Effect Of The Pioneer Plant And Its Community On Soil Aggregate Stability In The Riparian Zone Of The Three Gorges Reservoir

According to the seasonal operation of the Three Gorges Reservoir(TGR),the vegetation-root-soil relationships in the riparian zone has not been fully elucidated for the periodic water-level fluctuation and the strong heterogeneity of flooding stress.The mechanism of these relationships along hydrologic stress gradient is also still unclear.Thus,based on the TGR riparian zone,this study attempted to adopt both methods of in situ observations and mesocosm experiment.Selecting a typical tributary riparian zone(Baijixi)as the research object,the statistical modelling was used to establish the quantitative relations between soil aggregate stability and external factors(soil physicochemical properties,plant community characteristics and root characteristics).Furthermore,the influencing mechanism of pioneer plant(Cynodon dactylon(L.)Pers.)community diversity and functional characters on soil stability in the riparian zone was elucidated.Thereafter,based on the results of in-situ survey,the mesocosm experiment of C.dactylon and its community were conducted under different diversity gradient,and the influence on the stability of soil aggregates was analyzed from root traits and rhizosphere microorganisms.The result shows that:First,the dominant plants and the stucture of the plant community had distinct zonation in the riparian zone.Through the investigation from Baijiaxi riparian zone,plant species can be assigned to three functional groups,including annual herb,biennial herb and perennial herb.C.dactylon,a perennial clonal plant,was the pioneer species.Plant communities displayed an increasing trend of plant diversity(Shannon-Wiener index)and richness with the increase of water-level elevations.Moreover,the distribution of dominant species correlated with elevation gradient.The grass C.dactylon and Cyperus rotundus L.were the dominant species at the lower elevations from 150 m to 160 m.In addition,C.dactylon and Xanthium strumarium L.were the dominant species at the higher elevations from 160 m to 175 m.However,the peak values of aboveground biomass and belowground biomass appeared from the elevation of 160 m to 165 m.Root traits represented homogeneity along the vertical gradient of water level elevation.Second,given the influence of periodic water level fluctuation,the stability of soil aggregates in the riparian zone represented apparent zonation.Through the investigation on the soil aggregates aggregate in different water-level elevations,the stability of soil aggregates was strongly affected by periodic water level fluctuation.The soil aggregates stability increased with the increase of water level elevation,but there were significant differences in the aggregate stability indexes,such as the Geometric mean diameter(GMD)and the fractal dimension(D)appeared more affected by the different intensity of flooding,compared with the mean weight diameter(MWD).Large aggregate(>2 mm)and its Total carbon content(TC)significantly affected the stability of soil aggregate.Root length density(diameter <1.0 mm),root relative volume density and root surface area density were the key root functional traits that significantly affected the stability of soil aggregates.Third,the hypotheses of plant community diversity effect on soil stability were confirmed to shift and coexist along a spectrum of environmental change.In the lower elevations(150 m?160 m),there was no significant effect of plant growth on the soil aggregates stability before and after the flooding.The complementary effects of different species influenced the stability of surface soil aggregates,and the main functional species were the major contributors to the stability of subsurface soil aggregates in this area.In the elevations from 160 m to 175 m,Plant growth had a significant effect on the stability of soil aggregates after the drainage,and the plant community promoted the stability of soil aggregates through the complementary effect of different species.Fourth,the structure equation models(SEM)of in-situ data verified the hydrologic driving mechanism of plant on the soil aggregate stability in the riparian zone.In the SEM,the direct effect on soil aggregate stability by water flooding was stripped.Waterlevel fluctuation affected the distribution of dominant species and plant community diversity.Through the root system and soil cementation,the increase of species diversity promoted the agglomeration of large aggregate particles(>2 mm),thus increasing the stability of soil aggregates.Fifth,the result of the mesocosm experiment further revealed that several dominant plants from the riparian zone promoted the stability of soil aggregates mainly through complementation effect.The result also showed that improving the soil aggregates stability of several dominant plants in the riparion zone to sort as follows:X.strumarium,C.dactylon,C.rotundus L.and Echinochloa crus-galli(L.)P.Beauv.Meanwhile,roots played an essential role in increasing the soil aggregate stability.Clonal plants with stoloniferous roots(C.dactylon)and plant with straight roots(X.strumarium)had strong coupling effect on the stability function of aggregates.Moreover,with the increase of species richness,plant communities affected the soil aggregates stability by increasing root biomass and root functional traits.Sixth,the analysis of the rhizosphere microbial community in the mesocosm experiment suggested that,the increase of the richness of C.dactylon communities promoted the stability of soil aggregates by affecting the diversity of soil fungal communities of rhizosphere.The structure of bacteria and fungi community in different plant communities showed high host specificity.However,Proteobacteria and Actinobacteria were the main dominant species of bacteria community,and Ascomycota was the main dominant species of fungi community.With the increase of plant species numbers,the richness and diversity of soil bacteria were basically in line with the trend of increasing firstly and then decreasing.In contrast,soil fungus richness and diversity were gradually increased.The study revealed that the soil bacteria and fungi communities were significantly stable with soil aggregates stability,and TC in the water-stable aggregates(>2 mm),root length density of Poaceae and root traits of Xanthium.Furthermore,fungal community diversity index significantly promoted the stability of soil aggregates.In summary,the pioneer plant C.dactylon can effectively improve the soil structure in the riparian zone in the TGR.Under strong flooding stress,C.dactylon promoted the soil aggregates stability by root functional traits.Whereas,C.dactylon improved the soil aggregates stability by niche complementary effect with other species under low water flooding stress and no flooding stress.The results helped to disentangle the impact of community structure and composition on the ecosystem function in the process of the vegetation succession.It provided strong theoretical support for the ongoing evaluation of soil structural stability and scientific management under the phytoremediation mode in the riparian zone.Furthermore,it is of considerable significance for soil erosion control and ecosystem stabilization in the TGR.