Fluctuation Zone Research Articles

Water level–driven agricultural nonpoint source pollution dominated the ammonia variation in China's second largest reservoir

Rainfall-runoff and water flooding are the driving mechanisms of agricultural nonpoint source pollution (ANPSP), but existing research has hardly focused on water level–driven ANPSP. Danjiangkou Reservoir was the second largest reservoir in China, and its water quality was dominated by ANPSP. This study explored the effect of water level on water quality of Danjiangkou Reservoir and aimed to provide basis for water quality management of large reservoirs. The effect of water level–driven ANPSP on the concentration of reservoir ammonia was studied employing the methods of factor decomposition and multiple regression on a extensive time series data of reservoir ammonia, water level, rainfall, fertilizer usage, and inflow river ammonia. The long-term trend revealed the reservoir ammonia peaked in 2011 and the inflow river ammonia peaked in 2012 (Han River) and 2013 (Dan River), which indicated the success of point source control in the past 15 years and the dominant role of ANPSP in the reservoir ammonia in recent years. With the long-term trend series, the multiple regression results showed that 56% of the variation of the reservoir ammonia concentration was due to the water level (standardized regression coefficient 0.422), fertilizer usage (standardized regression coefficient 0.522), and inflow river ammonia (standardized regression coefficient 0.219). However, the rainfall was insignificant. The predominance of water level and fertilizer usage in explanation of the reservoir ammonia variation indicated that water level–driven ANPSP was the primary factor influencing the reservoir ammonia. The effect of water level was primarily reflected in the long-term variation of ammonia concentration rather than the seasonal variation within the year. This study showed that when compared with rainfall–driven ANPSP, water level–driven ANPSP had a greater impact on the reservoir ammonia. Water quality protection should center on the management of the water level–fluctuation zone.

Soil column experiment and numerical simulation research of Pb migration process in the groundwater fluctuation zone based on HYDRUS

Groundwater level fluctuation alters hydrodynamic conditions and hydrogeochemical action, thus affecting the migration and transformation of pollutants. This study investigated the effect of groundwater level fluctuations on Pb migration processes. We selected two typical media, coarse sand and medium sand, for the soil column experiments and numerical simulations of the Pb migration process in the groundwater level fluctuation zone. The soil column experiments showed that when the contaminants were distributed at a 30–40 cm depth in the soil column, the Pb migrated into the water in the range of 20–45 cm in coarse sand, while the range of Pb migration in medium sand decreased to 30 cm–40 cm. We constructed a numerical model of Pb migration in the aeration zone–saturation zone coupled with water transport and solute transport by HYDRUS-1D. The model simulated the migration process of Pb in different amplitude and frequency of water level fluctuation and initial concentration of pollutants. The results showed that the coarse sand column and medium sand column showed relatively noticeable changes in Pb concentration in water at 30–40 cm and 30–35 cm, respectively. The migration range and concentration of Pb in water are proportional to the medium particle size and the frequency of water level fluctuation, and inversely proportional to the magnitude of water level fluctuation. The Pb concentration in the water increased or decreased as the initial pollutant concentration of the medium increased or decreased. Its variation is basically the same as the variation of the initial contaminant concentration. The research results can provide a scientific basis for the remediation of heavy metal pollution in groundwater.

Responses of leaf functional traits to different hydrological regimes and leaf economics spectrum in the water level fluctuation zone of Three Gorges Reservoir, China.

A unique riparian ecosystem has been created as a result of anti-seasonal flooding after reservoir operations, which notably influences the distribution patterns of plant communities and their functional characteristics in the riparian zone. Plant functional traits which reflect the physiological and ecological processes of plants in particular ecosystems are crucial for indicating the variations in the ecosystem structure and function. To better understand the adaptation strategies of plants to hydrological changes and provide a scientific basis for the selection of species in the re-vegetation of the newly formed ecosystems, 14 leaf functional traits and leaf economics spectrum (LES) of 19 dominant plants under different hydrological conditions were investigated in the water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR). The results showed that anti-seasonal flooding has significant effects on the leaf functional traits of plants (P < 0.05). The net photosynthetic rate of annual plants was significantly higher than that of perennial plants (P < 0.05), and there was a significant correlation between leaf phenotypic and photosynthetic traits (P < 0.05). Canonical correspondence analysis showed that soil water content and available phosphorus were the main factors affecting the leaf function of dominant species, indicating that hydrologic factors were still important environmental factors affecting leaf functional traits of dominant species in the WLFZ. And annuals from the WLFZ have characteristics of thick leaves, high photosynthetic rate, short lifespan, and high nutrient concentrations, which make them close to the fast investment-return end of LES. On the contrary, perennials are close to the slow investment-return end of LES. The high productivity investment of annuals is better than the high defense investment of perennials for adapting to the special habitats in the WLFZ. These results indicated that different functional plants in the WLFZ of the TGR under different hydrological regimes can adopt different strategies by weighing the associations and trade-offs between their economic traits.

