Bioscience Methods 2025, Vol.16, No.6, 270-279 http://bioscipublisher.com/index.php/bm 2 73 stabilized, but also the vitality of the soil and the diversity of microorganisms have improved significantly (Iqbal et al., 2021; Urmi et al., 2022). Some studies simply replaced part of the chemical nitrogen source, approximately 30%-50% of the components, and replaced it with organic fertilizer. Not only did the land not become lean, but the fertilizer also lasted longer, and the yield and fertilizer efficiency did not decline. This "mixed and matched" fertilization approach is indeed more in line with the direction of ecological agriculture than a single route. It can also reduce the reliance on synthetic fertilizers and, incidentally, alleviate the problem of environmental degradation. 3.3 Smart fertilization and precision agriculture: emerging trends Intelligent fertilization sounds quite "high-end and sophisticated", but in fact, it has gradually been implemented. New tools such as remote sensing, AI, and variable fertilization systems have begun to help farmers actually examine the soil, measure nutrients, and apply fertilizers at regular intervals and in fixed quantities (Lu et al., 2020). In this way, less fertilizer was used, up to a quarter could be saved, the efficiency of nitrogen fertilizer increased, but the output did not suffer a loss. More importantly, greenhouse gas emissions can still be reduced significantly. However, this set of operations is by no means simple. Equipment, data and technical training are all indispensable. Farmers also need to adopt them gradually. However, in any case, this route points out a new direction for future rice cultivation - how to achieve high yields does not rely on excessive fertilization, but on smarter application methods. 4 Field Evaluation Indicators of Nitrogen-efficient Rice Varieties 4.1 Growth dynamics and population structure during the growing season To be honest, whether high-nitrogen-efficiency varieties are good or not, it is not enough to just look at the yield. How they grow in the fields and whether the population structure is stable or not all need to be taken into consideration. Under low-nitrogen conditions, some varieties can still maintain relatively strong root systems, and the biomass of the aboveground part is not bad either. The roots have long density and fast growth rate. However, this effect is sometimes also related to whether the planting density and the amount of nitrogen fertilizer applied are well coordinated. Too low density or too much or too little nitrogen fertilizer may disrupt the rhythm. Generally speaking, moderate nitrogen dosage and reasonable planting density are conducive to dry matter accumulation, an increase in the number of panicles, and a more reasonable leaf area, thereby forming a relatively ideal population structure (Zhu et al., 2023). Some NUE varieties also have good canopies, many tillers, and well-grown flag leaves. Photosynthesis is relatively stable throughout the growing season, and resource conversion keeps up (Li et al., 2024). 4.2 Nitrogen uptake, utilization efficiency, and recovery rate Absorbing a lot of nitrogen does not necessarily mean it is used well. If nitrogen cannot be recovered, it is still in vain. To truly determine whether a variety is highly efficient or not, it is necessary to look at several aspects including total absorption, utilization rate and recovery rate together. Especially when nitrogen application is reduced, the gap is easier to be seen (Wu et al., 2016; Lu et al., 2020; Wang et al., 2022). Indicators such as NUpE and NUtE are increasingly commonly used in the field nowadays. When viewed together with AE and PFP, they can better evaluate the overall performance. In fact, once the root system performs well, such as having long roots, a large enough root surface area, and strong oxidation capacity, the efficiency of nitrogen absorption and utilization will be easier to improve. Meanwhile, the activity of enzymes, such as nitrate reductase and glutamine synthase, also plays a key role. 4.3 Yield components and their relationship with economic traits Whether a variety is worth promoting depends on whether it is productive and stable. But where the output comes from, in the final analysis, it still needs to be examined separately. Factors such as the number of spikes, the number of small spikes, the grouting condition and the thousand-grain weight basically determine the quality of the foundation, and these are closely related to the efficiency of nitrogen fertilizer utilization. Interestingly, many high-NUE varieties have seen their yields increase instead of decrease even when nitrogen input is not so high, thanks to the improvement of these structural factors (redistribution during the grouting period also contributes).
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