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Transferring CO2 to soil organic carbon (SOC) is an important carbon sequestration mechanism that may mitigate global climate change. Grasslands can preserve 10-30% of total SOC stock. This study investigated the contribution of different soil particle-size fractions to SOC sequestration in grasslands, which play an important role for stabilization of SOC.

Soil layers at 0-10, 10-20, 20-40, and 40-60 cm were sampled from meadow steppe, typical steppe and desert steppe in Inner Mongolia, northern China in July and August, 2016. Bulk soils were fractionated by wet-sieving method to investigate SOC stock in different particle-size fractions. SOC was determined with ElementarVario MAX CNS analyzer after removing soil inorganic C.

The SOC stock in 0-60 cm depth was 163.0 Mg C ha(-1) in typical steppe soils, 127.3 Mg C ha(-1) in meadow steppe and 44.4 Mg C ha(-1) in desert steppe. The silt (2-53 mu m) and clay (< 2 mu m) fractions accounted for 62.2 and 19.2% in meadow steppe, 35.3 and 12.7% in typical steppe, 12.2 and 6.6% in desert steppe of total soil mass. The SOC stock in silt and clay fractions accounted for 59.8 and 32.0% of total SOC stock in meadow steppe, 58.9 and 19.8% in typical steppe, and 43.9 and 28.2% in desert steppe, respectively. The SOC concentration and stock were positively correlated with mean annual precipitation (MAP) and negatively correlated with mean annual temperature (MAT). Our study demonstrated that climatic factors and soil texture are main reasons for the difference of SOC stocks among three grassland types.

The distribution of soil particle-size fraction and its associated organic C showed distinct variations under three grassland types in Inner Mongolia. Management practices, such as changing grazing intensity in three grassland types, are important for the sustainable use of grasslands.