A downward CO2 flux seems to have nowhere to go

[摘要] Recent studies have suggested that deserts, which are a long-neglectedregion in global carbon budgeting, have strong downward CO₂ fluxes andmight be a significant carbon sink. This finding, however, has been stronglychallenged because neither the reliability of the flux measurements nor theexact location of the fixed carbon has been determined. This paper shows,with a full chain of evidence, that there is indeed strong carbon flux intosaline/alkaline land in arid regions. Based on continuous measurement of netecosystem CO₂ exchange (NEE) from 2002 to 2012 (except for 2003), thesaline desert in western China was a carbon sink for 9 out of 10years, 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 NEEestimates comes from the close agreement of NEE values obtained from thechamber and eddy-covariance methods. After ruling out the possibility ofchanges in C stored in plant biomass or soils, the C uptake was found to beleached downwards into the groundwater body in the process of groundwaterfluctuation: rising groundwater absorbs soil dissolved inorganic carbon(DIC), and falling groundwater transports the DIC downward. Horizontalgroundwater flow may send this DIC farther away and prevent it from beingobserved locally. This process has been called "passive leaching" of DIC,in comparison with the active DIC leaching that occurs during groundwaterrecharge. This passive leaching significantly expands the area where DICleaching occurs and creates a literally "hidden" carbon sink process underthe desert. This study tells us that when a downward CO₂ flux isobserved, but seems to have nowhere to go, it does not necessarily mean thatthe flux measurement is unreliable. By looking deeper and farther away, aplace and a process may be found "hidden" underground.

[学科分类] 地球化学与岩石