Effects Of Short-term And Long-term Simulated Nitrogen Deposition On Fine Root Biomass And Functional Traits In A Subtropical Evergreen Broad-leaved Forest

Fine roots are important organs for plant nutrient and water absorption.Atmospheric nitrogen（N）deposition affects the structure and function of fine roots by affecting soil nutrient availability.However,it is not clear how this effect varies with the duration of nitrogen deposition.In order to investigate the effects of simulated short-term and long-term nitrogen deposition on fine root biomass and functional traits,this study was carried out in the experimental platform of nitrogen deposition at the National Field Station of Sanming Forest Ecosystem in Fujian province.The nitrogen deposition plot was set up with three nitrogen addition treatments:control（CT）,low nitrogen（LN）and high nitrogen（HN）,with four replicates in each plot.Soil and fine roots were sampled during the growing seasons of 2015（short-term nitrogen addition,3 years）and 2021（long-term nitrogen addition,9 years）,and the related soil properties,fine root biomass and fine root functional traits were analyzed.（1）The results showed that soil total carbon（TC）,nitrogen（TN）,phosphorus（TP）,and soil C:N,N:P,C:P were not significantly affected by both short-term and long-term nitrogen addition.Soil total C was only significantly different between soil layers,while soil total N,total P,C:N,N:P and C:P were significantly different between soil layers and years.Short-term high nitrogen treatment significantly increased the contents of soil ammonium nitrogen（NH₄⁺-N）,while the contents of ammonium nitrogen and mineral nitrogen in soil under long-term high nitrogen treatment were significantly higher than those under short-term high nitrogen treatment.The soil nitrate nitrogen（NO₃^--N）content increased with the increase of nitrogen addition level in both short-term and long-term.Short-term and long-term nitrogen additon had no effect on soil available phosphorus（AVP）.There was no significant effect on soil microbial carbon（MBC）in both short-term and long-term nitrogen addition.Short-term LN treatment reduced soil microbial nitrogen（MBN）,but long-term nitrogen addtion had no effect on soil microbial nitrogen.Soil MBC:MBN ratio was not affected by nitrogen addition.Long-term LN treatment reduced the activity of leucine aminopeptidase（LAP）,but had no significant effect on N-acetylglucosaminidase（NAG）,β-glucosinase（BG）,and acid phosphatase（AP）.Factors such as precipitation and temperature are important disturbance factors affecting the results of nitrogen addition.（2）In terms of fine root biomass,the effect of nitrogen addition on fine root biomass was not affected by nitrogen addition time.HN treatment significantly reduced the fine root biomass（RBD）of 0-1mm,but both LN and HN significantly increased the2-5mm root biomass,suggesting that nitrogen application promoted root growth,but excessive N addition inhibited the growth of smaller diameters roots.The effect of N addition on fine root biomass was not affected by soil layers.LN treatment increased fine root biomass in different soil layers.（3）In terms of fine root morphological traits,short-term nitrogen addition increased the specific root length（SRL）and specific root area（SRA）of 0-1mm fine roots,but decreased the root tissue density（RTD）.Long-term nitrogen addition had no significant effect on specific root length,specific surface area and tissue density of fine roots with different diameters.The results showed that after short-term nitrogen addition,fine roots adopted the acquisitive strategy,while after long-term nitrogen addition,fine roots demand for nitrogen decreased,and adopted the conservative ecological strategy.The effect of N addition on fine root morphology was not affected by soil layers.（4）In terms of fine root chemical traits,short-term nitrogen addition had no significant effect on fine root carbon content（RCC）,but long-term nitrogen addition significantly increased fine root C content.The increase of carbon content was related to the conservative ecological strategy of fine roots.N and P contents in fine roots were not affected by nitrogen addition,but the interaction of years and diameter classes had significant effect on fine root nitrogen content（RNC）and phosphorus content（RPC）.The mechanism of nitrogen fertilization on carbon,nitrogen and phosphorus in fine roots is complicated,and is affected by various factors such as enzyme activity of fine roots,root exudates,and mycorrhizal infection.Long-term nitrogen addition had no significant effect on Ca and Al content in fine roots,but Ca and Al content in fine roots had significant difference among diameter classes.（5）In terms of physiological characteristics of fine roots,the nitrate reductase activity of fine roots increased with the increase of nitrogen application level,and the nitrogen assimilation ability of fine roots was enhanced,which prevented the accumulation of nitrate in the body and threatened the plants.Long-term nitrogen application decreased the activity of acid phosphatase（RAP）in fine roots,which was related to the inhibitory effect of nitrogen application on mycorrhizal fungi.（6）In 2015,soil total N and nitrate nitrogen were the main factors affecting the variation of 0-1mm diameter fine roots,and the higher the soil total N,the higher the fine root P content.In 2021,the variation of 0-1mm diameter fine roots was significantly affected by soil ammonium nitrogen content and N:P ratio.Ammonium nitrogen was the most important factor leading to the change of Al content in fine roots,while the soil N:P ratio was significantly negatively correlated with the specific root length and positively correlated with carbon content in fine roots.In this study,short-term and long-term simulated effects of nitrogen deposition on fine root biomass and functional traits were carried out to reveal the variation and mechanism of nitrogen deposition on fine roots in subtropical evergreen broad-leaved forest with nitrogen deposition time.It provides an underground ecological perspective for predicting the effects of atmospheric nitrogen deposition on structure and function of evergreen broad-leaved forest.