High quality and high efficiency fertilization of rice

Rice (Oryza sativaL.),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 (Denget al.2019).

The use of chemical fertilizer,especially N fertilizer,has historically played a critical role in the growth of rice yield (Zhanget al.2015;Woodet al.2020). However,poor nitrogen 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 (Zhanget al.2015,2020;Guoet al.2020;Huet 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 (Zhanget al.2015;Denget al.2019;Huet al.2020;Woodet al.2020).

The core of high quality,efficient and precise fertilization for rice is to improve the nitrogen use efficiency and rice quality depending on the characteristics of different varieties. There are three aspects to improve nitrogen use efficiency:breeding rice varieties with high nitrogen use efficiency,precisely managing nitrogen fertilizer application and adopting new fertilization methods for high efficiency fertilizers.

First,breeding rice varieties with high nitrogen use efficiency is one fundamental method to improve nitrogen use efficiency (Varshneyet al.2020;Liuet 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. Tianet 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 nitrogen fertilizer application is the fastest and most effective way to improve nitrogen utilization efficiency,which includes soil testing and fertilizer recommendation (Chenet al.2014),real-time and site-specific nitrogen management (Penget al.2006,2010),precise and quantitative fertilization (Linget 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 nitrogen rate,the nitrogen fertilizer ratio of base and tillering fertilizer to panicle fertilizer,and accurate leaf color diagnosis of nitrogen panicle fertilizer (Ling 2007). In recent decades,precision management of nitrogen fertilizer has been rapidly developing. There are three examples in this issue.

The first example is that total nitrogen 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.Nitrogen (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,Wanget al.(2021)find that main crop N rate significantly affects rice main crop. However,given N applied at 99 kg ha–1at 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 nitrogen 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 midseason hybridindicarice is still unclear. Zhanget al.(2021)conducted a field experiment to ascertain the critical grainfilling characteristics that contribute to rice milling quality,appearance quality and cooking and eating quality under different nitrogen 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-seasonindicarice under appropriate nitrogen applications.

The third example is the development of diagnostic techniques for panicle nitrogen fertilizer. Yaoet al.(2021)developed a new critical N dilution curve for hybridindicarice under the mechanical pot-seedling transplanting system.This curve is able to determine more accurately and reliably the nitrogen nutrition status in PMT of hybridindicarice 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 nitrogen use efficiency(Lamet al.2018;Liuet al.2020). Slow and controlledrelease fertilizer as a new type of fertilizer has been the research subject of researches worldwide (Chenet al.2018;Weiet 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 (Timilsenaet al.2015). Theoretically,it can greatly improve nitrogen use efficiency by making the supply of nitrogen 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 (Yuanet al.2016). However,explicit methods are particularly important that reduce the number of fertilizer applications and increase nitrogen use efficiency under the context of increasing agricultural labor costs (Keet al.2018;Liet al.2018;Miet al.2019;Sunet al.2019). The side-dressing placement fertilizer application of new highefficiency fertilizers meet this demand and become the latest breakthrough in improvement in nitrogen use efficiency(Zhanget al.2016;Panet al.2017).

For side deep placement of nitrogen fertilizer in paddy rice,Zhaoet al.(2021) conducted a field experiment and find that two fertilization models (RTN3RNR1) could achieve the dual goals of increasing grain yield and nitrogen use efficiency. These two fertilization models are worth further investigations.

For slow and controlled-release fertilizers suitable for rice,Wuet 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 nitrogen fertilizers for rice.

LI Gang-hua

Guest Editor

College of Agriculture,

Nanjing Agricultural University,

Nanjing 210095,P.R.China