Integrating Aquatic and Terrestrial Carbon Fluxes to Assess the Net Landscape Carbon Balance of a Highly Erodible Semiarid Catchment

The magnitude of the carbon (C) sink due to terrestrial primary production may be overestimated if the C losses through fluvial networks are not properly accounted for. In this study, we hypothesize that terrestrial-aquatic C transfers represent a major loss of the terrestrial C sink in semiarid catchments. To test the hypothesis, we assessed the net landscape carbon balance (NLCB) of a highly erodible catchment on the semiarid Chinese Loess Plateau by compiling terrestrial net ecosystem productivity (NEP) estimates and aquatic fluxes of downstream C export, emission of CO2 and CH4, and C burial. Our results showed that, despite the severe erosion and strong C mobilization, this catchment was a net C sink (NLCB = 48.3-54.8 Mg C km(-2) year(-1)). Downstream C export at catchment outlet and evasion of CO2 and CH4 only offset terrestrial NEP by 7.7%-8.5% due largely to low flow discharge and small water surface area of the stream network. Furthermore, C burial in reservoirs played a considerable role, reducing an additional 7.5%-8.3% of the NEP. Altogether, approximately 84% of the net terrestrial C production was stored within the catchment. Our study reveals that semiarid catchments with widespread erosion and low terrestrial production remain a net C sink, although the magnitude is lower than those found in other climate zones. Our study further advocates the need to integrate terrestrial-aquatic C transfer into a comprehensive net C balance accounting at the landscape scale. Ignoring the aquatic C loss will cause biased estimates of the overall C sink strength.