Effects Of Nitrogen Addition On Fine Root Morphology And Anatomy Of Dominant Shrubs In Xiaoxing'an Mountains

Numerous studies have been carried out over the past decades on the effects of ni-trogen deposition on terrestrial ecosystems,which have confirmed that N deposition caused soil acidification and changed soil availability resources.Fine roots,which were the most sensitive and active part of the plant and were an important organ for the ab-sorption of water and nutrients,and were severely affected by N deposition.However,there were not many studies on N deposition on fine roots,and there was no uniform conclusion on the mechanism of its effect.Therefore,in order to reveal the response of fine roots to environment changes,a simulated N deposition experiment was conducted in Fenglin National Nature Reserve,Heilongjiang.In this study,four shrubs(including Acanthopanax senticosu,Lonicera japonica,Spiraea salicifolia and Sorbaria sorbifolia)were studied,which included four nitrogen input levels,control(CK,0 g·N·m^-2·a^-1),low nitrogen(LN,2.5 g·N·m^-2·a^-1),medium nitrogen(MN,5.0 g·N·m^-2·a^-1),and high nitrogen(HN,7.5 g·N·m^-2·a^-1),and each treatment was replicated three times.Fine roots were sampled at the soil surface in mid-July during the 2019 plant growing season,and the root systems were divided into 1st-5rd fine root to investigate the effect of N addition on fine root morphology(fine root length,diameter,and surface area),anatom-ical structure(stele diameter,cortical thickness,ratio of stele to root diameter,and tra-cheid characteristics),and fine root stoichiometry(C,N,and P and the ratio of them)of four dominant shrubs in conjunction with their physiological function and structure.The main results showed that:(1)Low nitrogen addition increased the fine root diameter,fine root length,and surface area of 1st-5rd fine roots of the four shrubs,but HN treatment produced inhibi-tion,what's more,intra-and interspecific variation in fine root morphological indices among species.Moreover,the changes in fine root diameter were correlated with stele diameter by association with anatomical structures.This indicated that fine root mor-phology changed with anatomical structure under the N addition treatment.(2)Treatment,species,root order and their interaction all had significant effects on fine root morphology and anatomical structure.N addition promoted the physiological growth of the four shrubs,and the number of tracheid and total tracheid area were sig-nificantly increased,and it was the change in tracheid that led to the increase in stele diameter.N addition significantly increased the ratio of stele to root diameter which im-proved fine root transport capacity.LN treatment increased the cortical thickness of1st-3rd fine root,while cortical thickness decreased in the HN treatment,and the rate of nutrient uptake by the root system was accelerated,and the number of cortex layers were the factor that caused the change of cortical thickness.(3)N addition decreased fine root organic carbon content,but increased total N and total P content.Moreover,the fine root N:P ratio was below 14,so plant growth was mainly N-limited.In this study,the morphological characteristics as well as anatomical structure of fine roots under N addition were significantly different among tree species,treatments,and root orders.These results contribute to an indepth understanding of the nutrient up-take strategies of tree fine roots and have important theoretical implications for the physiological and ecological functions of fine roots.