Water Level Fluctuation Zone Research Articles

Identification and prevention of agricultural non-point source pollution risk based on the minimum cumulative resistance model

Agricultural non-point source pollution (AGNPS) risk prevention and control is more conducive to reducing costs than post-treatment. Therefore, how to effectively identify risks is a key problem to be solved. The study area is located in the NANTUO small watershed of the Three Gorges Reservoir Area in China, on the side of the Yangtze River, where agriculture is developed with many kinds of plantings. There is a water level fluctuation zone (WLFZ) with seasonal elevation difference up to 30 m due to the water level regulation of the Three Gorges reservoir, which is a hot area of AGNPS research. On the basis of daily water level observation in NANTUO small watershed, based on the theory of “source” and “sink”, this study explored the land use change in different seasons with the help of high-resolution remote sensing image (According to the change of water level, it can be divided into two periods: non-submergence period (low water level period from the beginning of June to the end of September) and submergence period (high water level period at other time)). With the help of “source” identification (Types of land use that produce pollutants) and resistance surface calculation index system (Minimum cumulative resistance model, MCR model), the risk level and risk transmission path (The easiest path for pollutants to move to water) of AGNPS in WLFZ were analyzed. Combined with the idea of ecological corridor construction and pollution control, the risk change of regional AGNPS under the optimization of land use mode (The cultivated land types within 50 m and 100 m (Landscape optimization zone) around the water area are adjusted to forest land respectively in two scenarios (Q1 and Q2)) was analyzed as well. Based on our results, 1) the difference in the water areas between the non-submergence and submergence periods was 3.79 km2, and the areas of grassland, farmland, and forest land in the non-submergence period increased by 0.62, 0.85, and 0.35 km2, respectively. The “source” land in this region was mainly sloping farmland with slope above 6°. 2) The calculation results of the minimum cumulative resistance base surface for each source land showed the characteristics of “high values in the west and dispersedly distributed, low values in the east and continuously distributed”. 3) The distribution of the resistance surface during the non-submergence and submergence periods was basically the same; the high-risk zones were widely distributed throughout the region, while the low-risk zones were mainly distributed in the eastern mountains. 4) The number of risk transmission paths followed the order of a4 (farmland with slopes between 6° and 15°) > a5 (farmland with slopes between 2° and 6°) > a1 (rural settlements) > a3 (farmland with slopes between 15° and 25°) > a6 (farmland with slopes ≤ 2°) > a2 (farmland with slopes above 25°), and about 90% of the risk transmission paths were distributed in the lower levels (i.e., level 1, level 2, level 3). The proportion of risk transmission paths in the lower levels during the submergence period was higher than that during the non-submergence period. 5) The proportions of high risk, relatively high risk, medium risk, low risk, and no risk in the study area varied respectively following the rules of 72.79%→70.07%→66.56%, 16.85%→18.16%→18.3%, 6.24%→7.21%→9.3%, 2.25%→2.36%→3.32%, and 1.87%→2.2%→2.52% under the intact status of non-submergence period (Q0), Q1 scenario, and Q2 scenario. 6)The maximum values of pollution transmission path resistance of “source” land under the intact status of non-submergence period (Q0), Q1 scenario, and Q2 scenario followed the order of Q0 < Q1 < Q2. The results showed that the seasonal regulation of water level by artificial water conservancy project has a negative impact on AGNPS, and the MCR model was effective in the study of AGNPS risk and transmission path identification in small area. The results indicated that the wider the landscape optimization belt was, the stronger the barrier effects on pollutants would be and the more difficult it was for the pollutants to reach the water. The increase in the width of the landscape optimization belt could significantly reduce the proportion of low-level transmission paths, which indicated that the construction of ecological corridor had a good positive effect on the water environment protection.

Bioavailability of organic phosphorus in the water level fluctuation zone soil and the effects of ultraviolet irradiation on it in the Three Gorges Reservoir, China

Ultraviolet (UV) irradiation is an abiotic pathway for the transformation of complex phosphorus (P) components into inorganic P in ecosystems. To explore the effect of UV irradiation on organic P (OP) bioavailability in the water level fluctuation zone (WLFZ) soil, we collected representative soil samples from WLFZ of the Pengxi River, a tributary of the TGR, China. We determined the contents of different forms of OP in the WLFZ soil through sequential extraction. The bioavailability of different forms of OP and the effect of UV light were characterised using a combination of enzymatic hydrolysis and UV irradiation. The OP contents of the different extracts (Po) were ranked as NaOH-Po > NaHCO3-Po > H2O-Po, whereas those of enzymatically hydrolysable organic P (EHP) were ranked as NaOH-EHP > NaHCO3-EHP > H2O-EHP. UV irradiation was found to improve OP bioavailability, as demonstrated by increased levels of UV-sensitive P (UV-P) and EHP in the extracts after irradiation. The total contents of bioavailable Po in extracts were 5.6–35.3% higher after UV irradiation than before irradiation. Thus, the effect of UV irradiation on the OP bioavailability and release activity cannot be neglected in TGR WLFZ soil.

