Analysis Of Different Vegetation Types And Soil Carbon And Nitrogen Isotopic Compositions And Influencing Factors On The Loess Plateau

The Loess Plateau is a typical ecological fragile region in China.In order to ensure the sustainable development of the ecology in this region,a series of ecological engineering measures have been implemented on the Loess Plateau.Returning cropland to forest and grassland is a scientific and regional ecosystem project,the aboveground biomass increase during the process of vegetation restoration,however,the impact on soil carbon and nitrogen pools and the carbon and nitrogen cycles of the entire system is controversial.So,the study takes carbon and nitrogen isotopes as research tools,analyzing their distribution and influencing factors on the Loess Plateau,revealing the impacts of vegetation restoration on the carbon and nitrogen cycle theoretically and giving useful and meaningful suggestions on the construction of a healthy and sustainable ecosystem in practice.In this study,we investigated the?¹⁵N and?¹³C in grassland,shrnb land and forest land on a typical north-south transect of the Loess Plateau and collected the soil and foliar samples of all existing vegetation restoration vegetation types and a typical hillslope in Liudaogou watershed.The objectives of this study were:?1?to investigate distribution characteristics of?¹⁵N and?¹³C in soil and foliar;?2?to analysis the differences of the slope?¹⁵N and?¹³C;?3?to explore the main controlling factors of?¹⁵N and?¹³C by using the method of Patrial Lease Squares Regression analysis?PLSR?,Analysis of Variance,Geostatistics and Structural Equation Modeling?SEM?;?4?to probe the effects of different vegetation restoration ages on?¹⁵N and?¹³C under different land cover types.The main results can be drawn as follows:?1?The contents of?¹⁵N and?¹³C in soil and foliar under three different land use types of forestland,shrub land and grassland were measured and analyzed in the North-South transect of the Loess Plateau?Yangling,Changwu,Ansai,Mizhi and Shenmu?.The results showed that the average values of soil?¹⁵N in forestland,shrub land and grassland were 5.82‰,5.58‰and 4.77‰,respectively;the average of foliar?¹⁵N were0.55‰,0.54‰and-0.97‰,respectively;the average of soil?¹³C were-11.05‰,-12.73‰and-16.47‰,respectivelt;the average of foliar?¹³C were-27.43‰,-27.02‰and-27.97‰.The contents of soil?¹⁵N and?¹³C were higher than it in the foliars,meanwhile,the contents of soil and foliar?¹⁵N and?¹³C in the forestland and shrub land were higher than it in the grassland.By establishing PLSR models,we found the mean annual precipitation?MAP?,soil total nitrogen?STN?and soil organic carbon?SOC?were the main influencing factors of soil?¹⁵N;MAP significantly affects foliar?¹⁵N and soil?¹³C;the main control factor of foliar?¹³C was soil carbon-nitrogen ratio?SCN?.?2?Based on the investigation of the restoration of existing vegetation in the watershed of Liudaogou,the plants were divided into 6 categories according to the different plant functional types?PFTs?:C₃-herb,C₄-herb,nitrogen-fixing grasses,grasses,shrubs and forests.The natural isotopic content of different PFTs were analyzed,and the results showed there were significant differences in nutrient and natural isotopic content in foliars and soil of different PFTs.Soil and foliar organic carbon and total nitrogen contents in shrub land were significantly higher than those in other plots;the organic carbon in soil and foliar of C₄-herb were the lowest.The content of?¹³C in foliar of C₄-herb was greater than that in other plots significantly.The trend of foliar?¹³C,soil and foliar?¹⁵N were basically the same andtheorderwereasfollows:shrub>forest>nitrogen-fixing grasses>C₃-herb>grasses>C₄-herb.There were positive correlations between foliar?¹³C and soil?¹³C,soil?¹⁵N also increased with the increase of foliar?¹⁵N.?3?To study the effects of slope position and profile on soil and foliar nutrients,carbon and nitrogen isotope distribution,we selected typical vegetation restoration slopes in the watershed of Liudaogou.The results showed that the foliar carbon increased gradually with the decrease of the slope position,the foliar nitrogen,soil organic carbon and total nitrogen all increased first and then decreased.The contents of foliar?¹³C and?¹⁵N showed a trend of first decreased and then increased with the decrease of slope position,while soil?¹³C and?¹⁵N showed a trend of first increased and then decreased.Geostatistical analysis was used to obtain the best fitting model of the semivariance of the contents of?¹³C and?¹⁵N in the soil and foliar on the hillslope.Soil?¹³C is an Exponential model,soil?¹⁵N and foliar?¹³C are Spherical model,and foliar?¹⁵N is a Gaussian model.Soil organic carbon and total nitrogen decreased first and then stabilized with the increasing of soil depth,while soil?¹³C and?¹⁵N increased first and then stabilized.?4?Analyzing the soil and foliar nutrient and isotope content of different vegetion restoration ages in the watershed of Liudaogou,we found that with the increase of vegetation restoration ages,soil total nitrogen in forestlands,soil total nitrogen and organic carbon in shrub lands and grasslands increased significantly,soil organic carbon in forestlands increased first and then decreased.However,the differences in foliar carbon and nitrogen were not significant between different vegetation restoration ages.The soil and foliar?¹⁵N under all land use types,the soil?¹³C of shrub land and grassland showed a trend of increase first and then decreased with the increase of negetation restoration ages.Based on SEM,we found that the vegetation restoration ages had the biggest influence on soil?¹³C and foliar?¹⁵N in grasslands,soil?¹³C and?¹⁵N and foliar?¹³C in shrub land,soil and foliar?¹⁵N in forestland.