黄土高原半干旱区土壤干层水分恢复研究

A study on water restoration of dry soil layers in the semi-arid area of Loess Plateau

黄土高原土壤干层是一个重要的生态环境问题,研究干层土壤水分的恢复对正确指导黄土高原退耕还林还草,实现该区土地的可持续利用具有重要意义。研究在黄土高原半干旱区的固原县,选择了将紫花苜蓿翻耕后3a、12a的坡耕地,对其土壤干层的水分恢复状况进行了分析。发现二者土壤干层水分最大恢复深度分别为3m、4.8m,但土壤水分含量在中效水及其之上的主要恢复层深度分别为2m、2.2m。苜蓿翻耕3a和12a后2m以上土层土壤平均湿度都能恢复到易效水或极易效水的水平,可以满足1年生农作物的生长需求而不会进一步恶化土壤水分生态环境。但即使苜蓿翻耕12a后土壤水分,也不能满足林木和多年生豆科牧草正常生长的水分需求。

A dry soil layer is defined as any layer within the soil profile that maintains a persistent low water content, which generally results from negative evapo transpiration equilibrium. Nearly all soils under perennial leguminous crops and re vegetated forests on Loess Plateau contain dry soil layers, which constitutes a serious obstacle to the sustainable land use of Loess Plateau of China. The formation, properties and distribution of dry soil layers in the Loess Plateau has been extensively studied, but little research has been conducted regarding soil water restoration in dry soil layers once the plants which have caused the dry soil layer are die or turn to annual crops. The purpose of this study was to examine soil water restoration at 2 sites where the land use was changed from alfalfa to annual crops for 3, and 12 years. The soil water of one site on which alfalfa had been growing for 11 years and one farmland site (row crop) were also tested as controls. All the sites are on a slope gradient of approximately 12.5 degrees. The soil water was measured in the soil profiles to depth of 9 9 m. Soil water samples were tested at intervals of 0 1 m to a depth of 1 m, 0 2 m from 1 to 3 m, and 0 3 m at 3 and deeper. The soil samples were extracted using a subsoil probe and oven dried for testing soil moisture contents. Results were reported in percent gravimetric soil water content (θ g). The study area was located at the northwestern hilly part of the Loess Plateau with an annual precipitation of 433 8mm. Because of the thickness of the loess deposits of the study area, there was no relationship between soil water and groundwater. Thus, the soil water restoration of the dry soil layer was totally dependent on rainfall. The results show that the maximum water restoration depths of the 3 years and 12 years farmland after alfalfa were 3m, 4 8m. But the main restoration depths (within which the soil content was equal or exceeds the medium available water content to plants)of the 3 years, 12 years farmland after alfalfa were 2.0m and 2 2m with average soil moisture of 13 7% and 12 5%, march higher than the soil moisture of the corresponding layers of 11 year alfalfa (9 6% and 9 2%), but close to the soil moisture of corresponding layers of continuous farmland (15 3% and 14 7%). From the view of the availability of soil water content to plants, the soil moistures of main restoration layers of both 3 years and 12 years farmland after alfalfa were reached easily available water content or the easiest available water content. Because much previous study has verified that the main water consumption layer by annual crops is within 2m but that by forest and artificial perennial leguminous plants exceed 8m in the area, the water contents of the 3 years and 12 years farmland after alfalfa can meet the needs for growing annual crops without further deteriorating soil water ecosystems, but it can′t meet the needs of planting trees or perennial leguminous plants for their normal growth even after 12 years of transferring from perennial alfalfa to annual crops.