Absorption And Utilization Of Fertilizer Nitrogen By Rubber Tree(Hevea Brasiliensis) Under Ethrel Stimulation

Natural rubber is an important industrial raw material, which is mainly produced by rubber tree (Hevea brasiliensis). The global and domestic demand of natural rubber is growing with economy development year by year. To meet the increasing demand for natural rubber, the ideal solution is to improve rubber yield per unit area. As an important practice to enhance rubber yield, Ethrel stimulation has been wildly applied in rubber plantations in China and other natural rubber producing countries. Ethrel is believed to increase rubber yield through enhancing latex biosynthesis and prolonging latex flow, which are usually accompanied by more nutrients loss with latex extraction, particularly nitrogen (N) element. More nitrogen fertilizer dosage is required for the compensation of the loss through latex harvesting.N among all nutrients is central to living systems. Suitable N content in rubber tree is an essential facet of latex production and tree growth. It is believed that rational theological basis could be provided for adequate N fertilization through the study of rubber tree absorption and utilization characteristics of fertilizer-N (FN) under Ehtrel stimulation tapping, to keep nutrient balance and realize efficiency production purposes.Therefore, a field experiment was carried out, in which15N labeled N fertilizer was used, to study the effects of Ethrel stimulation on N fertilizer absorption and utilization by rubber tree. The possible effects of N fertilizer in the following year on tree growth and latex production were also examined later on. Following results and conclusions were obtained in this study.1. Increased N loss while N use efficiency (NUE) improved under Ethrel stimulation tapping.Dry rubber yield (DRY) and total solid content (TSC) were greatly higher in Ethrel stimulation tapping than those in conventional tapping. An obvious increase of N effusion was found in simulation tapping, the loss rate was1.15times as that of conventional tapping.It was found that rubber tree absorption and utilization of FN were influenced by phenology no matter it was tapped under stimulation tapping system or conventional tapping system. The rate of FN distribution and transportation in latex was significantly higher under stimulation tapping than that under conventional tapping, before October. The amount of FN in latex harvested rose from October to December in both Ethrel Stimulation tapping and conventional tapping. In contrast to early period of the year, the late season of the year saw a higher FN distribution and transportation rate in latex under conventional tapping than stimulation tapping. The nutrient use efficiency (NUE) was measured with per unit weight of DRY against per unit weight of FN for latex production. The results showed that higher NUE was achieved under stimulation tapping than under conventional tapping.FN occurred in latex at different speeds which were in relation to N fertilizer application time. Slower appearance of FN in latex was observed in the first application of N fertilizer in the early period of the year while quicker presence of the FN noted in the following two N fertilizer applications. The difference in the present time of FN in latex could be ascribed to the development of the tree root system and soil moisture status. Seasonal variation of FN in latex abstracted from the trees under conventional tapping and stimulation tapping were considered to be related with the N nutrient cycle in rubber trees.2. The correlation between latex N content and physiological parameters of latex production enhanced under stimulation tappingIt was found that sucrose content (SUC), inorganic phosphorus (Pi), thiol content (R-SH) and Mg2+content were higher under stimulation tapping than those under conventional tapping. These parameters were varying in different seasons.The correlation coefficients between N content and SUC, Pi and R-SH were significant under stimulation tapping while only SUC and R-SH were significantly correlated with N content under conventional tapping. It implied that the correlation between latex N content and physiological parameters of latex production was enhanced under stimulation tapping. FN content in latex was significantly correlated with SUC, Pi and R-SH under both stimulation tapping and conventional tapping. Both total N content and FN content in latex had no relationship with Mg2+content under both tapping systems.3. The transportation and distribution of FN to different parts of rubber tree varying with phonological variationDifferent parts of rubber tree had varied utilization capacities of FN. The transportation and distribution of FN to tender tissues and organs were preferential in vigorous growth period. Higher abundance of5N and higher Ndff%(nitrogen derived from fertilizer) were observed in tender tissues and organs such as inflorescence, tender apical bud and tender leaf flushes in this period, which indicated that FN was more favourable for partition into these tissues. In contrast, FN was transported into storage parts of rubber tree before leaf fall during wintering. Higher abundance of15N and higher Ndff%were determined in phloem of trunk, bark of twigs (1-year-old), bark of twigs (2-year-old), wood of collateral and wood of twigs (2-year-old). These results demonstrated that phloem and bark were important parts for N storage.As the biggest biomass among all organs of rubber tree, the tree trunk could store and reserve most FN absorbed by the plants. Rubber roots took up and transported FN into above-ground part while obtained and reserved FN from N back-flow process before defoliation.4. Soil residual rates, distribution and balance regime of FNN application brought additional N into low layers of soil around the fertilization hole. No obviously difference was found between different soil layers of fertilizing holes which were filled with N fertilizer at early stage of the year and midyear, probably due to strong leaching. On the contrary obvious FN was observed in20-40cm layer of the soil around the fertilizing hole in late application treatments due mainly to weak leaching,.Taking fertilizer fate into consideration, the recovery rate of FN applied at early stage of the year was the lowest among the three application treatments; whilst higher recovery rate at midyear and late-year application treatments. N lose was observed in all three treatments of N fertilizer application. The pathways of N lose might involve N removal by littler fall, ammonia volatilization, runoff lose and leaching out of the root region.5. FN still detectable in latex during the following2years after fertilizationFN could still play certain parts in later regeneration of rubber tree in ensuing years after fertilizer application, which suggested that residual effects of FN should be taken into consideration in N management and fertilization decision making. The seasonal variation of FN effusion along with latex harvest between stimulation tapping and conventional tapping was evident, which could be governed by the mechanism of N cycling in rubber tree. From the results of3-year-observation, it was determined that the end of September or the beginning of October was a critical point for rubber tree to switch from transportation of N upward to crown to withdraw N into storage organs before wintering.6. Remobilization and utilization of storage FN in rubber tree for growth and photosynthesis next yearApparent FN was observed in the structures and parts of twigs. FN could be stored in twigs and reutilized in vegetative growth. The distribution rate of FN in the phloem of twigs was the highest as compared with xylem and cortex because it is an important part for N storage. The Ndff%value in1-year-part of twigs was significantly higher than that in2-yeat-part of twigs, which indicated that obviously transportation of FN from storage N in2-yeat-part of twigs into1-year-part of twigs occurred. N storage played an important role in new shoot development.However, the proportion of FN content in rubber leaf was low, and no close relationship was found between FN content in rubber leaf and photosynthesis parameters. FN seemed not a crucial factor which determined the photosynthesis capacity of rubber leaf. Its contribution rate to photosynthesis capacity accounted for about6%only that of total N in rubber leaf. Based on the above findings, it could be concluded that higher N loss in association with higher latex yield under Ethrel stimulation tapping may also enhance the absorption and utilization of fertilizer N to certain extent, and that adequate supply of N fertilizer to rubber tree could compensate for those N loss through latex extraction to ensure normal growth and latex regeneration as well as nutrient storage capacity of the crop while residual effects of N fertilizer should be taken into consideration when fertilization decision is made to avoid or minimize any fertilizer waste and pollution to soil and water environment. The present findings provided certain new theological basis for N management in rubber plantations particularly under Ethrel stimulation tapping, which could be of certain interests not only to natural rubber scientists but also to plantation managers as well.