Changes In Tree-ring Stable Nitrogen Isotope Ratios And Tree Growth In The Heihe River Basin

The evolution of nitrogen cycle in global ecosystem is deeply influenced by human activities and climate change.The determination of the nitrogen isotope ratio of each component of the forest ecosystem can provide an explanation for the mechanism of the nitrogen cycle inside and outside the plants,thereby deepening our understanding of the nitrogen cycle on the ecosystem scale,and improving nitrogen scientific management capabilities.The nitrogen cycle change indicated by the tree-ring stable nitrogen isotope ratio（δ15N）is closely related to the climate and environmental conditions.Its characteristics reflect the dynamic changes of the long-term forest ecosystem nitrogen cycle and compensate for the lack of site monitoring data.Measuring the stable nitrogen isotope ratios of tree rings in forest ecosystems can provide long-term information for understanding regional nitrogen cycle characteristics,quantifying nitrogen conversion processes,and exploring the mechanism of occurrence,thereby deepening our understanding of nitrogen cycles and forest management capabilities.The Heihe River is the second largest inland river in China.There are obvious differences in the ecological landscape environment characteristics of different sections of the river basin.As an ecologically sensitive area in arid and semi-arid regions,the Heihe River Basin has become a key focus area for many studies on dendroclimatology and dendrohydrology.However,little attention has been paid to the effects of human activities,mainly agricultural fertilization,on nitrogen cycle and tree growth in a large scale.The Heihe River is the second largest inland river in China.There are obvious differences in the ecological landscape environment characteristics of different sections of the river basin.The upper reach is a forest water conservation area,which is rich in vegetation and is less affected by human activities;the middle reach gathers 96%of the agricultural crops in the basin;the Ejina Basin grows unique riparian forests and shrubs,presenting the landscape of a desert natural oasis.As an ecologically sensitive area in arid and semi-arid regions,the Heihe River Basin has become a key focus area for many studies on dendroclimatology and dendrohydrology.However,little attention has been paid to the effects of human activities,mainly agricultural fertilization,on nitrogen cycle and tree growth in a large scale.In this study,tree ring samples of Qinghai spruce from Sidalong（SDLA）and Dayekou（DYK）in the mountain forests of the Heihe River Basin and Populus euphratica from Langxinshan（LXS）riparian forest were collected,and the radial growth trend of the tree rings of Heihe tree rings was measured;the pre-processing of the stable nitrogen isotope（δ15N）samples of different tree rings were compared and explored.Measure the long-term δ15N and nitrogen content（N%）of tree rings on a long-term scale,and understand the characteristics which indicated by tree-ring δ15N;In-depth assess the characteristics of nitrogen emissions from human activities in the Heihe River Basin and the dynamic changes of the growth of Populus euphratica in riparian forests affected by multiple factors.The radial growth changes of tree rings can evaluate the growth of forests in the Heihe River Basin.In the perspective of 815N of tree rings,we discussed the dynamic effects of changes in nitrogen cycle characteristics on riparian forests growth,filling the gap of high resolution in nitrogen isotope data in the Heihe River.Finally,it can provide a new idea for in-depth understanding the long-term anthropogenic nitrogen deposition of agricultural activities in the Heihe River Basin.The conclusions are as follows:（1）The growth characteristics of Qinghai spruce at two sampling sites in the mountainous area of the Heihe River Basin show good consistency.The standard chronology contains more climate information.The chronological width index and Base Areal Index（BAI）also showed a significant downward trend from 1830 to 1930.In recent decades,it has shown an upward trend,indicating that the growth of Qinghai spruce is accelerating in recent decades.For Populus euphratica,the annual ring index and BAI changed relatively little during the more than 100 years from 1900 to 2012.The fluctuation of growth characteristics was relatively small,and the impact of the climate environment was relatively minor.（2）Three different pretreatments（chemical treatment,hot water treatment,bulk wood）were carried out on the tree ring samples of Qinghai spruce for two consecutive periods（1975-1978 and 1986-1990）.The results of the tree ring δ15N showed that the treering δ15N of the three treatments has consistent continuous change trend after the generalization correction,the environmental nitrogen signal retained after chemical pretreatment is less,and the signal retained by the bulk wood sample is more.Therefore,the chemical pretreatment method may be more suitable for tree species that grow in a high nitrogen input environment.For samples with unclear wood chemical properties and whether chemical treatment can remove the physiological driving mode,bulk wood samples should be used for the measurement and research of δ15N in tree rings to retain a more complete environmental nitrogen cycle signal.（3）The tree-ring δ15N of Qinghai spruce in the mountain area of the Heihe River showed a significant and continuous downward trend（-44.7%/10a,p<0.001;-28.9%/10a,p<0.001）,indicating a relatively close nitrogen cycle in the forest ecosystem of the Heihe mountainous area;the tree-ring 815N of Populus euphratica on the riparian of the Heihe River showed a significant upward trend（2.4%/10a,p<0.05）,indicating that the growth of Populus euphratica in the downstream riparian forest was affected by agricultural emissions in the middle reaches for a long time,and the nitrogen cycle of the riparian forest was relatively open.However,whether Qinghai spruce or Populus euphratica,except for the 3-5 years close to the bark,the tree ring N%does not fluctuate significantly with time.What is more noteworthy is that the tree-ring N%and δ15N showed a significant negative correlation at the three sampling points（r=-0.952,-0.852,-0.766,p<0.001）.（4）Nitrogen deposition does not have a significant regulating effect on Qinghai spruce in the Heihe mountainous area;while for Populus euphratica,the nitrogen carried by the river is the most important factor regulating the growth of trees.We use δ18O,δ15N,and intrinsic Water-Use Efficiency（iWUE）（estimated by tree-ring δ13C）as proxies for climate factors,nitrogenous substances,and carbonaceous substances,combined with the annual streamflow in the Heihe River Basin,we have deeply explored its impact on the growth of riparian forest trees.Further,understanding the multiple regulatory factors affecting the dynamic growth of Populus euphratica in riparian forest with 1978 as the breaking point,we found that the growth of Populus euphratica was mainly regulated by nitrogen before 1978,and the growth of Populus euphratica was jointly regulated by carbon and nitrogen by the effect of CO2 fertilization after 1978.