Climate and soil properties drive soil organic and inorganic carbon patterns across a latitudinal gradient in southwestern China

Purpose
Drylands account for 47.2% of land area and contain 15.5% of global carbon (C). However, the variation in organic and inorganic C stocks across latitudinal gradients in arid and semiarid shrubland ecosystems remains understudied, and we lack in-depth understanding of the main drivers of C variation at this spatial scale.

Methods
Here, we sampled soils from 95 sites across a latitudinal gradient to explore both the latitudinal patterns and potential drivers of soil organic carbon density (SOCD) and soil inorganic carbon density (SICD). We also assessed variation in SOCD and SICD down the soil profile, by sampling soils at four depths (0 – 10 cm, 10 – 20 cm, 20 – 30 cm, and 30 – 50 cm).

Result
Both SOCD and SICD exhibited a binomial relationship with latitude (P < 0.01). Soil properties accounted for the greatest variation in SOCD, with the most important explanatory factor being exchangeable calcium, followed by mean annual temperature, pH, plant diversity, and silt content. Soil pH and plant diversity were more important in explaining variation in SOCD in the subsoil (> 20 cm depth) than the topsoil. For SICD, soil properties explained the greatest variation at all depths. Soil pH explained the most variance in SICD, followed by exchangeable calcium and mean annual temperature in the topsoil (i.e., 0 – 10 cm and 10 – 20 cm). In the subsoil (i.e., 20 – 30 cm and 30 – 50 cm), exchangeable calcium was the most important predictor, followed by soil organic carbon, mean annual temperature, and pH.

Conclusion
Our study shows that soil properties are a strong predictor of latitudinal patterns of soil organic and inorganic C in arid and semiarid shrubland ecosystems. We also identified differences in potential drivers of SOCD and SICD with depth, advancing our understanding of large-scale patterns of C storage in arid and semiarid soils.