Occurrence, sources and seasonal variation of PM2.5 carbonaceous aerosols in a water level fluctuation zone in the Three Gorges Reservoir, China

Periodic water impoundment and seasonal flood events result in a 30 m water level fluctuation zone (WLFZ) in the Three Gorges Reservoir Region (TGRR), China. In order to assess the occurrence, sources and seasonal variation of airborne carbonaceous pollutants, a total of thirty-nine PM2.5 samples, twenty-two from August 2017 (summer: low water level ~135m) and seventeen from January 2018 (winter: high water level ~175m) were collected consecutively at a rural site in WLFZ in the TGRR. The results showed that OC, EC, char, soot, 16 PAHs and n-alkanes were higher in winter (mean: 9.17 μg/m3, 4.69 μg/m3, 4.25 μg/m3, 0.45 μg/m3, 19.8 ng/m3 and 210 ng/m3, respectively) than in summer (mean: 6.55 μg/m3, 1.70 μg/m3, 1.37 μg/m3, 0.32 μg/m3, 6.13 ng/m3 and 79.1 ng/m3, respectively). Compositional differences suggested air-soil/plant exchange as a source for 2-3-rings PAHs in summer and biomass burning for 4-5-rings in winter. Diagnostic ratios and PCA suggested more vehicular emission for PAHs in summer. Plant waxes contributed 18.2% and 26.2% of the n-alkanes in summer and winter, respectively, implying a relatively greater contribution from petroleum residues. The specific ratios and relative abundance of hopanes and steranes indicated mixed sources from both vehicle exhaust and petroleum residue; and petroleum residues contribute more steranes in summer compared with winter. As revealing basic information on the formation mechanism of carbonaceous aerosols, this study could contribute to a further understanding of their environmental geochemical behavior in the WLFZ of TGRR.

Research on recently occurred reservoir-induced Kamenziwan rockslide in Three Gorges Reservoir, China

A landslide occurred in Kamenziwan, Zigui, the Three Gorges Reservoir area, located about 43 km from the dam on December 10, 2019. Due to a successful early warning system, no casualties were reported following the Kamenziwan landslide. However, roads, citrus groves, water supply lines, and 280-V power lines were damaged, resulting in a direct economic loss of 5.8 million RMB. This landslide is a newborn, wedge-shaped (apparent dip) red-layer rockslide, controlled by double-slip surface and covers an area of about 2.1 × 104 m2 and a volume of about 42 × 104 m3. The sliding mechanism includes leading-edge traction in the early stage and middle-rear gravity displacement in the late stage. The landslide movement was rapid, with a maximum sliding velocity of about 0.15 m/s, a movement time of more than 15 min, and a movement distance of about 18 m. Based on our investigation, the Kamenziwan landslide is not triggered by the reservoir water level and rainfall; indeed, this landslide has no obvious triggering factors. The results from our study suggest the long-term water level fluctuation in the Three Gorges Reservoir area has eroded the key anti-slip section in the middle-lower part of the Kamenziwan landslide, greatly accelerating its evolution process. The landslide that occurs “naturally”, due to the degradation of rock mass in the water level fluctuation zone, may be an important path for the development of new landslides in the later stage of water impoundment. The Kamenziwan landslide is of great reference value for understanding the development of a new type of reservoir-induced landslide in the Three Gorges Reservoir area. Comprehensive studies on this landslide help to identify potential geohazards, similar to Kamenziwan landslide, in the early stage of their development.

Influence of Sediment Characteristics on Heavy Metal Fraction Distribution in the Water-Level Fluctuation Zone of the Three Gorges Reservoir Area, China

The concentrations and fractions of heavy metal in sediments at different altitudes in the water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) were determined. The Pearson correlation analysis was used to determine the differences between heavy metal fractions and physicochemical properties. The results showed that Cr and Cu were mainly horizontally and vertically distributed in the residual (RE) fractions with the highest stability and relatively low ecological risk. Pb and Zn were mainly in the Fe-Mn oxide-bound (Fe-Mn) fractions, which can be reduced to a bioavailable state when the redox potential decreased or the oxygen was severely depleted in the aquatic environment. There were significant differences in the heavy metal fractions and risks in sediment in the three different altitudes of the TGR. The physicochemical characteristics of the sediment indicated that the sediments in the low altitude area had rough forms and large pores. In addition, heavy metals at low altitudes were likely to be released into the water during antiseasonal water storage, causing secondary pollution, which greatly increased the mobility of heavy metals and ecological risks to the environment.

Evaluation of Hg methylation in the water-level-fluctuation zone of the Three Gorges Reservoir region by using the MeHg/HgT ratio.

