Temporal And Spatial Evolution And Mechanism Of Ecosystem Carbon Budget In Xing'an Type Permafrost Region

In the context of global warming,the processes,changes and mechanisms of carbon budget and carbon cycle in permafrost regions brought about by permafrost changes are important basic scientific issues for national carbon neutrality.Permafrost is an important part of the cryosphere,which is one of the five spheres of the Earth system.It is mainly distributed in high latitudes and high altitudes and is extremely sensitive to global climate change,accounting for about 25%of the earth's land surface in the northern hemisphere.Over the past few decades,most permafrost regions have experienced accelerated thawing due to a warming climate.Permafrost degradation can change forest ecosystems and soil microbial activity,change soil respiration,and then affect the carbon source/sink effect of permafrost ecosystems,forming positive feedback on climate change,thereby accelerating global warming and triggering a series of issues such as hydrology,carbon cycle,ecological environment and engineering infrastructure,which have seriously affected people's lives and regional socio-economic development.The Xing'an Permafrost(XAP)in northeastern China is located in the transition zone between permafrost and seasonally frozen soil,and is located on the southern edge of the permafrost around the Arctic.It is more sensitive and vulnerable to climate and environmental disturbances.Affected by latitude and altitude,its changes and response processes are more complex and diverse,with regional and global representation,and it is a unique sensitive area for studying the ecological and environmental effects of permafrost degradation.In the context of permafrost degradation,what are the carbon source/sink effects of XAP vegetation,soil,and ecosystems?Has something changed?Research is still lacking.On the basis of summarizing and commenting on the research on the impact of permafrost change on ecosystems and carbon cycle at home and abroad,this thesis took XAP as the study area,and used multivariate data such as air temperature,ground radiation,CO₂ concentration,soil temperature,land use cover/change data,etc.,using field observation,model simulation,trend analysis,GIS spatial analysis,control experiments and other methods to analyze the change characteristics of the XAP natural environment,and simulate the 1982-2019 XAP growing season vegetation CO₂ absorption,soil respiration CO₂amount and ecosystem net carbon fluxes,analyzed its temporal and spatial distribution and change characteristics,clarified the change law of carbon source/sink effect in XAP ecosystem,and revealed the mechanism of its change.The main contents and conclusions of this paper are as follows:(1)Since the 1980s,the temperature in the XAP growing season has increased significantly,with an increase of 0.565?,which shows a significant increase in the whole region in space,with an average temperature increase rate of 0.362?/10a.The three types of permafrost regions all(2)showed a significant increasing trend,and the permafrost region in the island-shaped thawing zone had the largest temperature increase rate,which was 0.403?/10a.Since the 1980s,the surface temperature at 0 cm and the soil temperature at 15 cm,20 cm,40 cm,80 cm,and 160 cm in the XAP growing season have increased significantly,and the heating rate is between0.392?/10a and 1.074?/10a,respectively.The surface temperature heating rate was the fastest,increasing by 2.689?.The three types of permafrost regions all increased significantly,and the surface temperature increase rate in the island permafrost region was the largest,which was1.172?/10a.Since the 1980s,the growth of XAP vegetation has improved significantly,the vegetation development period has been advanced and the end period has been delayed,and67.378%of the area has shown a significant increase trend,with an increase rate of 0.032/10a,and the NDVI index has increased by 0.089.The vegetation growth in different types of permafrost regions has significantly improved for more than 80%of the area.Since the 1980s,the land use/cover of XAP has undergone great changes,mainly characterized by an increase in the area of arable land and forest land,and a decrease in grassland and other use types.Among them,the area of cultivated land continued to expand,increasing by 21302.77 km~2.The forest types changed significantly,with the largest area of coniferous forest to shrub forest,which was 1991.72 km~2,concentrated in the north-eastern part of the study area,and the rest of the forest types changed less.It reflects the impact of human activities on land use change in XAP,as well as the change of permafrost degradation on the natural succession of forest land types.