Effects Of Simulated Groundwater On Water And Salt Characteristics Of Soil-Tamarix Chinensis And Tamarisk Growth In The Yellow River Delta

Tamarix chinensis is a constructive species in the Yellow River Delta.Moisture and salinity are the main factors limiting its growth,result its degradation by varying degrees,and even promoting the formation of low-quality and low-efficiency forests.The groundwater depth and salinity are the main limiting factors of groundwater influencing the growth of plants based on the"source-bank"relationship of salt and water in groundwater and soil-plant each.The groundwater depth in the coastal area is relatively shallow,and the mineralization of groundwater is high,mainly salt salinity,due to the influence of precipitation,evaporation and other factors during year,the mineralization of groundwater also shows some changes by brackish water,saline water and salt water.However,it is unclear that the water-salt migration processes in different media of groundwater-soil-plants and how they affect of tamarisk growth under different groundwater depths and salinity.Therefore,it is difficult to answer the production practice problems that need to be solved,such as serious degradation of Tamarix in the saline-alkaline land on the muddy coast and low survival rate of afforestation.In this study,the key issues affecting the growth and distribution of Tamarix chinensis on the shale-habitat habitat in the Yellow River Delta were focused,taking soil columns planted with T.chinensis,six groundwater depths（0.3,0.6,0.9,1.2,1.5 and 1.8 m）were simulated under four mineralization by fresh water（control experiment）,brackish water,saline water and salt water respectively.In order to explore the spatio-temporal distribution characteristics of water and salt in different soil profiles and Tamarix chinensis tissues under different groundwater depths,clarify the turning-point groundwater depth and the abrupt-change soil layer where water and salt underwent significant changes in the"groundwater-soil-T.chinensis"system,determine the response of water and salt migration in groundwater-soil-plant under different mineralization,and its effect on tamarisk growth,find out the main parameters of water and salt in the groundwater-soil-tamarisk system that determine the growth of Tamarix chinensis.The results can provide scientific data and technical support for the prevention of secondary salinization of soil under shallow groundwater conditions and the water and the renewal and rejuvenation of Tamarix community in coastal areas.The main results are as follows.（1）Spatio-temporal characteristics of soil water and salt in soil-Tamarix chinensis under different groundwater depthsAt different groundwater levels under salt mineralization,with increasing soil depth,the transport of soil water,salt and salt ions varied greatly.With an increasing soil depth,when the groundwater depth was shallow with 0.3 m and 0.6m,the relative soil water content（RWC）of soil columns was first decreased and then increased,and it decreased at the groundwater depth of medium level（0.9m and 1.2m）and deep level（1.5m and 1.8m）.While soil salt content was decreased by soil depth increasing at the groundwater depth of shallow level and medium level,and it first decreased and then increased at deep groundwater depth.With the increase of soil depth,Na⁺and Cl^-decreased first and then increased,while K⁺and NO₃^-gradually decreased.Ca²⁺,Mg²⁺,and SO₄^2-decreased first and then increased at deep levels.There is a negative correlation with soil depth;CO₃^2-+HCO₃^-gradually increases.The medium groundwater level is a significant change of water and salt in the soil.1.2m is the highest groundwater level that the groundwater can rise and keep the topsoil wet under planting conditions,and the turning point of soil water and salt content occurred at level of groundwater 1.2 m,under which soil salinity and soil solution absolute concentration of soil profiles were both the highest;while the groundwater level of 0.9 m was the turning-point groundwater level at which soil salt ions showed significant changes in the T.chinensis-planted soil column.Under salt mineralization,the salt ions in the T.chinensis-planted soil column were closely related to the groundwater level.The topsoil（0-10cm）salt and salt ions were significantly more affected by the groundwater depth than other soil layers.As affected by groundwater evaporation,salt content and all the salt ions except CO₃^2-+HCO₃^-showed patterns of surface aggregation to different degrees.However,due to the impact of root uptake,the contents of salt ions rapidly decreased in the root distribution layer（20–50cm soil layer）,which rendered a turning-point layer that had significantly lower salt contents than those in the surface soil layer;Planting of T.chinensis could effectively reduce the accumulation of salt ions in the 20–50cm soil layer with a concentrated root distribution.This result suggested that the planting depth of T.chinensis should be greater than 20cm under saline mineralization.At the groundwater depth of shallow and medium,the fluctuations of RWC,S_C,Na⁺,Cl^-in topsoil were significantly greater than other soil layers,while at deep water level,the movement of water and salt were due to the limitation of soil capillary action,the fluctuation of topsoil layer was small,the root distribution layer or deeper soil layers fluctuated greatly.Affected by evaporation and root absorption,the rapid change of water and salt in topsoil occurs from April to September,and the soil water and salt parameters were relatively stable from September to April of next year.During two year study,the RWC of the topsoil fluctuates seasonally,but the final and initial water content had little change,however,at the conditions of no rainfall and irrigation,the topsoil salt and main ions had continued to accumulate.