In the recent decade, the hydroelectric reservoir is identified as a methylmercury (MeHg) hotspot and gained much attention. The artificial water level management in the Three Gorges Reservoir (TGR) in China formed a water-level-fluctuation zone (WLFZ) undergoing flooding drying rotations annually. However, the mercury (Hg) methylation and major geochemical driving factors at different elevations in the WLFZ remain unclear. Here we use total Hg (HgT) normalized MeHg (MeHg/HgT ratio) to evaluate Hg methylation degree in a one-year field study at 155, 165m elevations in the WLFZ and with >175m elevation as the reference. Results demonstrate that MeHg/HgT ratio at the WLFZ could reach 4.1% in soils, and both 155 and 165m elevations have a higher Hg methylation degree than the >175m elevation. However, the differences in MeHg/HgT ratios both in soils and waters between 155 and 165m elevations are not significant. This indicates the influence of different submerging periods on the MeHg/HgT at the WLFZ elevations is not observed. The significant correlation between the MeHg/HgT ratio and soil organic carbon (SOC) content implies a MeHg retention in re-exposed soils after flooding. Decoupling of MeHg/HgT ratios between submerged soil and overlying water are found at both elevations and therefore make MeHg/HgT in waters alone cannot be used to evaluate Hg methylation degree in this study. The calculation of HgT and MeHg partitioning coefficient (Kd) found an immobilization of MeHg by submerged soils at the WLFZ during the flooding period. Major geochemical factors, determined through principal component analysis (PCA), in affecting Hg methylation are the redox cycling of sulfur and the distribution of organic matters in the WLFZ.

Responses of AOA and AOB activity and DNA/cDNA community structure to allylthiourea exposure in the water level fluctuation zone soil.

Ammonia oxidation is mainly performed by ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). Allylthiourea (ATU) has been found to specifically inhibit ammonia oxidation. However, the effect of ATU on AOA and AOB transcription has been infrequently studied. In the present study, we examined the responses of AOA and AOB activity and DNA/cDNA community structure to ATU exposure. The ammonia oxidation activity in the 100-mg/L ATU group was 4.3% ofthat in the control group after 7days. When exposed to ATU, the gene abundance of AOA was favored compared with that of AOB, and there were no statistically significant differences in the abundance of AOB amoA in DNA and cDNA between the two groups. Compared with the control group, the gene abundance of AOA significantly increased by 5.23 times, while the transcription of AOA significantly decreased by 0.70 times. Moreover, the transcriptional ratio of AOA in the ATU group was only 0.05 times as highasthat in the control group. ATU selectively affected AOB and completely inhibited Nitrosomonas europaea and Bacterium amoA.22.HaldeII.kultur at the genetic level. Under ATU exposure, all AOA clusters were transcribed, but three AOB clusters were not transcribed. Our results indicated that the ammonia oxidation potential of the soil of water level fluctuation areas, based on ATU inhibition, was associated mainly with AOA amoA gene abundance and AOB community shifts in DNA and cDNA.

Rhizosphere Bacterial Community Structure and Predicted Functional Analysis in the Water-Level Fluctuation Zone of the Danjiangkou Reservoir in China During the Dry Period.

The water-level fluctuation zone (WLFZ) is a transitional zone between terrestrial and aquatic ecosystems. Plant communities that are constructed artificially in the WLFZ can absorb and retain nutrients such as nitrogen (N) and phosphorus (P). However, the microbial community composition and function associated with this process have not been elucidated. In this study, four artificially constructed plant communities, including those of herbs (Cynodon dactylon and Chrysopogon zizanioides), trees (Metasequoia glyptostroboides), and shrubs (Salix matsudana) from the newly formed WLFZ of the Danjiangkou Reservoir were evaluated. The bacterial community compositions were analyzed by 16S rRNA gene sequencing using a MiSeq platform, and the functions of these communities were assessed via Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analysis. The results showed that the bacterial communities primarily comprised 362 genera from 24 phyla, such as Proteobacteria, Acidobacteria, Actinobacteria, and Gemmatimonadetes, showing the richness of the community composition. Planting altered the bacterial community composition, with varying effects observed among the different plant types. The bacterial community functional analysis revealed that these bacteria were primarily associated with six biological metabolic pathway categories (e.g., metabolism, genetic information processing, and environmental information processing) with 34 subfunctions, showing the richness of community functions. The planting of M. glyptostroboides, S. matsudana, and C. dactylon improved the metabolic capabilities of bacterial communities. N- and P-cycling gene analysis showed that planting altered the N- and P-cycling metabolic capacities of soil bacteria. The overall N- and P-metabolic capacity was highly similar between C. dactylon and C. zizanioides samples and between S. matsudana and M. glyptostroboides samples. The results of this study provide a preliminary analysis of soil bacterial community structure and function in the WLFZ of the Danjiangkou Reservoir and provides a reference for vegetation construction in this zone.

