The Effects Of Temperature Increasing On Ecosystem CO₂ Flux Of Different Vegetations In Permafrost Regions Over The Upper Reaches Of Heihe River Basin

Permafrost plays an important role in the carbon cycle of global terrestrial ecosystems, which acts as an important carbon pool. Under the global warming sceneries, permafrost degradation can cause the soil organic carbon stored in permafrost to be decomposed and released into the atmosphere as greenhouse gases（CO₂, CH4）. It can have a positive feedback to climate change. Soil organic carbon storage and its potential release in permafrost regions are important issues, however current studies mainly focus on high latitudes. Qinghai Tibetan Plateau（QTP） with the wide distribution of permafrost is sensitive to climate change., It may have strong degradation with global warming, which may have more carbon emissions than that in the high latitude permafrost regions. However, it is largely unknown about the carbon emission and its response to climate change in the permafrost regions. In this paper, the permafrost region over the upper reaches of the Heihe River Basin is selected as the study area. The study aims to estimate the ecosystem respiration rate（ERR） of different vegetation types in permafrost regions of QTP during the growing seasons, and the effects of soil temperature and moisture on CO₂ flux. We mainly conducted the following researches through field observations and laboratory experiments during the growing seasons in 2014 and 2015:（1） measured the ERRs of three different vegetation types and analyzed their relationship with environmental factors in permafrost regions over the upper reaches of the Heihe River Basin,;（2） analyzed ERR and net ecosystem exchange（NEE） in control plots and warming plots using the Open Top Chambers（OTCs）;（3） monitored the carbon fluxes in different stages of thaw slump（no slump, slumping and slumped） and used the method of space instead of time, analyzed the potential of carbon release in the process of permafrost degradation;（4） monitored the temporal and spatial changes of vegetation over the upper reaches of Heihe river Basin during 2013-2015 at the regional scale based on MODIS-NDVI remote sensing data, and analyzed the relationship between NDVI and carbon flux. The main results are shown as follows:During the growing seasons in 2014 and 2015, ERR began to increase from June, and reached a maximum in July and August, and then gradually decrease. The study area had the strong potential of carbon emission, the ERR of alpine meadow was the highest（average 3.71 μmol CO₂ m-2 s-1）, followed by that of alpine steppe is 3.58 μmol CO₂ m-2 s-1, and the ERR of alpine wet meadow was the lowest（average 2.81 μmol CO₂ m-2 s-1）. The ERR was positively correlated with soil temperature. The temperature sensitivity values（Q10） of the ERR in the alpine wet meadow and alpine meadow were higher than those in the alpine steppe. ERRs of different vegetation types had different relationship with soil moisture. There were a significantly negative correlation between the ERR and soil moisture in the alpine wet meadow and a positive correlation in alpine meadow and alpine steppe.The OTC warming results showed that the average soil temperature below 10 cm has increased 1.31℃ and 0.76℃ in alpine wet meadow and alpine meadow from June to September, respectively. There was no significant effect on soil moisture content. The ERR of alpine wet meadow and alpine meadow increased by 30.0% and 66.9%, respectively. Due to OTC warming, the increases in NEE at EBO #A, EBO #B and PT5 were 112 %, 147% and 167%, respectively. The NEE results suggest that the ecosystems in the permafrost regions of the upper reaches of the Heihe River Basin may act as carbon sinks during summer warming.Through monitoring the carbon flux in the Eboling Mountain with different stages of thaw slumps over the upper reaches of the Heihe River Basin, it showed that the ERR in the thaw slumped stage（4.29 μmol CO₂ m-2 s-1） was higher than that（3.98 μmol CO₂ m-2 s-1） in the stage of no slump. The ERR of the bare soils in the stage of thaw lumped was lowest（1.02μmol CO₂ m-2 s-1）. Used the method of space instead of time, it was concluded that the carbon flux can decrease from no slump to the bared soils during thaw slumps and increase from the bared soils to the new vegetation of thaw slumped stage.Based on MODIS-NDVI remote sensing data, the NDVI values in south-east was higher than north-west in the upper reaches of Heihe Basin. About 58% regions of NDVI values decreased from 2013-2015, which was mainly located in mid-western and south-eastern. In the upper reaches of Heihe River Basin, there was a significantly positive correlation between the NDVI values and carbon flux. The soil carbon flux increased with the changes of NDVI values.This paper monitored the ERRs of three different vegetation types in warming conditions and thaw slumps in the permafrost regions in the upper reaches of the Heihe River Basin. In addition, we evaluated the carbon release potential with global warming. Usually, there are many complicated environmental factors that affect the carbon flux in permafrost regions, and the various environmental factors can affect with each other. In future studies, we will monitor more carbon fluxes in different situations in order to understand the dynamics of ecosystem respiration and estimate the carbon flux in the permafrost regions of the QTP. It will provide more evidence for the permafrost in the QTP can play an important role in the carbon cycle of global terrestrial ecosystems.