Effects Of Different Nitrogen Deposition Levels On Fine Root Longevity In Metasequoia Glyptostroboides Plantation

The gradual increase of nitrogen deposition leads to the continuous growth of nitrogen and quick nitrogen cycle in the soil,which affects the growth of trees and fine roots.To carry out the study of nitrogen effects on plant root growth dynamic settlement,will help us more clearly understand the underground forest eco physiological processes(especiAlly root effect),to achieve scientific management under global change,has important theoretical significance and application value to improve forest productivity.In this study,Metaseguoia glyptostroboides plantation as the research object,using minirhizotron technique in situ to observe the dynamic process of birth,growth,aging and death under different nitrogen treatments,using Kaplan-Meier method to estimate the longevity of different diameter class,fine root order,soil depth and birth season,to explore the effect of different levels of nitrogen deposition on fine root longevity,the results are as follows:(1)With the increase of diameter class and order,the mean lifespan,median lifespan and cumulative survivorship of M.glyptostroboides fine root are significantly increased,except the order between three and four order,indicating the diameter and order are the main factor to influence the heterogeneity of longevity.(2)With the increase of soil depth,the number,mean lifespan,root diameter reached a maximum at 30~45cm.The cumulative survivorship of surface(0~15cm)was significantly lower than the deep(15~60cm),but there was no significant difference between deep roots.(3)The number,mean lifespan and median lifespan of M.glyptostroboides fine root were different in different seasons.The median lifespan of fine root cohort in different seasons were winter>autumn>spring>summer.There were significant differences among the spring,summer and winter,and there was no significant difference between the three groups and the autumn.(4)By comprehensive analysis Cox regression model showed that,the influence of root diameter,branch order,season of birth and depth of soil to root longevity,the results show that the root diameter,branch order,season of birth and depth of soil had significant effects on the longevity of M.glyptostroboides fine root.Influence factors of fine root longevity,according to different degree from large to smAll are branch order,season of birth,root diameter and depth of soil.(5)The effects of different nitrogen levels on fine root longevity were different.In low nitrogen condition,the number,mean diameter,lifespan were increase,but in the high levels of nitrogen deposition,the influence were decresse even negative.The response of different diameter class fine roots to nitrogen availability were not consistent.N1 significantly prolonged longevity of the smAll diameter(<0.3mm),but N4 significantly shortened the sm All diameter(<0.3mm);N3 and N4 significantly reduces longevity of the second diameter class;N1 significantly prolonged the root longevity of the fourth diameter class.We find difference by compare longevity of different fine root order: N1 was significantly prolonged the lifespan of first-order and fourth-order,N4 significantly shortened the root lifespan of first-order,second-order and third-order.While the other two levels of nitrogen deposition had no significant effect on fine root lifespan.Compared with the control,the fine root cumulative survivorship of 0~15cm and 45~60cm were significantly increased,and the longevity of fine root was significantly prolonged in the four nitrogen treatments,but the effects of different concentrations of nitrogen were different.In the 15~30cm soil layer,the fine root longevity of N1 and N2 was significantly higher than that of the control plots,and the fine root longevity of N3 and N4 was not significantly different from that of the control.In the 15~30cm soil layer,there was no significant effect of four nitrogen deposition levels on fine root longevity.N1,N2 and N4 levels of nitrogen deposition significantly prolonged the longevity of spring and summer,and only N1 level of nitrogen deposition significantly prolonged longevity of autumn,N4 levels nitrogen deposition significantly shortened the longevity of winter.By comprehensive analysis Cox regression model showed that,the mortality risk of N1 was significantly lower than the control,while N2 and N3 were not significant difference with control,the mortality risk of N4 was about double than the control.With the increase of diameter and root order,the mortality rate of fine roots significantly decreased,and the fine root longevity in autumn and winter was significantly higher than that in spring.However,the mortality rate of fine root was higher than that of surface layer under nitrogen condition.The influence factors of fine root longevity according to different degree from large to smAll are root order and diameter,birth season,levels of nitrogen deposition,soil depth.