Effect Of Nitrogen Addition On The Morphological Charactertistics Of Fine Root Of Dominant Tree Species In Warm Temperate Forests

With a dramatic increase in the rate of atmospheric nitrogen deposition(Nitrogen deposition),a serious impact on the ecological effects of forest ecosystems.As the major organ of a plant body to obtain nutrients,water and minerals,fine root is the root with a diameter of less than 2mm,which are important not only for plant functioning but also for the energy flow and the material circulation through the ecosystem.Plenty of researches on the effect of atmospheric nitrogen deposition on the morphological characteristics of fine root have been done.Yet,there are still many biases in the research results as a result of the fact that most of the previous studies were based on understory nitrogen additions,ignoring the potential influence of canopy uptake and retention of nitrogen.This study was conducted on the field control experiment platform of "canopy and understory simulation of nitrogen deposition" in the National Field Scientific Observatory of Forest Ecosystem in Dabie Mountain,Henan Province,China,in this study,we selected fine root from three dominant tree species(Liquidambar formosana Hance,Quercus acutissima Carr,Quercus variabilis Blume)as the subjects.And the purpose is to find out the effects of different concentrations(50 kg N ha-1yr-1 and 25 kg N ha-1yr-1)of canopy(CAN)and understory(UAN)addition of nitrogen on the dynamic characteristics of biomass,morphological characteristics and stoichiometry composition of fine root of dominant tree species in warm temperate deciduous broad-leaved mixed forests.The main results were as follows:(1)Nitrogen addition treatments reduced fine root biomass at different growing stages of the Liquidambar formosana Hance and Quercus variabilis Blume,and reduced fine root biomass at the end of the growing season of the Quercus acutissima Carr.High canopy nitrogen addition(CAN50)significantly reduced fine root biomass of the Liquidambar formosana Hance during the non-growing season.Contrast analysis showed that compared to canopy nitrogen addition,understory nitrogen addition significantly reduced the fine root biomass of the Quercus acutissima Carr in the non-growing season.In terms of temporal dynamics,fine root biomass of the three dominant tree species reached highest in the non-growing season,followed by the growing season and lowest at the end of the growing season.(2)Compared with transport roots(fourth and fifth-order root),absorptive roots(first,second and third-order root)are more sensitive to nitrogen addition.The CAN25 treatment increased the mean diameter and tissue density of absorptive root and decreased the specific root length and the specific surface area of absorptive root during the growing season in both the Liquidambar formosana Hance and the Quercus acutissima Carr.In contrast,the CAN25 treatment decreased the average diameter and tissue density of fine root and increased the specific root length and the specific surface area at the end of the growing season.The contrast analysis showed that compared to the canopy nitrogen application method,the understory nitrogen application method treatment significantly increased the average diameter of intact root and significantly decreased the specific root length and the specific surface area of fine root of Liquidambar formosana Hance at the end of the growing season.(3)Nitrogen treatments generally increased the total nitrogen content of fine root of the dominant tree species and decreased the total carbon content and carbon to nitrogen ratio of fine root.Contrast analysis showed that neither the nitrogen addition method nor the nitrogen addition concentration had a significant effect on the fine root stoichiometric fraction of the dominant tree species.(4)Correlation analysis showed that,under nitrogen addition treatments,the mean diameter of fine root of the dominant tree species was significantly negatively correlated with the specific root length and the specific root surface area),and the specific root length was significantly positively correlated with the specific surface area.In addition,fine root biomass was significantly and positively correlated with soil pH value and water content under nitrogen addition treatments,respectively.In summary,nitrogen addition had a negative effect on fine root biomass of dominant tree species,tending to reduce fine root biomass.The morphological characteristics of absorptive roots(root class 1-3)was more sensitive in response to nitrogen addition than the transport roots(root class 4-5).The specific root length of fine root of dominant tree species was highly relevant with specific surface area.Nitrogen addition of the dominant tree species tended to increase the total nitrogen content of fine root,and decrease the total carbon content and carbon to nitrogen ratio of fine root.The above results are adaptive strategies adopted by plants in response to environmental changes,and these results have some theoretical implications for the ecological functions of fine root in forest ecosystems.