Effects Of Nitrogen Application Level On Photosynthetic Nitrogen Use Efficiency Of Soybean

The distribution characteristics of nitrogen in leaves play a crucial role in enhancing the photosynthetic rate and photosynthetic nitrogen use efficiency（PNUE）of crops.There exists a positive correlation between PNUE and nitrogen physiological use efficiency,hence,enhancing leaf PNUE can significantly improve crop nitrogen use efficiency and yield.Proper use of nitrogen fertilizer may enhance soybean growth and yield by optimizing nitrogen distribution in both photosynthetic and non-photosynthetic systems,as well as improving PNUE and photosynthetic rate.However,there is a need for more analysis on the mechanism of nitrogen application levels on soybean leaf PNUE and yield improvement,specifically from the perspective of nitrogen allocation in the photosynthetic system.Therefore,this study was conducted in Heshan Farm,Heihe City,Heilongjiang Province from 2021-2022.Jinyuan 55 and Keshan 1 soybean varieties were treated with different nitrogen levels as urea(N₀-urea 0kg/hm²,N_0.5-urea 60kg/hm²,N₁-urea 120kg/hm²,N_1.5-urea 180kg/hm²).The relationship of nitrogen application level on photosynthetic and non-photosynthetic components and PNUE of soybean leaves,changes of nitrogen in photosynthetic system of soybean leaves at different leaf ages,changes of light and nitrogen attenuation in vertical canopy distribution of crop population,changes of nitrogen allocation in canopy photosynthetic system,and limiting factors of soybean photosynthesis and PNUE were studied by plot comparison.The results provided theoretical basis for improving photosynthesis,PNUE and photosynthetic yield of soybean under nitrogen nutrition.The results and conclusions are as follows:1.With the increase of nitrogen application level,chlorophyll content,nitrogen content per unit area and nitrogen content per unit mass of soybean at seedling stage showed an increasing trend;The photoenzyme content and activity of the two test varieties changed with the increase of nitrogen application.The content and activity of Rubisco enzyme of Jinyuan 55 under N₁treatment were the highest,while that of Keshan 1 under N_1.5treatment were the highest.Regulating the ratio of nitrogen distribution in the photosynthetic system and the non-photosynthetic system within a certain range can improve the photosynthetic nitrogen use efficiency and photosynthetic rate of plants.PNUE,carboxylated system components,electron transport components and nitrogen allocation ratio of non-photosynthetic system firstly increased and then decreased with the increase of nitrogen application level,while nitrogen components of phototrapping system showed a downward trend with the increase of nitrogen application level.Correlation analysis showed that carboxylated component and electron transport component nitrogen were positively correlated with PNUE（R²=0.82151,R²=0.59025,R²=0.846601,R²=0.99941）.The PNUE decreased with the decrease of nitrogen allocation ratio between phototrapping system and non-photosynthetic system.PNUE increased first and then decreased with the increase of nitrogen application level.Reasonable nitrogen application level could regulate PNUE by changing the nitrogen allocation ratio between photosynthetic system and non-photosynthetic system.Low nitrogen levels led to an increase in the proportion of nitrogen allocation in the non-photosynthetic system of soybean seedling leaves,and a decrease in the proportion of nitrogen allocation in the photocarboxylation system and electron transport system,resulting in a decrease in the efficiency of photosynthetic nitrogen consumption in soybean seedling leaves.The high nitrogen level caused the nitrogen transfer from photosynthetic system to non-photosynthetic system,which may be the reason for the decrease of PNUE.2.According to the whole growth period of the second compound leaf,it was found that the nitrogen content and chlorophyll content per unit area of leaves increased at first,then decreased,and reached the maximum at V₃-V₄stage,and gradually decreased after V4 stage.The increase of nitrogen application level is beneficial to the increase of nitrogen,chlorophyll and carbohydrate content in leaves.The carboxylation rate（Vcmax）and electron transfer rate（Jmax）showed an overall upward trend with the increase of nitrogen application level.Sufficient nitrogen application can increase the maximum carboxylation rate and electron transfer rate of CO₂,which is beneficial to the conversion of CO₂to carbohydrates,thus increasing the photosynthetic rate of leaves.Compared with before the leaves were fully unfolded,the photosynthetic nitrogen content of leaves increased rapidly after the leaves were fully unfolded.After the leaves were fully unfolded,the photosynthetic nitrogen content increased first and then decreased.Nitrogen application levels significantly changed nitrogen distribution in carboxylation,electron transport and light capture systems in the photosynthetic system.In the period of highest photosynthetic rate（V₃-V₄）,nitrogen distribution in photosynthetic tissues of N₀,N_0.5,N₁and N_1.5treatments of Jinyuan 55 and Keshan 1 was 41.04%-49.75%,46.12%-53.55%,47.72%-53.55%,44.65%-55.60%,47.50%-58.22%,51.07%-56.89%,52.10%-59.70%and 47.79%-61.13%,respectively.Compared with low nitrogen treatments,middle and high nitrogen treatments increased leaf PNUE of two tested varieties at different growth stages,and reached a significant difference level at V₃-V₄stages.After V4,leaves stopped growing and photosynthetic nitrogen content correlated positively with photosynthetic rate,but the high proportion of light-harvesting system,the low proportion of electron transport system and carboxylation system in photosynthetic nitrogen components were the key factors limiting PNUE.After V4,leaves stopped growing and photosynthetic nitrogen content correlated positively with photosynthetic rate.3.Studies on leaves at different sections in R₆stage showed that the appropriate nitrogen application level was conducive to the improvement of photosynthetic rate of leaves in the middle and lower part of the canopy,slowed down the senescence and chlorophyll degradation of leaves in the lower part of the canopy,and promoted the improvement of photosynthetic capacity of soybean population.With the decrease of canopy height,the limiting factors of PNUE changed from carboxylation system and electron transfer system to light trapping system and electron transfer system.The PNUE of lower canopy leaves could be significantly improved by increasing nitrogen application level.4.The yield of N₁treatment was the highest,which increased by 64.68%and 68.75%,60.7%and 91.9%compared with N₀treatment,respectively.The increase of nitrogen application level can significantly promote the increase of 100 grain weight,and the increase of 100 grain weight in the upper and middle segments of each treatment is greater than that in the lower segment.Correlation analysis showed that there was a positive correlation between photosynthetic system nitrogen content and yield at each section.