Soil organic carbon in Ultisol landscapes: Influence of erosion rates, soil development, depth, and hillslope properties

Although soils represent one of the largest global carbon reservoirs, soil organic carbon (SOC) stocks remain poorly quantified across many landscapes, limiting effective land management aimed at carbon sequestration in the pedosphere. Here, we report SOC stocks from 30 soil pits (sample n = 212) in low-order watersheds located in a slowly eroding (<20 m/Ma), humid-subtropical landscape that is the type-area for Ultisols. Neither SOC density nor SOC profile stock varies systematically with aspect or hillslope position, unlike in drier, faster eroding landscapes. Instead, SOC variance is primarily related to soil development: well-developed soils–characterized by deeper profiles, higher clay content, and greater concentrations of pedogenic iron–exhibit the highest SOC stocks (2.11 × 10 3 kgC m −2 ). These carbon-rich soils are found in first-order watersheds adjacent to major regional watershed divides exhibiting minimal tributary incision. Although erosion rates vary modestly across this landscape, they appear to strongly influence soil development and, in turn, SOC retention. SOC stocks are higher in watersheds with lower 10 Be-derived basin-averaged erosion rates (9.61 ± 0.16 m/Ma) than in faster-eroding basins (14.1 ± 0.3 m/Ma) near major streams. Additionally, SOC stocks remain high below 30 cm depth in all watersheds, indicating potential underestimation when only surface soils are sampled. Overall, SOC stocks increase with decreasing watershed erosion rate, emphasizing the importance of soil stability in SOC retention – particularly in slowly-eroding, deeply-weathered Ultisols – and provide a foundation for improving carbon sequestration models in similarly stable, humid-subtropical landscapes. • SOC stock increases with decreasing watershed-scale erosion rates. • Pedogenic Fe and clay content are strong predictors of SOC density. • SOC stock varies more strongly with erosion rate than hillslope aspect or position. • Deep SOC storage (>30 cm) in the Piedmont is significant and likely underappreciated.