『Abstract
　Grazing exclusion (GE) can increase the carbon (C) storage of semiarid grasslands in China; however, little is known about the effect of long-term GE on the C storage of soil fractions. On the basis of a chronosequence of grassland restoration from free grazing (FG) to 28-year GE, we investigated the dynamics of total C and N in soil fractions in order to assess the impact of long-term GE on the soil fractions. The results revealed that long-term GE had a marked impact on soil particle-size fractions and their total C and N concentrations. The C associated with sand and clay in the 0-10 cm soil layer exhibited a significant logarithmic increase as period of GE increases. The increase in C storage with the introduction of Ge was considerable, especially in the sand and clay fraction of 0-10 cm soil layer. The C:N ratios of the whole soil and of the sand and silt increased significantly with grassland restoration. Considering the accumulation of total C in the whole soil and soil fractions and their logarithmic increase during the GE chronosequence, we suggest that by implementing GE, the temperature grasslands of northern China could facilitate significant C storage on decade scales; however, when subjected to GE for 2 decades or more, these grassland soils would have a lower potential to further accumulate the C newly derived from litter and dead roots.

Keywords: C:N ration; Grazing exclusion; Particle-size fraction; Semiarid grassland; Carbon』

1. Introduction
2. Materials and methods
　2.1. Study area
　2.2. Site selection and soil sampling
　2.3. Particle-size fractionation and chemical analysis
　2.4. Calculations and statistics analysis
3. Results
　3.1. Particle-size distribution
　3.2. C concentration and storage in soil fractions
　3.3. N concentration and storage in soil fractions
　3.4. Changes in the C:N ratios of soil fractions
　3.5. Relationship between C and N concentration and the amount of clay plus silt
4. Discussion and conclusion
Acknowledgments
References