中文摘要：

以西安市唐长安城墙遗址公园绿地及城郊曹家堡农田为研究对象, 探讨城郊耕地转变为城市绿地后土壤容重、土壤碳库及碳同位素特征。结果表明, 城市绿地表层0～30 cm土壤容重低于耕地, 30～50 cm土层高于耕地。城市绿地及耕地0～50 cm土壤有机碳（SOC）平均含量分别为6.4 g·kg^-1和7.6 g·kg^-1。与耕地相比, 城市绿地建成10年后, SOC平均含量明显降低。城市绿地C/N明显高于耕地, 这与农田大量氮肥的投入密切相关。城市绿地及耕地土壤δ^13C值的变化范围分别为：-26.88‰～-21.18‰和-22.99‰～-21.12‰；平均值为-23.40‰和-21.92‰。城市绿地δ^13C值显著低于耕地, 这很好地反映了地表植被变迁及土壤含水量的变化：城郊农田为小麦 （C3植物）、玉米 （C4植物） 轮作, 土壤δ^13C值反映的是C3、C4植物混合的结果；而城市绿地则为纯C3植物, 且不定期浇灌, 土壤水分条件较好。我们的研究结果表明, 农田转变为城市绿地后, 土壤碳库减少, δ^13C偏负, 二者主要受控于土地管理方式的变化及地表植被的变迁。

英文摘要：

Taking the urban green land of Chang′an City Walls of the Tang Dynasty and suburban farmland in Xi′an city as test objectives, we study the characteristics of soil bulk density, soil carbon storage and carbon isotope composition. Our results indicate that： soil bulk density in urban green land is lower than in farmland at the top 30 cm while soil bulk density is higher than in farmland at the depth of 30 cm to 50 cm. The average soil organic carbon （SOC） content in urban green land and farmland is 6.4 g·kg^-1 and 7.6 g·kg^-1 respectively. Compared with farmland, the average SOC content declines in urban green land conversion from farmland about 10 years ago. However, C/N in urban green land is higher than in farmland, in which fertilizer is input. The δ^13C value ranges from -26.88‰ to -21.18‰ in urban green land and from -22.99‰ to -21.12‰ in farmland, which averages -23.40‰ and -21.92‰ respectively. The δ^13C value in urban green land is significantly lower than that in farmland, which reflects the changes of vegetation and soil water content in the two study sites. The main vegetation is wheat （C3 plant） and maize （C4 plant） in suburban farmland and thus the δ^13C value reflects the mixing result of C3 and C4 plant. However, there is only C3 plant in urban green land and soil moisture is relatively higher than in farmland due to irrigation. Our results indicate that both soil carbon storage and the δ^13C value declines during the conversion from farmland to urban green land, mainly controlled by the changes in land management and vegetation.