Effects On Nitrogen Storage And Reutilization In Young Rubber Trees Caused By Different Nitrogen Supply Rates

Nitrogen storage and remobilization play important roles in the construction of new organs of deciduous trees.Rubber plantations are important economic forests in the tropic area of China.By far,there is still no systematic study on the transportation,distribution and reutilization of stored nitrogen in rubber trees.The relationship between stored nitrogen reutilization and the level of nitrogen application is still unclear.Therefore,a single-factor randomized block design field experiment was carried out with three nitrogen rates(low nitrogen 28 g/plant,medium nitrogen 56 g/plant,and high nitrogen 84 g/plant),in which 2-year-old rubber tree saplings were used as the research object and stable isotope 15N labeling technique was used.The growth differences and nitrogen uptake,utilization and distribution characteristics of rubber tree saplings were studied,while the parameters of rainfall,daily average temperature,mature leaf nitrogen content,chlorophyll content were dynamically monitored.The results are expected to provide theoretical basis for the development of reasonable nitrogen fertilizer application for rubber trees.The main results are as follows:1.During the annual growth cycle of rubber trees,the total nitrogen content of leaves showed a regular change,and the whole showed a gradual decline trend.The absorption capacity of fertilizer nitrogen(Ndff value)was significantly different in different periods,and all showed N84>N56>N28.The Ndff value of the leaves increased with time and then decreased slowly.After November 15,there was a significant decrease.Nitrogen began to reflow,and about 30%of the nitrogen returned to the inside of the tree.The chlorophyll content of leaves had obvious seasonal dynamics,which was closely related to the change of total nitrogen.However,under different nitrogen application rates,there was no significant difference in leaf chlorophyll content;Chlorophyll content was affected by climatic factors,with rainfall and temperature,both have significant or extremely significant correlations.As the nitrogen reflux of the leaves decreased,the nitrogen content decreased,and the RuBisCO activity also decreased significantly,and the two showed a significant positive correlation.2.In the deciduous period,the total nitrogen content in trunk phloem was the highest among all organs and tissues of the rubber tree,followed by the annual branches while the lowest was seen in the main xylem.The Ndff values in the organs were significantly different,and the fine roots had the strongest ability to regulate the 15N in the fertilizer.Secondly,the annual branch had the smallest Ndff value of the main root.The nitrogen absorbed by the rubber tree was mainly distributed in the trunk,perennial branches,side branches,main roots and pericarp.These tissues are important nitrogen nutrient storage for the rubber trees in the dormant period.The distribution rate in the trunk was the largest among the storage organs.The low,medium and high nitrogen treatments were 35.18%,37.68%and 36.47%,respectively.Other organs even had a stronger ability to modulate the nitrogen of the fertilizer,and the amount of nitrogen absorbed by the fertilizer was still relatively small due to the small biomass.The nitrogen use efficiency decreased with the increase of nitrogen application rate,which was 47.55%,46.83%and 39.09%,respectively.The fertilizer nitrogen taken away by the leaves was about 8.83%,and the residual rate of 15N in each treated soil was 26.13%,20.14%and 19.97%which decreased with the increase of nitrogen rates.3.In the second year after fertilization,from the sprout time to the period when new grow leaves become stable,the total nitrogen content of the leaves showed a trend from high to low.The content of different treatments was close,and the average value decreased from 73.87 g·kg-1 to 25.34 g·kg-1.There was a significant difference in the Ndff values of each treatment blade.The overall trend was higher and then slowly decreased with the prolongation of time.The treatment ranges of N28,N56 and N84 were 9.56%～14.66%,20.31%～26.69%and 34.81%～44.70%,respectively.The total nitrogen content and Ndff value of the trunk skin were all higher in the early stage,and then gradually decreased.The nitrogen content of the leaves and trunk skin from the fertilizer was larger,indicating that the fertilizer nitrogen stored in the rubber tree had a significant reuse process during its early growth.4.In the second year after fertilization,the first awning stage of the rubber tree was measured.It was found that the total nitrogen content of the leaves was the largest,while the lower one was determined in fine roots.and its content in main xylem was the lowest.The organ’s abilities to accumulate nitrogen were different.The Ndff value of the same organ was in the order of N84>N56>N28,which increased with the increase of nitrogen application rate.The distribution abilities of nitrogen storage were significantly different in different parts of rubber tree.The nitrogen in the leaves was the highest,followed by the trunk xylem.and the smallest in the phloem.At this stage,the storage nitrogen in each organ was preferentially transported to younger tissue,and the Ndff observation value was higher;about 50.60%～53.98%of the nitrogen storage was transported to the new organs,which plays an important role in the construction of new organs.Compared with the dormant period,the nitrogen content of the fertilizer stored in the trunk decreased by about 51.20%,and the root system decreased by about 30.17%.The nitrogen stored in the plant had been obviously remobilized,and the proportion of transferred and reused fertilizer nitrogen reached 47.31%,50.46%and 52.03%.