（2）Spatio-temporal characteristics of tamarisk water and salt in soil-Tamarix chinensis under different groundwater depthsUnder salt mineralization,groundwater depth had more influence to water content of branches than leaves,there was no significant correlation between leaves water content and groundwater depth,so there was no significant changes during the growth cycles,the difference of water content in branches was little,except the period of seedling stage（scale branches）,water content of this period was slightly higher than that of other periods.At all of the groundwater depths,the salt cations in the same tamarisk tissues were Na⁺>Ca²⁺>K⁺>Mg²⁺,and the anions were mainly Cl^-and SO₄^2-.With the increase of groundwater depth,the content of Na⁺and Cl^-decreased first and then increased.The changes of SO₄^2-in leaves were same to Ca²⁺and Mg²⁺,and the the highest content of branches and leaves at the midium groundwater depth.At all of the groundwater depths,the contents of Na⁺and Cl^-in leaves at strong growth period was more than the period of semi-dormant,and the content of Na⁺and Cl^-in leaves had the same trend except the period of seedling stage（scale branches）.（3）Distribution characteristics of water and salt in soil-Tamarix chinensis under different groundwater salinityAt the groundwater depth of 1.2m,the groundwater salinity significantly affected the parameters of water and salinity in soil profiles and tamarisk tissues.With the increase of soil depth,the content of RWC increased,while S_C decreased first and then increased.The effect of salinity on the topsoil layer and the shallow soil layer were greater than that of subsoil,the contents of salt decreased in the root distribution layer with all groundwater salinity,and salt was assembled significantly on the surface.With the increase of groundwater salinity,the content of soil water and salinity in the whole soil columns increased by fresh water>brackish water>saline water>salt water.For Tamarix chinensis,the effect of groundwater salinity on salt content of tamarisk branches and leaves was greater than that of water,and groundwater salinity had no significant effect on water content of tamarisk leaves and branches,the content of water and the main salt ion in leaves were both more than that of shoots.In the same tissues of tamarisk,the content of Na⁺and Cl^-were all showed by tend of fresh water>brackish water>saline water（salt water）,and no significant difference among them,and salinity had little effect on K⁺and Ca²⁺and Mg²⁺.（4）Response of Tamarix chinensis growth to groundwater depth and salinityAt different groundwater levels under saline mineralization,the annual fresh and dry above-ground biomass and root biomass of tamarisk were all increased first and then decreased,In two years growth,the fresh above-ground biomass and dry weight increased most at the groundwater depth of 1.2m,0.9m and 1.5m next,and 0.3m was the worst.With salt and water stress time lengthened,the above-ground biomass of tamarisk decreased in the following year.Therefore,at saline mineralization,it is suggested that the planted depth of burial be 0.9-1.5m.It is suggested that the groundwater depth of Tamarix chinensis should between 0.9m and 1.5m when considering above-ground biomass of tamarisk.At different groundwater salinity under the groundwater depth of 1.2m,the fresh and dry above-ground and underground（total root）biomass of tamarisk were all decreased with the increase of salinity,for the underground biomass,the salinity effects on the change ranges of taproot are greater than those of lateral and capillary roots.（5）Coupling relationship of water-salt parameters in groundwater-soil-Tamarix and the drive for tamarisk growthThe coupling of different water and salinity parameters in groundwater-soil-Tamarix and their effects on Tamarix chinensis biomass were different.At the groundwater depth from0.3m to 1.8m,the above-ground biomass of tamarisk was significantly negatively correlated with groundwater salinity（P<0.01）,and groundwater salinity significantly affected the above-ground biomass of tamarisk.However,water content of leaves and branches had no significant correlation with groundwater depth（P>0.05）,and significant positive correlation between C_S and S_C（P<0.01）,significant positive correlation between C_S and RWC（P<0.05）,and a significant negative correlation between RWC and groundwater depth（P<0.01）.The above-ground biomass of tamarisk was used to evaluate the growth of Tamarix chinensis.The stepwise regression equations for various water and salinity parameters of groundwater-soil-Tamarix chinensis,the analysis result showed that the above-ground biomass increased with C_S,groundwater salinity,and water content of tamarisk branches,and increased by groundwater depth gradually.The main factors determining the above-ground biomass of Tamarix chinensis were different under different groundwater depth.When the groundwater depth was shallow by 0.3m and 0.6m,S_C and groundwater salinity and depth,and the Na⁺content of tamarisk were determined the above-ground biomass of Tamarix together,and it had a positive effect with groundwater depth,but a negative effect with S_C and groundwater salinity.While the medium groundwater level（0.9m,1.2m）,it is most affected by groundwater salinity;and when the depth is（1.5m,1.8m）,the main fators affecting above-ground biomass of Tamarix were groundwater salinity and Cl^-content in leaves,However,Cl^-content of leaves had a significant positive correlation to soil water content and groundwater salinity,and a significant negative correlation to groundwater depth.Therefore,according to factors affecting above-ground biomass of tamarisk under different groundwater depths,and combined planting depth,different regulatory measures could be implemented to control the growth of tamarisk.