| Soil organic carbon（SOC）and total nitrogen（TN）in grasslands play an important role in the global carbon and nitrogen biogeochemical cycle.To manage and improve the grasslands,which are vulnerable to degeneration,it is critical to estimate the SOC and TN content of grasslands and well understand its controls.The agricultural-pastoral ecotone of northern China has the dual functions of production and ecology.It contains rich grassland resources and serves as an ecological protection barrier for the central and eastern regions.However,climate change and human activities have made the ecological environment of this place increasingly fragile,and the restoration of the grassland ecosystem has become an important issue.The grassland ecosystem in the agricultural-pastoral ecotone of northern China was taken as an example in this thesis.Firstly,based on an extensive field surveys and multiple statistical approaches,we studied the spatial distribution characteristics of SOC and TN in northern China.Secondly,we explored their correlations with climate factors（mean annual temperature and mean annual precipitation）,biological factors（vegetation coverage,vegetation height,species richness,and Shannon-Weiner index）,soil physical factors（soil texture,soil specific surface area,and soil water content）,soil chemical factors（amorphous aluminum,amorphous iron,soil p H,soil total sulfur,and soil total phosphorus）,and human activity intensity（HAI）.In addition,the structural equation model was used to quantitatively analyze the effects of soil properties and HAI on SOC and TN.Lastly,measured data,data collected from published literature,and the second national land survey data were combined to analyze the distribution characteristics of the ratio of carbon to nitrogen,the distribution characteristics of SOC and TN density,and calculate the SOC and TN storage potential.The study results provided valuable information for improving our understanding of the SOC,TN,and global C and N cycle in response to climate change and human activities.The major conclusions were as follows:（1）The spatial distribution of SOC showed a trend of low in the west and high in the central and northeast,which was consistent with the distribution of precipitation.The spatial distribution of TN showed a trend of high in the central and low in the west and northeast.Affected by human activities and climate,soil nitrogen loss occurred in the northeast.（2）The profile distribution of SOC and TN was affected by soil physical factors and soil chemical factors.With the increase of soil depth,the correlation between soil physical factors and SOC and TN showed a decreasing trend,while the correlation between soil chemical factors and SOC and TN showed an increasing trend.Besides,deep SOC and TN storage potential were greater.（3）Soil physical factors（soil texture and soil specific surface area）,soil chemical factors（amorphous iron,amorphous aluminum,and soil total sulfur）,human activity intensity and climate factors（mean annual temperature）were the main factors affecting SOC changes.Soil chemical factors（amorphous iron,amorphous aluminum,and soil total sulfur）and biological factors（vegetation coverage）were the main factors affecting TN changes.The main influencing factors of SOC and TN changes at different soil depths were different.（4）The ratio of carbon to nitrogen in the study area was within 2–23,showing a spatial distribution characteristic of high in the middle and low in the west,which was related to vegetation,climate,and human activities.The density of SOC and TN was higher in the central and northeast of the study area,and the mean of SOC and TN density changes were 0.70 kg C m–2 and 0.10 kg N m–2,respectively.The storage of SOC and TN were 792.55 Tg C and 83.41 Tg N,respectively.The realistic storage potential of SOC and TN were 406.96 Tg C and 54.96 Tg N,respectively.In general,the soil in the study area played the role of carbon sink and nitrogen sink in the past 40years. |
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