Effects of planting Tamarix chinensis on shallow soil water and salt content under different groundwater depths in the Yellow River Delta

Groundwater is the main limiting factor that affects the growth and spatial distribution of vegetation in the Yellow River Delta, China. We carried out an experiment to investigate the effects of a Tamarix group and control group (bare soil, 0–20 cm) on shallow soil moisture, salt and salt ion distribution at various groundwater depths under brackish water salinity, and we simulated six different phreatic water depths (0.3–1.8 m). The results show that deeper groundwater depths decreased the soil water content, and the soil water content of the control group declined dramatically when the depth exceeded 1.2 m. However, when the phreatic depth exceeded 1.2 m, Tamarix presence mitigated the reduction in soil water content. A turning point in the water depth was also observed, and Tamarix exhibited water retention effects. The soil salt content presented converse S type as the phreatic water depth increased; Tamarix presence mitigated the reduction in soil salt content better than that in the control group. T. chinensis exhibited its maximum salt reduction range at a groundwater depth of 1.2 m. In the soil of the Tamarix group, the cations consisted mostly of Na⁺ and Ca²⁺, while the anions consisted mostly of Cl⁻ and SO₄²⁻. The soil Cl⁻/SO₄²⁻ ratio tended to decline along with the rise in the phreatic water depth. T. chinensis plantings resulted in easier leaching of shallow saline soils, which remained in a stable desalting state when the phreatic water depth was shallow (0.3–1.2 m); the shallow layers of the saline soil were converted to chloride saline soil. When the phreatic water depth was >1.2 m, the shallow soil was in a balanced state, and the main salt in the shallow soil consisted of sulfate salt; the soil salt content decreased, and the soil environmental quality improved. Therefore, when seedlings of T. chinensis are planted in coastal saline-alkali soil under brackish water salinity, the local phreatic water level should not be <1.2 m, as T. chinensis can effectively improve saline-alkali soils.