Soil organic carbon increases under intensive agriculture in the Central Sands, Wisconsin, USA

Soil organic carbon (SOC) plays a crucial role in maintaining fertility and productivity in sandy soils. This study mapped the spatial variability of SOC concentration, A-horizon thickness, and SOC stocks from the Central Sands in Wisconsin. Soil samples were collected from three different areas (area A, B, and C) that were sampled through grid sampling (GS, n = 100), conditioned latin hypercube sampling (cLHS, n = 100), and random sampling (RS, n = 150) schemes. Average SOC concentration of the A-horizon from soil sampling area A, B, and C were 6.1, 7.1, 8.3 g kg− 1, respectively. The mean A-horizon thickness for agricultural soils was 28 cm compared to 15 cm under adjacent grassland. Regression kriging was selected as prediction model where ECa and local topographic information (i.e., slope gradient, slope aspect, elevation, wetness index, altitude above channel network etc.) were used as predictors. We observed an increased SOC content, SOC stock, and A-horizon thickness with ECa and wetness index. SOC from area B had the strongest spatial dependency (NSR = 0.64) followed by area A (NSR = 0.72), whereas that from area C was the weakest (NSR = 0.78). Compared to SOC content and A-horizon thickness prediction, SOC stocks prediction had the maximum uncertainty. Predicted SOC stock (t ha− 1) ranged from 28 for sampling area A to 40 for B, and 59 for area C. These high SOC stocks are the result of decade long intensive agriculture with high amount of nitrogen input and irrigation. It has resulted in deep A-horizon and high SOC stocks. This study found that SOC stocks in the Central Sands could be estimated from A-horizon thickness (R2 ~ 0.5).