Abstract:

Objective: The major objectives of this study are to clarify the annual dynamics of energy fluxes and the energy balance in the Otindag Sandy Land of Inner Mongolia, reveal the seasonal and diurnal patterns of carbon fluxes, and explore how environmental factors influence carbon fluxes in this ecosystem. Method: An eddy covariance system and meteorological instruments were used to continuously in situ observe the carbon fluxes, energy fluxes, and environmental factors in the elm savanna ecosystem of the Otindag Sandy Land in Inner Mongolia throughout 2021. The nighttime partitioning of net ecosystem exchange (NEE), the energy balance closure method, and the Random Forest algorithm were used to calculate and analyze the variation characteristics of carbon fluxes, energy balance, and the impacts of environmental factors. Result: 1) The study found that in 2021, latent heat and sensible heat accounted for 48.6% and 41.5% of the net radiation in the elm savanna ecosystem, respectively, with an energy closure of 73%. 2) The annual NEE of the elm savanna was ?231.77 g·m^?2, the gross primary productivity (GPP) was 849.79 g·m^?2, and the ecosystem respiration was 603.36 g·m^?2. The study area served as a carbon source during January, February, November, and December, and a carbon sink during the other months of the year. The carbon sink strength was highest in July and August, with values of ?75.17 g·m^?2 and ?73.75 g·m^?2, respectively, accounting for 64.1% of the annual carbon sequestration. On a daily scale, carbon flux exhibited a typical unimodal variation, with a peak carbon sink of 0.220 g·m^?2 in summer. 3) At the annual scale, air temperature was the primary factor influencing carbon fluxes. At the seasonal scale, total solar radiation was the dominant environmental factor affecting NEE in spring, summer, and winter, with relative importance values of 47%, 37%, and 35%, respectively. In autumn, soil water content was the most influential factor, contributing relative importance values of 58%. For GPP, soil water content was the most significant driver across all seasons, accounting for 72% in spring, 46% in summer, 61% in autumn, and 32% in winter, followed by total solar radiation. Conclusion: The elm savanna has a much stronger carbon sequestration capacity than other grassland ecosystems in the same region, with soil water content being a key environmental driver of carbon fluxes. In the ecological restoration of the Otindag Sandy Land and the Horqin Sandy Land, the recovery of native elm savanna vegetation can provide dual benefits of sandy land rehabilitation and carbon sequestration increase.

Key words: elm savanna, eddy covariance, net ecosystem exchange (NEE) of carbon, gross primary production (GPP), flux footprint