Characteristics of humidity and temperature variations and CO2 exchange of mobile dunes at different space-time scales in Horqin sandy land, China

Abstract

Terrestrial carbon cycle plays a key role in driving climate change and ecosystem carbon balance. Understanding the variations of humidity and temperature and CO2 exchanges are meaningful to reveal the law and mechanism of regional carbon cycles in deserts. We examined the near surface humidity, temperature variations, and CO2 exchanges by eddy covariance and Bowen ratio systems in a typical mobile dune of Horqin sandy land. We analyzed the relationships between water-heat and CO2 exchanges of 0 to 10 m vertical height at daily and seasonal scales were analyzed. The results showed that the vertical variations of near surface temperature ranged from 0.4 ℃ to 2 ℃ and decreased with the increases of height from April to September, but with an opposite pattern in other months. The seasonal variation of air relative humidity was greater than 40%. During the growing season of 2018, the averaged daily net ecosystem carbon exchange (NEE) was -0.02 mg·m-2·s-1. The annual averaged daily NEE was 0.003 mg·m-2·s-1, indicating that the mobile dunes were carbon sources at the whole year scale. The vertical differences of temperature and humidity well fitted the NEE. The inflexion points of the fitting curve were at 10% humidity and 0.5 ℃ temperature, respectively. At the scalem of the year, the NEE fitting result of temperature was better than that of humidity, with the inflexion points at 17 ℃ and 65% humidity, respectively. In the growing season, the near surface vertical temperature difference was negative, which would inhibit CO2 absorption of mobile dunes. The circumstances of high humidity would promote the absorption of atmospheric CO2. Across different time and vertical height, the variations of humidity and temperature were closely related to CO2 exchanges, which affected carbon sink and source of mobile dunes. Carbon budget was more sensitive to temperature than humidity.