Transport characteristics of salt ions in soil columns planted with Tamarix chinensis under different groundwater levels

The groundwater level is the main factor affecting the distribution of soil salinity and vegetation in the Yellow River Delta (YRD), China, but the response relationship between the spatial distribution of soil salt ions and the groundwater level in the soil-Tamarix chinensis system remains unclear. In order to investigate the patterns of soil salt ions responding to groundwater levels, in the ‘groundwater-soil-T. chinensis’ system. Soil columns planted with T. chinensis, a constructive species in the YRD, were taken as the study object, and six groundwater levels (0.3, 0.6, 0.9, 1.2, 1.5 and 1.8 m) were simulated under saline mineralization. The results demonstrated the following: As affected by groundwater, Na(+) and Cl(-) were the main ions in the T. chinensis-planted soil column, with a trend of decreasing first and then increasing by the increase of soil depth. However, the contents of K(+) and NO(3)(-) gradually decreased and CO(3)(2-)+HCO(3)(-) gradually increased. As affected by groundwater evaporation, all the salt ions except CO(3)(2-)+HCO(3)(-) exhibited different degrees of surface aggregation in the 0–20 cm layer. However, due to the impact of root uptake, the contents of the salt ions rapidly decreased in the root distribution layer (20–50 cm soil layer), which rendered a turning-point layer that was significantly lower than the surface soil layer; such decreases in ion contents showed a relatively large rate of variation. In the whole T. chinensis-planted soil column, with increasing groundwater level, the contents of Na(+), Cl(-), Ca(2+), Mg(2+), and NO(3)(-) all tended to first decrease, then increase and decrease again, but the content of CO(3)(2-)+HCO(3)(-) first decreased and then increased. Therefore, the 0.9 m groundwater level was the turning point at which the main salt ions underwent significant changes. The contents of Na(+), Cl(-), Ca(2+) and Mg(2+) in the T. chinensis planted soil column exhibited moderate variability (14.46%<CV<86.46%), with a relatively large degree of variability across the 20–50 cm root-concentrated distribution layer and the surface soil layer. However, the K(+) content exhibited greater variability (CV>111.36%) at most groundwater level except less than 0.9 m. Therefore, planting T. chinensis could effectively reduce the accumulation of salt ions in the 20–50 cm soil layer with a concentrated root distribution, suggesting that the planting depth of T. chinensis should be greater than 20 cm under saline mineralization. This study can provide references for the control of soil secondary salinization and the management of T. chinensis seedling cultivation under saline mineralization.