Effects Of Long-term Nitrogen Addition On The Morphology,anatomical Structure And Stoichiometry Of Fine Roots Of Three Tree Species In Broad-leaved Korean Pine Forest

As the main organ for nutrient and water absorption,fine roots were the most active part of the plant root system and were particularly sensitive to soil environment.Nitrogen（N）deposition changed the soil environment,and would certainly affect the structure and function of fine roots.In this study,in May 2011,four different N addition treatment were established in broad-leaved korean pine forest in the Lesser Khingan Mountains,Northeastern China,which were control(0g·N·m^-2·a^-1),low nitrogen treatment(2.5g·N·m^-2·a^-1),medium nitrogen treatment(5g·N·m^-2·a^-1)and high nitrogen treatment(7.5g·N·m^-2·a^-1),each treatment included3 replicates respectively.The main dominant tree species（Pinus koraiensis,Abies nephrolepis and Acer ukurunduense）were selected and the root systems of three tree species were dug out in July 2019,to determine the change of fine roots traits under different N addition,which included fine root stoichiometry,fine root diameter,root length,root surface area,cortex thickness,cortical cell traits,stele diameter,ratio of stele to root diameter and conduit traits.Discuss the effects of different levels of N addition on the morphological traits,anatomical structure and stoichiometry of the fine root of the three tree species,understanding the variation and trade-offs of fine root traits of the three tree species under long-term N addition.It was the great significance for understanding and predicting the physiological function changes of plant roots under global climate change.The main findings were as follows:（1）Long-term low N and medium N addition significantly increased fine root diameter,length and surface area of three tree species.Long-term high N addition significantly increased Abies nephrolepis fine root diameter,length and surface area.high N addition significantly increased fine root diameter but markedly inhibited fine root length and surface area in Acer ukurunduense;high N treatment remarkably inhibited fine root diameter,length and surface area in Pinus koraiensis.（2）Long-term Low N and medium N treatments significant increased three tree species fine root cortex thickness,cortical cell layer number,cortical cell diameter,stele diameter,ratio of stele to root diameter.As well as the tracheid traits of Pinus koraiensis.Long-term high N treatment significantly increased root cortex thickness,cortical cell layer number,and stele diameter in Abies nephrolepis and Acer ukurunduense;high N addition significantly decreased stele diameter,tracheid number in Pinus koraiensis.The main reason for the increase of cortical thickness was the change of cortical cell diameter,and the change of central stele diameter was due to the change of conduit traits;the changes of cortical thickness and stele diameter jointly drove the variation of fine root diameter.（3）Long-term low N and medium N treatments significant increased fine root TN content and significantly decreased fine root C:N in all three tree species.High N addition markedly increased fine root TOC content in Pinus koraiensis.（4）Among the three tree species,there were significant differences in the response of fine root length and surface area of different tree species and different root orders to N addition;the fine root diameter,cortical thickness,cortical cell traits,and stele diameter of different tree species and different root orders has certain regularity by low N and medium N addition.While the response to high N addition was more complex and had no consistent regularity.In conclusion,long-term N addition was able to significantly affect fine root morphological traits,anatomical structure,and stoichiometry,but this effect was regulated by tree species,root order,and different N treatment levels.N addition can drive the increase of fine root diameter by increasing the stele diameter,so that the root system can enhance its transport capacity under the condition of increasing available N in the soil,which reflects the trade-off between root structure and function.These results suggest that fine roots of the three tree species optimize their function through strategies that adjust morphological traits,anatomical structure,and stoichiometry in response to changes in the soil environment.