Effects Of N Fertilization On Root Anatomy,Morphology,Nutrients And Mycorrhizal Colonization In Six Temperate Tree Species

Root functional traits play key roles in water and nutrient absorption,photosynthetic metabolites,turnover and decomposition in trees,and have great contribution to carbon(C)and nutrients cycles in forest ecosystem.As the most important trait,root anatomy has close correlations with other functional traits,such as morphology,nutrient concentrations and mycorrhizal colonization.However,the response of root anatomy following increasing nitrogen(N)availability is unclear,and its correlations with other functional traits have always been ignored in the previous studies.In such case,we chose six temperate tree species,Juglans mandshurica(Jug),Fraxinus mandschurica(Fra),Phellodendron amurense(Phe),Larix gmelinii(Lar),Pinus koraiensis(Pin)and Picea koraiensis(Pic),at the same site condition in plantations,sampled the first five order roots from surface soil layer for two years(2014 and 2015)of N fertilization,and measured their anatomical and morphological traits,C and N concentrations,and as well as mycorrhizal colonization.We got the following results:(1)Only the first three order roots in all six species were found to have intact cortex,the cortical thickness and cortical cell traits were generally decreased but insignificantly following N fertilization in these root orders.In contrast,stele radius in the first five order roots significantly increased with increasing N availability,with average increments of 19.6%and 28.9%in 2014 and 2015,respectively.Among the species for this trait,the highest increments were occurred in Fra(2014)and Phe(2015),and the lowest increments were observed in Lar(2014)and Pic(2015).Redundancy analysis(RDA)revealed that the increase of stele radius is mainly caused by significantly increased conduit diameter(49.7%)and conduit number(21.2%).N fertilization rather promoted the development of cortical tissue than the development of stele and conduit,indicating that root transport was improved by high N availability.(2)N fertilization generally increased root diameter,but decreased SRL and tissue density in the first five order roots in six tree species.During the treatments,the effects of fertilization on these morphological traits were similar between two years within species,but varied largely among species.Among species,the changes of root diameter and SRL by averaged the first five order roots were the largest in Fra,and the smallest in Pic.By contrast,the changes of tissue density depended on species and treatment years.Based on RDA analysis,we found that the significant increase of root diameter is mainly caused by significantly enlarged stele radius after fertilization.These results indicate that the changes of root morphological traits have close association with anatomical traits following N fertilization.(3)N concentration in the first five order roots increased,but C concentration and C:N ratios decreased,in six tree species following N fertilization.Differences of C and N concentrations were found relatively largely among species when compared to that between fertilization periods.The changes of N concentration by averaged the first five order roots ranged from 4.8%(Lar)to 27.1%(Jug)among species,while in C:N ratios it ranged from 5.1%(Pic)to 21.1%(Fra).In comparison,C concentration slightly changed among species(only 0.2%?1.2%).In each species,N concentration decreases while C concentration increases with ascending root order,such patterns in the control and in fertilization treatment are similarly.(4)Mycorrhizal colonization rate and degree decreased in the first order roots following N fertilization in all six species,such effects were significant in some species.For colonization rate,significant decreases were only observed in Lar and Pin.For colonization degree,it was unchanged in Lar,but significantly decreased in Fra,Phe,Pin and Pic for two years of fertilization.Such reduction was significantly in 2015(17.4%)but not in 2014(8.3%)for species of Jug.RDA analysis indicates that mycorrhizal colonization rate and degree are generally positively related to cortical thickness,cortical cell diameter and cortical cell layer in the first order roots.Considering the increase of N concentration but the decrease of mycorrhizal colonization in fine roots after N fertilization,the dependency of roots on mycorrhiza may decrease at high nutrient availability in tree species.Overall,our results revealed that the responses of anatomy,morphology,nutrients and mycorrhizal colonization of fine roots obviously differed among species,treatment years and root orders following N fertilization.The close relationships between anatomy and morphology,C and N concentrations and mycorrhizal colonization in fine roots suggest that tree physiology strongly link with root functional traits.These results have important significances on deeply understanding the mechanism of root dynamics under environmental changes.