Articles | Volume 11, issue 22
Research article
17 Nov 2014
Research article |  | 17 Nov 2014

A downward CO2 flux seems to have nowhere to go

J. Ma, R. Liu, L.-S. Tang, Z.-D. Lan, and Y. Li

Abstract. Recent studies have suggested that deserts, which are a long-neglected region in global carbon budgeting, have strong downward CO2 fluxes and might be a significant carbon sink. This finding, however, has been strongly challenged because neither the reliability of the flux measurements nor the exact location of the fixed carbon has been determined. This paper shows, with a full chain of evidence, that there is indeed strong carbon flux into saline/alkaline land in arid regions. Based on continuous measurement of net ecosystem CO2 exchange (NEE) from 2002 to 2012 (except for 2003), the saline desert in western China was a carbon sink for 9 out of 10 years, and the average yearly NEE for the 10 years was −25.00 ± 12.70 g C m−2 year−1. Supporting evidence for the validity of these NEE estimates comes from the close agreement of NEE values obtained from the chamber and eddy-covariance methods. After ruling out the possibility of changes in C stored in plant biomass or soils, the C uptake was found to be leached downwards into the groundwater body in the process of groundwater fluctuation: rising groundwater absorbs soil dissolved inorganic carbon (DIC), and falling groundwater transports the DIC downward. Horizontal groundwater flow may send this DIC farther away and prevent it from being observed locally. This process has been called "passive leaching" of DIC, in comparison with the active DIC leaching that occurs during groundwater recharge. This passive leaching significantly expands the area where DIC leaching occurs and creates a literally "hidden" carbon sink process under the desert. This study tells us that when a downward CO2 flux is observed, but seems to have nowhere to go, it does not necessarily mean that the flux measurement is unreliable. By looking deeper and farther away, a place and a process may be found "hidden" underground.

Short summary
We have discovered and verified a new CO2 sink in one of the most unlikely places on earth-a saline desert located in a desert-oasis ecotone. The observed downward CO2 flux was not found in either plants or soil, but in groundwater. Fluctuation of the groundwater table moves the dissolved inorganic carbon in soil, which originated from soil/root respiration, downward into the groundwater body and forms a carbon sink hidden under the desert.
Final-revised paper