CO₂ flux characteristics of the grassland ecosystem and its response to environmental factors in the dry-hot valley of Jinsha River, China

Abstract. The dry-hot valley of Jinsha River is distinguished by prolonged drought and high temperatures, making it a distinct non-zonal hot island habitat in the global temperate zone. It is an ideal location for studying changes in plant carbon budget under sustained drought and high-temperature conditions. However, there is currently a dearth of reports on CO₂ flux variations within plant ecosystems in this region. The study quantitatively analyzed the characteristics of CO₂ flux variation in the grassland ecosystem in this region and its response mechanisms to environmental factors using continuous observation data obtained from static assimilative chamber. The results indicate that both the environmental factors and CO₂ flux variations in grassland ecosystems exhibit significant seasonal characteristics. During the dry season (March to May), the grassland acts as a carbon source, exhibiting a daily average CO₂ flux of 0.1632 μmol·m^-2·s^-1, which cumulative CO₂ emissions for each month were 18.64 g·m^-2, 15.96 g·m^-2, and 20.64 g·m^-2, respectively. The ecosystem showed noteworthy carbon absorption characteristics during the rainy season (August to October), with a daily average CO₂ flux of -0.1062 μmol·m^-2·s^-1, which cumulative CO₂ absorption for each month were 6.42 g·m^-2, 24.41 g·m^-2, and 5.14 g·m^-2, respectively. Throughout the year, the ecosystem was a weak carbon source, emitting an annual cumulative CO₂ of 0.7078 t·ha^-1·a^-1, demonstrating carbon-neutral traits. In terms of environmental factors, there was a robust negative correlation exists with CO₂ flux between photosynthetically active radiation during the rainy season (R= -0.578, P<0.01). The daily CO₂ flux in different seasons was positively correlated with precipitation and relative humidity (P<0.01), and negatively correlated with air temperature, soil temperature and vapor pressure deficit (P<0.01). The diurnal variation of CO₂ flux in dry season was mainly affected by relative humidity, while that in rainy season was mainly affected by relative humidity and vapor pressure deficit. The variation of CO₂ flux was most influenced by soil water content, relative humidity, and vapor pressure deficit at both daily and monthly scales throughout the year. The influence of temperature factor on CO₂ flux changes at different time scales is generally weak.