Discussion on article “Calculation of atmospheric CO2 sink formed in karst processes of karst-divided regions in China”

A few calculated results and two important karst carbon sink data quoted in the article are being questioned. It is argued for: (1) A few basic parameters being used in the article are inaccurate. The calculated results are perhaps over-estimated in 3~4 times, and possibly 200 times larger for north China karst region and the buried karst region, respectively. (2)Because of intrinsic shortage of standard tablet method as a karst carbon sink determining tool, the “global karst carbon sink”(6.08×108 tC/a)obtained earlier is possibly overestimated by a factor of4 or so.(3)Estimated values for newly proposed items of the global water circulation model for carbon sink are thought to be significantly overestimated. The net global water circulation carbon sink is probably only a little larger than that owing to carbonate weathering of rocks. (4)Simplified formula for carbon sink calculation used in the article has accounted for only the DIC originated from atmosphere/soil CO2 corrosion but not for all of their origins. Therefore it should be corrected for non-atmosphere/soil CO2 corrosion further. At a global scale the correction factor is believed to be about 0.65. Consequently all results at this scale obtained earlier should be reduced by about 35%. (5) Actual global karst carbon sink flux eventually is probably of the order of 0.1±0.02 GtC/a, which is about6 times smaller than what is believed by authors of this article. Taking long term carbon cycle into account, given atmospheric carbon cycle nowadays is even in a strongly disturbed state, the net global karst carbon sink is probably much less than the order of0.01 GtC/a. It should not be considered as an important atmospheric carbon sink, especially comparing to the figure of photosynthesis (60 GtC/a).