(3)Since the 1980s,the vegetation in the XAP growing season has continued to be a carbon sink,and the carbon sink capacity has been significantly enhanced.From 1982 to 2019,the annual average of CO₂ uptake by vegetation in the XAP growing season was 7.144×10¹¹kg,showing a significant upward trend at 0.444×10¹¹kg/10a.From 1982 to 2019,the CO₂ uptake by vegetation in XAP increased by 1.687×10¹¹kg,82.25%of the entire XAP area has a significant increase in CO₂absorption by vegetation.The average values of vegetation CO₂ uptake in the island-shaped permafrost area,the island-shaped thaw area,and the large continuous permafrost area were4.974×10¹¹kg,0.997×10¹¹kg,and 1.173×10¹¹kg,respectively,and they were all positive values,continued to be a carbon sink,and the carbon sink capacity was significantly enhanced,with the change rates of 0.273×10¹¹kg/10a,0.078×10¹¹kg/10a and 0.093×10¹¹kg/10a,respectively.Except for the Hulun Lake area in the southwest of the island-shaped permafrost area,which showed a significant decrease,the CO₂ absorption of vegetation in other types of permafrost areas increased significantly.The CO₂ uptake by vegetation increased significantly in each month of the growing season,with the most obvious increase trend in July and September.(4)Since the 1980s,the soil respiration in the XAP growing season has been a carbon source,and the carbon source capacity has been significantly enhanced.From 1982 to 2019,the annual average of CO₂ released by XAP soil respiration was 0.786×10¹¹kg,and the change rate was0.48×10¹⁰kg/10a.From 1982 to 2019,the amount of CO₂ released by XAP soil respiration increased by 1.824×10¹⁰kg.The average values of CO₂ released by soil respiration in the island-shaped permafrost area,the island-shaped thaw area,and the large continuous permafrost area were5.456×10¹⁰kg,1.100×10¹⁰kg and 1.308×10¹⁰kg,respectively,and the change rates were0.298×10¹⁰kg/10a,0.083×10¹⁰kg/10a and 0.100×10¹⁰kg/10a,all continued to be the carbon source,and the carbon source capacity was significantly enhanced.From 1982 to 2019,the amount of CO₂released by soil respiration increased significantly in 82.29%of the entire XAP area,and the significantly reduced area accounted for 2.23%of the entire area,located in the southwest of XAP.From 1982 to 2019,the amount of soil respiration CO₂ increased significantly in each month of the XAP growing season,and the most obvious increase was in July and September.(5)Since the 1980s,the ecosystem in the XAP growing season has remained a carbon sink,and the carbon sink capacity has been significantly enhanced.From 1982 to 2019,the annual average CO₂ uptake of the ecosystem in the XAP growing season was 6.358×10¹¹kg,and the increase rate was 0.396×10¹¹kg/10a.From 1982 to 2019,the CO₂ uptake of the XAP ecosystem increased by 1.504×10¹⁰kg.The average net carbon fluxes of the ecosystems in the island-shaped permafrost area,the island-shaped thaw area,and the large continuous permafrost area were4.428×10¹¹kg,0.887×10¹¹kg,and 1.043×10¹¹kg,respectively,and the change rates were 0.243×10¹¹kg/10a,0.070×10¹¹kg/10a,and 0.083×10¹¹kg/10a,all showed a significant increase trend,and they all continued to be carbon sinks,and the carbon sink capacity was significantly enhanced.During the 1982-2019 XAP growing season,the area of the ecosystem with a significant increase in CO₂ absorption accounted for 82.11%of the total area,and the area with a significant decrease accounted for 2.36%of the total area,located in the Hulun Lake area in the southwest.From 1982 to2019,the monthly CO₂ uptake of the ecosystem in the XAP growing season was positive and increased significantly.(6)The decrease of NDVI in the southwest of XAP is probably related to the fact that the degradation of the permafrost interferes with the balance between the hydrothermal process and the growth of grassland vegetation in the permafrost area,causing grassland degradation and poor growth,thus reducing the increase rate of the carbon sink effect in the southwest of XAP.Ground radiation was the main factor that reduced the rate of increase in the carbon sink effect in the southeastern XAP.