Characteristics and influencing factors of carbon fluxes in winter wheat fields under elevated CO2 concentration

Elevated carbon dioxide (ECO₂) concentration has profound impacts on ecosystem carbon fluxes, with consequent changes in carbon sequestration and its feedback to climate change. Agroecosystem plays an essential role in global carbon sequestration. However, it is not well understood how the carbon fluxes of agroecosystem respond to increasing atmospheric CO₂ concentrations. In this study, an in-situ 2-year field experiment was conducted using open-top chamber with treatments including ambient CO₂ concentration (CK) and ambient plus 200 μmol mol⁻¹ (T) to investigate the characteristics and main factors influencing carbon fluxes during the 2017–2019 winter wheat growing seasons. Results showed that the dynamics of CO₂ fluxes under different treatments had similar seasonal trends, with the peak flux observed at the heading-filling stage. Compared to the CK, T treatment increased the cumulative amount of CO₂ (CAC) by 17.2% and 24.0% in 2017–2018 and 2018–2019 growing seasons, respectively. In addition, the seasonal CAC was highly dependent on treatment and varied with year, while there was no interactive effect of treatment and year (p > 0.05). ECO₂ concentration increased the biomass of wheat by an average of 8.28% over two growing seasons. There was a significant positive correlation between biomass and CAC, with biomass elucidating 52% and 76% of the variations in CAC under CK and T treatments, respectively. A good correlation was found between net ecosystem exchange (NEE) and environmental variables under different treatments. During the pre-milk ripening period, the NEE mainly depended on photosynthetically active radiation (PAR) and air temperature (Ta), while NEE was mainly controlled by PAR and soil water content (SWC) during the post-milk ripening period. Overall, the findings presented here demonstrate that the carbon exchange in wheat fields under different treatments serves as carbon sequestration, while ECO₂ concentration enhances the capacity of winter wheat fields to act as carbon sinks, which may have feedback to the climate system in the future.