Effects of Herbaceous Plants on Methylmercury Net Production and Release From Sediment After Flooding in the Water-Level Fluctuation Zone of the Jialing River.

To explore the effects of herbaceous plants on mercury (Hg) behaviors after flooding in the water-level fluctuation zone (WLFZ) of Jialing River, three typical local plants (D. pyramidalis, A. philoxeroides and C. dactylon) and their in-situ sediments were collected for flooding experiments to study the Hg dynamics of water and sediment in different treatments. The results showed that flooding increased sediment MeHg concentrations and flooded plants (especially for the D. pyramidalis) promoted this process. Similarly, the highest dissolved MeHg level and proportion of MeHg to total Hg (%MeHg) were observed in plant-water treatments in the presence of D. pyramidalis, indicating the potential for the methylation of Hg in the water body influenced by the decay process of herbaceous plants. These results suggest that the herbaceous plant D. pyramidalis contributes more to Hg methylation and release in the WLFZ of the Jialing River than other plants studied.

Drone-based investigation of natural restoration of vegetation in the water level fluctuation zone of cascade reservoirs in Jinsha River

The reservoir water level fluctuation zone (WLFZ) is a new and fragile ecosystem that is gaining attention with the construction of large and medium-sized hydropower plants. Compared to the natural riparian zone, it has a greater drop in water level, longer inundation time, more intense impact from alternating wet and dry conditions, and a wider impact on ecological security. The Jinsha River basin is located in the upper reaches of the Yangtze River in China, and several world-class large-scale hydropower projects with dam heights over 100 m have been built, forming a large area of reservoir WLFZ, however, due to the short time since their construction, there are few related studies. In this paper, fixed sample plots were set up in the typical WLFZs of each large reservoir in the Jinsha River basin. In response to the problem of the precipitous terrain and poor accessibility of the Jinsha River basin, a combination of small UAV surveys and field research in July 2020 was used to draw vegetation cover maps and extract topographic data for each site, and quantitatively analyse the community composition, dominant species types, area, coverage, spatial distribution patterns and environmental factors of tolerant vegetation using spatial superposition analysis, neural network models, landscape pattern indices and typical correlation analysis. The results showed that the original drought-tolerant vegetation in the arid river valley WLFZ has evolved into amphibious herbaceous vegetation, with trees and shrubs disappearing and species composition tending to be simpler. 44 species of plants, mainly in the Asteraceae and Gramineae families, were extant, 61% of which were also reported in the Three Gorges Reservoir WLFZ. The water level variation showed convergence in the natural screening process of suitable species in the WLFZ. Moreover, even in the dry valley WLFZs, flood stress showed a more significant filtering effect on vegetation species than drought stress. The vegetation in the WLFZ showed an obvious band-like aggregated distribution along the water level elevation gradient, and the vegetation coverage along the flooding gradient is as follows: upper part of the WLFZ >> middle part > lower part, and mainly concentrated in the gentle area with slope less than 35°. Flooding stress, drought stress and soil substrate deficiency were the main limiting factors for vegetation recovery in the WLFZ. The vegetation restoration of the WLFZ should be adapted to local conditions, and the dominant role of native species should be emphasized. At the early stage of the restoration of the WLFZ, native species should be selected for artificial planting to accelerate the formation of vegetation cover, and gradually advance downwards along the gradient of water level elevation, while for areas of the WLFZ with slopes greater than 35° and large topographic relief, biological engineering measures should be used to help plant establishment, and after a certain stable cover has been formed, natural restoration should be the main focus.

