| Phoebe zhennan S.Lee is an evergreen plant that is particularly popular in China,especially in the southwest part.This tree species is also well known for landscaping and timbering and its timber is an important raw material for high valued-added products,e.g.,furniture,engineering structural wood,etc.However,the distribution area and the timber production of P.zhennan are gradually depleted due to long-term high intensity deforestation.And the immature technology of cultivation and management for P.zhennan seedlings and plantations has restricted the commercial development of this precious tree species.Fortunately,fertilization has the potential to ameliorate nutrient limitation on low-fertility soils and enhance seedling growth.However,literature on effects of fertilization on P.zhennan is less abundant and fertilization levels and fertilizer types were not enough and little is known about optimal fertilization rates and its effects on growth of P.zhennan seedlings.Thus,it is important to understand optimal nutrient management of P.zhennan seedling.In this study,different fertilization rates of three local fertilizers,i.e.,compound fertilizer(CF),oil cake(OC),and duck manure(DM)were used in order to find out the optimal fertilizer type and rates for the growth of P.zhennan seedlings and to elucidate the effects of fertilization on leaf photosynthetic characteristics,soil fertility and the growth of P.zhennan seedlings.These fertilizers were applied at seven rates based on the nitrogen content:0(control),150,300,450,600,750,and 900 mg N plant-1 yr-1,and designated as N0,N1 N2,N3,N4,N5,and N6,respectively.All the fertilizers were equally divided into three parts,and each part was applied into soil in 2014,respectively;four parts divided equally were applied in 2015,respectively.We expect the results of this study could be used as inputs to fertilization strategy of P.zhennan seedlings.The main results were as follow:(1)The total height,basal diameter growth and biomass of P.zhennan seedlings showed an initial increase followed by a decreasing trend with increasing fertilization rates in all the three fertilizers throughout the test period.The maximum value of total height,basal diameter and biomass occurred at level N3 for CF,and N4 for OC,and N5 for DM.The total height,basal diameter increment and biomass at the three levels for corresponding fertilizer were significantly increased by 72.8%,97.8%,181.2%,48.1%、53.9%、78.2%,and 118.6%,196.7%,320.9%,respectively,as compared to NO.Additionally,the growth of P.zhennan seedlings at N6 for CF was inhibited while that for OC and DM could be promoted over time.Generally,application of DM showed the greatest promoting effect on the growth of P.zhennan seedlings,when comparison was conducted between fertilizer types,CF acted the worst.The regression analyses between fertilization rate and morphological growth suggested that the optimal fertilization range of CF,OC and DM to current year and one-year-old P.zhennan seedlings should be applied at rate of 1.7～1.9,16～22,91～132 g-plant-1 yr-1(calculated as pure N 405～446,478～652,607～883 mg N plant-1 yr-1),respectively,and 2.7～3.6,27～34,149～223 g plant-1 yr-1(calculated as pure N 658～862,812～1001,1000～1493 mg N plant-1 yr-1),respectively.(2)Foliar N,K contents in fertilized P.zhennan seedlings were significantly increased and then decreased,and gradually increased respectively,with the addition of increasing amount of fertilizers with time.In CF fertilization,N6 was the only level giving significant less N contents compared to NO during the whole test period.The application of CF induced the reduction of foliar P contents,showing a decreasing tendency with the increasing fertilization rates over time,while a parabolic pattern was shown as the rates of OC,DM increased.At the end of fertilization(November in 2015),ratio N/P of leaves treated with CF,OC and DM were increased with increasing fertilization level,and the variation range was 3.43～8.90,3.27～6.12,3.43～6.10,respectively.(3)The leaf chlorophyll contents(Ch1),net photosynthetic rate(Pn),stomatal conductance(Gs),and Transpiration rate(Tr)of P.zhennan seedlings treated by the three fertilizers all fell after rising with the increase of N rate.The maximum of Ch1,Pn,Pnmax appeared at N3 for CF,N4 for OC,N5 for DM,respectively.Chl,Pn,Gs,Tr,Pnmax at N6 level in CF treatment was markedly reduced while these in OC and DM treatments were significantly increased,indicating that application of organic fertilizer(DM,OC)could function better in promoting the growth of P.zhennan seedlings.(4)Application of all three fertilizers increased the contents of ammonium nitrogen(NH4+-N),nitrate nitrogen(NO3--N),total nitrogen(TN),soil oranic matter(SOM),available phosphorus(AP),and available potassium(AK).The promoting effect on the soil available nutrients P.zhennan seedlings grew was DM>OC>CF.(5)Micorbial biomass nitrogen(MBN)contents,soil urease activity,phosphatase activity and sucrase activity were significantly increased treated by the three fertilizers during the whole test period while the response of microbial biomass carbon(MBC)and ratio MBC/MBN(Bc/BN)to the three fertilizer types were less sensitive.Generally,the promoting effects on soil microbial biomass and enzyme activity were DM>OC>CF.Overall,SOM,soil urease activity,sucrase activity,Chl,foliar N contents played key roles in the growth of P.zhennan seedlings.It is suggested that N source is an important influencing factor on the growth of P.zhennan seedlings.The present study found that highest N rates(1800 mg N yr-1)under the treatment CF had a negative impact on P.zhennan seedlings,whether growth traits or photosynthesis,however,under the treatments OC or DM,there was a positive effect.Combined these leaf physiology indices,soil fertility with growth fitting results,we could conclude that 1.7～1.9,16～22,91～432 g plant-1 yr-1 and 2.7～3.6,27～34,149～223 g plant-1 yr-1 were optimal ranges for fertilizer CF,OC and DM to current year and one-year-old P.zhennan seedlings,respectively.In addition,the best fertilization strategy of current year and one-year-old P.zhennan seedlings was DM with 91～132,149～223 g plant-1 yr-1(calculated as pure N 607～883,1000～1493 mg N plant-1 yr-1),respectively,which could help optimize the growth and photosynthesis of P.zhennan seedlings.The present findings are considered to be valuable for improving productivity of P.zhennan seedlings,at least in the experiment area under study. |
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