Abstract

Soil inorganic carbon (SIC) is an important component of total terrestrial carbon stocks. However, compared to soil organic carbon, our understanding of its spatial distribution and drivers is relatively poor. In the literature, the ratio of precipitation to evapotranspiration is usually cited as the main driver of SIC distribution worldwide, but confounding factors make it difficult to assess this relationship. Chile has a unique geography with a large climatic gradient along a relatively homogeneous geological setting, which is why it is considered a natural laboratory. We collected a dataset of 511 soil profiles from the literature and modelled the SIC content using climatic and relief information within the digital soil mapping framework. We used SHAP to determine if the model captured sound relationships and to estimate the marginal contribution of each covariate to the model output. We estimated a total SIC stock to 1 metre depth around 1.4 Pg. Additionally, we found three total annual precipitation thresholds of spatial and pedological significance at 20, 250 and 500 mm total annual precipitation (TAP). Below 20 mm, SIC tends to be low in abundance and on the surface, contrary to reports in current global studies that overestimate SIC content in hyperarid regions. Between 20 and 250 mm SIC is ubiquitous in the landscape, occurring in all geomorphological positions. Between 250 and 500 mm TAP, SIC occurs in flat and concave parts of the landscape and almost disappears on slopes. A clear threshold occurs at 500 mm TAP, above which the concentration of SIC decreases rapidly. This work provides a model of carbonate distribution in a climatic gradient that covers almost the entire range of precipitation on the planet, contributing to a better understanding of the spatial variability of an essential component of the carbon cycle.