Bacterial and archaeal compositions and influencing factors in soils under different submergence time in a mercury-sensitive reservoir.

Soils in the water-level-fluctuating zone (WLFZ) of Three Gorges Reservoir (TGR) inundated by water for different periods of time are confirmed to have disparate characteristics to mercury (Hg), and thus it is of great significance to further investigate microbial compositions and influencing factors. The objective of this study was to compare bacterial and archaeal richness, α-diversities and compositions, as well as affecting variables, especially Hg concentrations, among soils under different submergence time-SI (inundated soil), SS (semi-inundated soil), SN(non-inundated soil) and SSe (sediment)-based on high throughput sequencing. Results showed that sediment had significantly higher bacterial and archaeal richness and α-diversities than the other soil types. Anaerolinea and Aeromonas, as well as Altiarchaeales, Nitrosoarchaeum, and Methanosarta were dominant in SSe, while sharply decreasing in the other soil types, with significant difference among groups. An unclassified genus in SCG critically predominating in SI, SS and SN, drastically reduced in SSe, with extremely significant difference among groups. Bathyarchaeota and Nitrososphaera, both dominating in SSe, decreased dramatically and almost vanished in SI and SN. All the variables except pH posed a significant positive effect on bacterial and archaeal compositions in SSe, while opposite effect in the other three soil types. MeHg and THg concentrations had relatively weaker effects on microbial compositions comparing to variables like NH4+, CEC, OM and SO42+.

三峡水库消落区植被在差异性水淹环境中的分布格局

PDF HTML阅读 XML下载 导出引用 引用提醒 三峡水库消落区植被在差异性水淹环境中的分布格局 DOI: 10.5846/stxb201811032382 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点研发计划课题（2017YFC0505301）；国家自然科学基金项目（31770465） Distribution pattern of vegetation in water-level fluctuation zone of the Three Gorges Reservoir as affected by differential flooding regimes Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:人工水库修建引发的差异性水文节律是决定消落区植被群落格局的主要因素，高强度水淹环境中水淹胁迫是影响植被的重要因子而低强度水淹环境中物种竞争是影响植被的重要因子。为了探究差异性水淹环境中三峡水库消落区植物的水淹耐受能力及光资源竞争能力（植物株高）对植被群落分布格局的影响，对三峡水库典型消落区不同水淹强度下生长的植被进行了研究，结果表明：（1）典型消落区调查共发现有植物41种，其中高耐淹低竞争能力型植物4种，其生物量在所有物种生物量中的占比达70.99%，低耐淹高竞争能力型植物23种，其生物量占比为28.02%，低耐淹低竞争能力型植物14种，生物量占比不足1%，消落区内无高耐淹高竞争能力型植物物种分布；（2）高耐淹低竞争能力型植物在水淹强度大的消落区区域占优，低耐淹高竞争能力型植物在植物物种竞争压力大的消落区区域占据主导，低耐淹低竞争能力型植物在消落区中仅有零星分布；（3）消落区植被生物量格局随着高程增加呈现出先增加后减少的趋势。研究差异性水淹环境对三峡水库消落区植被分布的影响，可以为深入理解消落区植被分布格局的形成机制和大型水库消落区植被恢复与重建提供理论依据。 Abstract:The different hydrological regimes caused by artificial reservoir construction is the main factor affecting the patterns of community vegetation in the water-level fluctuation zone. The flooding stress in the high-intensity flooding area is an important factor affecting vegetation, but species competition in the low intensity flooding area is a key stress factor affecting vegetation. Plants growing under different flooding regimes in the typical water-level fluctuation zone of the Three Gorges Reservoir were studied in order to explore the effect of plant flooding tolerance and optical resource competitiveness (plant height) on vegetation distribution patterns. The results showed the following:(1) A total of 41 plants species were distinguished and analyzed in the studied area, including 4 plants with high submergence tolerance and low competitiveness, making up 70.99% of total biomass; 23 plants with low submergence tolerance and high competitiveness, making up 28.02% of total biomass; 14 plants with low submergence tolerance and low competitiveness, making up less than 1% total biomass; and no plants with high submergence tolerance and high competitiveness in the water-level fluctuation zone; (2) The plants with high tolerance and low competitiveness dominated in the water-level fluctuation zone with high flooding intensity, those with low tolerance and high competitiveness dominated in the water-level fluctuation zone with high competitive pressure, and plants with low tolerance and low competition were only sporadically distributed at the shorelines; (3)The biomass pattern of vegetation along an inundation gradient showed a tendency to increase first and then decrease. This study could help us understand the mechanisms of vegetation distribution patterns at the water-level fluctuation zone of the Three Gorges Reservoir and provide basic knowledge on vegetation restoration and reconstruction in this zone. 参考文献 相似文献 引证文献

