National-scale distribution and its influence factors of calcium concentrations in Chinese soils from the China Global Baselines project

Abstract

The China Geochemical Baselines (CGB) project provides geochemical baselines for nearly all chemical elements, including Ca, by sampling transported soils at a density of 1 sample per approximately 3000 km2 across nearly all of China. A total of 3382 top samples (TS) were collected at depths of 0–25 cm and 3380 deep samples (DS) at depths >100 cm. The median (background) values of Ca in the TS and DS were 1.96% and 1.84%, respectively. On the spatial distribution maps, Ca concentrations generally decreased from northwestern to southeastern China. The natural climate, especially the aridity/humidity status, primarily influences this national-scale distribution. The median Ca values were ranked as follows: arid (4.61% in the TS and 4.15% in the DS) > semi-arid (2.73% in the TS and 2.55% in the DS) > sub-humid (1.37% in the TS and DS) > humid (0.56% in the TS and 0.52% in the DS) areas. In the arid areas of northwestern China, evaporite minerals can be deposited in soils, resulting in a regional high-Ca background. In the humid regions of southeastern China, soil water readily leaches Ca, leading to a regional low-Ca background. The highest Ca concentrations sparsely distributed in northwestern China (>90% quantile, Q90) and the higher Ca contents in the semi-arid and sub-humid areas (Q75–Q90) are linked to the parent Ca-bearing rocks. While no high Ca concentrations exist in both the TS and DS in southeastern China, although carbonate rocks are widely distributed. The lowest Ca concentrations distributed in southeastern China (<Q10) are predominately a result of continuous exposure to human-induced acid rainfall. Both natural and anthropogenic factors control the Ca concentrations and distribution in Chinese soils.