Effects of land-use types on the vertical distribution of fractions of oxidizable organic carbon on the Loess Plateau, China

The oxidizability of soil organic carbon (SOC) influences soil quality and carbon sequestration. Four fractions of oxidizable organic carbon (very labile (C-1), labile (C-2), less labile (C-3) and non-labile (C-4)) reflect the status and composition of SOC and have implications for the change and retention of SOC. Studies of the fractions of oxidizable organic carbon (OC) have been limited to shallow soil depths and agroecosystems. How these fractions respond at deep soil depths and in other types of land-use is not clear. In this study, we evaluated the vertical distributions of the fractions of oxidizable organic carbon to a soil depth of 5.0 m in 10 land-use types in the Zhifanggou Watershed on the Loess Plateau, China. Along the soil profile, C-1 contents were highly variable in the natural grassland and shrubland I (Caragana microphylla), C-2 and C-4 contents were highly variable in the natural grassland and two terraced croplands, respectively, and C-3 contents varied little. Among the land-use types, natural grassland had the highest C-1 and C-2 contents in the 0-0.4 m layers, followed by shrubland I in the 0-0.1 m layer. Natural grassland had the highest C-4 contents in the 1.0-4.5 m layers. Natural grassland and shrubland I thus contributed to improve the oxidizability of SOC in shallow soil, and the deep soil of natural grassland has a large potential to sequester SOC on the Loess Plateau.