Carbon export in the land-to-ocean aquatic continuum (LOAC) of China

It has long been recognized that terrestrial ecosystems are not isolated from other earth systems with all the absorbed carbon being permanently sequestered on land. Inland water systems (e.g., streams, rivers, lakes, and reservoirs) are an important component of the global carbon cycle, functioning as active reactors that transport and transform large quantities of terrestrially derived carbon. Strong interactions between terrestrial ecosystems and inland waters indicate that a portion of the carbon sequestered on land by vegetation can be transported to the ocean through inland waters, the land-to-ocean aquatic continuum (LOAC). Therefore, the transport, transformation, and redistribution of terrestrial carbon along this continuum will change the land carbon sink strength. A comprehensive understanding of the magnitude and significance of carbon transfer in the LOAC in modulating the net landscape carbon balance is of paramount importance for an accurate assessment of carbon budget. In this work, we systematically examined the carbon transport in the LOAC of the entire China, including carbon export into the ocean, carbon burial within inland waters, and carbon emissions into the atmosphere. Our results show that the flux of carbon transported into the ocean and buried within Chinese inland waters was 40-45 Tg C yr^-1 and 10-15 Tg C yr^-1, respectively. In addition, the flux of carbon emissions (as CO₂ and CH₄) from Chinese inland waters was in the range of 100-105 Tg C yr^-1. The total carbon flux entering Chinese inland waters was estimated at 150-160 Tg C yr^-1 with carbon emissions being the largest transport pathway (63-70% of the total). Compared with the simultaneous terrestrial carbon sink in China, this terrestrial-aquatic carbon export could offset China’s terrestrial carbon sink capacity by up to 25%. Our results highlight that the terrestrial-aquatic carbon export must be integrated into future national-scale carbon budgets.