Contribution of Carbon Dioxide Concentration to the Diurnal Variation in Land Surface Carbon Dioxide Uptake From the Atmosphere.

Gross primary productivity (GPP) is the key pathway for CO₂ uptake by terrestrial ecosystems. Diurnal variation in atmospheric CO₂ concentration ([CO₂]) can reach 5%-15%, yet the extent to which (CO₂) contributes to diurnal GPP variations across biomes and how this contribution varies across climate gradients remain unclear. Here, we used global half-hourly eddy covariance observations to quantify the importance of (CO₂) in driving diurnal GPP variations and investigated the environmental factors influencing this response. On average, the relative contribution of (CO₂) to diurnal GPP variations, calculated through multivariate regression, was 11% across all sites, comparable to the contributions of vapour pressure deficit (13%) and soil moisture (11%). We also observed systematic differences in the contribution of (CO₂) to GPP variability across ecosystems with different plant functional types and climate conditions. Variation in the contribution of (CO₂) to GPP is primarily driven by temperature-related variables. At long-term sites, we identified a significant upward trend in (CO₂) influence on GPP over time, suggesting an increasing role of (CO₂) in explaining GPP variations. This study quantified the importance of (CO₂) to the diurnal variations of GPP across globally distributed ecosystems and highlighted that it makes a significant contribution to diurnal GPP patterns.