Abstract: Carbon exchange capacity in soil-vegetation-atmosphere system and its response to land use change is one of the key issues concerning global climate change research. Net Ecosystem Exchange (NEE) is a key feature of the ecosystem carbon exchange. In this study, the carbon flux under enclosed and grazing sandy grassland was studied by chamber-based gas measurement system to reveal the influence of land use change on NEE; thereafter we analyzed the relationships between NEE and environmental factors in Horqin sandy grassland. The results showed that: (1) NEE in 17-year enclosed grassland was significantly lower than that in 22-year enclosed; and NEE in 22-year enclosed grassland was relatively lower (P<0.01) than that in grazing grassland. This implied that carbon sequestration capacity would be boosted with enclosure of sandy grassland. (2) During the growing season, enclosed grasslands showed as carbon sinks, while grazing grassland indicated as a carbon source. (3) Providing only considering the carbon sequestration in sandy grassland, long-term enclosure should not recommended since carbon sequestration capacity in 17-year enclosed grassland was higher than that in 22-year enclosed grassland, although carbon sequestration in 22-year enclosed grassland lasted relatively longer during growing season. (4) The relationships between environmental factors and NEE in different grasslands were not straightforward, linear combination of environmental factors were able to respectively explain the variations of NEE by 39.5% in 22-year enclosed grassland, 32.1% in 17-year enclosed grassland, and 21.2% in grazing grassland. This exerted important significance for the effective management and scientific utilization of sandy grassland resources in the context of climate change.

Key words: sandy grassland, net ecosystem exchange, chamber method, enclosure, grazing