Effect Mechanism Of Nitrogen And Phosphorus Addition On The Growth Of Machilus Pauhoi And Cunninghamia Lanceolata Mixed Forest

Machilus pauhoi is a precious evergreen broad-leaved native tree species in China,with important ecological and economic value.Cunninghamia lanceolata is a valuable coniferous tree species in China.Vigorously developing artificial forests of M.pauhoi and C.lanceolata lanceolata has significant implications for increasing the strategic reserves of precious timber resources in our country and alleviating the shortage of such resources.In recent years,although many artificial forests of M.pauhoi are mixed forests with conifers,mainly mixed with C.lanceolata,the interaction mechanism between M.pauhoi and C.lanceolata is not clear,especially regarding their interactions after nitrogen and phosphorus addition.Therefore,this study took the pure M.pauhoi forest（M.P（Pure））,pure C.lanceolata forest（C.L（Pure））and their mixed forest（Mixed（M.P&C.L））in Yanping District,Nanping City,Fujian Province as research objects.Four addition treatments including single nitrogen addiltion(N,100 kg·hm^-2·a^-1),single phosphorus addiltion(P,10kg·hm^-2·a^-1),nitrogen and phosphorus addiltion(NP,kg·hm^-2·a^-1,10 kg·hm^-2·a^-1)and control treatment（CT）were used to conduct nutrient addition experiments.Through field investigation and sample collection,the effects of nitrogen and phosphorus addition and mixed planting on soil physical and chemical properties,microbial community structure of forest,functional characteristics of fine roots and leaves of trees were analyzed to clarify the resource balance strategy of M.pauhoi and C.lanceolata affected by nitrogen and phosphorus addition and mixed.The structural equation model was combined to analyze how nitrogen and phosphorus addition and mixed planting affect the growth regulation mechanism of M.pauhoi and C.lanceolata through resource balance strategy from aspects such as soil physical and chemical properties of forest land,microbial community structure,absorption root,leaf traits functional traits.The research results have preliminarily revealed the nutrient utilization and resource balance strategy of M.pauhoi and C.lanceolata after nitrogen and phosphorus addition and mixed,as well as their regulation mechanism on the growth of forest trees,providing theoretical basis for nitrogen and phosphorus nutrient management of M.pauhoi and C.lanceolata,scientific construction of mixed forests of M.pauhoi and C.lanceolata.The main findings of this research are as follows:（1）Mixed C.lanceolata significantly increased the soil p H,nitrate nitrogen content（NO₃^-N）,ammonium nitrogen（NH₄⁺-N）,mineral nitrogen content（MN）,STP,available phosphorus（SAP）,microbial nitrogen content（MBN）,microbial N/P ratio（MBN:MBP）and biomass of Gram-positive bacteria（G+）,Gram-negative bacteria（G-）and actinomycetes（ACT）.It also reduced the contents of soil total carbon（STC）,soil total nitrogen（STN）,soil total carbon-phosphorus ratio（SC:P）,soil total N/P ratio（SN:P）,microbial carbon（MBC）,microbial carbon:nitrogen ratio（MBC:MBN）,microbial carbon:phosphorus ratio（MBC:MBP）as well as acid phosphatase activity（AP）,β-glucosidase（βG）andβ-N-acetylglucosaminidase（NAG）.NP mixed addiltion significantly increased the soil total phosphorus content（STP）and NO₃^-N of M.pauhoi pure plantation,while reducing the AP of M.pauhoi pure plantation.（2）Mixed with C.lanceolata significantly alters the structure of soil bacterial and fungal communities,increases the Ace and Chao indices of soil bacteria and fungi in forest soil,and reduces the coverage index.There are significant differences in bacterial WPS-2 among different N,P addition and mixed treatments.The effect of N and P addition on soil bacterial and fungal diversity was not significant.（3）Mixed planting significantly increased the specific root length（SRL）and specific root area（SRA）of M.pauhoi absorption roots,but decreased its root diameter（AD）and root carbon content（RCC）.However,it had no significant effect on the phenotypic and nutrient characteristics of C.lanceolata absorption roots.The addiltion of N significantly increased the SRL and SRA of M.pauhoi transport roots in mixed forests,while the addiltion of N and P significantly reduced the AD and RCC.M.pauhoi and C.lanceolata have different resource trade-off strategies for absorbing root,with M.pauhoi having a higher capacity for acquisition（higher SRL,SRA,and RP）and C.lanceolata having a more conservative strategy（higher RN,RTD,and RNP）.