不同泥沙埋深对几种一年生草本枯落物分解及养分动态特征的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 不同泥沙埋深对几种一年生草本枯落物分解及养分动态特征的影响 DOI: 10.5846/stxb201912032625 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 重庆市建设科技项目（城科字2019第1-4-2）；重庆市科技兴林首席专家团队项目（TD2019-2）；中央林业改革发展资金科技推广示范项目（渝林科推2017-12）；国家国际科技合作专项（2015DFA90900） Effects of different simulative sediment depths on litter decomposition and nutrient dynamic change of several annual herbaceous plants Author: Affiliation: Fund Project: This research was financially supported by the Chongqing Municipality Housing and Urban Construction Committee (No. Chengkezi 2019 1-4-2), Chongqing Municipality Key Forestry Research Project (No. TD2019-2), Forestry Extension Project of China Central Finance (No. Yulinketui 2017-12) and International Sci-Tech Cooperation Project of Ministry of Science and Technology (No. 2015DFA90900). 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:为探究三峡消落带泥沙淤积条件下一年生草本枯落物的分解及养分变化特征，通过模拟实验，采用分解袋法，研究不同泥沙埋深对4种一年生草本分解及养分动态的影响。所选择的三峡消落带4种一年生优势草本植物包括马唐（Digitaria sanguinalis）、稗（Echinochloa crusgali）、狼杷草（Bidens tripartita）、苍耳（Xanthium sibiricum），设置了无掩埋（0 cm）、中度掩埋（5 cm）和深度掩埋（10 cm）3个泥沙掩埋深度，试验周期为180 d。结果表明：（1）在整个实验期间，4种一年生草本枯落物的分解速率均呈现先快后慢的特点，与0 cm泥沙埋深处理相比，5、10 cm埋深下4种一年生草本枯落物的分解速率均显著降低；（2）在分解结束时，4种一年生草本枯落物C含量变化不大，P元素处于释放状态，而在泥沙掩埋下N元素处于富集状态；（3）除马唐和稗枯落物的P含量变化外，4种一年生草本枯落物C、N、P含量均表现为0 cm掩埋处理显著低于5、10 cm处理组；相反，4种一年生草本枯落物K含量则表现为0 cm掩埋处理显著高于5、10 cm处理组。研究表明消落带大量泥沙淤积抑制枯落物的分解和C、N、P元素释放，有利于降低对三峡库区水体富营养化及营养物质悬浮的贡献率，一定程度上，对库区水质保护有正面效应。 Abstract:To explore traits related to litter decomposition and the nutrient change of annual herbaceous species under the condition of sediment deposition in the Three Gorges Reservoir. The effects of different sediment depths on ground litter decomposition and nutrient dynamic characteristics of four annual dominant herbaceous species (Digitaria sanguinalis, Echinochloa crusgali, Bidens tripartita, and Xanthium sibiricum) in the water-level fluctuation zone of the Three Gorges Reservoir were investigated using the litter-bag method. Three sediment depths included no burial (0 cm), moderate burial (5 cm) and deep burial (10 cm), were treated with an experimental period of 180 days. The results showed that:(1) During the experiment period, the decomposition rate of litters of four annual herbaceous species was fast at first and then slow. Compared with 0 cm sediment depth treatment, the decomposition rate significantly decreased in 5 cm and 10 cm. (2) When the decomposition ended, the carbon (C) content of four annual herbaceous species litters remained stable, the phosphorus (P) element was released, while the nitrogen (N) was enriched under sediment burial. (3) Except for changes in phosphorus content of D. sanguinalis and E. crusgali litter, the content of carbon, nitrogen and phosphorus of four annual herb litter in 0 cm was significantly lower than that in 5 cm and 10 cm treatment groups. On the contrary, the potassium (K) content of four annual herbs species litters in 0 cm was significantly higher than that in 5 cm and 10 cm treatment group. The results revealed that a large amount of sediment deposition in the water-level fluctuation zone inhibited the decomposition and C, N, and P release of the litter, which is beneficial to reduce the contribution of litter decomposition to water eutrophication and nutrient suspension in the Three Gorges Reservoir, and to a certain extent, has a positive effect on water quality protection. 参考文献 相似文献 引证文献

生态袋护坡技术在三峡水库消落带植被恢复中应用的可行性研究

生态袋护坡技术在三峡水库消落带植被恢复中应用的可行性研究

三峡库区消落带池杉-土壤碳氮磷生态化学计量特征

三峡库区消落带池杉-土壤碳氮磷生态化学计量特征

Carbon, nitrogen, and phosphorus ecological stoichiometric characteristics in leaves and fine roots of &lt;i&gt;Taxodium distichum&lt;/i&gt; in the water-level fluctuation zone of the Three Gorges Reservoir region

