Responses Of Soil Physical And Chemical Properties And Plant Community To Water Level Change In The Water Level Fluctuation Zone Of Canyon Landform Area Of The Three Gorges Reservoir

Based on the monitoring data of soil physical and chemical properties and plant community before experiencing water level fluctuating(2008)and after experiencing many times of water level fluctuating(2009,2012,2015)in the water level fluctuation zone(WLFZ)with the altitude of 155-175 m,located at Wushan and Zigui section of the Three Gorges Reservoir(TGR),the temporal and spatial dynamics of soil properties and species composition and structure of plant community under the long-term influence of water level fluctuating were revealed.Based on the investigation and analysis of the plant community after experiencing 7 times of water level fluctuating(2015),the niche characteristics and interspecific association of the dominant plants were studied.And then the Canonical Corresponding Analysis(CCA)was used to quantitatively analyze the relationships between the quantitative characteristics of plant community and the soil physical and chemical properties,the hydrological regime characteristics of the TGR.The aim was to provide the scientific basis for ecological restoration and ecological prevention and control in the WLFZ of canyon landform area of the TGR.The results were as follows.(1)The soil density in Wushan plot increased with the increasing times of water level fluctuating,and the total porosity,capillary porosity,non-capillary porosity,maximum water capacity,capillary water capacity and field water capacity decreased.While the soil density,capillary porosity,capillary water capacity and field water capacity in Zigui plot decreased,and the total porosity,non-capillary porosity and maximum water capacity increased.Soil physical indicators in different soil layers showed Wushan > Zigui before experiencing water level fluctuating(2008),but soil physical indicators exhibited Zigui > Wushan in a certain year after experiencing 1 time(2009),4 times(2012)and 7 times(2015)water level fluctuating,except for the capillary porosity,capillary water capacity and field water capacity of 10~30cm.Differences of soil physical indicators in different soil layers and different elevation zones were reduced with the increasing times of water level fluctuating.(2)The contents of total nitrogen,available nitrogen,available potassium and organic matter in Wushan plot and Zigui plot gradually decreased with increasing times of experiencing water level fluctuating,and the contents of total phosphorus,total potassium and available phosphorus,and pH value exhibited the opposite trend.The contents of total nitrogen,total potassium and available phosphorus in different soil layers displayed Wushan > Zigui before experiencing water level fluctuating and after 1 time of water level fluctuating,however,they transformed into Zigui > Wushan after experiencing 4 times and 7 times of water level fluctuating.Differences of soil chemical indicators in different soil layers and different elevation zones gradually reduced with increasing times of experiencing water level fluctuating.Compared with the nutrient classification standard of the national second soil census,the contents of total phosphorus and available phosphorus were extremely high grade in Zigui plot and the content of available phosphorus was high grade in Wushan plot after experiencing 7 times of water level fluctuating,and other indicators were in the middle or below grade.Phosphorus was the most nutritious element that had increased during the process of experiencing water level fluctuating.(3)The content of cadmium increased with increasing times of water level fluctuating,the contents of copper,zinc,chromium firstly increased and then decreased,and the content of plumbum in different plots have changed.Soil heavy metals in different soil layers showed Wushan > Zigui before experiencing water level fluctuating,but the content of cadmium after experiencing 1 times,4 times,7 times of water level fluctuating and the contents of zinc and chromium after experiencing 4 times,7 times of water level fluctuating displayed Zigui > Wushan.The contents of soil heavy metals were not significant differences in different soil layer and different elevation zones with increasing times of water level fluctuating.Results of geocumulative index showed that cadmium,copper and zinc were unpolluted grade before experiencing water level fluctuating,but the content of cadmium was strong-extremely polluted grade and the contents of copper and zinc were moderate-polluted grade after experiencing 7 times of water level fluctuating.The contents of chromium and plumbum were always in an unpolluted state before and after experiencing water level fluctuating.The potential ecological risk of cadmium was minor grade before experiencing water level fluctuating,while it was extremely high grade after experiencing 4 times and 7 times of water level fluctuating.The potential ecological risks of other soil heavy metals were always minor grade before and after experiencing water level fluctuating.The potential ecological risk of cadmium was the highest in the WLFZ.