毛乌素沙地沙丘土壤含水量特点——兼论老固定沙地上油蒿衰退原因

CHARACTERISTICS OF SOIL MOISTURE CONTENT ON SAND DUNES IN MU US SANDY GRASSLAND:WHY ARTEMISIA ORDOSICADECLINES ON OLD FIXED SAND DUNES

土壤表面干沙层在冬春少雨季节较厚 ,流动沙地的又明显地大于固定沙地的。流动沙地地表干沙层以下的土壤含水量一般大于同深度固定沙地的 ,特别是在根系密集分布的土层。冬季或早春土壤含水量比较低 ,雨季的明显较高 ,雨季结束后 ,土层中的水分在重力的作用下会向深层运动 ,导致根系层土壤含水量下降。在植物生长季 ,缺少降雨时 ,固定沙地的土壤含水量会因植物的蒸腾消耗而变得很低 (1%～ 2 % ) ,而流动沙地的不受或受此影响很小。对土层持水能力的分析表明 ,土壤结构影响降水的有效性。固定沙地相对粘重的表土层影响降雨对灌木根系层土壤水分的补充。大于 1mm的降雨对流动沙地土壤水分的补给就有效果 ,但小于 5 mm的降雨基本上会被固定沙地的表土层吸收 ,对根系层土壤水分的补给效果不大。一般的小雨就有利于根系分布较浅的草本植物的生长 ,而只有较大的降雨才能有效地补给灌木根系层的土壤水分。然而 ,在固定沙地大暴雨还会因发生地表径流而降低其补给土壤水分的有效性。由此可知 ,固定沙地土壤结构对根系分布较深的灌木和半灌木的生长不利 ,而有利于其庇护下的草本植物的生长 ,并促使固定沙地油蒿群落在封闭的条件下向气候顶极的本氏针茅草原的演替。

Soil moisture content of a drifting sand dune and a fixed sand dune in the Mu Us sandy grassland of Inner Mongolia was measured over a period of one year. The surficial dry soil layer was significantly thicker in the winter and spring than in the summer and autumn. This dry soil layer was significantly thicker during the winter and spring, and slightly thinner during the summer and autumn, on the drifting sand dune than on the fixed dune. Soil moisture content, generally being lower in the winter and spring than in the summer and autumn, was higher on the drifting sand dune than on the fixed sand dune. The difference of the soil moisture content between the drift sand dune and the fixed sand dune was less in the winter and spring than in the plant growing season. The fine texture of upper soil layer on the fixed sand dune appeared to reduce the replenishment of moisture to the soil of the shrub root layer. The measured results and calculation suggest that rains of more than 1 mm are able to effectively replenish soil moisture on drifting sand dunes, but rains less than 5mm usually permeate no more than 5 cm from the soil surface on fixed sand dunes. Heavy rains may result in water runoff on fixed sand dunes. In combination with the relatively high frequency of small rains in the area, the soil texture and moisture content of fixed sand dunes seem to favor herbs over semi shrubs. This may be a main reason leading to the succession from Artemisia ordosica community to climax steppe in less grazing condition.