黄土高原水土保持林对土壤水分的影响

Soil moisture dynamics of water and soil conservation forest on the Loess Plateau

黄土高原植被恢复的限制因素主要是土壤水分,植被与土壤水分关系的研究对黄土高原植被恢复具有重要意义。2008年7月1日至2009年10月31日间采用EnviroSMART土壤水分定位监测系统以每30min监测1次的频度,对晋西黄土区刺槐人工林地、油松人工林地、次生林地的土壤水分变化进行了研究。研究得出:次生林地0—150 cm土层中平均蓄水量为331.95mm,刺槐人工林地为233.85 mm,有整地措施的油松人工林地为314.85 mm,刺槐人工林比次生林多消耗的98.10 mm土壤水分主要来源于80 cm以下土层。次生林主要消耗0—80 cm土层的水分,而人工林不但对0—80 cm土层水分的消耗量大于次生林,对深层土壤的消耗也较次生林大,这将有可能导致人工林地深层土壤的"干化"。在土壤水分减少期(11—1月)刺槐人工林土壤水分的日均损耗量为0.86mm、油松人工林为0.82 mm、次生林为0.84 mm。土壤水分缓慢恢复期(2—5月)刺槐人工林地土壤水分的恢复速度0.90mm/d,油松人工林地为0.53mm/d、次生林地为0.79 mm/d。土壤水分剧烈变化期(5—10月)刺槐人工林地土壤水分含量的极差为95.71mm,油松人工林地为179.1mm,次生林地为72.03mm。在干旱少雨的黄土高原进行植被恢复时,应多采取封山育林等方式,依靠自然力量形成能够与当地土壤水资源相协调的次生林,是防止人工植被过度耗水形成"干化层"、保障水土保持植被持续发挥生态服务功能的关键。

Soil moisture was the limiting factor for vegetation rehabilitation on the Loess Plateau in China. Understanding the relationship between vegetation and soil moisture was important to soil and water conservation. During the period of July 1, 2008 and October 31, 2009, we measured soil moisture dynamics of Robinia pseudoacacia plantations, Pinus tabulaeformis plantation and the secondary forests every 30 minutes on the Loess Plateau in western ShanXi Province, China. The results showed that the average soil water storage in the 0--150cm soil profile under the secondary forest is 332 mm, 234 mm under Robinia pseudoacacia plantations, and 315 mm under Pinus tabulaeformis plantation with land preparation. The low soil moisture in the Robinia pseudoacacia plantations was due to water consumption not only from the 0--80 cm soil layer but also from below the 80 cm soil layer. In contrast, secondary forests consumed soil water mainly from upper layer of the soil （0--80 cm） and used precipitation to keep balance between forests consumption and soil water. The year round change of soil moisture can be divided into soil moisture decreasing stage, slowly recovering stage and dramatically fluctuating stage. During soil decreasing stage from November and January, the soil moisture decreases at similar rate under all three forests （ from 0. 82---0. 86 mm/d）. During February and May, which is the stage of soil moisture slowly recovering, the daily recovering pace in Robinia pseudoacacia plantations is 0.90mm/d, Pinus tabulaeformis plantation 0. 53mm/d, secondary forest 0. 79mm/d. During May and October, which is the stage of soil moisture dramatically fluctuating, the recorded range of soil moisture in Robinia pseudoacacia plantations is 95. 71mm, Pinus tabulaeformis plantation 179. lmm, secondary forest 72. 03mm. Land was prepared as narrow terrace before Pinus tabulaeformis plantation, which could increase infiltration and soil moisture during rainfall, resulting much higher soil moisture range during the dramatically fluctuating stage. Soil profile of 0--150cm could be divided into dramatic changing layer, weak changing layer and relatively stable layer according to coefficient of variation of soil moisture. The dramatic changing layer was 0--40 cm to Robinia pseudoacacia plantation, while 0--30 cm soil to both secondary forest and pine artificial forest. The coefficient of variation soil moisture in secondary forest was less than 0.01 in 60--150 cm soil profile, which was the relatively stable layer and obviously higher than Robinia pseudoacacia plantations and Pinus tabulaeformis plantation. The coefficient of variation of 80--150 cm soil moisture under the secondary forest was less than 0.05, which means soil moisture changes little below 80cm and thus the secondary forest seldom used the soil water below 80cm. This research suggests that more ＂secondary forest should be promoted instead of plantation to avoid excessive consumption of soil water and prevent ＂dry layer＂ formation in soil profile on the Loess Plateau.