Carbonate weathering-related carbon sink fluxes under different land uses: A case study from the Shawan Simulation Test Site, Puding, Southwest China

In the study of global climate change, a major focus of research into the carbon cycle is to determine the fate of missing carbon sinks. Carbonate weathering-related carbon sinks as a result of water‑carbonate-CO2-aquatic phototroph interactions may make a major contribution. Establishing optimal land uses, which determine soil CO2 concentrations and water supplies for carbonate dissolution, may be a feasible and effective way to increase the sink potential. Elucidating both the hydrological and the hydrochemical behavior under different land uses is critical for rational planning of land use changes to increase the carbon sink. Given the complexity within natural karst catchments, the Shawan Simulation Test Site was established at Puding, Southwest China, to simulate the influence of land uses with controlled carbonate test beds - bare rock, bare soil, crop land, grass land, shrub land. Soil CO2 concentrations, hydrochemical parameters (pH, major ion concentrations) and the 'spring' (artificial drain) discharge were intensively measured from Sept. 1, 2015 to Aug. 31, 2016 to investigate the carbon and water responses to different land uses in different seasons. In the vegetated land uses (crop, grass or shrub), DIC increased due to the increase of soil CO2 resulting from stronger microbial activities and root respiration in summer and autumn growing seasons. In the bare rock and soil cases, there was also an increase in DIC in summer and autumn, due to decomposition of prior organic matter within soils and/or rock pores. The average DIC concentration ranking, high to low, was grass land, shrub land, crop land, bare soil, bare rock. Soil CO2 concentration was thus the dominant of DIC concentration, which is a key multiplier of carbon sink fluxes (CSF = 0.5 ∗ [DIC] ∗ RD, where RD is depth of runoff, [DIC] is DIC concentration, and 0.5 because in carbonate dissolution, only half of the [HCO3−] is of atmospheric carbon origin). However, runoff depth ranked almost in reverse order, i.e., from high to low, bare rock, bare soil, crop land, shrub land, grass land. The CSF ranking, from high to low, was grass, crop, shrub, bare rock, bare soil. A new parameter, LCIC (Land use Change Impact on CSF) is defined to compare the impacts of land use change on [DIC] and RD, and evaluate their combined effects on CSF. Compared to bare rock, the absolute values of LCIC (| LCIC | s) were > 1, and CSFs were larger for the three tanks with vegetation cover; CSF is smaller for bare soil, | LCIC | < 1. Finally, it was found that grass land may constitute an optimal land-use for increasing the carbonate weathering-related carbon sink that is critical for carbon management to counter global warming.