为探究三峡库区消落带落羽杉（Taxodium distichum（L.） Rich.）叶片与细根C、N、P生态化学计量特征，于2018年7月对忠县三峡库区消落带植被修复示范基地3个水淹处理组（深度水淹组，DS；中度水淹组，MS；浅淹对照组，SS）的落羽杉进行叶片与细根采集，并调查其生长特征，测定分析落羽杉叶片与细根C、N、P生态化学计量特征及相关性.结果表明：（1）各水淹处理组落羽杉叶片C含量表现为SS组 > DS组 > MS组，且SS组显著高于MS组；细根C含量在不同水淹处理组之间均无显著性差异.（2）细根N、P含量表现为DS组 < MS组 < SS组的协同增长趋势；叶片N、P含量则表现为DS组 > MS组 > SS组的协同降低趋势，且叶片N、P含量分别约为细根N、P含量的3倍和2倍.（3）各水淹处理组的叶片C/N、C/P比值均显著或极显著低于细根；叶片N/P比值均高于细根，且其比值相对恒定，均表现为SS组 > DS组 > MS组的趋势.（4）相关性分析表明，叶片P含量与细根N含量呈显著负相关关系，叶片N/P比值与细根N/P比值呈显著正相关关系.研究表明，落羽杉叶片与细根在生长和代谢过程中具有整体性，养分和光合产物在地上与地下部分之间分配权衡，以维持营养的动态平衡，使落羽杉具有内稳性，从而更好地适应和响应三峡消落带水位变化.;To explore the C, N, and P ecological stoichiometric characteristics of leaves and fine roots of Taxodium distichum (L.) Rich in the water-level fluctuation zone of the Three Gorges Reservoir (TGR) region. The C, N, and P content and their ratio in leaves and fine roots of T. distichum in Zhong County under three submergence treatments—Deep Submergence (DS), Moderate Submergence (MS), and Shallow Submergence (SS) were measured and analyzed in July 2018. The results showed that: (1) The C content in leaves of T. distichum showed a trend of SS>DS>MS group, and SS group was significantly higher than that in MS group; there was no difference in C content of fine roots under three submergence treatments. (2) The N and P content in fine roots showed a synergistic growth trend of DS < MS < SS group; the N and P content in leaves showed a cooperative decrease trend of DS>MS>SS group, and the N and P contents of the leaves was about 3 times and 2 times of that in fine roots, respectively. (3) The C/N and C/P ratios of the leaves were significant or extremely significantly lower than those of fine roots in submergence treatment group; the N/P ratio was higher than that of fine roots, and its ratio was relatively constant. Moreover, the N/P ratio between leaves and fine roots showed a trend of SS > DS > MS group. (4) Correlation analysis showed that there was a significant negative correlation between leaf P content and fine root N content, and the leaf N/P ratio was significantly positively correlated with fine root N/P ratio. This work demonstrates that the leaves and fine roots of T. distichum have integrity in the process of growth and metabolism, the nutrients and photosynthetic products are distributed between the above- and below ground parts to maintain the dynamic balance of nutrients, so that the T. distichum has homeostasis and thus adapt well to and respond to changes in the water-level fluctuation zone of the TGR region.

Vegetation of the water-level fluctuation zone in the Three Gorges Reservoir at the initial impoundment stage

The water-level of Three Gorges Reservoir first reached a normal storage level 175 m in October 2010, resulting in a large-area water-fluctuating zone with an elevation difference of 30 m (145–175 m). We used the Geographic Information System (GIS) technology, the field quadrat survey, the species importance value calculation, and literature data to assess the natural environment situation, the vegetation regeneration status, and the ecological benefits of the dominant plant species in terms of mitigating agricultural non-point source pollution of the WLFZ in the Three Gorges Reservoir at the initial impoundment stage. The results showed that: (1) the WLFZ area presented an obvious trend of increasing first and then decreasing from west to east, and the exposed area at the elevation intervals of 145–175 m, 155–175 m, and 165–175 m accounted for 20, 37.49, and 42.51% of the total area, respectively; (2) the WLFZ area gradually decreased with increasing gradient, and the exposed area at the gradient intervals of <15°, 15–25°, 25–35°, 35–60°, and >60° accounted for 60.90, 18.62, 12.03, 8.06, and 0.39%, respectively, of the total area. (3) The vegetated area increased with increasing water level elevation, and the vegetation coverage area at elevation intervals of 145–155 m, 155–165 m, and 165–175 m accounted for 4.96, 16.09, and 44.77%, respectively, of the total area of the corresponding elevation interval. The vegetated area gradually decreased with increasing gradient, and at the sections Jiangjin-Changshou, Fuling-Yunyang, and Fengjie-Yiling, the vegetated area accounted for 15.30, 71, and 12.70%, respectively, of the total vegetation coverage area. (4) The important value calculation results indicate that the dominant species significantly differed among the different elevations in the three belt transects of the Yangtze River main stream (Zhongxian section), the Xiaojiang River (Kaizhou section), and the Xiangxi River (Guizhou section). The dominant annual herb species were Digitaria sanguinalis (Linn.) Scop., Conyza canadensis (Linn.) Cronq., Bidens frondosa Linn., Echinochloa crusgali (Linn.) Beauv., and Setaria viridis (Linn.) Beauv., while the dominant perennial herb species were Cynodon dactylon(Linn.) Pers. and Cyperus rotundus Linn.. (5) The water level elevation was the most important factor affecting vegetation restoration. At the initial impoundment stage, the naturally regenerating plant species Cynodon dactylon(Linn.) Pers. and Cyperus rotundus Linn. could be used to prevent and control regional agricultural non-point source pollution, combined with the establishment of plant hedges using the native species Morus alba L. and Vitex negundo Linn.. We provide a scientific basis for the development of measures to mitigate agricultural non-point source pollution in the Three Gorges Reservoir area, using ecological restoration.

