The Heihe River is the second longest inland river in China. The eco-environmental problems of the oasis in the lower reaches of Heihe River have markedly deteriorated over the past several decades because of the comprehensive effects of a number of factors. These factors include the continuous decrease in the discharge water of the Heihe River, human and animal population growth, increase of socio-economic activities, and climate warming. Water deliveries have been conducted by the Heihe River Bureau since 2000 to address the existing eco-environmental problems. Several scientific and realistic issues concerning this significant artificial water delivery project include the response process and mechanism of the eco-environment involving vegetation restoration and reconstruction brought about by water deliveries in the lower reaches of Heihe River. The responses of soil water and salinity variation to water deliveries were presented based on the investigation of soil and plant plots in 2001 and 2010. This paper aimed to evaluate the variance in the soil moisture and soil-soluble salt content before and after water deliveries in the lower reaches of Heihe River. The relationship between soil moisture, as well as soil-soluble salt content, and vegetation growth variables was also determined.
The results indicated that the soil moisture content (SMC) increased at different depths after water deliveries in Heihe River. The rate of increase varied with the different types of communities and could be arranged in the following order: Populus euphratica＞Tamarix chinensis＞Herbal. Moreover, the soil-soluble salt content (SSC) increased after water deliveries at different depths because of the strong evaporation effects. The rate of increase could be arranged in following the order: Sophora alopecuroides＞T. chinensis＞Haloxylon ammodendron＞P. euphratica. In addition, it could be found in the conversion of soil salinity type from slightly saline soil dominated by HCO₃^- and Na⁺ to medium and intense saline soil mainly dominated by SO₄^2-, Cl^- and Ca²⁺. The average density and crown, with P. euphratica community as an example, increased with increasing SMC of less than 6%, in which these values ranged from 40 m² to 70 m² and 0.5 plants per 100 m² to 5 plants per 100 m² at an SMC of more than 6%, respectively. Similarly, the average density ranged from 0.5 plants per 100 m² to 5 plants per 100 m² with an SSC of more than 6%. However, the average crown did not decrease with an SSC of more than 6%. Therefore, we conclude that the "self thinning" process of the P. euphratica community, rather than the salt, caused stress to the plant growth.