Effects Of NH₄⁺-N/NO₃^--N Combined Application On Growth And Physiology Characteristics Of Machilus Pauhoi Seedlings

In order to explore the preference for NH₄⁺-N or NO₃^--N of Machilus pauhoi seedlings under the background of global N deposition,we planned to take 1-year-old M.pauhoi seedlings as the research object and the local(Anxi,Quanzhou)hilly red soil as the substrate.A controlled experiment with 7 different NH₄⁺-N/NO₃^--N combined treatments(The ratio of ammonium to nitrate is 1:9(T1),3:7(T2),5:5(T3),7:3(T4),9:1(T5)and 2 control treatments(CK1,CK2))was conducted,and the growth and leaf phenotypic,nutrient and physiological traits of M.pauhoi seedlings after applying different forms of N addition in December 2019 and August 2020,respectively.Moreover,the main nutrient traits of potting soil were investigated and measured after N addition.The growth and physiological traits of M.pauhoi seedlings after different NH₄⁺-N/NO₃^--N ratio combined addition were analyzed,and the effects on NH₄⁺-N/NO₃^--N absorption characteristics were clarified.The optimal plan of NH₄⁺-N/NO₃^--N combined application suitable for the growth of M.pauhoi seedlings were found.The research results can provide a theoretical basis for further research on the influence of atmospheric N deposition with different forms on the growth of M.pauhoi seedlings,and provide theoretical and technical references for research on the effect of atmospheric N deposition on the subtropical evergreen broad-leaved tree species.The main research results were as follows:(1)The combined treatments of NH₄⁺-N/NO₃^--N within one year had a significant effect on the relative growth rate(RGR)of the ground diameter(GD)and plant height(TH)of M.pauhoi seedlings,but had no significant effect on the RGR of biomass.GD was the highest in the CK1 treatment;TH was the highest in the CK2 treatment;RGR was the highest in the T3(NH₄⁺-N:NO₃^--N=5:5)treatment.Although the differences in GD,TH and RGR of the five NH₄⁺-N/NO₃^--N combined treatments were not significant,they were all the highest in the T3 treatment,and showed a gradual downward trend with the addition of NH₄⁺-N and NO₃^--N,indicating that compared with other combined treatments,the NH₄⁺-N/NO₃^--N ratio of 5:5 was more conducive to the growth of M.pauhoi seedlings.However,it still requires further observation and research.The treatments with NH₄⁺-N/NO₃^--N ratios of 1:9 and 9:1 were less suitable for the growth of M.pauhoi seedlings,due to the lower growth rates of GD,TH and biomass.And the higher concentrations of NO₃^--N would cause the death of M.pauhoi seedlings.(2)The combined treatment of NH₄⁺-N/NO₃^--N had a significant effect on the morphological traits of M.pauhoi seedlings at 4 months of fertilization,but had no significant effect on them after fertilization for 1 year.at 4 months of fertilization,the leaf area(LA)and specific leaf area(SLA)of M.pauhoi seedlings were higher in the T1(NH₄⁺-N:NO₃^--N=1:9)treatment,but the leaf dry matter content(LDMC),leaf tissue density(LTD)and leaf relative water content(LRWC)were lower.The leaf morphology in the T5(NH₄⁺-N:NO₃^--N=9:1)treatment was opposite,characterized by lower LA and SLA,as well as higher LDMC,LTD and LRWC.The leaf of M.pauhoi seedlings in the T1 treatment at 4 months of fertilization had a higher resource utilization efficiency,but lower nutrient investment for leaf construction.The leaf in the T5 treatment had a lower resource utilization efficiency,but higher nutrient investment for leaf construction.Different NH₄⁺-N/NO₃^--N combined treatments had little effect on the morphological traits of M.pauhoi seedlings after 1 year.(3)The combined application of NH₄⁺-N/NO₃^--N had certain effects on the nutrient of M.pauhoi seedlings for 4 months and 1 year.at 4 months of fertilization,the leaf C content(LC)treated with NH₄⁺-N/NO₃^--N ratio of 1:9,5:5 and 7:3 was higher,and the content of organic compounds was relatively high.One year after fertilization,the leaf N content(LN)was significantly lower in the T1 treatment than that in other treatments,not conducive to the increase of LN.The leaf P content(LP)and C:P(LCP)were not significantly different for 4 months and 1 year.at 4 months of fertilization,leaf LN and LP as well as the coordination of C,N and P elements were relatively high.One year after fertilization,the C utilization efficiency of M.pauhoi seedlings treated with T1 and T5 was relatively high.The leaf in the T1 treatment was restricted by both N and P or both no shortage,while the other 4 treatments were mainly restricted by P.(4)The combined treatments of NH₄⁺-N/NO₃^--N had no significant effects in the leaf SPAD values,transpiration rate(Tr)and stomatal conductance(Gs)after 4 months and 1 year.at 4 months of fertilization,leaf photosynthetic rate(Pn),water use efficiency(WUE)and photosynthetic N use efficiency(PNUE)were significantly higher in the T1 treatment than those in other treatments,but lower in the intercellular CO₂ concentration(Ci),However,there was no significant difference in Pn,WUE and PNUE among different treatments at 1 year.(5)The combined treatments of NH₄⁺-N/NO₃^--N had significant effects on soil N content(TN),C:N(SCN),C:P(SCP),N:P(SNP)and p H value,but no significant effects on soil C content(TC)and P content(TP).In the T4(NH₄⁺-N:NO₃^--N=7:3)and T5 treatments,potting soil had higher NH₄⁺-N and NO₃^--N contents,and its soil had a higher accumulation of N content.N addition with different N forms had little effect on TC,TP and SNP.The rhizosphere p H was lower in the NH₄⁺-N treatment,and the opposite effect was in the NO₃^--N treatment.(6)After applying N addition with different NH₄⁺-N/NO₃^--N ratio for 1 year,the soil physical and chemical properties had a certain correlation with leaf morphology,nutrients and photosynthetic traits of M.pauhoi seedlings.TN,SCN,and p H had a greater impact on leaf traits.The potting TN and NO₃^--N had a greater impact on the RGR.The GD was significantly positively correlated with SCN,but significantly negatively correlated with TN.TH was significantly negatively correlated with soil NO₃^--N content.LN and LCN had a greater impact on the RGR.The GD was significantly negatively correlated with SPAD values,but significantly positively correlated with Ci.The TH was significantly negatively correlated with LC.RGR It was not related to leaf traits.In summary,among the five NH₄⁺-N/NO₃^--N combined treatments,the T3treatment(NH₄⁺-N:NO₃^--N=5:5)may be more helpful to the growth of M.pauhoi seedlings,and the treatment with the high ratio of NO₃^--N would cause M.pauhoi seedlings death.The application of different N forms within one year had a greater impact on the main traits of potting soil and leaf nutrients,but had less effect on the morphological traits and photosynthetic physiological traits of M.pauhoi seedlings.