Analyzing spatio-temporal changes and trade-offs/synergies of gross ecosystem product based on water-energy-food nexus

The value of the ecosystem's ultimate goods and services for human welfare and long-term economic and social development is known as the gross ecosystem product (GEP). For the study of GEP accounting, the suggested water-energy-food (WEF) nexus offers a fresh viewpoint. This work aims to build a GEP accounting index system based on WEF, investigate its spatio-temporal evolution characteristics, and assess trade-offs and synergies between and within the water, energy, and food subsystems. Using the Three Gorges Reservoir area (TGRA) as an illustration, the findings revealed that, firstly, the comprehensive benefit of GEP based on WEF showed an upward trend in TGRA. Still, it was worth noting that the total production of the food ecosystem decreased. Secondly, the GEP based on WEF in five periods showed a spatial pattern of "high east and west, low middle." Thirdly, the Pearson correlation coefficient indicated that the GEP trade-off relationships based on WEF were dominant in TGRA, with the strongest trade-offs between AQV, SCV, APV, and LEV. In addition, in bivariate local spatial autocorrelation, the value of the six ecosystem service function relationships was dominated by the trade-off relationship, and the distribution of trade-offs and synergies showed significant heterogeneity at the county scale in the TGRA. Finally, hot spot analysis showed that the hot spots of the gross water and energy ecosystem products were scattered in the tail area of the study area. In contrast, the hot spots of the gross food ecosystem product were concentrated in the belly region. The findings of this study provided a basis for the scientific formulation of territorial spatial pattern optimization for water, energy, and agricultural resources in the TGRA and can more accurately reflect the status of the ecological environment and changes of WEF over time. Moreover, this paper also gives full play to the growth advantages of shipping and aquatic products, implements effective soil erosion prevention and control measures, and establishes water-saving mechanisms and other measures in terms of water resources. Subregional plans for industrial structure and strengthening of waste gas and wastewater treatment facilities regarding energy resources are developed. Implement the cultivated land protection system and promote the superiority of crop varieties and other measures in terms of food resources.

[Biochar or Straw Substituting Chemical Fertilizer Increase the Risk of Phosphorus Loss in Subsurface Runoff in Sloping Farmland]

The purple soil slope farmland is an important agricultural land in southwest China but is also one of the main sources of agricultural non-point source pollution in the Three Gorges Reservoir area. Taking reasonable measures to control the loss of soil nutrients is of great significance to the treatment of non-point source pollution in the region. Here, a three-year (2018-2020) field runoff experiment was conducted to monitor and evaluate the phosphorus (P) loss in sloping farmland via surface runoff (i.e., surface flow, 0-20 cm) and subsurface runoff (i.e., subsurface flow, 20-60 cm), with five treatments including no fertilization (CK), conventional fertilization (CF), optimal fertilization (OF), biochar combined with 85% of OF (BF), and straw combined with 85% of OF (SF). The results showed that fertilization application reduced the sediment yields and surface runoff flux but increased the subsurface runoff flux. The total loss flux of phosphate (PO3-4-P), total phosphorus (TP), and particulate phosphorus (PP) in surface flow were the highest in the BF treatment and the lowest in the SF treatment. All fertilization applications increased the P loss fluxes in subsurface flow relative to that in CK. The highest PO3-4-P and TP loss flux in subsurface flow was found in the BF (213.88 g·hm-2 and 694.54 g·hm-2, respectively) treatment, followed by that in the OF and SF treatments. Redundancy analysis (RDA) results showed that surface runoff flux and biochar application were the main factors contributing to increased P loss in surface flow, and subsurface runoff flux was the main factor contributing to increased P loss in subsurface flow. In summary, the SF treatment reduced the amount of sediment yield and surface runoff flux in sloping farmland of purple soil and was the most effective for controlling P loss, whereas the risk of subsurface runoff flux requires further attention.

[Characteristics and Risk Evaluation of Heavy Metal Contamination in Paddy Soils in the Three Gorges Reservoir Area]

