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 179.1mm, 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.