Abstract: Urbanization is one of the main factors that influence the regional ecosystem. However, the paucity of observation on the soil organic carbon (SOC) beneath urban impervious surface area (ISA) limits the estimation on the implication of urban land conversion in regional carbon cycle, especially in the arid regions. In this paper, urban land covers (water, ISA, greenspace, cropland, and bare ground, remnant desert) and land cover conversion of Urumqi, Xinjiang, China was estimated using the models of Vegetation-Impervious surface-Soil for urban land covers, linear spectral mixture analysis, and decision tree classifier based on Landsat TM/ETM of 1990 and 2010. The impact of urban land conversion on the ecosystem carbon storage was estimated based on the SOC and vegetation carbon density that obtained from the field observation and literatures. The dominant land conversion was the ISA expansion, which was tripled during 1990-2010, mostly sprawling in the north, and infilling in the southwestern Urumqi. The ISA expansion mainly occupied the bare soil/remnant desert (62%) and cropland (27%). It was found that more than 95% of urban ecosystem carbon stored in the soils, and 48% of which was in the soil under ISA. Land cover conversion during 1990-2010 resulted in a total of 25% carbon loss, in which converting of bare soil/remnant desert and cropland into ISA explained 68% of the total SOC loss and 63% of the total VEGC loss, whose spatial distribution was consistent with the spatial distribution of land conversion. Urban ecosystem carbon sequestration could be improved by intensive management and reasonable planning of the proportion between urban ISA and green space, which could make up for part of carbon loss caused by land conversion.

Key words: arid area ecosystem carbon storage, land-cover change, soil organic carbon, Urumqi, vegetation carbon