Adaptive Regulation Of Nitrogen Threshold On Carbon And Nitrogen Metabolism In Soybean

Soybean, the main grain and oil crop in China, is also an important nitrogen-fixingcrop. Nitrogen, the key element of protein, nucleic acid and enzymes of soybean, isthe most important factor influencing soybean growth and yield. There are threesources of nitrogen during the whole growth periods of soybean: nitrogen come fromsoil, fertilizers and nitrogen-fixing. Among them, nitrogen-fixing occupied nearly halfof the total nitrogen, so it is still necessary to apply nitrogen fertilizers.Nitrogen-fixing of soybean is greatly influenced by nitrogen fertilizers, thus how toapply nitrogen fertilizers is one of the key factors to influence nitrogen-fixing. Whenless nitrogen fertilizers are applied, the process of nodulation and nitrogen-fixing willbe effected indirectly by the poor growth of soybean plant. When more nitrogenfertilizers applied, the ability of nodulation and nitrogen-fixing will degrade due to thesufficient nitrogen can meet the nitrogen requirement of soybean plant, which causednitrogen inhibitory. This study aims to find the most suitable nitrogen level and atthe same time maximize the ability of nitrogen-fixing.In this study, six levels of nitrogen were set in2011using sand culture method toinvestigate the soybean growth, nodulation and nitrogen-fixing of nodulating andnon-nodulating soybean(two species of nearly isoline soybean), and selected thethreshold value of nitrogen inhibitory during the process of soybean nitrogen-fixingby using Subtraction Calculating Method. In the year of2012, three nitrogenapplication levels of soybean based on the nitrogen repression threshold value of ourprevious experiments using soil culture method were set to investigate the adaptation response of plant growth, metabolism of carbon and nitrogen, and the yield ofsoybean to nitrogen inhibitory. Our results are as follows:1. We found the best nitrogen level of nodulating soybean is54mg urea/kg sandwhen selected threshold, while it is216mg urea/kg sand in non-nodulation soybean.We concluded that54mg urea/kg sand is the threshold value based on ourexperimental situation by Subtraction Calculating Method.2. The metabolism of carbon and nitrogen inside soybean was regulated byexogenous nitrogen level. In most conditions, the total soluble sugar and sucrose inthe various parts of plant, carbon accumulation contents, the ratio of chlorophyll a/band other related index of carbon metabolism together with soluble protein contents ofleaves, ureide contents, nitrogen accumulation contents and other related index ofnitrogen metabolism were higher when nitrogen level was54mg urea/kg thannitrogen level was108mg urea/kg. We concluded that critical nitrogen level was theoptimum condition for carbon and nitrogen accumulation of soybean plants and bestfor the accumulation of carbohydrate and protein.3. Root growth character reached the highest value under the condition of criticalnitrogen levels. In this study, each index of root morphology, carbon and nitrogenaccumulation of roots, the number of nodule and the weight of dry nodule had thehighest value on the nitrogen level of54mg urea/kg. We concluded that54mgurea/kg is the optimum nitrogen level for promoting soybean root growth.4. Soybean yield components had obvious differences among different nitrogenlevels. Compared with nitrogen inhibitory level,100-seed weight in soybeanincreased16%and grain weight per plant（ripening） increased22%under thecondition of critical nitrogen level. Besides, in most cases, biomass of shoot and totalplant, pod number per plant, pod shell and the content of soluble sugar and sucrosewere higher at the critical nitrogen level. We concluded that the best nitrogen level forthe growth of soybean, the development of soybean pod and yield component wascritical nitrogen level.In summary, when different allogenic nitrogen levels were integrated with theadaptation response of plant growth, the development of root system, carbon and nitrogen accumulation, and yield components, we concluded that in the condition ofour experiment, critical nitrogen level(54mg urea/kg) was the optimal nitrogen levelfor soybean growth、development and yield formation, and also the optimal nitrogenlevel for nitrogen-fixing.