Response of soil organic carbon spatial variability to the expansion of scale in the uplands of Northeast China

Soil organic carbon (SOC) plays an important role in maintaining and improving soil fertility and quality as well as mitigating climate change. Understanding SOC density spatial variability is fundamental for describing soil resources and predicting SOC. Three categories were used to create spatial scales: administrative category (county, city, province and region scale), soil taxonomic category (family, sub-group, great group and order scale) and soil type (zonal soil and azonal soil)-administrative category. Soil organic carbon density variability and its response to the expansion of scales in the topsoil (0–20 cm) and soil profile (a depth of 1 m) layers in the uplands of Northeast China were examined based on coefficient of variation (CV) values using data of 1041 profiles obtained from the Second National Soil Survey of China. The results depicted that SOC density variability increased not only in the topsoil layer but also in the soil profile layer with the expansion of scales in all categories. In the administrative category, there was a strong logarithmic relationship between upland areas or administrative areas and mean SOC density CV. Though mean SOC density CV within each soil order increased from family to order, the trend and range of increase varied greatly. Soil organic carbon density variability for zonal and azonal soils was similar in terms of trends but different in terms of rate with increasing scale from county to region. A strong logarithmic relationship between upland area and mean SOC density CV was also observed. These relationships indicated that reducing upland area by five orders of magnitude would halve the CV. Therefore, when estimating the SOC pool in uplands, both administrative and soil type scales should be considered in the sampling design, especially for azonal soil.