Abstract: The photosynthesis of green plants can absorb carbon dioxide and release oxygen,which helps keep the atmosphere balance of CO₂ and O₂. Measuring the value of carbon fixation and oxygen release is very important for understanding the ecological system as well as its change. The cost of atmospheric CO₂ treatment and their dynamic changes were analyzed based on remote sensing and meteorological data in northern Shaanxi. Light energy utilization model was used to estimate Net Primary Productivity(NPP)with MODIS/NDVI data, and then NPP was converted to the amount of CO₂ fixation through photosynthesis equations. Finally,the values of CO₂ fixation and the least cost of atmospheric CO₂ treatment in northern Shaanxi ecosystem were computed by using forestation cost method. Results show as follows:(1)The cost of CO₂ treatment from high to low was: desert > steppe > cultivated vegetation > meadow > forest. Therein,forest,bushwood and grass belonged to ecological vegetation which could produce a certain economic benefit;while other vegetation belonged to consumptive vegetation.(2)Due to the otherness of different land cover,the mean cost of CO₂ treatment increased gradually from south to north part of northern Shaanxi,which is because that the vegetation in south part of northern Shaanxi belongs to the ecological type,while vegetation in north and central part of northern Shaanxi belongs to the consumptive type.(3)The change rate of total cost increased from east to west and from southeast to northwest. And the growth rate of southern part of northern Shaanxi is slower than that in north and central part of northern Shaanxi. The cost of atmospheric CO₂ treatment presents a significant difference between south and north region in northern Shaanxi,which is mainly due to the otherness of different land cover. South part of northern Shaanxi which belongs to semi-humid region has relatively high vegetation,therefore,the vegetation can not only fix more CO₂ but also can produce a certain economic benefit. However,the concentration of CO₂ showed a significant increase in north part of northern Shaanxi due to the development of energy industry and its processing industry in the ecologically vulnerable area.

Key words: NPP, carbon fixation, carbon capture and storage, cost accounting, northern Shaanxi