Soil organic carbon in deep profiles under Chinese continental monsoon climate and its relations with land uses

•Vertical SOC distribution and related factors in 33 21-m profiles were investigated.•SOC stocks in upper 1-m layers account for only 9% of total carbon in 21-m profiles.•SOC storage at depth (5–21 m) was significantly higher under forest than under cropland.•Within and below the rooting zone, the main factors affecting SOC were different.•Land use and rooting characteristics significantly affect SOC in deep soils.

We collected soil samples from depths between 0 and 12–21 m at 33 sites across the Chinese Loess Plateau in order to determine the vertical distributions and storage of soil organic carbon (SOC), as well as to test the hypothesis that SOC in deep soils (below 5 m) is greater under forest than under permanent cropland. The overall distributions of SOC within a profile were divided into three sub-layers: 0–2, 2–14, and 14–21 m, with significantly different (P < 0.01) mean SOC values of 3.28 ± 2.39, 2.07 ± 0.79, and 1.56 ± 0.57 g kg−1, respectively. In the deep soil layer (5–21 m), SOC storage was significantly higher (P < 0.01) under forest (47 ± 0.43 kg m−2) than under cropland (38 ± 0.44 kg m−2). Within the rooting zone, the factors affecting SOC variation were root length, pH and clay content; below the rooting zone, the factors were soil water content, pH and clay content. Land use and rooting characteristics significantly affected the magnitude and vertical distribution of SOC within both shallow and deep layers. Therefore, changes in land use can alter SOC storage in deep soils, which can have important consequences for global climate change.