Water-level alterations modified nitrogen cycling across sediment-water interface in the Three Gorges Reservoir.

Water-level regime alteration-associated redox fluctuation plays a primary role in governing exchange and transformation of nitrogen (N) in water-level fluctuation zones (WLFZs), while few understanding of how hydrological regimes under reservoir operation affected N cycling across the sediment-water interface (SWI), giving rise to uncertainties in reservoir N nutrient management. Batch microcosm simulation experiments with intact sediment cores from WLFZs of the Three Gorges Reservoir (TGR) were conducted for 24days to identify holistic flooding-drying process mechanism on N-cycling patterns. Our results showed a distinct transition of N-cycling mode across the SWI, shifting from biological denitrogen loss dominated in initial period of flooding to enhance endogenous N retention. A dramatic source-sink switch of nitrous oxide (N2O) occurred in the first 1.5days during the flooding period. However, combined accelerating migration of NH4+-N from sediment to overlying water, and subsequently enhanced transformation of NH4+-N to NO3--N formed from flooding to drying rotation, thereby increasing N loading to overlying water. The reason for this investigation could be attributed to intensive N loss through coupled nitrification and denitrification in oxic-anoxic microenvironments after flooding. With oxygen replenishment from atmosphere during drying phase, persistent ammonification of organic N in sediments provided sufficient source of NH4+-N for the formation of NO3--N fraction in a more oxic overlying water. Therefore, water-level regime alteration by reservoir operation was capable of weakening N removal from water body and lengthening internal N turnover time across redox-variable SWI. These findings elucidate new understanding of holistic hydrological regime mechanisms on N cycling across SWI and provide insight to biogenic N nutrient management for improving the green credentials of hydroelectric reservoir.

Characteristics of organic phosphorus fractions in soil from water-level fluctuation zone by solution 31P-nuclear magnetic resonance and enzymatic hydrolysis

Phosphorus (P) is an essential nutrient element for biological growth that can contribute to eutrophication in aquatic ecosystems. Water trophic status and algae growth are primarily related to the content of bioavailable P, which is primarily related to enzymatically hydrolysable organic P(EHOP) and dissolved inorganic P(IP). In this study, soil samples from the water-level fluctuation zone (WLFZ) were collected from a tributary of the Three Gorges Reservoir (TGR) to characterize the properties of organic P(OP) fractions using solution 31P-nuclear magnetic resonance (NMR) and enzymatic hydrolysis. 31P-NMR showed that orthophosphate was the main part of the bioavailable P in the WLFZ soil and accounted for 80.4% of the NaOH-EDTA extractable total P (NaOH-EDTA TP), while phosphate monoester accounted for 60.5% of NaOH-EDTA extractable OP (NaOH-EDTA OP). The soil properties and replenishment from the mainstream of the Yangtze River to the Pengxi River have a certain effect on the content and distribution of P forms in the WLFZ soil of the tributary. The EHOP accounted for 28.1% of the NaOH-EDTA OP, and a significant positive correlation was observed between labile monoester P and EHOP and organic matter (OM). The water-soluble OP(H2O-OP), bicarbonate-extractable OP(NaHCO3-OP), and Fe- and Al-associated OP(Fe/Al-OP) were significantly hydrolyzed by phosphatase and thus exhibited great release potential. The ranking of the bioavailability of OP was Fe/Al-OP > H2O-OP > NaHCO3-OP. Phytate-like P were mainly found in H2O-OP and NaHCO3-OP, which indicated that periodic submersion–emersion cycles promoted the release of phytate-like P from Fe/Al-OP into the water column of the TGR. These observations suggest that when the external P input was effectively controlled, a huge risk of release of the internal OP from the WLFZ soil, and the biogeochemical cycling of the bioavailable P played an important role in maintaining the eutrophication of the reservoir.

