## 1 Introduction  
Forests cover approximately 4.2 billion hectares globally, accounting for 30% of the total land area [1]. Forests sequestrate carbon primarily through tree growth, which are the largest carbon sinks among terrestrial ecosystems [2]. The global forest carbon pool was estimated as 861 Pg carbon [3]. Over 100 Pg of carbon is stored in the biomass of Amazonia forests [4]. The annual land biosphere sink was estimated to be approximately 0.20-0.25 Pg carbon in China during the past decades, forests being the major carbon sink by approximately 0.18 Pg yr\({}^{-1}\)[5, 6].  
Global forest carbon sink gradually increased during the past decades, but the magnitude varied among different regions [6, 7]. Terrestrial carbon sinks are mainly distributed in the mid- and high latitudes of the Northern Hemisphere [6]. Pine forests were a strong carbon sink by approximately 740.3 g C m\({}^{-2}\) in the Northeast China [8]. Annual NEE was estimated as \(-\)502.1 g C m\({}^{-2}\), accounting for about 32% of gross primary product in broad-leaved mixed forests [9]. Annual NEE flux of spruce forests was a robust carbon sink of 545.99 g C m\({}^{-2}\) on the northeast Tibetan Plateau [10]. NEE was estimated to be approximately \(-77\) g C m\({}^{-2}\) yr\({}^{-1}\) in a semi-arid shrubland of Northern China [11]. Boreal forests were a minor CO\({}_{2}\) sink of 64.01 g m\({}^{-2}\) yr\({}^{-1}\) in the northeast of China [12]. However, a typical southern taiga memorose spruce forest was found to be a source of atmospheric CO\({}_{2}\)[13]. Weak net CO\({}_{2}\) sinks in three years were reported to approximate 31.1 g m\({}^{-2}\) in a coniferous forest of Central Siberia [14]. Understanding land biosphere fluxes has been hampered by sparse data coverage and long-term eddy covariance observations are scarce.  
Climate change and environment factors are major drivers of carbon sinks [6], such as NEE being restricted by soil drought, vapor pressure deficit (VPD) being over 1.0 kPa, and canopy conductance significantly decreased, which depressed carbon NEE [15]. The vapor pressure deficit ranged from 0.13 to 1.88 kilopascal due to extreme temperature events, which reduced boreal forest carbon uptake [12]. Air temperature was the driving factor for NEE, followed by photosynthetically active radiation in boreal forests [10]. After the surface soil temperature exceeded 1 \({}^{\circ}\)C, forest ecosystems became persistent net CO\({}_{2}\) sinks [14]. The variety of soil moisture regimes provide non-uniformity in the response reactions of the CO\({}_{2}\) ecosystem-atmosphere exchange concerning climate anomalies [13]. Winter warming and decreases in autumn and winter precipitation may induce spring drought, which impairs carbon sequestration of shrubland [11]. In conclusion, the determined factors were significantly distinct in different regions.  
The Qilian Mountain is a vital ecological security barrier role in Western China and the Tibetan Plateau [10], as is has carbon sink and biodiversity conservation functions. Previous studies have focused on forests' primary production variations; however, studies of net ecosystem exchange magnitude on CO\({}_{2}\) fluxes and its driving mechanism are extremely lacking. Thus, we consecutively monitored CO\({}_{2}\) fluxes through the tower-based eddy covariance method from 2021 to 2022, and two scientific hypotheses were validated. Firstly, we maintain that NEE was weak in 2022 compared with that in 2021 because of the higher air temperature in 2022. Secondly, air temperature was a more important driving factor for NEE than precipitation.