Distribution of soil organic carbon (SOC) concentrations across the study area.

<p>Distribution of soil organic carbon (SOC) concentrations across the study area.</p

Parameters of the semivariogram models estimated for soil organic carbon (SOC) concentrations at different soil depths.

<p><i>R²</i>, coefficient of determination.</p

Isotropic semivariograms of soil organic carbon (SOC) concentrations at different soil depths.

<p>Isotropic semivariograms of soil organic carbon (SOC) concentrations at different soil depths.</p

Selected soil physical properties at different soil depths.

<p>BD, bulk density; CV, coefficient of variation; Stones, >2 mm; Sand, 2–0.2 mm; Silt, 0.2–0.002 mm; Clay, <0.002 mm. Mean values followed by the same letter are not significantly different (α = 0.05) at different soil depths using the LSD method.</p

Correlation analysis between soil organic carbon (SOC) concentrations and soil physical properties at different soil depths.

<p>BD, bulk density; ** denotes significance of correlation at <i>P</i><0.01.</p

The location of Gansu Province and the Heihe River Basin, China (a), the study site in the Heihe River Basin (b), and the soil-sampling points in the study area (c).

<p>The location of Gansu Province and the Heihe River Basin, China (a), the study site in the Heihe River Basin (b), and the soil-sampling points in the study area (c).</p

Comparison of SOC concentrations and SOC stocks in other desert areas in China.

<p>Comparison of SOC concentrations and SOC stocks in other desert areas in China.</p

Verification of interpolation reliability for soil organic carbon (SOC) concentrations at different soil depths.

<p>ME, mean error; RMSE, root mean square error; RMSSE, root mean square standardized error.</p

Stepwise multiple linear regression of soil organic carbon (SOC) concentrations with selected soil variables at different soil depths.

<p>MSE, mean squared error; <i>R²</i>, coefficient of determination; ** Denotes significance of correlation at <i>P</i><0.01.</p

Coefficient of variation of soil organic carbon (SOC) concentrations at different re-sampling areas.

<p>Coefficient of variation of soil organic carbon (SOC) concentrations at different re-sampling areas.</p