中文摘要：

为揭示黄土丘陵区退耕还林土壤固碳过程及其变化机制,采用物理分组法探讨了安塞纸坊沟退耕15~45 a刺槐与柠条林地土壤砂粒、粉粒、黏粒截存有机碳的效应与速率。结果表明,对比坡耕地,两种退耕林地土壤颗粒结合碳含量均随退耕年限延长显著增加,并且表层0~10 cm土壤增幅最高,10~60 cm各土层增幅基本接近。退耕15~45 a期间,刺槐与柠条林0~20 cm土层均以粉粒碳密度增速最高,分别达0.21、0.11 Mg/（hm2·a）,砂粒碳和黏粒碳增速相近,平均分别为0.13、0.06 Mg/（hm2·a）。同样的变化发生在0~60 cm土层,但各颗粒碳密度增速为0~20 cm土层的1.6~2.5倍。按此增速到退耕45 a时柠条林地砂粒碳、粉粒碳、黏粒碳相比坡耕地分别增大了2.6、1.1、0.8倍,刺槐林地则分别增大了8.3、2.2、2.8倍,并且对总有机碳累积贡献的平均比率为：砂粒碳（23%）等于黏粒碳（26%）且均小于粉粒碳（51%）。此外碳库管理指数比碳库活度与土壤总有机碳库变化有更显著的线性相关性。综上分析,该区域退耕刺槐林比柠条林土壤有更强的固碳效应,两种林地均以粉粒碳为主要固碳组分,以砂粒碳周转速率最快。

英文摘要：

In the loess hilly region of China,a large amount of arable land has been replaced by artificial forests,resulting in increased storage of soil organic carbon（ SOC）. To elucidate the process and mechanisms of SOC sequestration following conversion of cropland to forest,the storage and sequestration rates of SOC associated with sand,silt and clay was measured by physical fractionation on Robinia（ Robinia pseudoacacia L.） and Caragana（ Caragana korshinskii） lands that have been stood on former cropland for 15 ~ 45 a. Compared with cropland,the content of carbon in all soil particle-size fractions on all afforested lands was significantly increased with the stand time. Moreover,the highest increase of carbon was in the top soil layer（ 0 ~ 10 cm） and similar increase amplitude of carbon was found in 10 ~60 cm soil layers. During the 15 ~ 45 a afforestation on former cropland,the highest annual sequestration rates of carbon stock in 0 ~ 20 cm soil depths was in silt,which were 0. 21 Mg /（ hm2·a） and0. 11 Mg /（ hm2·a）,and similar sequestration rate of carbon was in sand with average value of0. 13 Mg /（ hm2·a） and in clay was 0. 06 Mg /（ hm2·a） on Robinia and Caragana lands,respectively.The same changing trend also occurred in 0 ~ 60 cm soil depth,and sequestration rate of carbon associated with all mineral particles was 1. 6 to 2. 5 times higher than those in 0 ~ 20 cm soil depth.Following the annual increase rates,the carbon stocks in sand,silt and clay were varied by 2. 6,1. 1 and0. 8 times on Caragana land,and those were varied by 8. 3,2. 2,2. 8 times on Robinia land respectively after 45 years conversion of cropland to forest. The average percentage that contributed to accumulation of total SOC in different mineral particles was in the order： silt carbon（ 51%） clay carbon（ 26%） =sand carbon（ 23%）. Additionally,there was a more significant linear correlation between SOC content and carbon management index（ CMI） than the linear correlation between SOC content and