Legume Genomics and Genetics 2025, Vol.16, No.6, 253-269 http://cropscipublisher.com/index.php/lgg 257 fertilizer application by 20%, the root-crown ratio of the soybean population became more reasonable, the root system vitality was enhanced, and the aboveground biomass and yield were the same as or even slightly higher than those of conventional fertilization. This shows that "reducing nitrogen and increasing potassium" is a fertilization adjustment strategy that conforms to the physiological characteristics of legume crops, which helps to give full play to the advantages of legume root nodules in nitrogen fixation and avoid resource waste and negative effects caused by excessive nitrogen fertilizer. In actual production, the N-P-K ratio should be different under different soil fertility conditions: plots with low fertility need to appropriately increase phosphorus and potassium inputs, while plots with high fertility, especially those with rich nitrogen residues in the previous season, should control the amount of nitrogen fertilizer to avoid inhibiting nitrogen fixation. By evaluating nutrient abundance and deficiency through soil testing and formulating a balanced fertilization plan according to local conditions, it can simultaneously promote the formation of a strong root system and a lush canopy of legume crops, and achieve the coordination and unity of underground nitrogen fixation and high-yield growth above ground. 3.2 Regulation of nitrogen fixation activity and absorption efficiency at different topdressing periods The timing of fertilization has no less impact on nutrient utilization efficiency and crop growth than the amount of fertilizer applied. For legumes, how to timely supplement the nutrients required in the middle and late stages while ensuring nitrogen fixation in the early stage is the key to fertilization strategy. Studies have shown that providing an appropriate amount of nitrogen source in the early growth period helps legume seedlings to establish and grow in the early stage, but if nitrogen is applied too early and too much, it will inhibit nodule formation and reduce the potential for biological nitrogen fixation due to increased inorganic nitrogen levels in the soil (Ciampitti et al., 2021). Therefore, it is generally recommended that nitrogen fertilizer should not be applied excessively at sowing. The nitrogen input in the base fertilizer for legume sowing is usually controlled at around 20-30 kg/ha to promote seedling growth without inhibiting nodules. As the crop grows, nodule nitrogen fixation gradually meets most of the plant's nitrogen needs during the vegetative growth stage. However, after flowering and podding, the demand for nitrogen in legumes increases dramatically, and nodule activity may decrease due to factors such as soil drought and nutrient deficiency. At this time, the appropriate amount of nitrogen fertilizer is beneficial to increase the protein content and yield of grains. The experiment compared the effects of topdressing nitrogen fertilizer at the early flowering stage (R1) and the grain filling stage (R5) of soybeans, and found that nitrogen application at the grain filling stage had a significantly higher yield-promoting effect than nitrogen application at the early flowering stage, while topdressing nitrogen too early at the early flowering stage often had no significant effect on yield. In a multi-year experiment in the United States, a single nitrogen application at the early flowering stage did not increase soybean yield, while delaying the application of the same amount of nitrogen fertilizer to the grain filling stage had a significant yield-increasing effect. It can be seen that the nitrogen supply "window period" for leguminous crops such as soybeans is in the middle and late reproductive growth. At this time, exogenous nitrogen will not significantly weaken nodule nitrogen fixation (because the plant has accumulated a large amount of nitrogen fixation capacity and nitrogen fixation is weakened in the late growth stage), but can make up for the lack of nitrogen fixation and nitrogen supply. Khalili et al. (2024) suggested that applying 60 kg/ha of nitrogen fertilizer in two times: 1/2 as basal fertilizer and the remaining 1/2 as topdressing at the pod setting stage, combined with the application of an appropriate amount of sulfur fertilizer, can significantly increase soybean seed yield and quality. The topdressing period also has an important impact on nutrient absorption efficiency. Generally speaking, phosphorus and potassium fertilizers should be applied early so that crops can fully utilize them throughout the growth period; while nitrogen fertilizers should be applied in stages or slow/controlled release technology should be used to match the absorption peaks of crops at different stages. In the field experiment in Qiqihar in this study, a small amount of nitrogen topdressing was carried out at the initial flowering stage and the grain filling stage of soybeans. The results showed that the number of pods and the weight of 100 grains treated with topdressing at the grain filling stage were higher than those of the control without topdressing, and the yield increased by about 5%, while the yield increase effect of topdressing at the initial flowering stage was not obvious. This is consistent with the above rule, that is, the response of soybeans to nitrogen is mainly in
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