Effects Of Nitrogen Addition On δ¹⁵N In The Xing'an Larch Forest In The Greater Khingan Mountains

A large amount of N-containing compounds（NOx）are discharged into the atmosphere,causing a sharp increase in the amount of N deposition.N deposition affects the structure and function of the ecosystem,and it has an impact on the material cycle of the ecosystem（C-N cycle）and other biogeochemical cycles.The degree of impact is affected by the initial N state of the ecosystem.At present,there are few studies on N subsidence in northern forest ecosystems in my country.Therefore,it has important theoretical and practical significance to carry out research on the influence of N subsidence on Larix olgensis forest in the Greater Khingan Mountains.Theδ¹⁵N of plants and soils is increasingly used as a comprehensive indicator reflecting the nitrogen status of the ecosystem,and can reflect the impact of nitrogen deposition on the C-N cycle of the forest ecosystem.In order to reveal the response of the N cycle of Larix olgensis forest ecosystem to the increase of N sedimentation in the Greater Khingan Mountains,this study took the Larix olgensis forest in the Nanweng River Nature Reserve of the Greater Khingan Mountains as the research object,set up artificial N addition experiments,and measured different N addition treatments.Plant-soilδ¹⁵N and N content and other indicators are designed to understand the N cycle response mechanism of Larix gmelinii forest under the background of N subsidence and to judge the current N state of the ecosystem.The research results help to reveal the changes in the carbon fixation capacity of the Larix olgensis forest ecosystem in the Greater Xing’an Mountains under the background of global changes,and lay the foundation for scientific assessment of the adaptability of the northern forest ecosystem to increased N deposition.The main experimental results are as follows:（1）Under the different treatments of N addition,theδ¹⁵N and N content of plant leaves increased significantly and the treatment of medium and high N addition significantly higher than the control and low-N addition treatment（P<0.01）.However,there was no significant difference inδ¹³C of leaves under different N treatments（P>0.05）.N addition changed the C content of leaves.Under high N addition treatments,the C content was higher than that of the control treatment,but the difference between the treatments was not significant（P>0.05）.（2）Under the different treatments of N addition,theδ¹⁵N and N content of the fine roots of plants increased significantly（P<0.01）.However,unlike the leaves,the C content of fine roots decreased.Under the high N addition treatment,the C content of fine roots was significantly lower than that of the control treatment（P<0.05）.And there was no significant difference of fine rootδ¹³C between different treatments（P>0.05）.（3）The content ofδ¹⁵N and N in the tree core showed an increasing trend with the increase of N addition,but the difference was not significant（P>0.05）.Theδ¹³C of the tree core gradually became negative with the increase of N addition,and the difference between the treatments was not significant（P>0.05）.There was no significant difference in the C content of tree core among different treatments（P>0.05）.（4）The organic layer（Oa+e）δ¹⁵N,N content and NH₄⁺-N are all significant under different N addition treatments Increase（P<0.05）,and although the content of NO3^--N increased,the difference was not significant（P>0.05）.The content ofδ¹³C and C in the organic layer has no significant difference between different treatments（P>0.05）.（5）Theδ¹⁵N and N content of the soil mineral layer increase with the increase of N addition,but each The difference between treatments was not significant（P>0.05）.NH4⁺-N and NO3^--N under high N addition treatment were significantly higher than the control treatment（P<0.05）.There is no significant difference between theδ¹³C and C content of the mineral layer under different N addition treatments（P>0.05）.N addition significantly changed theδ¹⁵N and N content of plant leaves,fine roots and soil organic layers,and the enrichment factor was close to"0"under high N addition.Therefore,we believe that the leavesδ¹⁵N and N can reflect the effectiveness of N.Compared with the mineral layer,the soil organic layer is more sensitive to N addition.