Research On The Carbon Source/sink Characteristics Of The Ecosystem On The Southern Slope Of The Qilianmountains And Its Effect Factors Of Carbon Pool

In the south slope of the Qilian Mountains,located at the northeastern edge of the Qinghai-Tibet Plateau and in Qinghai Province,In the past,the influencing factors of ecosystem carbon sequestration in this region mainly considered temperature,precipitation,land use,altitude and other factors,but rarely mentioned soil pore parameters,soil conductivity,soil inorganic minerals and soil microbial community.Based on this,this paper combined field investigation and indoor research,using literature collection,field sample survey,The carbon pool density values of overground and underground parts of different ecosystem types were obtained.Based on the land use data of the southern slope of Qilian Mountain from 2000 to 2020 and the carbon storage module of In VEST model,the spatial and temporal patterns of carbon storage of the ecosystem in the southern slope of Qilian Mountain were calculated.sources/sinks in the study area were identified,and selected the Amidongso sub-watershed to study the influencing factors of soil carbon pools.Soil physical and chemical properties,soil microstructure and soil microbial community structure were used to study the influencing factors of soil system carbon pools in the main ecosystems of the south slope of Qilian Mountains,which provide basic theoretical support for the accurate evaluation of carbon income and expenditure and carbon pool stability in the ecosystems of the south slope of Qilian Mountains.The main results of the study are as follows:（1）The total carbon density of vegetation and litter varied greatly among ecosystem types,with 69.7 t/hm~2 for woodland,15.3 t/hm~2 for scrub,14.9 t/hm~2 for grassland,and 4.3 t/hm~2 for cropland,showing that woodland had the highest carbon density and cropland had the lowest total carbon density.Among them,the forest vegetation carbon density is 44.8 t/hm~2 in the aboveground part and 17.1 t/hm~2 in the underground part.The organic carbon density of the shrub vegetation is 7.6 t/hm~2above ground and 7.7 t/hm~2 below ground;the organic carbon density of the grassland vegetation is 7.5 t/hm~2 above ground and 7.4 t/hm~2 below ground;the carbon density value and underground carbon density value are 3.2 t/hm~2 and 0.6 t/hm~2,respectively.With the carbon storage module in the In VEST model,the ecosystem’s organic carbon storage in Qilian Mountains’south slope was 292705 million t in 2000,290865million t in 2010,and 293519 million t in 2020.The carbon storage in the study area has increased by 814000t in 20 consecutive years,showing the transformation process from a weak carbon source to a carbon sink.The fastest increase in carbon stock is mainly concentrated in the central and central-eastern parts of the study area.（2）From 2000 to 2020,the NPP value of vegetation showed a fluctuating trend of increasing,then decreasing and then increasing,with an overall trend of slow increase.Among them,the difference between the NPP of woodland vegetation,shrub vegetation,and grassland vegetation maintained a relatively stable range,showing that the NPP of woodland and shrubland vegetation was significantly more extensive than that of grassland vegetation.The difference in NPP between forest and shrub vegetation ranges from about 10～18 gc/m~2.The NPP value of grassland vegetation is about 70～80 gc/m~2,smaller than the former two.（3）The particle size analysis revealed that the most soil types in the woodland were sandy loam,silt loam,loamy sand,sandy loam,and loam;the soil types in the scrub ecosystem were mostly silt loam with a small portion of sandy loam;most of the grassland was silt loam.The correlation analysis showed that the correlation between soil organic carbon density and environmental factors was basically the same as that between organic carbon content and environmental factors.The main influencing factors on soil organic carbon content and density distribution were determined by principal component analysis as soil water content,pore size（1.0～5.0mm）,pore size（>5.0mm）,microagglomerates（>0.25mm）,microagglomerates（0.05mm～0.25mm）,bacterial shannon index.It was further revealed that soil water content,soil pore diameters of 1.0-5.0 mm,and>5.0 mm provided a channel for water and soil organic carbon to move down for the stable retention of organic carbon in the soil,and the micro-aggregates were better for soil organic carbon.The carbon and its cementation are stable to provide the conditions.In terms of influencing factors of soil organic carbon pool,a three-in-one monitoring system of"inorganic nutrients-minerals-microbes"is proposed.（4）Identification by soil light thin-section and scanning electron microscopy（SEM）showed that:The occurrence state of organic matter in different soil depths is not consistent.The surface layer has the highest content of amorphous organic matter and coarse particle size.They have distinct boundaries with mineral grains.There are apparent plant fiber textures;the organic matter content in the middle layer is slightly lower,and the particle size is finer.Some organic colloids are mixed with powdered quartz and feldspar debris;the deep layer has the lowest organic matter content and the smallest particle size.Most organic matter colloids have unclear boundaries and they are mostly mixed with silt quartz and feldspar.They are distributed in aggregates.The soil organic carbon occurrence mode in the middle and deep parts of forest soil is“organic-inorganic colloid”,while the occurrence mode of soil organic carbon in the middle and deep parts of shrub and grassland soil is“mechanical mixing+organic-inorganic colloid”.