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.