Continuous Measurements of Temporal and Vertical Variations in Atmospheric CO2 and Its δ13C in and above a Subtropical Plantation

Atmospheric CO2 dynamics in forest ecosystems are dependent on interactions between photosynthesis, respiration, and turbulent mixing processes; however, the carbon isotopic composition of atmospheric CO2 (δ13C) is not well established due to limited measurement reports. In this study, a seven-inlet profile system with a Picarro analyzer was developed to conduct continuous in situ measurements of CO2 and its δ13C in and above a subtropical plantation from 2015 to 2017. Results showed that ecosystem CO2 concentration was the lowest in the afternoon and reached its peak at dawn, which mirrored variations in its δ13C in and above the canopy. Inverse seasonal variations were apparent between CO2 and its δ13C in and above the canopy, and δ13C was positive during the peak growing season and negative at other times. Diel and seasonal variations in ecosystem CO2 and its δ13C were mainly affected by the vapor pressure deficit, followed by photosynthetic active radiation, temperature, and the enhanced vegetation index in and above the canopy; however, environmental and physiological factors had reverse or no effects near the forest floor. Nocturnal gradients of vertical variations in atmospheric CO2 and its δ13C were greater than diurnal variations due to weak turbulent mixing under more stable atmospheric conditions overnight. These results implicate that photosynthesis and respiration dominated CO2 dynamics above the canopy, while CO2 recycling by photosynthesis and turbulent mixing changed CO2 dynamics in the canopy.