Integrated Global Estimation of Terrestrial Carbon Efflux

Abstract

Carbon efflux (C_efflux) makes up 20% of total greenhouse gases emitted globally. Accurate global C_efflux is critical for projecting and responding to future global warming. However, current C_efflux estimates remain incomplete due to the omission of C_efflux released from the dissolution of inorganic carbon in calcareous soils (C_{efflux-calcareous}) and dissolution of lime in acidic soils (C_{efflux-acidic}). Here, we present a global terrestrial C_efflux that incorporates C_{efflux-calcareous}, C_{efflux-acidic}, and soil respiration. Using 7562 field measurements from 2481 publications and machine learning, this study revealed that C_{efflux-acidic} and C_{efflux-calcareous} contributed 0.29% and 0.09%, respectively, to an overall C_efflux of 96.52 Pg C year^-1 (95% confidence interval, 91.2-101.9 Pg C year^-1). Previous estimates of C_efflux from soil respiration underestimated 0.4% of C_efflux. Structural equation modelling (SEM) revealed that C_efflux was directly driven by heterotrophic respiration, autotrophic respiration, dissolution of soil inorganic carbon and liming, while being regulated indirectly by land productivity (gross primary productivity, roots and litter), soil properties (soil acidity and soil inorganic carbon content), and anthropogenic activities (nitrogen fertiliser application). Soil acidification must be viewed as a dual challenge impacting both crop yields and global warming threats. Therefore, the study emphasises the need to incorporate neutralising soil acidity into soil carbon dynamics models, which was previously overlooked.