Karst carbon sink mechanism and its contribution to carbon neutralization under land- use management.

The chemical weathering process of carbonate rocks consumes a large quantity of CO2. This has great potential as a carbon sink, and it is one of a significant pathway for achieving carbon neutrality. However, the control mechanisms of karst carbon sink fluxes are unclear, and there is a lack of effective and accurate accounting. We took the Puding Shawan karst water­carbon cycle test site in China, which has identical initial conditions but different land use types, as the research subject. We used controlled experiments over six years to evaluate the mechanisms for the differences in hydrology, water chemistry, concentrations and fluxes of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). We found that the transition from rock to bare soil to grassland led to increases in the DIC concentration by 0.08-0.62 mmol⋅L-1. The inorganic carbon sink flux (CSF) increased by 3.01-5.26 t⋅C⋅km-2⋅a-1, an increase amplitude of 30-70 %. The flux of dissolved organic carbon (FDOC) increase by 0.28 to 0.52 t⋅C⋅km-2⋅a-1, an increase amplitude of 34-90 %. We also assessed the contribution of land use modifications to regional carbon neutrality, it indicate that positive land use modification can significantly regulate the karst carbon sink, with grassland having the greatest carbon sequestration ability. Moreover, in addition to DOC from soil organic matter degradation, DOC production by chemoautotrophic microorganisms utilizing DIC in groundwater may also be a potential source. Thus, coupled studies of the conversion of DIC to DOC processes in groundwater are an important step in assessing karst carbon sink fluxes.

Organic carbon source tracing and the BCP effect in the Yangtze River and the Yellow River: Insights from hydrochemistry, carbon isotope, and lipid biomarker analyses.

Autochthonous organic carbon (AOC) formed by biological carbon pump (BCP) in surface waters may serve as a significant carbon sink. The locations, magnitudes, variations and mechanisms responsible for the terrestrial missing carbon sink by BCP are uncertain, especially in large river systems. In this study, hydrochemical characteristics, carbon isotope compositions of dissolved inorganic carbon (DIC) and organic carbon (OC), n-alkane homologues and C/N ratios of organic matter along the Yangtze River and the Yellow River were investigated to constrain the OC source and the significance of BCP effect. It was found that (1) DIC concentrations in the Yellow River were much higher than those in the Yangtze River, which was controlled primarily by the temperature effect; (2) AOC in the both rivers was characterized by lower C/N ratios and δ13CPOC values. Based on calculation of n-alkanes compounds, the AOC proportions ranged from 29 to 88% (49% on average, with a higher proportion (55%) in the rainy season than in the dry season (46%)) and 19-68% (41% on average; with a lower proportion in the rainy season (31%) than in the dry season (51%)) in the Yangtze River and the Yellow River, respectively, indicating intense aquatic production. Low dissolved CO2 concentration (6.17 µmol/L on average) of the Yangtze River limited the aquatic production and decreased the BCP effect in the dry season, indicated by lower AOC proportion. However, the BCP effect increased in the Yellow River in the dry season mainly due to the increased light penetration; (3) even in high turbidity riverine systems such as the Yellow River, the aquatic photosynthetic uptake of DIC could produce considerable AOC. These findings clearly show the formation of AOC by BCP in both the clear and high turbidity riverine systems, suggesting a potential direction for finding the terrestrial missing carbon sink.