|其他摘要||Urbanization has profound impacts on ecosystem carbon cycle. Impervious surface areas (ISA) cover about 63% of the urban area and 0.9% of the land surface in China, but the biogeochemical characteristics of the soil beneath the ISA is still in lack. As the result, global and regional carbon cycle studies had to rely on over-simplified assumptions to estimate the soil organic carbon pool (SOC) beneath the ISA (SOCISA). Validities of these assumptions directly affected the assessments of the urbanization effects on regional/global carbon dynamics. To reveal the biogeochemical characteristics of the ISA soil, to evaluate the various assumptions in former studies, and to quantify the effects of ISA construction on SOC, this study, based on 100-cm-depth soil profiles taken from 27 ISA sites and the nearby pervious surface area (PSA) in the Northern Tianshan Urban Cluster in a dryland of the northwestern China, compared the soil bulk density (BD) and SOC among ISA, PSA, and the original soils undisturbed by urbanization. Meanwhile，Employing Landsat TM images acquired in 2000 and Landsat OLI images 2014, the Linear Spectral Mixture Analysis was applied to retrieve the land cover fraction images of ISA, vegetation and remnant desert/bare soil of Northern Tianshan Urban Claster. Based on SOC density data determined through field observations and literature reviews, we have evaluated the urbanization, especially the ISAexpansion on the soil orgnic Carbon pool. The main conclusions are as followes:
(1) The mean regional soil bulk density BDISA=1.55±0.02 g cm-3, which was significantly higher than BDPSA=1.45±0.03 g cm-3 about 7% (p<0.01); SOCDISA = 5.74±0.40 kg C m-2 kg C m-2, which was significantly lower than SOCDPSA =8.69±0.75 kg C m-2 about 33.9% (p<0.01). The BDISA decreased significantly as the soil depth idcreased (p<0.05); however, the BDPSA did not show a significant vertical trend. Both of the SOCDISA and SOCDPSA declined as the soil depth increased significantly.
(2) The mean regional original BDO=1.41 g cm-3，which was lower than that of both urban soil; mean regional SOCDO=6.52 kg C m-2，which was slightly higher than SOCDISA, but significantly lower than SOCDPSA. SOCDO declined with the increasing soil depth. Although SOCISA was close to the mean SOCDO in the study region, the vertical patter of their BDO and SOCDO did not match, our simulations indicated that it is possible to predict the changes in SOCD during ISA construction if we considered the SOCD loss due to topsoil removal and soil disturbances as well as the effects of soil compaction on SOCD.
(3) The land-cover composition of Northern Tianshan Urban Cluster did not change obviously from 2000~2010, urban ISA was the dominant land-cover type. However, ISA of this region increased about 41% during this time period, which was converted large area of cropland and remnant desert. At the single city in Northern Tianshan Urban Cluster, the land-cover chage was same as the regioan land-cover pattern. Further analysis of the spatial pattern of land-cover change revealed that the impervious surface mostly sprawled outskirt displacing remnant desert/bare ground and cropland, and infilled cicyspace converting urban greenspace.
(4) A total of 4.96~4.99 Mt and 4.45~4.49 Mt soil organic carbon was stored in the metropolitan area of Northern Tianshan Urban Cluster in the year of 2000 and 2014, respectively. Due to the huge area, soil organic carbon beneath the ISA was the dominant soil carbon pool in 2000 and 2014, ocuupying 41% and 64% of total soil organic carbon, respectively. The Northern Tianshan Urban Cluster lost 0.50~0.51Mt C, which was about 10.3% of the total soil organic carbon storage in 2000, due to the land-cover change during 2000~2014. The main reason of the soil carbon loss was caused by the conversion of remnant desert and cropland into ISA.
(5) Because different urban land-cover types could have very different carbon storage, it is important to quantify the detailed “from-to” change trajectory information in land-use change detection. To this end, our study used the decision tree classifier to reveal the impact of complex land-covere change on urban soil organic carbon storage. This study choose the typical city of Northern Tianshan Urban Cluster—Urumqi as the case study to address the question, and increase the time period to 1990-2014. The result showed that ISA tripled from 1990-2014 displacing remnant desert and cropland. The city lost 24% of C stock from 1990-2014. About 68% of the soil organic carbon loss was caused by the conversion of remnant desert and cropland into ISA, mainly in the northern city.|