中文摘要：

以位于西南大学实验农场的长期免耕试验田为研究对象,通过0—60cm分层采集土壤样品的方法,探讨了不同耕作方式———冬水田平作（DP）、水旱轮作（SH）、垄作免耕（LM）及垄作翻耕（LF）对土壤轻组有机碳的影响。结果表明,重组有机碳是土壤总有机碳的主体,约占土壤总有机碳的69.56%—95.66%,在土壤剖面上随土壤深度的增加其分配比例逐渐升高;其次是自由轻组有机碳,约占土壤总有机碳的5.03%—26.43%,从土壤表层向下,其分配比例迅速下降;闭合轻组有机碳最低,仅占土壤总有机碳的1.37%—4.93%,其分配比例随土壤深度的变化不明显。在0—60 cm土壤深度内,不同耕作方式下自由轻组有机碳的平均含量为LM（4.36 g/kg）〉DP（2.11 g/kg）〉LF（1.74 g/kg）〉SH（1.46 g/kg）,相应的有机碳分配比例分别为17.1%、14.0%、12.2%和11.3%;闭合轻组有机碳的平均含量为LM（0.82 g/kg）〉DP（0.51 g/kg）〉LF（0.36 g/kg）〉SH（0.34g/kg）,相应的有机碳分配比例分别为3.36%、3.45%、2.71%和3.00%。因此,在西南地区紫色水稻土上实行垄作免耕能提高轻组有机碳的含量及其分配比例,从而改善土壤有机碳质量。另外,与土壤总有机碳、重组有机碳及闭合轻组有机碳相比,自由轻组有机碳对耕作方式的变化最敏感,是指示土壤有机碳变化的良好指标。

英文摘要：

Physical fractionation of soil organic carbon（SOC） has been useful in distinguishing specific carbon pools that are responsive to management and identifying the physical control of organic matter.Density fractionation allows the separation of SOC fractions from different locations in the soil and degree of association with mineral particles.The free light fraction carbon（F-LFC） includes loose organic particles in the soil and particulate organic carbon adhering to the exterior of secondary organomineral complexes,which is chemically very close to litter and has generally a very fast decomposition rate.The occluded light fraction carbon（O-LFC） is the fraction of uncomplexed organic matter that is trapped and physically protected within secondary complexes（intra-aggregate organic carbon）.This fraction shows a more advanced degree of decomposition and a slower turnover.The heavy fraction carbon（HFC） is composed mainly of highly decomposed materialand has a very slow decomposition rate.Changes in these different fractions,especially in the low-density fractions,may be more sensitive to management-induced impacts on soil than the total SOC or the complicated heavy fraction organic carbon.However,there are few direct data quantifying these two light fractions and their contributions to total SOC storage as related to changes in tillage practices. The impact of tillage on SOC fractions has been of great interest in recent years.The tillage experiment was located at the farm of Southwest University（30°26′N,106°26′E） in Chongqing since 1990.In this paper four tillage treatments including conventional tillage with rice only system（DP）,conventional tillage with rotation of rice and rape system（SH）,no-till and ridge culture with rotation of rice and rape system（LM）,and ridge culture with rotation of rice and rape system（LF） were selected as research objectives to measure the contents of different soil carbon fractions.Soil sampling at the depths of 0—10,10—20,20—30,30—40,