Characteristics and Dfferences of Dissolved Oxygen Stratification in Different Tributaries of Three Gorges Reservoir During Impoundment Period

Taking the Xiaojiang and Xiangxi Rivers, two typical tributaries of the Three Gorges Reservoir, as examples, this study analyzed and compared the hydrodynamic, thermal stratification, and temporal and spatial differences in dissolved oxygen (DO) and their responses to the water storage process in the two tributaries through field monitoring at different stages of the 2020 impoundment period. The results showed that:① at the initial stage of water storage, the DO in the surface layer of the Xiaojiang River was higher (7.00-13.00 mg·L-1) due to atmospheric reoxygenation and phytoplankton photosynthesis, and the oxycline appeared in the water depth of 3-5 m. A large area of anoxia (DO<2.00 mg·L-1) or even an anaerobic sublayer occurred in the water below 5 m. The DO in the Xiangxi River could be divided into three layers vertically:oxygen-rich surface water (8.00-12.00 mg·L-1), middle water (6.00-8.00 mg·L-1), and low-oxygen bottom water (4.00-6.00 mg·L-1). ② Thermal stratification provided a stable physical environment, whereas the upstream inflow and vegetation decomposition in the water-level fluctuation zone increased the content of organic matter, which likely increased the oxygen consumption which was conducive to the formation of an anaerobic bottom layer. In the Xiangxi River, the risk of hypoxia in the bottom water body was low because of the oxygen replenishment from the long-term downslope-bottom density current.③ Continuous monitoring also showed that the storage of the reservoir played a significant role in the replenishment of DO in tributaries, which effectively and rapidly improved the anaerobic phenomenon in the Xiaojiang River. In the Three Gorges Reservoir, it is feasible to ameliorate the water ecological problems such as anoxia and anaerobic conditions in the tributaries via reservoir operation. This study aids understanding of the characteristics and differences of DO stratification in different tributaries of the Three Gorges Reservoir, which can provide theoretical and technical support for reservoir ecological operation.

Transparent soil model test of a landslide with umbrella-shaped anchors and different slope angles in response to rapid drawdown

Umbrella-shaped anchors, as new stabilizing structures, have been increasingly adopted in engineering practice due to their simple structure, ease of installation, and high load capacity. Reservoir operation, especially during rapid drawdown, affects the long-term stability of reservoir bank slopes as this process affects the stress, seepage and deformation characteristics of the water-level fluctuation (WLF) zone. Thus, the characteristics of the deformation and failure mode of WLF zones reinforced by umbrella-shaped anchors under rapid drawdown conditions are important for the design and construction of anchoring measures. Five slope-anchor scale models based on the Yingpan landslide were established as transparent soil models considering different slope angles and drawdown rates. Displacement, velocity and macroscopic deformation data were obtained using particle image velocimetry (PIV) technology. The results indicated that the deformation process of both reinforced and unreinforced slopes under rapid drawdown conditions could be divided into four phases: initial, shallow sliding, multisliding and stable phases. In the anchor-reinforced models, loss of soil particles occurred when the water level dropped to reach the anchor heads, causing the anchor plates to loosen and slope deformation to accelerate, especially near the lower row of cables. The anchor heads failed first, and progressive slope sliding ensued. Under rapid drawdown conditions, soil liquefaction and subsequent loosening and sliding occurred, generating a far larger zone of accumulation than the zone of collapse. This indicates that soil liquefaction is another important factor of the link between rapid water level drawdown and slope instability causing local collapses. Combining the experimental results and 47 sets of field monitoring data, it could be concluded that the slope stability decreases with increasing slope angle of the WLF zone. This occurs because a higher water level is needed for deformation initiation in the WLF zone in the case of a higher slope angle.

Specific characteristics of the microbial community in the groundwater fluctuation zone.

Groundwater level fluctuation is a common natural phenomenon that causes alternate changes in oxygen, moisture, and biogeochemical processes in sediments. Microbes are sensitive to these environmental changes. Therefore, a specific microbial community is proposed to form in the groundwater fluctuation zone (GFZ). The vertical distributions of microbial abundance, diversity, and functional microbes and genes in sediment profiles were investigated, focusing on the GFZ, using high-throughput 16S rRNA gene sequencing, qPCR, and the Functional Annotation of Prokaryotic Taxa (FAPROTAX) approach. The relationships between chemical variables and microbial community structure were investigated by redundancy analysis (RDA). Results showed that the microbial abundance and microbial community richness and diversity were higher in the sediments of the GFZ. The nitrate reducers prefer to stay just below the groundwater level in the GFZ. The predominant microbes in the GFZ functioned as nitrifiers and Fe-oxidizers. The specific community in the GFZ is mainly related to NO3- and Fe(III) in the sediment. Consequently, the biochemical processes nitrification and Fe- and Mn-oxidation sequentially happen above the nitrate-reduction zone near the groundwater level in the GFZ. These results provide new knowledge in the biogeochemistry cycle of the GFZ and its disturbance on the vertical distribution and transport of biogenic elements and contaminants.

