| The Qinghai–Tibet Plateau has a huge soil carbon stocks,which plays an important role in the global carbon cycle and is a great support for China to achieve carbon peaking and carbon neutrality goals,as well as a sensitive area and key zone for global climate change.Land use has an important impact on the turnover of soil aggregates and the distribution and renewal of organic carbon fraction,which is the main site of organic carbon fixing and its spatial heterogeneity also affects the distribution of organic carbon fraction,and different carbon fraction has different carbon storage capacity and ecological service function.As a key subsurface process,the turnover of soil organic carbon is still being explored.In order to understand the influence of different land uses on soil organic carbon,this study selects five different land uses（bareland,farmland,wetland,shrubland,and plantation）on the Tibetan Plateau to investigate the influence of land uses on the stability of soil aggregates,the distribution characteristics of soil organic carbon,and its carbon stable isotope distribution characteristics.On this basis,we analyzed the flow of carbon in soil aggregates and organic carbon components,and finally understood the migration,distribution and transformation of carbon in soil organic carbon components.The main conclusions are as follows:（1）Land use practices can affect the stability of soil aggregates.Land use practices affect both mechanical and water stability of soil aggregates,among which water-stable aggregates are more sensitive and better reflect the structural stability of soil aggregates in Tibet.Agricultural farming increases the content of soil macroaggregates with mechanical stability but not water stability;soil aggregates in planted forests and wetlands have higher water stability characteristics.（2）The aggregate fraction is the main factor affecting soil organic carbon content in the Qinghai-Tibet Plateau region.Soil organic carbon content did not differ significantly among aggregates,while aggregates with particle size<1 mm accounted for a larger proportion,so small aggregates were the main source of soil organic carbon contribution.Soil organic carbon content was highest in wetlands and lowest in bareland.Active organic carbon was highest but less stable in wetlands,most stable in plantation soils,and least active in bareland.（3）The fractionation of organic carbon fractions in aggregate differed significantly in different land use practices.In bareland and farmland,the effect of aggregate particle size on the organic carbon of each fraction was not significant,while in wetland,shrubland and plantation there was significant fractionation of organic carbon of each fraction in aggregates.The organic carbon content of soil physical fractions under different land uses showed that LOC>MOC>POC,and soil EOC showed highly significant positive correlation with TN,TC,SOC and LOC,indicating that organic carbon was highly correlated with active organic carbon fractions and soil carbon and nitrogen contents in the Qinghai-Tibet Plateau region.The correlation trends were more similar for the original soil and large aggregates,indicating that large aggregates play a dominant role in the main properties of soil organic carbon in the Qinghai-Tibetan plateau region.（4）Multiple pathways exist for the transformation of soil organic carbon.Soil organic carbon flows in different particle size aggregates and between different fractions.Theδ13C of aggregate particle size and organic carbon fraction increases sequentially from MOC in large aggregate to MOC in small aggregate,so MOC in small aggregate is the end point of C flow. |
