Photosynthesis (GPP), the largest CO 2 flux from the land surface, i s currently estimated with considerable uncertainty between 100-175 Pg C yr -1 . More robust estimates of global GPP could be obtained from the atmospheric b udgets of other tracers such as, the oxygen isotopic composition (δ 18 O) of atmospheric CO 2 or carbonyl sulphide (COS). However, estimating GPP using thes e tracers hinges on a better understanding of how soil microbes modify th e atmospheric concentrations of CO 18 O and COS at large scales. In par ticular, understanding better the role and activity of the enzyme Carbonic Anhydrase (CA) in soil microbes is a critical factor u nderpinning the successful implementation of these tracers in global scale models. We addr essed this knowledge gap by measuring the exchange of CO 18 O and COS between soil microcosms and the atmosphere from over 60 sites covering a range of biome s across Europe and the USA. This novel dataset has led to the development of a new mechanistic framework that can be easily i mplemented in mult i-tracer Earth system models to predict variati ons in soil CA a ctivity across the terrestrial land surface. Using this multi-tracer approach we provi de independent estimates of globa l GPP constrained by the atmospheric budgets of CO 2 , CO 18 O and COS.