Photosynthetic-nitrogen Use Efficiency And Influencing Factors Of Different Species In Southern Subtropical And Tropical Area Of China

China has launched a massive afforestation in subtropical area from 1980 s,and created a large number of Pinus massoniana and Cunninghamia lanceolata pure forests.But these forests became reduced in production,had serious disease and pest,and reduced in biodiversity and soil fertility in recent years.Mixing broad-leaf trees into artificial coniferous forest or rebuilding these forests into mixed forest used other tree species could be better ways to avoid these adverse effects.Growth light conditions and soil nitrogen and phosphorus content were key factors in limiting broad-leaf trees growth when proceeding mixed plantation or reforestation.Photosynthetic nitrogen-use efficiency(PNUE)has been considered as an important leaf trait to characterise species in relation to their leaf economics,physiology,and survival strategy.PNUE also been considered as an important leaf trait in explaining leaf economics.Research on PNUE and its influenced factors,and their changes in different growth light conditions and soil nitrogen and phosphorus content in forestation species could provide theoretical foundation in tree species selection under different planting conditions.Tropical forests accounting for about half of the worldwide forest cover,playing an extremely vital role in the global carbon fixing and cycling.Despite such prominence,factors influencing PNUE in tropical forests still unclear.Tropical forests are preference hotspots of Fagaceae and Leguminosae tree species in the world,which also can be found in Jianfengling tropical montane rain forest,Hainan Island,China.Most of the tree species in Fagaceae were dominant species which importance value was higher than Leguminosae,and higher PNUE usually lead to higher competitiveness.Research on difference in PNUE and influenced factors of Fagaceae and Leguminosae tree species could help us explain their competitiveness and coexistence mechanism.In this study,four tree species(i.e.Dalbergia odorifera,Erythrophleum fordii,Castanopsis hystrix and Betula alnoides)in Guangxi Pingxiang Experimental Center of Tropical Forestry were chosen,and cultivated with three growth light,three soil nitrogen and phosphorus content gradients;five Leguminosae species(i.e.Ormosia fordiana,O.semicastrata,O.balansae,Pithecellobium clypearia and Pi.lucidum)and five Fagaceae species(i.e.Lithocarpus fenzelianus,Ca.fissa,Cyclobalanopsis phanera and Cy.patelliformis)in Jianfengling tropical montane rain forest were chosen.Licor-6400 portable photosynthesis system and common physical and chemical analysis methods were used to determine PNUE and influenced factors(i.e.leaf nitrogen allocation in different cell component,leaf photosynthetic capacity and CO2 conductance and so on)of these tree species.Analyzed the reason of difference in PNUE and influenced factors between tree species,and the reason of adaptable difference in PNUE and influenced factors under different growth light conditions,soil nitrogen and phosphorus contents.The main results were as follows:1.Under same growing conditions(full growth light,high soil nitrogen and phosphorus contents),B.alnoides seedling leaves had the highest PNUE up to 120.54 ?mol·mol–1·s–1,followed Ca.hystrix(112.01 ?mol·mol–1·s–1).Two nitrogen fixing trees D.odorifera and E.fordii had lower PNUE,valued 52.64 ?mol·mol–1·s–1 and 45.92 ?mol·mol–1·s–1,respectively.Nitrogen fixing tree species had lower PNUE first attributed to their significantly higher leaf nitrogen per area(Narea)and leaf nitrogen per mass(Nmass),but lower fraction of the total leaf nitrogen allocated to the photosynthetic apparatus(PP),especially allocated to Rubisco(PR)and bioenergetics(PB)were key factors that reduced their PNUE.The influence of mesophyll conductance(gm)to PNUE was difference between species.2.PNUE of D.odorifera and B.alnoides seedling leaves were significantly reduced as the soil nitrogen contents decreased,the reduction of PNUE in E.fordii and Ca.hystrix were not remarkable.PNUE of D.odorifera in low soil nitrogen treatment(LN)was 28.99% lower than high soil nitrogen treatment(HN),and PNUE of B.alnoides in LN was 25.67% lower than HN.Narea of D.odorifera and E.fordii were no significant decreased with the reduction of soil nitrogen,but Narea of Ca.hystrix and B.alnoides were significantly reduced in LN than HN(-23.53% and-26.21%,respectively).The reduction of PR and gm in LN significantly reduced PNUE of D.odorifera,and the reduction of PR,PB and gm in LN significantly reduced PNUE of B.alnoides,the