Abstract: This experiment used spatially distributed sites which had been fenced for 0, 3, 5, 7 and 10 years in desert steppe grassland in Yanchi County of Ningxia, to form a time series, for field and laboratory study of soil C dynamics. Variables measured included: total soil organic carbon (SOC), soil particulate organic carbon, soil dissolved organic carbon and soil readily oxidizable organic carbon for 10 cm increments of soil depth to 40 cm at each site. The results revealed significant differences (P<0.05) in total organic carbon at 5-10 cm, 10-20 cm and 20-40 cm soil depth, linked to the number of years of grazing exclusion. The SOC content ranged from 2.24 g·kg^-1 to 4.52 g·kg^-1, and in general increased with increased enclosure time. Thus, values were higher in desert steppe grasslands of 7 and 10 years grazing exclusion. There were no significant effects of grazing exclusion on soil particulate organic carbon (P>0.05). The contents of soil dissolved organic carbon were highest in grazed grassland (0.59, 0.49 and 0.56 g·kg^-1, respectively, for 0-5 cm, 5-10 cm and 20-40 cm soil depths). Values decreased initially after grazing exclusion, (with lowest values recorded after 5-7 years), and then increased with longer duration of exclusion. The soil readily oxidizable organic carbon content was highest (2.48 g·kg^-1) in desert steppe grassland subject to 7 years grazing exclusion. In summary, 7 years is a recovery point in the process of natural restoration and succession of degraded desert steppe by grazing exclusion.

Key words: desert steppe, enclosure, soil organic carbon, soil active organic carbon