Spatial distribution of caliche nodules in surface soil and their influencing factors in the Liudaogou catchment of the northern Loess Plateau, China

Quantifying the distribution of caliche nodules in a catchment is a basis toward understanding soil erosion and soil degradation as well as soil-water processes in the soil containing caliche nodules. In this study, field surveys and soil sampling were coupled with spatial and statistical analyses along with image processing to investigate the spatial distribution of caliche nodules and their influencing factors in the Liudaogou catchment, the northern Loess Plateau of China. Results showed that more caliche nodules were present at the top of the mountain, mountain ridges, roadsides, edges of ravines, and other high elevation regions, as represented by comparatively higher caliche nodule coverage (CNC, 11.26-18.28%). Caliche nodules were rarely present in low elevation areas such as the terraced cropland and check dam, as represented by lower CNC (<2.55%). More than 65% of the total caliche nodules had a diameter of 10-50 mm. The caliche nodule coverage and diameter showed strong spatial dependence. The CNC was significantly related to the slope gradient and vegetation cover (P < 0.01), and the caliche nodule diameter (CND) was significantly related to the elevation (P < 0.01). Two simple models were established to predict the CNC and CND values given the parameters of slope gradient and vegetation cover. The validation of the models showed relatively high accuracy. These findings quantify caliche nodule distribution in a catchment for the first time, and may benefit the assessment of soil erosion and soil degradation in this special soil in the Loess Plateau.