Effects Of Nitrogen Addition On Non-structural Carbohydrate Content And δ¹³C In Different Organs Of Pinus Tabulaeformis Seedlings

Carbohydrate is the main product of plant photosynthesis.Non-structural carbohydrate（NSC）is an important energy supplying substance in plant growth and metabolism.It provides a carbon buffer when plant respiration,growth and other physiological needs are not synchronized with photosynthesis.The change of NSC not only reflects the dynamics between assimilation savings and growth investment allocation,but also further affects plant growth and response to environmental changes.The effect of global nitrogen（N）deposition on plant NSC content has become a research hotspot in this field.The aim is to reveal the response mechanism of carbon balance and material cycle of ecosystem to the change of soil available N,and to propose valuable strategies for plantation cultivation and management.Based on the pot experiment of N addition for one-year-old Pinus tabulaeformis seedlings,four levels of N addition(0,3,6,12 g N?m^-2?a^-1)were set up.The plant organs were divided into five organs:leaves,branches,roots with different diameters（0-1mm,1-2mm,>2mm）.The contents of NSC and its components（soluble sugar and starch）in different organs of Pinus tabulaeformis seedlings were studied by periodic sampling and pulse labeling.The process and mechanism of photosynthetic carbon distribution and the influence ofδ¹³C in carbohydrates.The main conclusions are as follows:（1）The seedling height and biomass were significantly increased by high concentration of nitrogen(12 g N?m^-2?a^-1).At N₁₂ level,the seedling height and biomass were 26.62%and 28.57%higher than those without nitrogen application,respectively.With the increase of nitrogen addition level,root-shoot ratio decreased,leaf weight ratio and aboveground biomass increased,which indicated that the investment of Pinus tabulaeformis on root biomass decreased relatively with the increase of available N content in soil,and nitrogen addition was beneficial to the increase of aboveground biomass of plants.The changes of seedling height,biomass and distribution pattern indicated that Pinus tabulaeformis seedlings could adapt to the changes of external environmental conditions by adjusting the distribution of biomass between ground and root,leaf and stem.（2）The contents of soluble sugar,starch and NSC in 1-2mm roots were significantly higher than those in leaves,branches,0-1mm roots and>2mm roots because of the different functions and functions of different tissues in plant growth,and showed the size relationship between 1-2mm roots>leaves>branches and>2mm roots>branches and trunks in all tissues.The contents of soluble sugar,starch and NSC in various organs of Pinus tabulaeformis seedlings had significant differences in sampling time.The content of NSC and its components in the latter two samples decreased significantly,it showed that the above indicators were time-dynamic.With the increase of nitrogen addition level,the change trend of soluble sugar,starch and NSC contents in different organs of Pinus tabulaeformis seedlings was different,which indicated that different organs of Pinus tabulaeformis seedlings had different adaptability to nitrogen addition.Pinus tabulaeformis seedlings could adapt to the increase of nitrogen by changing the accumulation and distribution of NSC and its components in each organ.The degree of change varies with the level of nitrogen supply.The ratio of soluble sugar to starch in all organs was less than 1,and starch was the main component of NSC.With the increase of nitrogen level,soil pH value decreased,and with the increase of sampling time,the contents of soluble sugar,starch and NSC in Pinus tabulaeformis seedlings were mainly affected by pH.In conclusion,the effects of different nitrogen levels and sampling time on NSC content and its components（soluble sugar and starch）content in different organs of Pinus tabulaeformis seedlings were different.（3）¹³C stable isotope labeling showed that ¹³C content andδ¹³C in carbohydrates increased with the increase of nitrogen addition level,which indicated that short-term N addition was beneficial to plant assimilation of ¹³C and increased the distribution of ¹³C in carbohydrates in different organs of Pinus tabulaeformis.The value ofδ¹³C in soluble sugar,starch and cellulose showed some differences among different nitrogen treatments and organs,indicating that the assimilation of carbon and its distribution in different organs were affected by the physiological characteristics of plants and the changes of soil available N.In conclusion,there are significant differences in NSC and its components content and carbon distribution among different organs in Pinus tabulaeformis seedlings at different nitrogen levels,indicating that global N deposition may have different effects on allometric growth of different organs of trees,and thus affect coniferous forest ecosystems.