Can flux tower research neglect geochemical CO2 exchange?

This study examines the hypothesis that surface-atmosphere exchange of CO2 in terrestrial ecosystems always can be interpreted purely in terms of biological processes, neglecting geochemical cycling by karst systems that characterize 22 million km2 in the world [Yuan Daoxian, G., 1997. The carbon cycle in karst, Zeitschrift für Geomorphologie 108 (Supplementbände), 91–102]. Eddy covariance data of net CO2 fluxes are examined for two ecosystems over karstic substrates in contrasting climates in the North and South of Spain. A semi-arid matorral is found to behave similarly to previously studied ecosystems when well watered, but sizeable mid-day CO2 emissions during extended drought and plant senescence are found to be incompatible with ecophysiological interpretations of the flux. For a temperate pasture overlying an accessible cave, afternoon CO2 emissions in summer are likewise inexplicable in a biological context, but coincide with periods when ventilation is observed inside the subterranean cavity. These results suggest direct linkages at times in CO2 exchanges among atmosphere, ecosystems, and carbonate substrates which represent more than a tenth of the Earth's land surface.