石羊河下游民勤绿洲不同年限退耕地土壤物理性质变化研究

石羊河下游因水资源减少而退耕大面积土地，测试其物理性质，旨在为管理提供依据。选择不同退耕时期（1 年、2 年、3 年、4 年、5 年、8 年、15 年、24 年和31 年）土地，分层采集0~40 cm范围内土样，分析其容重、孔隙度和机械组成。结果表明：随着退耕年限增加，土壤粘粒呈减少趋势，粉粒增加，退耕4 年达到最大，之后则略有增减；土壤垂直剖面的土壤粒度分异性增大，而变化最大的土层为0~10 cm。退耕土壤容重随着退耕时间延长而增加，退耕3 年之后的不同深度的土壤容重及孔隙度的差异变小。随着退耕年限增加，土壤质地变粗，通透性增加，土壤质量下降。退耕3~4 年期间的土壤物理性质相对变化明显，是退耕地管理的关键时期。

Studies of Soil Physical Property of Different Abandoned Lands in the Minqin Oasis, Downstream of the Shiyang River

The vast area of farmland was abandoned at downstream of the Shiyang River because of decreased water recourse. To ensure the ecological safety of Minqin Oasis and to provide anagement basis for the abandoned land, soil physical properties were observed and studied. The authors analyzed land with abandonment time of 1 year, 2 years, 3 years, 4 years, 5 years, 8 years, 15 years, 24 years and 31 years. Samples were took at the 0-40 cm layer of soil to measure the bulk density, porosity and grain composition of soil in the different abandoned lands. Results showed that the tendency of clay content was decreasing, conversely, the fine sand increased at the layer of 0-10 cm of different abandoned lands. The changes of grain content reached a peak at the fourth year after the land was abandoned, then varied slightly. The variation of grain content of abandoned land was obvious with the extension of the abandonment year. The soil bulk density decreased and porosity increased with the extension of the abandonment year. The difference of porosity and soil bulk density at the range of 0-40 cm of different abandoned land decreased after the land abandoned for 3 years. The quality of soil was decreased, while the soil permeability was improved. The soil physical properties changed obviously in 3 to 4 years after abandonment, so it is the key time for land management.