Enrichment Effects Of Soil Nutrients And Salts Of Tamarix Chinensis Shrubs At Different Groundwater Depths In Yellow River Delta

Groundwater depth directly affects soil moisture,salinity and nutrient dynamics,which are closely related to vegetation growth,and is the dominant factor in determining vegetation growth,distribution,population succession,and species diversity.In order to investigate the distribution characteristics of Tamarix chinensis shrubs and its soil nutrient and salt enrichment effects under different groundwater depths in the muddy coastal zone of Bohai Bay.This study evaluated T.chinensis shrubs at five groundwater depths（GW1,GW2,GW3,GW4,and GW5,with average groundwater depths of 0.54 m,0.83 m,1.18 m,1.62 m,and 2.04 m,respectively）in a muddy area of Beihai new district,Binzhou city,Shandong Province,in the Yellow River Delta.Growth indexes,such as forest density and plant height,coverage and diversity were measured and analyzed in plots with T.chinensis shrubs.Environmental factors,such as groundwater depth,soil water and salt conditions,and soil nutrients were also measured.Through redundancy analysis and structural equation modelling,key factors influencing the growth distribution and diversity characteristics of T.chinensis shrubs were explained.The differences in enrichment rates of soil nutrients and salt by T.chinensis shrubs growth under different groundwater depths were explored.The response relationship between groundwater depth,vegetation quantity characteristics,and soil physicochemical properties was quantitatively described.The results indicate that:（1）The number of plant species at different groundwater depths was GW4>GW3>GW5>GW2>GW1.The Simpson index,Shannon-Weiner index,Pielou index and Margalef index of T.chinensis shrubs at GW4 depth are higher than those at other depths,with the highest plant diversity and more stable species structure.T.chinensis populations were all aggregated distribution,and the aggregation intensity increased and then decreased with increasing groundwater depth.The height,canopy,and coverage of the T.chinensis shrubs all increased and then decreased with increasing groundwater depth,reaching their maximum at GW4,and the forest density and aboveground biomass reached a maximum at GW3.The basal diameter of the T.chinensis shrubs decreased gradually with the groundwater depth.（2）The soil water content of T.chinensis shrubs at different groundwater depths differed significantly（P<0.05）and gradually decreased with increasing grounwater depth.The soil storage rainfall and effective storage capacity increased gradually with increasing groundwater depth.Soil bulk density ranged from 1.21 to 1.35 g/cm³,with the maximum at GW5.Compared to GW5,GW1,GW2,GW3 and GW4 decreased by 5.17%,10.40%,9.17%and 1.82%respectively.Soil capillary water storage,non-capillary water storage,total water storage,capillary porosity,non-capillary porosity,total porosity and porosity ratio of T.chinensis shrubs at different groundwater depths increased and then decreased with increasing groundwater depth,reaching a maximum at GW3.（3）The soil nutrient content of T.chinensis shrubs at different groundwater depths differed significantly（P<0.05）,with soil nutrient content increasing first and then decreasing with increasing groundwater depth,showed that GW4>GW5>GW3>GW2>GW1.Compared to GW1,GW2,GW3,GW4 and GW5 were found to increase by 47.10%,70.85%,121.43%and87.64%,respectively.In terms of horizontal distribution,the overall soil nutrient content of the0-20 cm soil layer showed that the roots>canopy center>canopy brim>interspaces,and showed"fertile island"effect.In terms of vertical distribution,the nutrient content of the soil at different positions from the T.chinensis at different groundwater depths showed a maximum at the surface layer（0-5 cm）,showed the phenomenon of nutrient surface aggregation.The enrichment ratios of soil nutrients at different groundwater depths were mostly greater than 1,indicated that the T.chinensis shrubs were enriching soil nutrients.The enrichment ratios of soil nutrients at different positions from the T.chinensis were generally root>canopy center>canopy brim.Soil nutrient enrichment rates at different groundwater depths were GW4>GW3>GW5>GW1>GW2.（4）The soil salinity content of T.chinensis shrubs at different groundwater depths differed significantly（P<0.05）,with the overall soil salinity content showing GW1>GW2>GW3>GW4>GW5.In the horizontal distribution,the"salt valley"or"salt island"effect was observed for soil salt ions at different groundwater depths in the 0-20 cm.It was mainly reflected in the enrichment of soil K⁺,Ca²⁺,Mg²⁺and SO₄^2-by T.chinensis shrubs,which formed"salt islands"under the canopy.It had the effect of migration,lowering and detoxification of soil Na⁺,Cl^-and salt content,forming"salt valley"under the canopy,while the change of HCO₃^-was not significant（P>0.05）.In terms of vertical distribution,the soil salt content at different positions from the T.chinensis shrubs at different groundwater depths showed a maximum at the surface layer（0-5 cm）,and decreased to different degrees with increasing soil depth,showing the phenomenon of salt aggregation.Soil Na⁺and Cl^-accounted for 52.77%-67.48%of the total salt ion content and were the key ions in the formation of the"salt valley"effect.The overall effect of T.chinensis shrubs on soil Na⁺,Cl^-and soil salt content transport was GW3>GW4>GW5>GW2>GW1.（5）From the redundancy analysis and structural equation modeling,it is clear that soil salt content,groundwater depth and soil available phosphorus are the main factors affecting plant diversity and the distribution of T.chinensis shrubs.Soil salinity directly affects the growth trait,spatial distribution pattern and diversity of T.chinensis shrubs,and indirectly affects plant diversity by affecting the growth of T.chinensis shrubs with a total effect of-0.92,-0.49,and-0.17,respectively.Soil nutrient content directly affects the growth of directly and indirectly affects plant diversity by affecting the growth of T.chinensis shrubs.The direct effect of groundwater depth on the growth and spatial distribution pattern of T.chinensis shrubs was-0.65 and-0.91,respectively,and the direct effect on plant diversity was not significant（P>0.05）,but mainly affected plant diversity indirectly by affecting soil salinity.In summary,soil salt content,groundwater depth and soil available phosphorus are the main factors affecting plant diversity,growth,and distribution of T.chinensis shrubs.The groundwater depth not only directly affects the growth and spatial distribution pattern of T.chinensis shrubs,but also indirectly affects it by influencing soil salinity and nutrients.The total effect of groundwater depth on the growth,distribution pattern and plant diversity of T.chinensis shrubs was 0.50,-0.50 and-0.05,respectively.The groundwater depth suitable for the growth of T.chinensis shrubs in this study was 1.18-1.62 m.Within this depth range the"fertile island"-"salt valley"effect of T.chinensis shrubs was strong,the growth condition of T.chinensis shrubs and associated plant diversity was high and the community was stable.