Photosynthetic acclimation of riparian plant Distylium chinense to heterogeneous habitats

Distylium chinense is an evergreen dominant shrub with strong adaptability under heterogeneous habitats, including natural riparian zone (NRZ), anti-seasonal water level fluctuation zone (ASWLFZ), and no water level fluctuation zone (NWLFZ) in the Three Gorges Reservoir Region (TGRR) in China. In order to understand the photosynthetic ecological adaptability and habitat adaptation strategies of D. chinense under heterogeneous habitats, its photosynthesis and chlorophyll fluorescence parameters were investigated. The results showed that photosynthetic characteristics were significantly different among heterogeneous habitats (p<0.05). The PSII potential activity and PSII maximum photochemical efficiency (Fv/Fm) of D. chinense in the ASWLFZ were the highest among heterogeneous habitats. Non-photochemical quenching coefficient (qN) in the NWLFZ was higher than that in the NRZ and ASWLFZ (p<0.05). Stepwise regression analysis showed that water use efficiency and stomatal conductance were correlated with specific leaf area (SLA), leaf area and leaf dry weight. Regression curve fitting and canonical correspondence analysis revealed that light intensity, soil water content, air humidity and temperature were the main factors affecting photosynthetic characteristics and leaf functional traits. Our results evidenced that D. chinense adapted to the high water content and low light intensity environments, especially in the valley waterfall habitats, by increasing stomatal conductance and net photosynthetic rate, reducing SLA and maintaining moderate qN and Fv/Fm with adopting a profligate water use pattern to improve photosynthetic productivity. Therefore, D. chinense has a wide photosynthetic acclimation to different environments, which provided baseline information for the conservation and ecological restoration in the newly formed hydro-fluctuation zone and other similar degenerative riparian ecosystem.

Role of the rhizosphere of a flooding-tolerant herb in promoting mercury methylation in water-level fluctuation zones

The water-level fluctuation zone (WLFZ) has been considered as a hotspot for mercury (Hg) methylation. Flooding-tolerant herbs are gradually acclimated to this water-land ecotone, tending to form substantial root systems for improving erosion resistance. Accompanying rhizosphere microzone plays crucial but unclear roles in methylmercury (MeHg) formation in the WLFZ. Thus, we conducted this study in the WLFZ of the Three Gorges Reservoir, to explore effects of the rhizosphere of a dominant flooding-tolerant herb (bermudagrass) on MeHg production. The elevated Hg and MeHg in rhizosphere soils suggest that the rhizosphere environment provides favorable conditions for Hg accumulation and methylation. The increased bioavailable Hg and microbial activity in the rhizosphere probably serve as important factors driving MeHg formation in the presence of bermudagrass. Simultaneously, the rhizosphere environments changed the richness, diversity, and distribution of hgcA-containing microorganisms. Here, a typical iron-reducing bacterium (Geobacteraceae) has been screened, however, the majority of hgcA genes detected in rhizosphere, near-, and non-rhizosphere soils of the WLFZ were unclassified. Collectively, these results provide new insights into the elevated MeHg production as related to microbial processes in the rhizosphere of perennial herbs in the WLFZ, with general implications for Hg cycling in other ecosystems with water-level fluctuations.

Migration Law of LNAPLs in the Groundwater Level Fluctuation Zone Affected by Freezing and Thawing

Freezing and thawing can cause dynamic fluctuations of the groundwater level, resulting in the migration and retention of LNAPLs. However, this process is difficult to observe visually, and a suitable simulation method for its quantitative calculation is lacking. In this study, a numerical simulation is established by coupling the HYDRUS-1D software and the TOUGH program to realize dynamic simulation of the entire process of soil temperature changes, water migration, water level fluctuation, and redistribution of LNAPLs during the freeze–thaw process. The results of the study show that the process of soil freezing and thawing causes water migration, which in turn causes groundwater level fluctuation, leading to the migration and redistribution of LNAPLs within the water level fluctuation zone. In this process, the soil particle size and porosity control the response degree and speed of the water level under freezing and thawing and the spatiotemporal distribution of LNAPLs by affecting the soil temperature, capillary force, and water migration. The migration ability of free LNAPLs is determined by their own density and viscosity; the retention of residual LNAPLs is affected by soil porosity and permeability as well as LNAPL viscosity. The results of this study can not only be used to develop a simulation method for the migration and retention mechanism of LNAPLs in cold regions but also serve as a scientific and theoretical basis for LNAPL pollution control in seasonal frozen soil regions.