reduction of PR slightly reduced PNUE of E.fordii and Ca.hystrix.E.fordii seedling generally less affected by changes of soil nitrogen content;less nitrogen fertilizer could be used in its seedling cultivating,and E.fordii seedling could be used in forestation area with low soil nitrogen content.Increasing nitrogen application could be useful for the other three kinds of tree seedlings in promoting their growth.3.PNUE of D.odorifera and Ca.hystrix seedling leaves significantly reduced as the amount of soil phosphorus decreased.PNUE of D.odorifera in low soil phosphorus treatment(LP)was 44.59% lower than high soil phosphorus treatment(HP),and PNUE of Ca.hystrix in LP was 25.67% lower than HP,the changes of PNUE in B.alnoides and E.fordii were not remarkable.Leaf phosphorus per area(Parea)of these four species were significantly reduced as the amount of soil phosphorus decreased,the Parea reduce proportion in LP than HP was 15.69% in D.odorifera,47.01% in E.fordii,23.30% in Ca.hystrix and 23.30% in B.alnoides.The reduction of soil phosphorus content also reduced leaf nitrogen content,photosynthetic capacity and gm of D.odorifera and Ca.hystrix.The reduction of PR and gm in LP significantly reduced PNUE of D.odorifera,and the reduction of PR,PB and gm in LP significantly reduced PNUE of Ca.hystrix.E.fordii and B.alnoides seedling were less affected by changes of soil phosphorus content;less phosphorus fertilizer could be used in their seedlings cultivating,and these seedlings could be used in forestation area with low soil phosphorus content.Increasing phosphorus application could be useful for other two tree seedlings in promoting their growth.4.PNUE of D.odorifera and E.fordii seedling leaves significantly increased as the growth irradiance decreased,PNUE of Ca.hystrix reduced as the growth irradiance decreased,PNUE of B.alnoides was highest in 40% full light treatment.Narea of these four species were significantly reduced as the growth irradiance decreased,mainly attributed to their reduced leaf mass per area(LMA).LMA of D.odorifera in 10% full light was only 42.67% of LMA in full light,LMA of E.fordii was 62.03%,LMA of Ca.hystrix was 57.50% and LMA of B.alnoides was 43.71%.E.fordii seedling leaves had the highest photosynthetic capacity in 40% full light,and the other three tree species had the highest photosynthetic capacity in full light.The changes of PR and PB were key factors that influenced PNUE in D.odorifera and B.alnoides between different growth light conditions,and the changes of PR,PB and gm were key factors that influenced PNUE in E.fordii and Ca.hystrix.In conclusion,for artificial pure forests where E.fordii seedlings are planted,forest gaps should be restricted,the other three kinds of tree seedlings should be planted in open forests environment.5.Leguminosae species of Jianfengling tropical montane rain forest had lower PNUE than Fagaceae species(45.78 ?mol·mol–1·s–1 and 62.02 ?mol·mol–1·s–1,respectively).Leguminosae species had lower PNUE first attributed to their significantly higher Narea and Nmass than Fagaceae species,but lower PP,especially lower PR and PB were key factors that decreased their PNUE.The influence of gm to PNUE was difference between species.Higher PNUE of species in Fagaceae might be one of the reasons to explain their higher competitive ability in tropical montane rain forest.Differentiated photosynthetic nitrogen use strategy could make Leguminosae and Fagaceae species make full use of soil nitrogen in same ecological system,and maximize carbon sequestration.6.Fraction of leaf nitrogen allocated to cell wall(PCW)and PR were high in Ca.hystrix and B.alnoides seedling leaves,and their Narea declined under low nitrogen treatment,which may implied that nitrogen in their leaves was not enough for both Rubisco and cell wall,thus trade-offs between their PCW and PR.PCW and PR were low in D.odorifera and E.fordii seedling leaves,and their Narea were no significant decreased with the reduction of soil nitrogen,then no trade-offs appeared.The significant negative correlation between PCW and PR in D.odorifera,E.fordii and Ca.hystrix seedling leaves under soil phosphorus treatments was not trade-offs between nitrogen in Rubisco and cell wall,the changes of PCW and PR was an adjustment of plant for adapting changes of soil phosphorus environment.The changes of PCW and PR under different growth irradiance environments also a kind of adjustment of plant for adapting changes of light.Trees in Jianfengling tropical montane rain forest also had low PCW and PR,nitrogen in leaves of these tree species was enough for both Rubisco and cell wall.