（4）Mixing significantly increased the specific leaf area（SLA）,leaf P content（LPC）,and decreased its leaf relative water content（LRWC）,leaf N content（LNC）,and leaf carbon-nitrogen ratio（LC:N）.Mixing significantly increased the leaf N resorption efficiency（NRE）and leaf P resorption efficiency（PRE）of M.pauhoi and C.lanceolata,and decreased the old leaf N concentrations（OLN）and old leaf P concentrations（OLP）of C.lanceolata.Nitrogen and phosphorus addition had no significant effect on the leaves phenotypic traits of M.pauhoi and C.lanceolata.Single addiltion of N significantly reduced the NRE and PRE,OLN and OLP of M.pauhoi and C.lanceolata in pure forest and mixed forest.Single addiltion of P significantly increased and decreased the PRE of C.lanceolata in mixed forest and pure forest,decreased the OLP of P M.pauhoi in pure forest,and increased the OLP of C.lanceolata in pure forest.The N and P addition significantly increased the LN:P of M.pauhoi in pure forest.M.pauhoi and C.lanceolata have conservative（high LDMC,LC:P,NRE and PRE）and acquired（high LNC,LPC,OLN,OLP and LTD）leaf-level resource strategies,respectively.Nitrogen addition significantly enhanced the leaf-level resource acquisition strategy of C.lanceolata,and mixing changed the leaf-level resource strategies of M.pauhoi and C.lanceolata to acquisition and conservation,respectively.（5）Mixed planting with C.lanceolata significantly promoted tree height（TH）,diameter at breast height relative growth rate（DBHRGR）,tree height relative growth rate（THRGR）and crown width relative growth rate（CWRGR）of M.pauhoi,while having no significant effect on C.lanceolata.There is a positive interaction between M.pauhoi and C.lanceolata due to M.pauhoi and C.lanceolata had different growth strategies.N addiltion,P addiltion and NP addiltion all increased the complementarity of C.lanceolata to M.pauhoi while reducing the complementarity of M.pauhoi to C.lanceolata.The complementarity of C.lanceolata to M.pauhoi was highest under N treatment,while that of M.pauhoi to C.lanceolata was highest under CT treatment.M.pauhoi and C.lanceolata had the highest DBH,TH and Volum in the mixed forest addiltion of N and CT treatments,respectively.Nitrogen and phosphorus addition increased the diameter at breast height（DBH）,TH,volume,crown width（CW）and their relative growth rates（RGR）in M.pauhoi,but had no significant effect on C.lanceolata.（6）Mixed planting had a significant negative effect on the NAG activity and bacterial coverage index in forest soil,which then had a significant positive effect on M.pauhoi growth.Mixed planting had a significant positive effect on the soil bacteria chao index,indirectly leading to weak positive effects on C.lanceolata growth.Nitrogen and phosphorus addition had a significant positive effect on the NO₃^-N content in forest soil,which in turn had a significant positive effect on the growth of M.pauhoi.Both Mixed planting and nitrogen and phosphorus addition had weak positive effects on NH₄⁺-N and NAG in soil,indirectly resulting in significant negative effects on C.lanceolata growth.In summary,there is a positive interaction between M.pauhoi and C.lanceolata attributed to the separation of resource trade-off strategies between these two tree species at the absorb root,leaf and growth levels.The effect of mixing on forest soil,microbial and tree growth,leaf and root functional traits was more significant than that of N and P addition.N and P addition and mixing modulated M.pauhoi growth by affecting N-related forest soil physicochemical properties（DON、NO₃^--N、NH₄⁺-N、NAG）and tree functional traits（LNC）,and C.lanceolata growth by affecting N-related forest soil physicochemical properties（SC:N、NH₄⁺-N）.In future research,a longer study on the complementarity between M.pauhoi and C.lanceolata is needed to provide theoretical support and scientific basis for the scientific management and management of M.pauhoi and C.lanceolata mixed plantation.