Soil Organic Carbon and Its Controlling Factors in the Lakeside of West Mauri Lake along the Wetland Vegetation Types

There is a huge carbon pool in the lakeside, which is sensitive to environmental changes and can very easily be transformed into a carbon source as land from the lake is reclaimed. In this paper, West Mauri Lake was employed as a case study to examine soil organic carbon (SOC) and its controlling factors along the lakeside. Four transects of land use (i.e., vegetation) types along the landward lakeside were identified as the fluctuation zone, the beach zone, the mesozoic farmland rewetting zone and the xerophytic farmland rewetting zone. With the increase in soil depth, SOC in the lakeside decreased significantly (p &lt; 0.05). SOC had an obvious seasonal variation (p &lt; 0.001), ranking in order: winter (December) &gt; spring (February) &gt; summer (May). Among the aforementioned transects, SOC density differed significantly (p &lt; 0.05), showing a significant increasing trend. Pearson correlation indicated that most soil physiochemical factors showed a significant correlation with SOC (p &lt; 0.01), except total chromium, total copper, total zinc and total phosphorus. The relationship between SOC density and total nitrogen (N) has an obvious “S” curve, and total N accounts for 81% of the variation of SOC, suggesting that total N is the main controlling factor of SOC in the lakeside. The significant difference in SOC along the different vegetation (land use) types implied that land use affects the SOC in the lakeside. The long-term accumulation of N fertilizer after the man-made reclamation and aquaculture obviously controls SOC in the lakeside of West Mauri Lake.

An Experimental Study on the Migration of Pb in the Groundwater Table Fluctuation Zone

As a result of fluctuations in the shallow groundwater table, hydrodynamic conditions change alongside environmental conditions and hydrogeochemical processes to affect pollutant migration. The study aimed to investigate the migration, adsorption, and desorption characteristics of Pb on fine, medium, and coarse sand in the water table fluctuation zone by using several laboratory methods, including the kinetic aspects of Pb2+ adsorption/desorption and water table fluctuation experiments. The results showed that the adsorption and desorption curves fit the Elovich equation well at a correlation coefficient above 0.9. In the adsorption and desorption kinetic experiments for fine, medium, and coarse sand collected and from the floodplain, the maximum adsorption capacity of Pb2+ was 2367 mg·kg−1, 1848 mg·kg−1, and 1544 mg·kg−1, respectively. The maximum desorption capacity of Pb2+ was 29.18 mg·kg−1, 62.38 mg·kg−1, and 81.60 mg·kg−1, respectively. In environments with pH greater than 4, the adsorption capacity was proportional to the pH, but the desorption capacity decreased as the pH increased in water. As the water table varied, the lowest pH occurred in the polluted medium we set initially. When the distance between the pollutants and sample solution grew further, pH increased, and the Pb2+ concentration decreased in the sample solution. In the column experiment of water table fluctuations on coarse sand, Pb2+ migrated nearly 5 cm upward from the original pollutant and migrated less than 10 cm downward from that. In our experiments on medium and fine sand, the upward and downward migration distances were &lt;5 cm. The groundwater table fluctuations, pH variation, and Pb concentration currently influence the migration of Pb.

Experimental and Simulation Research on the Process of Nitrogen Migration and Transformation in the Fluctuation Zone of Groundwater Level

