Effects Of Soil Fertility On Nutrient Resorption And Photosynthetic Nitrogen Characteristics During Nutrient Resorption Of Quercus Mongolica

It is a well-known way for the root system to obtain nutrients from the external environment.At the period of lignification,the growth of seedling height is slow,and the nutrients in the current year’s tissues gradually return to the perennial tissues,which shortens the time required for nutrient cycling,and enables the trees to maximize the use and maintenance of nutrients.Under better site conditions,trees can absorb more nutrients,and may not need too much nutrients to participate in the nutrient resorption.The nutrient utilization strategies of trees in different fertility sites need further study.During the lignification stage,the decrease of nitrogen concentration in the leaves due to nutrient reflux may affect the carbon assimilation ability of trees.Previous studies have found that trees still maintain high photosynthetic capacity in the lignification period,which is an important period for tree biomass accumulation.Then there may be at least two mechanisms at the lignification period to avoid the decline of photosynthetic capacity caused by the decrease of nitrogen concentration in leaves:The leaves can use more nitrogen directly in photosynthesis,or optimize the photosynthetic system nitrogen to improve photosynthetic nitrogen-use efficiency.The two mechanisms are controlled by the site fertility of tree growth at the same time.Therefore,it is of scientific value to study the mechanism of photosynthetic nitrogen distribution and carbon assimilation maintenance in lignification stage.Clarifying the above-mentioned problems is helpful for in-depth understanding of tree nutrient utilization strategies,and could provides a theoretical basis for formulating relevant cultivation measures.The objective of this study is Q.mongolica seedlings.Fertilization treatment was used to simulate the soil fertility of three gradients:low,medium and high.At the beginning of nutrient resorption（T₁）,the medium-term of nutrient resorption（T₂）and the end of nutrient resorption（T₃）,roots,stems,and leaves content of nitrogen（N）and phosphorus（P）were measured to analyze the nutrient resorption and the contribution of resorption to nutrient storage.Light response curve and carbon dioxide response curve of leaves at T₁and T₂were measured and analyzed the changes in the allocation of photosynthetic nitrogen during nutrient resorption process.The main results and conclusions are as below:1)Resorption efficiency and time of different elements are different.Resorption efficiency of N（48.63%）was higher than that of P（26.93%）,and N resorption（T₁）started earlier than that of P（T₂）.N resorption efficiency reached the highest under the condition of medium fertilizer（54.18%）and the lowest under the condition of high fertilizer（45.16%）.P resorption efficiency reached the highest under the condition of low fertilizer（43.20%）.The level of fertilization decreased gradually the P resorption efficiency to 17.46%.2)Different soil fertility has different effects on the N and P resorption contribution to seedling storage:N element resorption of Q.mongolica from leaves has the highest contribution rate to seedling storage under medium fertilizer conditions（18.65%）,and N absorbed by the roots on seedling storage contribution rate reached the lowest（81.35%）.With the increment fertilization,the contribution of P resorption to the reserves decreased to 46.40%,and the contribution of P absorption the reserves increased to 42.66%.3)During the period of leaf nitrogen resorption,the photosynthetic system nitrogen content and the proportion of total nitrogen in the leaves increased（T₂increased by 15.38%and 26.42%compared with T₁）.Among them,the light-harvesting components nitrogen content and the proportion of total nitrogen in the leaves decreased（T₂decreased by 23.31%and 4.08%respectively compared with T₁）.While the Rubisco components and the bioenergetics components nitrogen content increased（T₂increased by 27.40%and 14.35%respectively compared with T₁）,and the nitrogen distribution ratio of the that also increased（T₂increased by 38.32%and 26.51%respectively compared with T₁）.Fertilization significantly enhanced the photosynthetic capacity of leaves.It increased the nitrogen proportion of light-harvesting components at T₁and T₂by 75.76%and 111.11%.The photosynthetic nitrogen-use efficiency was increased 42.35%and 19.27%at T₁and T₂by fertilization,respectively.To sum up,during the lignification period,with the resorption of leaf N,the content and concentration of nitrogen in leaves of Q.mongolica decreased,the distribution of photosynthetic nitrogen and the photosynthetic nitrogen-use efficiency in leaves changed:the content and proportion of photosynthetic component n in leaves decreased,while the content and proportion of carboxylation component,electron transfer component and photosynthetic N increased;the photosynthetic nitrogen-use efficiency decreased.It was concluded that the leaves of Q.mongolica maintained strong photosynthetic capacity by optimizing and adjusting the distribution of photosynthetic nitrogen during the lignification period,but the final photosynthetic rate decreased,resulting in the reduction of photosynthetic nitrogen utilization efficiency.With the improvement of soil fertility,the degree of P nutrient resorption decreases:Q.mongolica will increase the nutrient recovery rate to adapt to its nutrient conditions in the face of the shortage of P nutrient supply,on the contrary,it does not need too much nutrient to participate in the resorption when the soil nutrient supply is relatively sufficient.There is a balance mechanism between nutrient absorption and return;with the increase of soil fertility,the N distribution of leaf light harvesting system increases,and this phenomenon does not change with time.