Short communicationSoil organic carbon on the fragmented Chinese Loess Plateau: Combining effects of vegetation types and topographic positions

- The SOC concentration declined over soil depths, regardless topographic positions or vegetation types.
- The SOC concentration of cropland varied evidently with topographic positions.
- Topographic positions and vegetation types have interactive influence on the SOC stocks.

The influence of vegetation coverage and topography on soil organic carbon (SOC) stocks has been intensively studies. However, very few of the studies have recognized the potential combining effects of vegetation types and topographic positions onto SOC distribution, especially on the Chinese Loess Plateau where vegetation recovery has generated complex combination of fragmented topography and vegetation coverage. This study systematically sampled soil cores (259) from four vegetation types (woodland, grassland, cropland, and orchard) at three topographic positions (tableland, slope and valley bottom). Each soil core was divided into three layers: surface soil (0-20 cm), subsoil (20-60 cm) and deep soil (60-200 cm). Our results show that: (1) the SOC concentration declined over soil depths, regardless topographic positions or vegetation types. The absence of ancient cultivation layers at the valley bottoms further made the SOC stocks deep to 200 cm there much less than the tableland with thick loess soil layers (8.3 kg km-2 vs. 13.4 kg km-2). (2) The SOC concentration of cropland varied evidently with topographic positions, with the greatest on the tableland (8.0 g kg-1), and the least along the slope (5.3 g kg-1). However, grassland was rather stable across the three topographic positions. (3) In addition, the SOC concentrations of the three vegetation types were comparable on the tableland (6.1 g kg-1), while differed noticeably at the valley bottoms (5.0 g kg-1). Overall, our findings in this study call for the account for each combination of topographic position and vegetation type, so as to properly assess regional SOC stocks for sustainable land use.