青海环湖地区草原土壤含水量及富集规律

【目的】研究青海环湖地区草原土壤水分运移与富集规律、土壤水分剖面分布模型、水分循环与水分平衡,揭示该地区土壤水库蓄水特点、土壤干层及其恢复条件,为该地区土壤水资源及草原植被保护、土壤水库建设和草原生态环境的可持续发展提供科学依据。【方法】利用轻型人力钻连续4年采取600多个土壤样品,采用烘干称重法测定土壤含水量。采用双环入渗法原位测定土壤入渗率,采用激光粒度仪分析土壤粒度,采用负压计原位测定土壤吸力。【结果】青海环湖地区的土壤剖面水分分布较为稳定,不论旱季还是雨季,约65%的水分富集在0—0.4 m土层中,0.6 m以下土层水分严重不足。该地区土壤吸力为0.17—0.42 MPa,土壤田间持水量为20%左右。0—0.4 m土层含水量一般为23%,大于田间持水量(20%),故存在约3%的重力水;土层0.6 m以下含水量仅约为6.5%。该地区0.6 m以下土层一般发育有不同等级的土壤干层,且土层厚度越大干层发育越严重。该地区0.4 m以下土层水分含量与深度之间的关系可以用幂函数模拟描述,模拟函数的增量曲线表明,在2009—2011年降水累积增加约50 mm的条件下,土壤含水量的增加量由0.4 m深度的约5%逐渐降低到0.8 m深度的约3%,0.8 m以下土层水分增加量不足3%。该地区土壤入渗率为1.3—3.0 mm·min-1,入渗率较高有利于降水向土壤水转化。该地区的土壤质地优良,但0.6 m以下土层含水量已接近或低于粉砂土无效水的含量(5%)。【结论】青海环湖地区气温低、土壤冻结期长,造成该地区土壤水分具有在土壤上部滞留和富集的突出特点。该地区土壤平均厚度不足1.5 m,导致该地区土壤水库的调蓄功能较弱。而土壤水分的上部滞留和富集增强了该地区土壤水库对浅根系草原植被的调蓄功能,并且具有抑制草原荒漠化发生的重要作用。青海环湖地区在2009—2011年降水量增加到400—420 mm的条件下,土壤水分表现出微弱的正平衡,薄土层中的土壤干层消失,而较厚土层中的土壤干层仍然存在。该地区土壤干层恢复速度很缓慢,恢复的水分增加量低于5%。土壤干层的发育和分布深度很小不仅指示出该地区生态系统较为脆弱,而且还指示出该地区不适于发展需水较多的乔木植被。

Soil Moisture and Enrichment Regularity of Steppe Soil in Qinghai Lake Area

【Objective】The aim of this study on the steppe soil in Qinghai Lake area is to explore the soil moisture migration and its enrichment characteristics, soil moisture distribution model, hydrological cycle and balance and reveal the characteristics of soil reservoirs, dried soil layer and its recovery conditions, and then provide a scientific basis for protecting soil water resources and steppe vegetation, construction of soil reservoir and sustainable development of grassland ecological environment.【Method】A series of experiments were performed to determine soil moisture, soil suction, infiltration rate and particle size. More than 600 soil moisture samples were collected by Light-duty Human Drill in 2009-2012. Drying weighing method was used to determine soil water content. Bicyclic infiltration method was used to determine soil infiltration rate in situ. Soil particle size was analyzed by Laser Particle Analyzer. Soil suction was measured by Tensiometers in situ.【Result】The distribution of soil water at soil profile has a certain stability, that is, in dry season or rainy season, the soil water in this area is enriched about 65% at 0-0.4 m depth and is quite shortage under 0.6 m depth. The soil suction in this area ranging from 0.17 MPa to 0.42 MPa suggests that field capacity of this area is about 20%. The infiltration rate of soil in this area varying from 1.3 mm·min^-1 to 3.0 mm·min^-1 shows that it is high and advantageous to precipitation into soil. The soil moisture is about 23% and 6.5% at 0-0.4 m depth and under 0.6 m depth, respectively. Field capacity of this area is about 20%, meaning that there is about 3% gravity water at 0-0.4 m depth of soil. The soil usually develops a dried soil layer with different levels under about 0.6 m in this area, and the greater soil thickness, the more seriously dried soil layer develops. Under 0.4 m depth of soil, the relationships between soil moisture and depth can be described by power function. The parameters of power functions (i.e. a, k) are able to well reflect the average level of soil moisture (a) and the degree of soil moisture decreased with the increase of depth (k). As the precipitation increased by about 50 mm from 2009 to 2011, the increment of soil moisture can be clearly reflected by the incremental curve of analogue function. The increment of soil moisture within the depth from 0.4 m to 0.8 m gradually decreased from 5% to 3% and was less than 3% under 0.8 m depth. The soil property is excellent in this area, but the soil moisture under 0.6m depth is close to or below 5% unavailable water of silty soil.【Conclusion】The retention and enrichment of soil water at 0-0.4 m depth is rare and this characteristic mainly depend on the low temperatures and long-term freezing of soil in Qinghai Lake area. The regulation function of soil reservoir is weak in this area due to the average thickness of soil is less than 1.5 m. To some extent, the retention and enrichment of soil water not only can enhance the regulation function of soil reservoir for vegetation in shallow root, but also can play an important role in inhibiting the occurrence of grassland desertification. The soil water balance in this area showed a weakly positive balance because of 400-420 mm annual precipitation from 2009 to 2011. Under this condition, the dried soil layer in thin soil was able to disappear completely, but could not in thick soil. The dried soil layer was recovered slowly in this area and its soil moisture recovered was less than 5.0%. Development of dried soil layer and its distribution with small depth not only show that the ecosystem in this area is more vulnerable, but also indicate this area is not suitable for growing arbor vegetation because it will consume more water.