Effects Of Nitrogen And Phosphorus Additions On The Internal Cycles Of Trace Elements In Chinese Fir Plantation Of Red Soil Hilly Region

Atmospheric nitrogen（N）deposition and soil phosphorus（P）limitation are the most important environmental problems in red soil hilly region of China,and the effects of N and P addition on the ecological process and productivity of Chinese fir forest have attracted much attention.Trace elements play an important role in the structure and function of ecosystem although the concentration of trace elements in plants is far less than trace elements.However,the effects of trace elements on the growth and productivity of Chinese fir are not clear,and the response degrees and mechanisms of the internal cycles of trace elements in Chinese fir to the atmospheric N deposition and soil P limitation in subtropical red soil hilly region are unknown.To reveal these mechanisms mentioned above,a filed experiment was conducted based on the field experiment platform of fertilization for Chinese fir plantation established by Qianyanzhou experimental station of Chinese Academy of Sciences in 2011.Six treatments including N1(50 kg N·hm^-2·a^-1),N2(100 kg N·hm^-2·a^-1),P(+50 kg P·hm^-2·a^-1),N1+P(50 kg N·hm^-2·a^-1+50 kg P·hm^-2·a^-1),N2+P(100 kg N·hm^-2·a^-1+50 kg P·hm^-2·a^-1)and CK（control）were carried out,and each treatment comprised five completely randomized zone group trials.Soil（rhizosphere and bulk soil）,root system（1-2 order roots,3-5 order roots）,fresh twigs（1 and 2 order twigs）,fresh leaves（1 and 2 order leaves）,litter（1 and 2 order branches and leaves litter）were sampled in summer and winter of 2016,and the concentration of trace elements（Fe,Mn,Cu,Zn）was measured to study the effect of N and P addition on each link of trace element internal cycles process.The objects of the study are to explore the effect of N and P addition on the internal cycles of trace elements in Chinese fir forest and reveal its mechanism,and try to understand the potential regulation mechanism of trace elements on the productivity and stability of Chinese fir forest.The findings of the study provide scientific basis for the development of biogeochemical cycle theory of trace elements,and help better cope with atmospheric N deposition and P limitation in subtropical red soil hilly region.The main results are as follows:（1）The effects of N and P addition on availability and rhizosphere effect of trace elements in soil were different.N addition inhibited the availability of Mn in soil.High N addition inhibited the availability of Fe in bulk rhizosphere soil and Mn rhizosphere effect,promoted Fe rhizosphere effect.P addition increased the availability of Mn in soil.Cu,Zn availability and rhizosphere effect were not sensitive to N and P addition.（2）The effects of N and P addition on trace elements of roots in different seasons were different.In 1-2 order roots,Mn and Zn were more easily accumulated in summer and Fe and Cu were more easily accumulated in the winter.N addition inhibited the absorption and transportation of Zn by roots in summer,and Zn content in 1-2 order roots was more sensitive to N addition when compared to 3-5 order roots.P addition inhibited the absorption of Fe by1-2 order roots in summer.The combination of high N and P addition promoted the absorption of Fe by 1-2 order roots in winter,and the absorption and transportation of Zn by roots in winter.（3）The effects of N and P addition on the trace elements of twigs in different seasons were different.Zn in 1 order twigs was more easily accumulated in the summer and Mn in1 order twigs was more easily accumulated in the winter.The addition of low N inhibited the migration of Fe,Cu,and Zn to the young twigs and Mn to the twigs in the summer.The addition of P inhibited the migration of Zn to the twigs in the summer.The addition of N,P alone and combination inhibited the migration of Mn to the twigs in the winter.（4）The effects of N and P addition on the trace elements in leaves in different seasons were different.Fe,Mn,Cu,Zn in 1 order leaves were more easily accumulated in the summer.P addition promoted the migration of Fe to 2 order leaves,but inhibited the migration of Zn to leaves.The combination of High N and P addition promoted the migration of Fe to the leaves in the winter.Mn and Cu in the leaves were not sensitive to the N and P addition.（5）The effects of N and P addition on the resorption efficiency and resorption proficiency of trace elements in twigs and leaves were different.The resorption efficiencys of Mn and Zn were more sensitive to N and P addition than that of resorption proficiencys,while Fe and Cu showed opposite trends.The resorption efficiencys of Fe and Zn in leaves were more sensitive to N and P addition than that of resorption proficiencys,while Cu showed opposite trend.The response patterns of Mn resorption efficiency and resorption proficiency in leaves were similar to that of N and P addition.（6）There are different coupling relationships in the internal cycles of different trace elements.In the process of Mn internal cycles,the concentrations of Mn in soil,roots,twigs and leaves showed good internal correlation,but the responses of Fe,Cu and Zn were varied with different source and sink.There were different correlations among trace elements in different organs and tissues of Chinese fir.There was a positive coupling effect between Fe and Zn in soil,1-2 order roots and 2 order twigs litter.There was a positive coupling effect between Mn and Cu in rhizosphere soil,2 order twigs and 2 order twigs litter,and there was an antagonistic effect between Mn and Cu in 2 order leaves.There was a positive coupling effect between Fe and other trace elements in 1 order twigs,and there was an antagonistic effect between Mn and Zn.In conclusion,the effects of N and P addition on supply of soil,absorption of roots,transportation and recovery of branches and leaves,and other internal cycles processes of trace elements are different.The internal cycles of trace elements in Chinese fir is controlled by both external supply and self-regulation.Under the background of atmospheric N deposition and alleviation of soil P limitation,the potential effect and mechanism of trace elements on the productivity and stability of subtropical Chinese fir forest need further research.