(4)At altitude 155-172 m,146 kinds of native plants belonging to 56 families were recorded before experiencing water level fluctuating.After experiencing 1 time,4 times,7 times of water level fluctuating,the native plants respectively reduced by 52.1%,67.8% and 74.0%,and the native plants that always existed in the WLFZ accounted for 18.5%(27 species).At the same time,there were 47 “newborn” species of 24 families,19 “newborn” species of 14 families and 11 “newborn” species of 9 families.At altitude 172-175 m 137 kinds of native plants belonging to 57 families were recorded before experiencing water level fluctuating(2008 and 2009).After experiencing 2 times(2012)and 5 times(2015)of water level fluctuating,the native plants respectively reduced by 71.5% and 78.8%,and the native plants that existed in the WLFZ were responsible for 12.4%(17 species).In the meantime,there were 12 “newborn” species of 11 families and 14 “newborn” species of 11 families.These “newborn” plants were constantly changing in the process of experiencing water level fluctuating.The dominant plants in the WLFZ were mainly the herbaceous plants of Gramineae and Compositae after experiencing water level fluctuating.The proportion of annual herbaceous and perennial herbaceous constantly increased with increasing times of experiencing water level fluctuating,while the proportion of shrub and arbor gradually decreased,and the species richness index,Shannon-Wiener diversity index and Pielou evenness index also gradually reduced.(5)Before and after experiencing water level fluctuating,the species composition of dominant plants was similar with increasing elevation,however,differences of their important values in different elevation zones were obvious.The proportion of annual herbaceous reduced with increasing elevation,while the proportion of shrub and arbor increased,but the proportion of perennial herbaceous along the elevation gradient was different in the meantime.The species richness index,Shannon-Wiener diversity index and Pielou evenness index increased with increasing altitude during the period.(6)After experiencing 7 times of water level fluctuating,the niche breadth of dominant plants in order of size were Eclipta prostrate > Digitaria chrysoblephara > Bidens polosa > Setaria viridis > Echinochloa crusgalli > Setaria gllauca > Xanthium sibiricum > Cynodon dactylon > Cyperus rotundus > Polygonum hydropiper.The larger niche proportional similarities were D.chrysoblephara vs.E.crusgalli,E.prostrata vs.X.sibiricum,B.pilosa vs.P.hydropiper,and the larger niche overlap values were B.pilosa vs.P.hydropiper,E.prostrata vs.X.sibiricum,C.dactylon vs.C.rotundus,and D.chrysoblephara vs.E.crusgalli.These results suggested plants of larger niche breadth had larger niche proportional similarity and niche overlap value.The proportion of niche proportional similarity and niche overlap value of more than 0.5 in the WLFZ were 51.1% and 66.7%,respectively,but they were 31.1% and 44.4% in the non-fluctuation zone.The niche breadth,niche proportional similarity and niche overlap value of most dominant plants in the WLFZ were greater than that of the same plant in the non-fluctuation zone,indicating that the niche differentiation between different dominant plants was lower,and the stability of plant community was worse.(7)After experiencing 7 times of water level fluctuating,the significant positive association was found among overall 18 main plants.Among 153 species pairs consisting of 18 main plants,78 species pairs showed a positive association and 75 species pairs showed a negative association in the chi-square test,while 74 species pairs showed a positive correlation and 79 species pairs showed a negative correlation in the Spearman rank correlation test.The species pair with Jaccard index of less than 0.2 accounted for 82.4% of the total species pairs,suggesting that most species pairs were not closely related.A larger proportion of negative association in the Chi-square test and the Spearman rank correlation test indicated the habitat was not stable and obvious competition existed among different plants.(8)Results of correlation analysis showed that the flood frequency and flood time had a significant effect on soil physical and chemical properties.Results of canonical corresponding analysis(CCA)showed that these environmental factors,including the flood frequency and time,the contents of phosphorus,cadmium,copper and zinc,the pH value,had a great influence on the important values of dominant plants,followed by the contents of nitrogen,potassium,organic matter,and the soil porosity and the water-holding capacity,the soil density and the flood depth of the impact were relatively small.The proportion of annual herbs was affected by the phosphorus content,and the proportion of perennial herbs was affected by the flood frequency and time and the pH value.The proportion of shrubs and arbors,and the species diversity indexes was affected by the contents of nitrogen,potassium,organic matter,the soil porosity and the water-holding capacity,but they mainly played a limiting role.