Rice (Oryza sativa L.), one essential staple cereal, feeds over 60% of the world’s population (FAO 2020). As the global population grows, improving rice yield becomes an effective strategy to achieve food security (Deng et al. 2019).
The use of chemical fertilizer, especially nitrogen (N) fertilizer, has historically played a critical role in the growth of rice yield (Zhang et al. 2015; Wood et al. 2020). However, poor N management, especially excessive fertilizer application rate, has caused a number of adverse effects, such as, rice quality deterioration, yield instability, environmental degradation, and so on (Zhang et al. 2015, 2020; Guo et al. 2020; Hu et al. 2020). Therefore, worldwide scientists have increasingly focused on how to increase the utilization rate and decrease the input of N fertilizer without compromising rice yield and quality (Zhang et al. 2015; Deng et al. 2019; Hu et al. 2020; Wood et al. 2020).
The core of high quality, efficient and precise fertilization for rice is to improve the N-use efficiency (NUE) and rice quality depending on the characteristics of different varieties. There are three aspects to improve NUE: breeding rice varieties with high NUE, precisely managing N fertilizer application and adopting new fertilization methods for high efficiency fertilizers.
First, breeding rice varieties with high NUE is one fundamental method to improve NUE (Varshney et al. 2020; Liu et al. 2021). However, nitrogen-efficient genes are still a long way from common use in rice breeding. For current varieties, especially hybrid rice varieties, the use of restorer line is undoubtedly more direct and effective. Therefore, it is worthwhile to explore restorer lines with a high yield and high NUE (HYHN) property, and their common agronomic traits. Tian et al. (2021) select a number of highly efficient HYHN-type restorer lines and find that the advantages of HYHN restorer lines include a high level of nutrient accumulation and distribution to the panicles, and smooth flows of nutrients along the transportation channels. This finding provides important guidance for the crossbreeding of existing varieties.
Second, precision management of N fertilizer application is the fastest and most effective way to improve NUE, which includes soil testing and fertilizer recommendation (Chen et al. 2014), real-time and site-specific N management (Peng et al. 2006, 2010), precise and quantitative fertilization (Ling et al. 2005), and so on. The precise and quantitative fertilization is the most typical among these approaches. It uses systematic technical methods and parameters for the determination of total N rate, the N fertilizer ratio of base and tillering fertilizer to panicle fertilizer, and accurate leaf color diagnosis of N panicle fertilizer (Ling 2007). In recent decades, precision management of N fertilizer has been rapidly developing. There are three examples in this issue.
The first example is that total N rate depends on rice varieties and cultivation methods. Ratoon rice cropping is an important component of the rice cropping system in USA, and has expanded to Asian countries in recent years. N is the most effective nutrient for promoting regrowth and development of ratoon tillers, and improving N use efficiency of ratoon rice production will likely enhance the economic sustainability of rice production. Based on an experiment test spanning several years, Wang et al. (2021) find that main crop N rate significantly affects rice main crop. However, given N applied at 99 kg ha–1 at pre-flood after main crop harvest, the yield of rice ratoon crop is not significantly affected by main crop N rate. In addition, neither main crop N nor ratoon crop N has a significant effect on the head rice yield of ratoon crop.
The second example suggests that N rate might be related to rice grain quality. Few studies have examined the relationship between grain-filling characteristics of superior and inferior grains, and the grain quality of mid-season hybrid indica rice is still unclear. Zhang et al. (2021) conducted a field experiment to ascertain the critical grain-filling characteristics that contribute to rice milling quality, appearance quality and cooking and eating quality under different N applications. The results indicate that the prolonging grain-filling duration and increasing grain weight at the maximum grain-filling rate of inferior grains contributed to improved milling quality, appearance quality and cooking and eating quality of mid-season indica rice under appropriate N applications.
The third example is the development of diagnostic techniques for panicle N fertilizer. Yao et al. (2021) developed a new critical N dilution curve for hybrid indica rice under the mechanical pot-seedling transplanting system. This curve is able to determine more accurately and reliably the N nutrition status in pot-seedling mechanical transplanting (PMT) of hybrid indica rice than the existing curves, which can improve the management of real-time and dynamic rice fertilization.
Finally yet importantly, the adoption of new fertilization methods for high efficiency fertilizers is the latest breakthrough in improvement in NUE (Lam et al. 2018; Liu et al. 2020). Slow and controlled-release fertilizer as a new type of fertilizer has been the research subject of researches worldwide (Chen et al. 2018; Wei et al. 2018). In order to meet the long-term nutrient needs of crops, this type of fertilizer slows down the release of nutrients by employing different coating materials and adding inhibitors (Timilsena et al. 2015). Theoretically, it can greatly improve NUE by making the supply of N fertilizer synchronized with the demand of rice crops, which is achieved by the usual practice of splitting application of fertilizer according to crop nutrient requirements (Yuan et al. 2016). However, explicit methods are particularly important that reduce the number of fertilizer applications and increase NUE under the context of increasing agricultural labor costs (Ke et al. 2018; Li et al. 2018; Mi et al. 2019; Sun et al. 2019). The side-dressing placement fertilizer application of new high-efficiency fertilizers meet this demand and become the latest breakthrough in improvement in NUE (Zhang et al. 2016; Pan et al. 2017).
For side deep placement of N fertilizer in paddy rice, Zhao et al. (2021) conducted a field experiment and find that two fertilization models (RTN3RNR1) could achieve the dual goals of increasing grain yield and NUE. These two fertilization models are worth further investigations.
For slow and controlled-release fertilizers suitable for rice, Wu et al. (2021) conducted experiments to examine the N release characteristics of seven different slow and controlled release fertilizers, and their impacts on rice grain yields and yield components, in order to provide a theoretical basis for their further use. They find that the N release characteristics of different types of slow and controlled release fertilizers were significantly different and proposed that the fertilizer type with a stronger N supply capacity and a longer effective duration was more conducive to dry matter accumulation at the later growth stage, thus promoting higher rice yield.
In summary, studies in this special focus engage in a new research topic on high quality, efficient and precise fertilization of rice. The findings offer valuable guidance and reference for the management of high quality and high efficiency N fertilizers for rice.