Effects of different groundwater depths on the distribution characteristics of soil-Tamarix water contents and salinity under saline mineralization conditions

- The interaction effects of water salt in groundwater-soil-plant system is proposed.
- Groundwater level significantly affects the migration of water salt in plant-soil.
- The increase of groundwater level is a primary factor to reduce soil water.
- 1.2 m groundwater depth is the turning level for the salt of plant-soil columns.

The objective of this study was to explore the correlations between Tamarix and soil water salinity parameters and groundwater depth under saline mineralization conditions and to reveal the transport rules and exchange effects of salinity in soil-Tamarix systems with different groundwater depths. Soil columns planted with Tamarix chinensis (T.-soil column) grown in the Yellow River Delta were chosen for modeling. Six different groundwater depths were simulated in the models. We measured and analyzed the water and salinity contents of different soil profiles with different groundwater levels and Na+ contents in new T. chinensis branches and leaves. The results showed the following: (1) under shallow and medium groundwater levels, with increasing soil depth, the differences in the water contents between the T.-soil column and the control soil layer gradually decreased. Under deep groundwater levels, the differences increased and then decreased. Planting T. chinensis decreased the soil water content, and higher groundwater levels corresponded with a greater decrease in the soil water content of the T.-soil column. With increasing groundwater depth, the depth at which the water content of the T.-soil column suddenly changed increased. When the groundwater levels were 0.6 m, 0.9 m, 1.2 m, 1.5 m, and 1.8 m, the critical depths at which the water contents suddenly changed in the T.-soil columns and control were 0.3 m, 0.5 m, 0.7 m, 1.0 m, and 1.3 m, respectively. (2) Although planting T. chinensis clearly decreased soil salinity, the amplitude of the decrease increased and then decreased with increasing groundwater depth. The maximum decrease was reached at a groundwater level of 1.2 m. Importantly, the groundwater level of 1.2 m was the critical level for increasing the capillary water in the T.-soil column and was the depth at which the water and salinity significantly changed (the water and salinity turning depths). When the groundwater depths were 0.6, 0.9, 1.2, 1.5 and 1.8 m, the water turning depths in the T.-soil columns and control were 0.3, 0.5, 0.7, 0.7 and 0.7 m, respectively. Under medium (0.9 m and 1.2 m) and deep (1.5 m and 1.8 m) groundwater depths, the minimum soil salinities in the T.-soil column occurred at depths of 0.5 m, 0.7 m, 0.7 m, and 1.0 m, respectively. (3) With increasing groundwater depth, the water contents of the T.-soil column, T. chinensis leaves, and new branches gradually decreased. The salinity of the T. chinensis leaves decreased and then increased, and the salinities of the T.-soil column and new branches increased and then decreased. Under different groundwater depths, the water contents of the T. chinensis leaves were significantly higher than the water contents of the branches, and the salinity contents of the leaves were significantly lower than the salinity contents of the branches. With increasing groundwater depth, the water transported from the branches to leaves of T. chinensis significantly increased, and the ratio of Na+ that migrated decreased and then increased.