藏东横断山区草地利用变化对土壤质量的影响

Impacts of Grassland-use Change on Soil Quality in the Hengduan Mountain Region,Eastern Tibet

草地利用变化导致的土壤质量演变已经引起了全球广泛关注,然而有关西藏高原这方面的研究目前还很少。为此,探讨藏东横断山区草地转变成坡耕地后土壤质量变化。选择3种土地利用类型:草地、短期耕作坡耕地、长期耕作坡耕地,采集土壤表层样品,并测定土壤理化性质。利用土壤退化指数(DI)与土壤质量综合指数(IQ)评价了3种土地类型的土壤质量。结果表明:草地垦殖后的短期耕作造成表层土壤容重、砂粒含量明显增大(容重:从1.11g/cm3增大至1.32g/cm3;砂粒含量:从43.3%增大至54.7%),粘粒、粉粒含量明显减小;长期耕作导致土壤表层厚度明显增加(从23.6cm增加到30.6cm),有机质与全氮含量显著降低(分别降低29%、22%);两种坡耕地全磷与全钾含量变化均不明显。短期、长期耕作坡耕地DI与IQ均低于草地,表明坡耕地土壤质量出现退化。利用DI与IQ评价出来的短期耕作坡耕地与长期耕作坡耕地土壤质量退化程度不一致,表明这两种指数不能等效地评价该区不同土地利用类型下的土壤质量。土壤质量综合指数被认为是评价本研究区土壤质量状况的一种更合理方法。据此断定,长期耕作坡耕地土壤质量退化较严重。

In spite of more concerns about soil quality evolutions induced by grassland-use change world widely, little has been known about these evolutions in Tibetan Plateau at present. The objective of this study was to discuss the changes in soil quality owing to the conversion of grassland to sloping farmland in the Hengduan Mountains of eastern Tibet. Three adjacent land-use types including grasslands, sloping farmlands with a short-term cultivation history （ about 3 years） and those with a long-term cultivation history （ about 50 years） were studied. Soil profile sampling and physical and chemical properties determination were conducted, and then soil qualities of the three land types were evaluated by using DI （ integrated soil quality index） and IQ （ soil degradation index）. Results showed that compared to the grasslands, soil bulk density and sand content significantly increased （ an increase from 1.11 g/cm^3 to 1.32 g/cm^3 and from 43.3% to 54.7%, respectively）, and the contents of clay and silt declined significantly in the short-term sloping farmlands. Compared to those in the grasslands, surface soil depth no- tably increased （ an increase from 23.6 cm to 30.6 cm）, while SOM and total N contents obviously declined in the long-term sloping farmlands （29% and 22% decrease, respectively）. Total P and K contents changed insignificantly in the two sloping farmlands. DI and IQ of the two sloping farmlands were lower than those of the grasslands, suggesting soil quality degradation after the conversion from grasslands to sloping farmlands. Evaluation results to the soil quality of the two sloping farmlands by using DI and IQ were inconsistent, confirming that the two methods were nonequivalent in evaluating soil quality levels under different land-use types. DI is regarded as a more reasonable method in evaluating the soil quality of this study area; thereupon the conclusion that more serious soil degradation of the long-term sloping farmlands was drawn.