Effect of environmental factors on regional soil organic carbon stocks across the Loess Plateau region, China

Accurate knowledge of regional soil organic carbon (SOC) stocks and the effects of environmental factors on SOC is crucial, both from the perspective of regional carbon budgets and appropriate landscape management of SOC. However, little information is available regarding the regional SOC stocks in the Loess Plateau region in China. Thus, the objectives of this study were to estimate the current regional SOC stocks and to analyze the relationship between SOC and pertinent environmental factors, i.e. precipitation, temperature, elevation, slope gradient, clay plus silt content (<20 μm) and land use. We investigated upper (0–20 and 20–40 cm) and deeper (0–100 and 100–200 cm) soil layers at 382 sampling sites across the entire Loess Plateau region (620,000 km2). Regional spatial distribution of soil organic carbon density (SOCD) was depicted in a map and SOC stocks were calculated for different soil depths using a geostatistical method. Analysis of variance (ANOVA) was used to analyze the effects of environmental factors on SOCD. Results showed that the mean SOCD was 2.64 kg C m−2 in the 0–20 cm soil layer and 4.57 kg C m−2 in the 0–40 cm soil layer, and it was estimated that 1.64 and 2.86 Pg (1 Pg = 1015 g) of organic carbon were stored in these soil layers, respectively. Estimates for deeper soil layers indicate that mean SOCD in the 0–100 and 0–200 cm layers was 7.70 and 12.45 kg C m−2, respectively, while the total organic carbon stocks amount to 4.78 Pg C (0–100 cm) and 5.85 Pg C (0–200 cm), respectively. Precipitation, temperature, elevation, clay plus silt contents and land use showed significant regional impacts on SOCD. Generally, SOC contents are higher in soils on mountains (with relatively high elevations and low temperatures) and valleys (with low elevations and high precipitation). The results also show that human activities have heavily affected SOC accumulation. Measured SOCD under cropland was relatively higher than under grassland and forestland. The study provides an overview of the current spatial pattern and stocks of SOC, as well as the effects of environmental factors on SOCD, across the entire Loess Plateau region and may be of further use in optimizing strategies for ecological restoration and regional SOC dynamic modeling as an important initial input.

► An intensive sampling of soils across the entire Loess Plateau region in China was conducted to determine soil organic carbon levels in upper (0–40 cm) and deeper (0–200 cm) soil layers.
► Geostatistical analysis and kriging was used to create a continuous surface across the region for soil organic carbon density for the upper soil layers.
► Relationships between environmental factors (i.e., precipitation, temperature, elevation, slope gradient, clay and silt contents (<20 μm), and land use type, and their interactions) and soil organic carbon density were analyzed to explain the distributions in the region.
► Notably, on the regional scale, soils under cropland had higher SOCD levels than under forest and grassland.