Abstract
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.
Highlights
Consistent enrichment of atmospheric CO2 concentration from preindustrial times due to human activities (Stocker et al, 2013), together with global climate change, have focused much scientific and public attention on carbon cycling (Siegenthaler and Sarmiento, 1993; Chapin et al, 2006; Houghton, 2007)
In recent years, a few studies have proposed that desert regions, which have long been considered negligible in global carbon budgeting, have strong downward fluxes into the ground and might be a significant carbon sink (Jasoni et al, 2005; Stone, 2008; Wohlfahrt et al, 2008; Xie et al, 2009)
By partitioning net ecosystem CO2 exchange (NEE) into GPP and Reco (Fig. 2b), it was found that 151.65 g C m−2 year−1 was assimilated by green plants and 126.65 g C m−2 year−1 was released in the form of ecosystem respiration, including autotrophic and heterotrophic respiration
Summary
Consistent enrichment of atmospheric CO2 concentration from preindustrial times due to human activities (Stocker et al, 2013), together with global climate change, have focused much scientific and public attention on carbon cycling (Siegenthaler and Sarmiento, 1993; Chapin et al, 2006; Houghton, 2007). Substantial studies have been carried out to locate this residual sink, but the uncertainties remain large (Schimel et al, 2001; Houghton, 2007), especially in territorial ecosystems where the carbon flux and the source–sink processes vary greatly in magnitude and mechanism from region to region, such as from rainforest to desert (Schimel et al, 2001; Heimann and Reichstein, 2008). The net carbon uptake in the Mojave Desert of the United States, based on flux-tower measurements, was reported to be around 100 g C m−2 year−1
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