Effects Of N Addition On Functional Traits And Growth Dynamics Of Fine Roots In Korean Pine Plantation

Under the background of increasing global N deposition,the effects of exogenous N addition on tree root growth are complex,and the changes of fine root traits and their driving mechanisms are still unclear.The uptake and transport of soil water and nutrient resources by fine roots is the basis of the growth,development and evolution of terrestrial plants,and their uptake and transport functions depend on a series of functional traits related to morphology,chemistry and anatomy.Therefore,the mechanisms of how fine roots adjust growth and functional traits in response to environmental change are crucial for understanding plant strategies in resource acquisition,growth,and competition,and for describing plant and ecosystem processes.N addition experiments were initiated in Korean pine plantations for 3consecutive years.Twenty plots（four treatments with five replicates）were assigned:Control,Low-N,Medium-N and High-N(0,20,40 and 80 kg N hm^-2 yr^-1).Fine roots in different soil layers and seasons were sampled to analyze the growth dynamics and functional traits（including fine root morphological,anatomical and chemical traits）of different functional categories in response to N addition,to provide a theoretical basis for adaptive management of Korean pine plantation under the background of atmospheric N deposition and even global change.The main research results are as follows:（1）N addition increased root tips diameter,specific root length,cortical thickness,and the ratio of cortical thickness to stele radius,but decreased the root tissue density of Pinus koraiensis.These results suggest that changes in root tips anatomy after N addition lead to changes in root morphological traits with potentially important effects on root physiological functions,such as increased water and nutrient uptake.N addition increased root tips nitrogen and phosphorus concentrations and root respiration rate,decreased root tips C/N ratio and non-structural carbohydrates,changed root tips chemistry and physiology,and regulated fine root metabolism to a certain extent.In the CK,the root tips mycorrhizal infection rate was 27.03%,and the low-N,medium-N and high-N treatments infection rates were 21.93%,16.06%and 9.81%,suggested that N addition reduced the root tips mycorrhizal infection rate.The root tips respiration was20.8,21.8,23.67 and 24.31 nmol O₂ g^-1 s^-1 in the four treatments,suggested that N addition increased the root tips respiration rate.These results suggest that roots respond to changes in soil nutrients and atmospheric environment and adjust fine root traits to meet water and nutrient requirements for plant growth,which is also an effective strategy for plants to adapt to habitat conditions over time.（2）The morphological,anatomical and chemical traits of absorptive and transport roots of Pinus koraiensis were studied under N addition in different seasons.It was found that there were significant seasonal differences between absorptive and transport roots.In spring and summer,plants tend to build roots with smaller root tissue density and larger specific root length to improve their nutrient and water absorption capacity.In autumn,plants tend to build roots with larger root tissue density and diameter to reserve nutrients and energy to resist winter stress and enhance root stress resistance.Driven by the addition of N,plants reinforce this pattern.When soil nutrient availability and habitat change under N addition and at seasonal scales,plants adjust their functional traits and resource acquisition strategies to maintain their normal growth and reproduction.（3）Studied the morphological,anatomical and chemical traits of the absorptive and transport roots of Pinus koraiensis in different soil layers under N addition,found that there were significant differences in the traits of the absorptive and transport roots among soil layers.In the shallow soil layer,N addition increased the absorptive roots specific root length,the ratio of cortex thickness to the stele radius,N and P concentrations,and decreased the root tissue density.The increased transport roots diameter,stele,the ratio of stele to root dimeter,the concentrations of nitrogen and phosphorus,which indicated that the absorption capacity of absorptive roots and transport capacity of transport roots were improved by N addition.In the deep soil layer,roots had larger root diameter,stele diameter,ratio of stele to root diameter,higher nitrogen and phosphorus concentrations,and smaller root tissue density.Study found that N addition did not change the deep root functional traits,and the self-regulation ability of plants made them still maintain the high exploration and exploitation ability and water absorption and transport ability in the deep soil layer,which is an important strategy to increase the water availability of trees and cope with seasonal drought.When N limitation was alleviated,plants increased the utilization efficiency of non-structural carbohydrates and reduced root NSC concentration and storage for aboveground biomass accumulation and belowground root construction.Fine roots showed different combinations of functional traits in different soil layers,indicating differences in root foraging strategies,while deep root characteristics were an important strategy to increase water availability for trees and to cope with seasonal drought.（4）N addition increased plant carbon allocation to roots,and changed the biomass ratio of absorptive and transport roots.The turnover rates of absorptive and transport roots were inconsistent in response to N addition.Total root biomass was 2.34,2.82,3.05 and 3.19 t·hm^-2in CK,low-N,medium-N and high-N treatments,respectively.N addition increased the absorptive and transport root biomass,increased the shallow root biomass,and changed the biomass vertical distribution pattern.The ratio of absorptive root biomass to transport root biomass was 0.45,0.40,0.36 and 0.39 in CK,low-N,medium-N and high-N treatments,respectively.N addition decreased the ratio of absorptive root biomass to transport root biomass,suggested that plants are more willing to allocate more carbon to long-lived roots,thus improving carbon use efficiency.The length production and mortality of fine root growth in Korean pine plantation were mainly concentrated in summer with obvious seasonality.The turnover rate of absorptive roots was 1.44,1.16,1.23 and 1.35 yr^-1in CK,low-N,medium-N and high-N treatments,respectively.N addition decreased the turnover of absorptive roots but had no effect on the turnover of transport roots.This inconsistent response pattern can be attributed to the functional and structural differences of the two fine roots,and also indicates a cost-benefit trade-off for plants.The lower C/N ratio and higher N concentration in fine roots suggest that N addition may delay root decomposition.Combined with the lower root turnover rate,N addition may increase soil carbon and nutrient retention and reduce nutrient cycling rate.In conclusion,the growth dynamics and functional traits of fine roots of Pinus koraiensis were significantly different and closely correlated between seasons and soil layers under N addition.When habitat changes,trees will adjust root functional traits and resource acquisition strategies to maintain their normal growth and reproduction.These results not only have important theoretical significance for understanding the dynamics of fine roots and their physiological and ecological functions under the condition of increasing N availability.It also has important theoretical contributions to understanding the changes of forest nutrients and tree growth after nitrogen addition and guiding forest management measures.