Date Published: April 12, 2019
Publisher: Public Library of Science
Author(s): Ximei Zhao, Jiangbao Xia, Weifeng Chen, Yinping Chen, Ying Fang, Fanzhu Qu, Jian Liu.
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 NO3- gradually decreased and CO32-+HCO3- gradually increased. As affected by groundwater evaporation, all the salt ions except CO32-+HCO3- 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-, Ca2+, Mg2+, and NO3- all tended to first decrease, then increase and decrease again, but the content of CO32-+HCO3- 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-, Ca2+ and Mg2+ in the T. chinensis planted soil column exhibited moderate variability (14.46%
Soil salinization is one of the most important land resources and environmental issues worldwide, and it urgently requires a solution [1–2]. With the increasing tension between population growth and natural resource management, the improvement and utilization of saline-alkali land resources have become the focus of research and attention in various countries around the world [3–5]. The Yellow River Delta (YRD) is one of the fastest land-forming estuarine deltas in China, as well as globally; and it is rich in natural resources and is an important reserve land resource . However, this region is experiencing high groundwater evaporation and a large shortage of fresh water resources. It has frequent seasonal droughts and a fragile ecological environment, and the severe soil salinization has become a bottleneck, restricting the sustainable development of agriculture and forestry in this region. Under the influences of regional natural and human factors, saline soils in different bioclimatic zones have different occurrence characteristics and evolutionary patterns . The salt composition and ion proportions of saline soils exhibit typical regional characteristics, and the salt accumulation and desalination processes are significantly different . Particularly in the climate zone of arid deserts, salt-containing parent rocks and parent materials, active surface water, and groundwater recharge are the forces driving the formation of saline soils . However, in the YRD, the groundwater depth is generally low due to seawater intrusion and sea level rise , and the shallow groundwater is the most sensitive factor and main source of water for the terrestrial saline-alkali vegetation during its key growth period along the muddy coast in this region [10–12]. The level and salinity of the groundwater control the contents and distributions of soil salts [13–15], which in turn affect the growth and development, distribution pattern, and community succession of the dominant vegetation in the YRD . Furthermore, the vegetation growth and distribution are the main factors determining the recharge of and dynamic variations in groundwater [15,17]. Therefore, this study considered the water-salt coupling effect in the soil-plant system caused by groundwater variations to examine the distribution patterns of the main salt ions in the soil-plant system from the perspective of the groundwater level, which is of great scientific significance in terms of the effective control of soil secondary salinization, the efficient utilization of groundwater resources, and the cultivation and management of saline-alkali plants.
Under different groundwater levels, the variation trends of soil salt ions were quite different with increasing soil depth, and some ions showed abrupt variation points. With an increasing soil depth, the Na+ and Cl- contents first decreased and then increased, exhibiting a pronounced surface aggregation pattern, and K+ and NO3- gradually declined, with a drastic variation in the surface soil layer and a small variation in the bottom soil layer. The Ca2+, Mg2+, and SO42- contents were negatively correlated with soil depth except the groundwater level of 1.5 m and 1.8 m. The CO32-+HCO3- content gradually increased.