Impact of long-term water level fluctuation on the distribution, transport, and fate of phosphorus in reservoir sediment.

Water level fluctuation zone (WLFZ) with changeable environmental conditions would affect the transport and release of nutrients in the sediment. To investigate the influence of long-term water level fluctuations on sediment phosphorus (P), a reservoir sediment partitioning method based on historical water level was developed and applied in the Biliuhe Reservoir. The reservoir sediment was divided into frequent fluctuation zone (FFZ), fluctuation zone of continuous droughts (DFZ), and continuously submerged zone (CSZ). Sediment cores in different zones were collected and P was analyzed. TP content in the surface sediment progressively increased from FFZ, DFZ, to CSZ except the 0-10-cm samples of D1, and TP content of sediment cores increased gradually with decreasing depth except D1 core. Ferric iron-bound P (Fe-P) and organic P (OP) exhibited uniform spatial variation with TP, while no clear patterns were found with regard to the other forms. Fe-P composed most of TP (54.67 ± 10.15%) and had a significant positive correlation with stable OP (5.55 ± 2.07%). The contributions of Fe-P, detrital apatite P (Det-P), and OP are stable among the four cores, indicating that the relationship between the various P forms is relatively stable despite variations in environmental conditions of WLFZ. The alternation of deposition and erosion under long-term water level fluctuation can promote P transport down-flow, which can be prevented by the continuous growth of plants in WLFZ. Thus, the reasonable use of the plant growth in WLFZ can control the P load of the reservoir.

Spatial variations of soil phosphorus forms and the risks of phosphorus release in the water-level fluctuation zone in a tributary of the Three Gorges Reservoir

The dynamics of soil phosphorus (P) in the water-level fluctuation zone (WLFZ) generally poses a great threat to the ecology of dam-formed reservoirs worldwide. Our study explored the spatial variations of soil P forms and the risks of P release in the WLFZ in a tributary of the Three Gorges Reservoir (TGR) in the upper reaches of the Yangtze River, China. Soil samples from multiple altitudes (145, 155 and 165 m) in the WLFZ, upland soils (175 m) as well as reservoir sediment samples were collected along seven transects in a tributary of the TGR (Pengxi River) in August 2016. Hedley's fractionation method was used to characterize the P forms, and the physical and chemical properties of the samples were determined. The distributions of the total P (TP) across the transects decreased in the order of sediments (675.9 ± 124.8 mg/kg), upland soil (658.9 ± 191.1 mg/kg), and WLFZ soil (613.9 ± 100.7 mg/kg). Similar distributions of the bioavailable P (Bio-P) were observed. Longitudinally, the TP and Bio-P of the WLFZ soil gradually decreased from the estuary to the upstream of the tributary. The spatial variations of P in the WLFZ soil in the tributary of the TGR was influenced by the soil particle size distribution (PSD) that, in turn, was a result of joint effects of the anti-seasonal hydrological regime, rainfall erosion and mainstream backwater. Long-term flooding increased P release from soil and decreased the soil degree of P saturation (DPS) throughout the WLFZ, especially in the lower portion of the WLFZ. According to Water-P, molar Al:Fe ratio and DPS, the current risk of soil P release throughout the WLFZ are very low. Monitoring should be prioritized at an altitude of 165 m and in the estuary of the WLFZ considering the high DPS and/or low Al:Fe ratio in these areas.

Production and migration of methylmercury in water-level-fluctuating zone of the Three Gorges Reservoir, China: Dual roles of flooding-tolerant perennial herb

Water-level-fluctuating zone (WLFZ) is a prevalent water-land ecotone favorable for mercury (Hg) methylation. The succession of flooding tolerance plants in WLFZ gradually changes the landscape, and also brings a new question worth understanding whether these plants would enhance methylmercury (MeHg) production in WLFZ and increase risks to the aquatic environment. Given bermudagrass (Cynodon dactylon (L). Pers) as the dominant perennial herb with high flooding-tolerance in WLFZ of the Three Gorges Reservoir (TGR), we conducted a comprehensive study to investigate its roles in the production and migration of MeHg in WLFZ by field observations and stable isotope tracer experiments. Results showed that both elevated MeHg levels and Hg methylation rates appeared in soil/sediment in bermudagrass growing area, implying that the growth of bermudagrass could significantly enhance MeHg production. However, MeHg migration from sediment to water was restricted during the flooding period of the TGR, as obviously higher partitioning coefficients of MeHg between the sediment and porewater (p < 0.05) and lower MeHg release fluxes were observed in vegetated area, indicating that the presence of bermudagrass instead probably decreased the water MeHg level. Whereas, it is noteworthy that elevated MeHg in soil/sediment induced by the bermudagrass could pose potential risks to the benthos and further to the TGR food chain.