白刺沙堆退化与土壤水分的关系

近几十年来,我国西北干旱区白刺沙堆退化严重,导致固定沙丘活化,流沙掩埋绿洲,造成了严重危害。如何尽可能长期保持白刺沙堆的稳定、防止白刺沙堆活化成为绿洲保护和沙漠化防治急需解决的问题。在多年野外观察的基础上,提出了"土壤水分收支不平衡所导致的土壤水分减少是白刺沙堆退化的主要原因"的研究假设。但是,由于缺少长期的野外观测试验,这个假设一直未被很好地证明。为了证明这个假设,在甘肃民勤的绿洲外围选择了雏形、发育、稳定和死亡四个退化阶段的白刺沙堆,于2008年1月至2012年6月利用中子水分仪和土壤烘干称重法对土壤水分进行了长期观测。结果表明:各样地的土壤含水量均呈现出2008年最大,2009年和2011年次之,2010年最小的趋势。年内变化是春季土壤含水量最低,夏季逐渐增加,随后逐渐减小。在不同发育阶段,雏形阶段的土壤含水量最大,且降水容易下渗。稳定和死亡阶段的白刺沙堆土壤含水量很低,降水难以下渗,只有大的降水事件发生时,水分才可以下渗。因此,稳定和死亡阶段白刺沙堆的土壤水分经常在植物的凋萎点之下,是造成白刺沙堆退化重要原因。证明了"土壤水分减少是白刺沙堆退化的原因"的研究假设。研究结果对今后的植物固沙实践活动会有积极的参考意义。

The degradation of Nitraria dunes and soil water in Minqin oasis

Nitraria dunes can fix a huge amount of moving sand and are one of the most effective ways to protect oasis from moving sand in Northwest China. In recent decades, Nitraria dunes in Northwest China have seriously degraded. As a result, the fixed sand is released, and the oasis is damaged again and again. How to sustain Nitraria dunes long term and prevent the Nitraria plant from dying is an important issue. Based on observation over time, we hypothesized that declining soil water content resulted from an imbalanced soil water budget leading to Nitraria dunes degradation. However this hypothesis hasn''t been proved satisfactorily for the lack of long-time field experiments. To prove this hypothesis, we observed soil water content from January 2008 to June 2012 in different degradation stages of Nitraria dunes in the marginal oasis of Minqin, Gansu Province. The methods we used to observe soil water content were the neutron moisture gauge method and weighting method after dried. Results showed that average volumetric soil water content was the highest in 2008 and the lowest in 2010.The variation of volumetric soil water content was the most obvious at the depth of 0-50 cm.The vertical variation pattern of volumetric soil water content was different in different degradation stages. At the depth of 0-90 cm, volumetric soil water content varied the greatest in the early growth stage and rapid growth stage for low water-holding capacity, high seepage power and high air permeability of soil. Three kinds of soil layers divided by soil water variation features occurred in these stages of the Nitraria dunes. These were sudden-change layer, stable layer and increasing layer. However, in the peak growth stage and senescence of Nitraria dunes, volumetric soil water content varied slightly because of crust on the Nitraria dunes surface. Average volumetric soil water content was lower in the early spring, gradually rising in the summer and declining in the autumn.The maximum soil water content occurred in July and September. Moreover, the variation of volumetric soil water content in different seasons was the most obvious at the depth of 0-30 cm. With increasing depth, the volumetric soil water content variation became less. During different degradation stages, the volumetric soil water content in the early growth stage was the highest because precipitation was easy to infiltrate. However, the soil water content was the lowest in the peak growth stage and senescence because precipitation cannot infiltrate without a significant precipitation event. And soil water content in the peak growth stage and senescence of Nitraria dunes was lower than their wilting coefficient. As a result, soil water content cannot satisfy the basic water demand for plant growth which was the principle reason why Nitraria dunes degraded. Through this study, it can be concluded that declining soil water content is the main reason why Nitraria dunes degraded. To take advantage of precipitation efficiently and prevent evapotranspiration on the surface of Nitraria dunes is the key to protecting Nitraria dunes and sand stabilization. This result will provide a positive influence on subsequent vegetation sand-control activities.