| With the high-speed development of the national economy,there is a global growing demand for wood,while forest area is declining year by year.Populus tomentosa is the main tree species for plain-greening,green corridor and timber base construction in North China as populus tomentosa is good in fast-growth,short rotation and fast nutrient depletion.Common forest soil is difficult to meet the needs of fast-growing and high-yield of populus tomentosa due to its poor condition of nutrient supply,low content of total nutrient and available nutrient,so fertilization is an important measure to construct fast-growing and high-yield plantation.In order to study the effects of different proportion of N,P,K on seedling growth and physiological properties and identify the optimum amounts and ratio of fertilization of hree hybrid clones of populus tomentosa, seedlings were treated with an experiment of fertilizer combinations of nitrogen,phosphorus and potassium of three different levels based on an orthogonal experimental design L9>(34) in pots,which provide a scientific basis for reasonable fertilization.And a field experiment was conducted with "311-B"D-saturation optimum design to study the effects of different proportion of N,P,K on seedling growth and physiological properties and identify optimum amounts and ratio of fertilization of clone-No S86 which performed better in pots experiment,which provide a reference for the construction of fast-growing and high-yield plantation.The general conclusions of this paper are listed as following:(1)By analyzing Indicators including height,diameter,biomass and net photosynthetic rate xtotal leaf area(PTL)comprehensively,we can draw conclusions:the optimum ratio of N,P,K of Populus tomentosa hybrid clone-No S86 was N(3 g)+P2O5(2.25 g)+K2O(0 g);The optimum ratio of N,P,K of populus tomentosa hybrid clone-No 46 was N(5 g)+P2O5(4.5 g)+K2O(1g);The optimum ratio of N,P,K of populus tomentosa hybrid clone-No 26 was N(7 g)+P2O5(2.25 g)+K2O(0.5 g).We studied the optimum amounts and ratio of fertilization for hybrid clones of populus tomentosa for the first time and realized the optimal allocation of genotype and fertilization,which had the potential to promote the industrial development of seeding propagation of populus tomentosa.(2)The total amount of optimal fertilization for clone-No S86,No 46 and No 26 were 5.25 g,10.5 g and 9.75 g respectively,while maximum height of clone-No S86,No 46 and No 26 were 227.95 cm,217.20 cm and 192.24 cm,so seedlings of clone-No S86 grew better and needed less amount of fertilization,it can be promoted as a superior clone by intensive cultivation to achieve good ecological and economic benefits.(3)The effects of different fertilizing treatments on net photosynthetic rate xtotal leaf area(PTL)were more significantly different than that on net photosynthetic rate and net photosynthetic rate xtotal leaf area showed a more significant correlation with growth,therefore evaluation in each treatment of populus tomentosa hybrid clones combined net photosynthetic rate with total leaf area was more reliable.Range of available light for fertilizing treatments(difference between light saturation point and light compensation point)was larger than control,fertilization can help to improve light environment adaptability of populus tomentosa hybrid clones.(4)Fertilization can effectively improve water-retaining capability in leaf of populus tomentosa hybrid clones.Fertilization had no significant effect on leaf water content,but the leaf water content in clones had significant difference,leaf water status of clone-No S86 was much better than clone-26 and clone-46.Carbon isotope ratio of leaf,stem and root showed no difference in fertilizing treatments,but showed significant difference in clones,carbon isotope ratio of leaf, stem and root for clone-No S86 and No 46 were significantly higher than clone-No 26,and therefore water use efficiency of clone-No S86 and No 46 were higher than clone-No 26. (5) N,P,K concentrations of leaves were the highest in organs of clone-No S86,No 46 and No 26.N concentrations of leaf,stem and root for No S86 were in order of leaves>stems>roots,K concentrations were in order of leaf>root> stem;N concentrations of leaf, stem and root for No-46 and No-26 were in order of leaf>root>stem,K concentrations were in order of leaf>root>stem.Nutrient concentrations in fertilizing treatments were generally higher than control and fertilization can effectively improve nutrient concentrations of leaf,stem,root.(6)By constructing multi-objective decision model and combining the growth and physiological indices, we can draw a conclusion that the optimum ratio of N,P,K of populus tomentosa hybrid clone No-S86 was N(313.712 kg·hm-2)+P2O5(166.812 kg·hm-2)+K2O(40.137 kg·hm-2)in field experiment.
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