Effects Of Different Nitrogen Application Methods And Levels On Plant Functional Traits In Forest At The Climatic Transitional Zone

It is a growing consensus that increased atmospheric nitrogen(N)deposition due to human activities has emerged as one of the most serious problems in global change.Nitrogen is one of the most important factors for forest ecosystem,atmospheric N deposition plays a key role in the structure and function of forest ecosystem.Considering to the close relationships between atmospheric N deposition and forest ecosystem,previous studies have reported the responses of plant and soil to simulated atmospheric N deposition in forest ecosystem based on the N addition experiment through understory.However,the method by understory N application ignored the potentially effect on some key processes occurring in forest canopy and resulted in an incorrectly assessment of the effects of N deposition on soils.To examine the differential responses of N cycling of soil and plant to understory/canopy N application in forest ecosystems,soil physi-chemical properties and leaf functional traits of dominant tree species were investigated on the"Canopy N application and increased precipitation"experiment established in Jigongshan National Natural Reserve in 2013.The experiment including five treatments:control,canopy application 25 kg N ha-1 yr-1,canopy application 50 kg N ha-1 yr-1,understory application 25 kg N ha-1 yr-1:and understory application 50 kg N ha-1 yr-1.Leaves of Celtis sinensis,Quercus variabilis,Liquidarmbar formosana,Acer buerger maple,and Lindera glauca were collected during mature and senescent periods to investigate the responses of specific leaf area,leaf N resorption efficiency,leaf carbon(C)and N contents,and soil physi-chemical properties to different N application methods and levels.The study will be in favor of the understanding of the response of forest ecosystem to atmospheric N application.Based on the measurements and analysis in 2016-2017,we found that N application methods had no effects on the soil physi-chemical properties and leaf functional traits.However,the infuences of N application levels on the physi-chemical properties of soil and leaf functional traits of different species were significant.Nitrogen application 25 kg N ha-1 yr-1 increased soil moisture by 14%.Nitrogen application 50 kg N ha-1 yr-1 increased soil moisture,soil total N content,and soil inorganic-N content by 30%,32.8%,and 15.3%,respectively,reduced soil C:N ratio by 22.5%.Nitrogen application 25 kg N ha-1 yr-1 improved C content of mature leaves of C.sinensis,C content of senescent leaves of C.sinensis,and N content of senescent leaves of L.formosana by 2.4%,14.9%,and 9.5%respectively,but decreased C content of senescent leaves of L.glauca by 8.5%.In addition,N application 50 kg N ha-1 yr-1 elevated C content of mature leaves of C.sinensis,N content of mature leaves of A.buerger maple,and N content of senescent leaves of L.formosana by 2.5%,12.3%,and 19.4%,respectively,but decreased C:N ratio of senescent leaves of L glauca and leaf N resorption efficiency of L.formosana by 15.4%and 16.8%,respectively.Our finding indicates that the increased soil total N and inorganic-N content under high levels of N application improves plant uptake ability on soil available N content,which caused elevated C and N content in leaves of both mature and senescent periods,thus reduced the leaf N resorption efficiencyThis study showed that N application levels affect N cycling of soil and plant in a temperate deciduous broadleaves forest in Central China,whereas the N application methods had no influences on soil physi-chemical properties and leaf functional traits.The study indicates that short-term atmospheric N deposition experiment through understory application in this region showed no differences with the method through canopy application/Long-term investigation is needed to examine whether the effect of canopy vs.understory N application on forest ecosystem is different.