The fluctuation of groundwater causes a change in the groundwater environment and then affects the migration and transformation of pollutants. To study the influence of water level fluctuations on nitrogen migration and transformation, physical experiments on the nitrogen migration and transformation process in the groundwater level fluctuation zone were carried out. A numerical model of nitrogen migration in the Vadose zone and the saturated zone was constructed by using the software HydrUS-1D. The correlation coefficient and the root mean square error of the model show that the model fits well. The numerical model is used to predict nitrogen migration and transformation in different water level fluctuation scenarios. The results show that, compared with the fluctuating physical experiment scenario, when the fluctuation range of the water level increases by 5 cm, the fluctuation range of the nitrogen concentration in the coarse sand, medium sand and fine sand media increases by 37.52%, 31.40% and 21.14%, respectively. Additionally, when the fluctuation range of the water level decreases by 5 cm, the fluctuation range of the nitrogen concentration in the coarse sand, medium sand and fine sand media decreases by 36.74%, 14.70% and 9.39%, respectively. The fluctuation of nitrogen concentration varies most significantly with the amplitude of water level fluctuations in coarse sand; the change in water level has the most significant impact on the flux of nitrate nitrogen and has little effect on the change in nitrite nitrogen and ammonium nitrogen, and the difference in fine sand is the most obvious, followed by medium sand, and the difference in coarse sand is not great.

Dam construction attenuates trace metal contamination in water through increased sedimentation in the Three Gorges Reservoir

Dam construction has a far-reaching impact on trace metal accumulation and the metal-induced quality of the aquatic environment. However, the long-term impacts of dam construction and impoundments on the spatial distribution of trace metals and water quality remain poorly understood. Here, we found that the concentrations of trace metals in the mainstream water of the world's largest reservoir, Three Gorges Reservoir (TGR), decreased after impoundment, while their concentrations and contamination in the sediments of the water-level fluctuation zone increased significantly, especially for anthropogenic sources of metals such as cadmium, lead, and zinc. The spatial and temporal variations of anthropogenic metals in the sediments revealed increased anthropogenic dominance in their distribution under current hydrological management, especially for the urban area of the upper TGR. Sediment fluxes, particle composition, and extreme climate modulated the distribution of trace metals in the sediments. The results demonstrate that human activities have increasingly determined the distribution and contamination state of trace metals in the mainstream TGR. However, in contrast to our previously thought, the anthropogenic discharge of trace metals did not adversely affect water quality. Our results indicate that dam construction in riverine systems attenuates trace metal contamination in water through sediment sorting and deposition.

Spatiotemporal variations of redox conditions and microbial responses in a typical river bank filtration system with high Fe2+ and Mn2+ contents

The redox conditions strongly affect redox kinetics, hydrochemical composition and microbial reactions under river bank filtration (RBF). For investigating the redox conditions, zonation factors, and microbial responses in a typical RBF with high Fe2+ and Mn2+ contents, three wells were established 20 m apart perpendicular to the south bank of the Songhua River with a constant flow rate of 400 m3/h. Redox zonation was identified from the redox sequences, redox products, reactions identified and seasonality. High-throughput sequencing, principal coordinates analysis, and redundancy analysis were used to identify the microbial responses to redox zonation. The results indicated that the RBF was stabilized after 21 d of water extraction with a 51%, 36% and 21% decrease of Fe2+, Mn2+ and NH4+ contents, respectively. Feature redox indices varied with strong regularity in the horizontal and vertical filtration directions, and the filtration area could be divided into O2/NO3−, Fe/Mn, and SO42− reduction zones. Seasonality resulted in markedly fluctuation of Fe2+ and Mn2+ contents in groundwater. Pseudomonas, Flavobacterium, and Limnobacter were the dominant bacteria in the O2/NO3−, Fe/Mn, and SO42− reduction zones, respectively. DO explained 21.1% of microbial variability in the fluctuation zone, while Mn2+ and Fe2+ explained 27.7%, and 26.2%, respectively, of microbial variability in the saturated zone.

Soil seedbank: Importance for revegetation in the water level fluctuation zone of the reservoir area

