soil to the demands of rice plants. A portfolio solution is needed in which integrative management should be established to reduce N loss and increase N use efficiency in rice production.