Overgrazing and soil carbon dynamics in eastern Inner Mongolia of China

Eastern Inner Mongolia of China is a typical ecotone between sandy forests and steppe. Little is known about the effect of overgrazing on carbon loss from soil in semiarid steppe and sandy forests of the north of China. The soil carbon parameters were measured in a 10,000 ha natural reserve in eastern Inner Mongolia of China (43 degrees 30'-43 degrees 36'N, 117 degrees 06'-117 degrees 16'E). Three situations were compared: primary protected (PP), moderately protected (MP) and highly degraded (HD). Soil and litter samples were recovered in spring and summer. Soil organic carbon (SOC) and CO2-C values decreased from the PP (9.23 kg m(-2) and 157 g m(-2)) to the HD (1.69 kg m(-2) and 57 g m(-2)) sites whereas the C mineralization rate increased toward the less restored sites (1.06-2.37). Surface-litter C was different in both sites under protection ( PP 648 and MP 408 g m(-2)), an was low at the HD site (17 g m(-2)). Leaves from woody species dominated the surface litter at the PP site, whereas grass material was predominant at the MP site. During summer, both CO2-C and SOC decreased, whereas the C mineralization rate increased. We calculated that C loss since the introduction of cattle into the forest was 77 M g ha(-1), reaching a total of 1.1 x 10(15) g for eastern Inner Mongolia. These values are higher than those caused by the conversion of steppe and other ecosystems into agriculture or cultivated pastures. The amount of C fixed at the PP site (650 g ha(-1) year(-1)) indicates that the sandy soils have a significant potential as atmospheric carbon sinks.