Differences And Their Influencing Factors In Soil And Vegetation Characteristics Between Water-level-fluctuation Zone And Riparian Zone In The Three Gorges Reservoir

Globally,there are more than 70,000 large dams to satisfy the increasing demand for hydroelectricity,flood mitigation and reliable water supplies.The cumulative effect of the dams on habitats and vegetation is becoming increasingly prominent.The Three Gorges Reservoir?TGR?reached the normal water level of175 m in 2010 with a flood control capacity of 22.15×10¹00 m³,which resulted in the formation of a water-level-fluctuation zone?WLFZ?with an area of 300.0km².The impoundment of the Three Gorges Dam like the most dams in the world also significantly altered the hydrological regime,physicochemical properties of sediments and soils,and the plant community structure in the WLFZ.Due to various soil types,high habitat heterogeneity,complex sediment characteristics,and great human disturbance,more data are needed to further understand the spatial variability of soils and sediments.However,results from many studies presented poor comparability due to the different testing methods,sampling sites in only in the WLFZ soil or in the sediment.The soil habitat,vegetation characteristicsin the WLFZ and the riparian zone were investigated from 2009 to 2017.The differences in nitrogen,phosphorus and metals fractions and their influencing factors in the sediment,the WLFZ soil and the upland soil were studied in situ experiments.The results were as follows:?1?Periodic flooding-drying may alter the physical and chemical properties of soil in the WLFZ and the riparian zone.From 2009 to 2017,the average of soil pH at the altitude of 150 m changed little.The contents of organic matter,total nitrogen,alkali hydrolyzed nitrogen and total phosphorus decreased,while the contents of available phosphorus increased.At altitude of 170 m,soil pH,the contents of organic matter,total nitrogen,alkaline hydrolyzable nitrogen decreased,but available phosphorus content increased.The soil pH in the WLFZ was lower than those in the riparian zone.Soil density,the concentration of available phosphorus and available potassium in the WLFZ were higher than those in the riparian zone.?2?Periodic flooding-drying may alter the plant community composition,the life form and the dominant species in the WLFZ and the riparian zone.There were 146 species of vascular plants,belonging to 113 genera of 38 families in the WLFZ.The plant families with the largest number of species were Compositae?17.12%?,Gramineae?16.43%?and Leguminosae?6.85%?respectively.The life forms present at the highest percentage were annual herbs?53.42%?and perennial herbs?38.36%?respectively.The dominant species in the WLFZ were Xanthium sibiricum,Cyperus rotundus and Cynodon dactylon.In the WLFZ,the key environmental variables affecting the important value of plant species were the elevation and the available nitrogen.There were 171 species of vascular plants,belonging to 155 genera of 69families in the riparian zone.The plant families with the largest number of species were Gramineae?11.11%?,Compositae?9.36%?and Rosaceae?5.26%?.The life forms have perennial herbs?42.69%?and annual herbs?25.73%?.The dominant species were Pogonatherum paniceum,Pterocarya stenoptera,Swida paucinervis and Melia azedarach.In the riparian zone,the main environmental variables affecting the importance value of species were elevation and soil organic matter.?3?The soil nitrogen forms and their contents changed during the period of flooding-drying-flooding.Compared with the non-flooded soil,the contents of total nitrogen?N?and total hydrolysable N in the flooded soil increased,while the contents of acid-insoluble N,ammonium N and amino acid N changed little,while the content of amino acid N and amino sugar N decreased.The contents of total nitrogen and total hydrolysable N increased with an increase of submergence depth.However,the contents of amino sugar N,ammonium N and nitrate N decreased.The nitrate reductase activity was negatively correlated with the content of total nitrogen and total hydrolysable N.The nitrite reductase activity was positively correlated with the content of total hydrolysable N,amino acid nitrogen and ammonia N.The hydroxylamine reductase activity was positively correlated with the contents of nitrate nitrogen.?4?During the period of flooding-drying-flooding,the concentration of phosphorus fractions in the WLFZ soil varied.Compared with the non-flooded soil,the contents of TP and Ca₈-P in the flooded soil decreased.The percentages of Ca₈-P to total phosphorus decreased by 16.05%,but the percentages of Ca₂-P and Fe-P in total phosphorus increased by 3.91%and 8.50%.The rank order of P species concentrations was Fe-P>Al-P>Ca₂-P>Ca₈-P.?5?The concentration of both TP and NaOH-P in the sediment of the mainstem exhibited an increasing trend from the end of backwater area to the TGD,and HCl-P declined.The average concentration of P species in the sediment was higher than those found for the upland soil and the WLFZ soil.HCl-P was the main P fraction in most investigated samples,accounted for more than 50.00%of the total.All phosphorus species in the upland soil demonstrated greater spatial heterogeneity than those in the WLFZ soil and the sediment.?6?The average concentration of Cd,Cu,Cr,Zn,As and Pb in the sediment were 1.06±0.29,60.87±24.24,106.18±11.04,157.95±28.23,13.21±3.30 and 68.19±25.34 mg·kg^-1 respectively.The concentration of total Zn in the sediment exhibited an increasing trend from the end of backwater area to the TGD,and the oxidizable fraction concentration of Cd,Cu,Zn declined.The oxidizable fraction concentration of Cr in the WLFZ soil and the upland soil also declined.The average concentration of Cd,Cu,Cr and Zn in the sediment was higher than those in the WLFZ soil and the upland soil.The potential risk coefficient for Cd was greater than that for other metals.This study clarified the differences in soil habitat,vegetation characteristics and their driving factors in the WLFZ and the riparian zone,revealed the longitudinal variation?from the end of the backwater area to the Three Gorges Dam?in phosphorus fractions and heavy metal fractions.The results are beneficial not only for determining the distribution of the nitrogen,phosphorus and metals being moved downstream but also for assessing the potential release risk in the sediment and the WLFZ soil.It may benefit the further understanding of the ecological evolution trends of large reservoirs.