Vegetation succession in the water level fluctuation zone (WLFZ) is driven by periodical water fluctuations, the mechanisms of response and synergistic evolution between aboveground vegetation (AGV) and soil seed bank (SSB) in the WLFZ remain unclear. To illustrate the response between AGV and SSB and the importance of SSB for revegetation in the WLFZ, the Three Gorges Reservoir (TGR) was taken as the target in the current study, and five sampling sites that lie in the center of reservoir and major tributaries of the TGR were selected. Plant community survey along the three water level gradients (i.e., 165–170 m, 170–175 m and 175–180 m) was conducted. Simultaneously, SSB samples on the topsoil (0–5 cm) were collected for germination experiments. A total of forty-nine species were observed from the SSB, belonging to 24 families and 47 genera, of which Asteraceae (8 species) and Poaceae (6 species) dominated. The number of total germinated seedlings species from soil samples from the sampling sites differed, i.e., was lower in Fengjie and Wushan compared to Yunyang, Gaoyang and Fengdu. The seed density in 165–170 m was significantly lower than that of 170–175 m and 175–180 m (p < 0.05). The Sørensen similarity coefficients between AGV and SSB tended to decrease with the increase of water level gradient, ranging from 0.04 to 0.42. SSB species composition was significantly associated with total carbon and total nitrogen contents of the soil (both p < 0.05). The SSB density was significantly negatively correlated with concentration of soil total nitrogen (p < 0.05), the species richness of SSB was significantly negatively correlated with soil pH value (p < 0.05). Hence, the relationship between the SSB and the soil habitat might be an important factor driving the construction of vegetation in the WLFZ. The correlation between dynamic of SSB and TGR hydrological regimes should be considered for revegetation in the WLFZ.

Functional Trait Diversity Shapes the Biomass in the Dam-Induced Riparian Zone

The construction of dams has caused a serious decline in riparian ecosystem functioning and associated services. It is crucial to assess the response of riparian plant communities to flooding stress for their conservation. Functional traits composition, functional diversity, and species diversity are commonly used to investigate the effect of abiotic stress on ecosystem functioning and services (i.e., biomass). Yet, how the functional traits respond to the flooding stress along a dam-induced riparian habitat remains unclear, and how biodiversity affects biomass still exists controversy. Accordingly, this study investigated the response strategies of functional traits subjected to the flooding stress and its correlation with aboveground biomass (AGB) in the water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR). We found that leaf traits and root traits showed a significant response to elevation, whereas they demonstrated different distribution patterns. Leaf traits showed acquisitive-conservative-acquisitive resource strategies along the flooding stress, while root traits shifted from species conservative resource to acquisitive resource strategies. AGB was found to be positively related to the community weighted mean (CWM) trait values for leaf dry matter content (LDMC) and negatively related to specific leaf area (SLA), but the AGB showed no relationship with the root traits. AGB accumulated greatly in the intermediate species diversity, and we also found a significant relationship between functional diversity and biomass within threshold values. Additionally, Rao’s exerted the most significant influence on the biomass, suggesting that the functional diversity index is a better indicator of biomass variation. The results obtained only partly supported the “mass ratio hypothesis” in leaf traits and mainly supported the “niche complementarity hypothesis”, which suggested that these two theories are not mutually exclusive at the early stage of vegetation community succession with an unstable community structure in dam-regulated riparian zones.

The assessment and source apportionment of metals in the water-level fluctuation zone of the upper reaches Yangtze mainstream

The assessment and source apportionment of metals in the water-level fluctuation zone of the upper reaches Yangtze mainstream

Variation in soil properties and its influence on the dynamic change of soil erosion resistance to overland flow in the water-level fluctuation zone of the Three Gorges Reservoir, China

The water level fluctuations, rainstorms, and plants could lead to the variation in soil properties in the water-level fluctuation zone (WLFZ) and thus may influence the soil erosion resistance (soil erodibility, Kr, and critical shear stress, τc). However, variation in soil properties and its influence on the dynamic change of soil erosion resistance to overland flow in the WLFZ of the Three Gorges Reservoir (TGR) still unclear and need to be further quantified. Therefore, soil samples from different elevations were collected along a slope profile at different times in the WLFZ of the TGR, then the soil properties and plant biomass were measured and the soil detachment capacity (Dc) were estimated by conducting a scouring experiment. Then the Kr and τc were obtained according to the linear relationship between Dc and shear stress. The results indicated that most measured soil properties suffered low variability but organic matter content showed the moderate variability. Kr varied from 0.10 to 0.17 s m−1 and τc varied in the range of 1.56–2.24 Pa for all sampling points, and both showed spatial and temporal variations. Kr decreased with increasing elevation until reaching a minimum value and then increased spatially, while it showed a decreasing trend or a decreasing trend followed by an increasing trend over time for different elevation segments temporally. τc increased with increasing elevation spatially and increased or showed an increase followed by a decrease over time for different elevation segments temporally. Kr was significantly influenced by clay, silt, and sand content, mean weight diameter and soil organic matter content, while τc was only affected by clay content. These research results highlight the dynamic change of soil erosion resistance and are helpful for understanding the spatial and temporal variations in soil erosion resistance in the WLFZ of the TGR area.