Soil Cd, Hg, Pb, As, Cr, Cu, Zn, and Ni of 12 districts in the Three Gorges Reservoir area (Chongqing section) were analyzed, and different evaluation methods were used to assess the degree of contamination, potential ecological risk, and human health risk of soil heavy metals in paddy soils. The results showed that the average values of all heavy metals except Cr in paddy soils in the Three Gorges Reservoir area exceeded the background values of soils in the Three Gorges Reservoir area, and the contents of Cd, Cu, and Ni in 12.32%, 4.35%, and 2.54% of the soil samples exceeded the screening values, respectively. The variation coefficients of the eight heavy metals were 29.08%-56.43%, which belonged to the medium and above-intensity variation levels and were influenced by anthropogenic activities. The eight heavy metals were contaminated in the soil, and 16.30%, 6.52%, and 2.90% of the soil Cd, Hg, and Pb were heavily contaminated. At the same time, the potential ecological risk of soil Hg and Cd were in the medium risk level on the whole. Wuxi County and Wushan County had relatively high pollution levels among the 12 districts, the Nemerow pollution index showed a moderate pollution level, and the comprehensive potential ecological risks were also at a moderate ecological hazard level. The results of the health risk evaluation showed that hand-mouth intake was the main exposure path of non-carcinogenic risk and carcinogenic risk. Soil heavy metals presented no non-carcinogenic risk for adults (HI<1), but 12.68% of the sites had non-carcinogenic risk for children (HI>1). As and Cr were the main influencing factors for non-carcinogenic and carcinogenic risks in the study area, and their total contributions to non-carcinogenic and carcinogenic risks were more than 75% and 95%, respectively, which was cause for concern.

[Structural Diversity and Its Temporal Variation in the Soil Bacterial Community Under Plantations of Taxodium distichum in the Riparian Zone of the Three Gorges Reservoir Area]

The riparian zone supports important ecological functions and acts as an ecotone connecting terrestrial and aquatic areas. Soil microbes under the revegetation of woody species are crucial to the biogeochemical cycle of nutrients. Here, soil samples were collected to examine the soil microbes during different emergence phases in 2019 (May:T1, July:T2, and September:T3) in the riparian zone of the Three Gorges Reservoir, China. The variations in the bacterial community were evaluated using high-throughput sequencing. The results showed that:during the emergence phases, soil properties such as pH value (pH), ammonium nitrogen (NH₄⁺-N), and nitrate-nitrogen (NO₃^--N) and soil enzymes changed significantly(P<0.05), and soil bacterial α diversity also changed with time. Except for the Chao1 index, the richness of rhizosphere soil bacteria showed T1>T2>T3, whereas the α diversity of non-rhizosphere soil bacteria showed T3>T1>T2. The redundancy analysis (RDA) test implied that soil urease, NH₄⁺-N, pH, and NO₃^--N were the key factors structuring the microbial community. Proteobacteria and Acidobacteria were the two dominant components among the 60 phyla that were detected in the soil. Based on phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt2) prediction, metabolism was the basic function of soil bacterial communities of Taxodium distichum; in the secondary functional layer, the metabolic pathways related to carbon, nitrogen, and phosphorus mainly included amino acid metabolism, carbohydrate metabolism, lipid compound metabolism, and energy metabolism, and the relative abundance of each metabolic function had a certain time difference in different periods. These findings could help us better understand how soil microbes change after restoring vegetation in the Three Gorges Reservoir area.

On the coordination in diversity between water environmental capacity and regional development in the Three Gorges Reservoir area

Water environment capacity has drew the attention of policymakers and stakeholders to sustainable development, and its dynamic changes are ultimately impacted by population, capital, and industrial clusters under regional development. Previous research, however, has not been able to completely comprehend it. In this paper, the authors use the Coupling Coordination Degree model and the Geodetector model to study the temporal and spatial evolution of water environment capacity and its driving mechanism based on regional development represented by regional function including urbanization function, ecological function, and agricultural function using the Three Gorges Reservoir area on county scale as a case study from 2000 to 2015. The results showed that (1) compared with 2000, 2005, and 2010, the water environment capacity of the whole reservoir area in 2015 was significantly improved. (2) The urban functions of each district and county are increasing in different years, and the dynamic changes of ecological and agricultural functions are obviously different. (3) The water environment capacity of districts and counties in the head area. There are significant disparities in the relationship between water environment capacity and regional function in various regions. Differences in water environment capacity are largely influenced by ecological function and the interaction driver of the proportion of agricultural function and urban function, which are typically the biggest of all the components. This suggests that regional development is a top priority in order to improve the operability of the water environmental capacity through more regulation, rules, and planning.

Landslide Displacement Prediction Based on Multivariate LSTM Model

There are many frequent landslide areas in China, which badly affect local people. Since the 1980s, there have been more than 200 landslides in China with a death toll of 30 or more people at a time, economic losses of more than CNY 10 million or significant social impact. Therefore, the study of landslide displacement prediction is very important. The traditional ARIMA and LSTM models are commonly used for forecasting time series data. In our study, a multivariable LSTM landslide displacement prediction model is proposed based on the traditional LSTM model, which integrates rainfall and reservoir water level data. Taking the Baijiabao landslide in the Three Gorges Reservoir area as an example, the data of displacement, rainfall and reservoir water level of monitoring point ZG323 from November 2006 to December 2012 were selected for this study. Our results show that the displacement prediction results of the multivariable LSTM model are more accurate than those of the ARIMA and the univariate LSTM models, and the mean square, root mean square and mean absolute errors are the smallest, which are 0.64223, 0.8014 and 0.50453 mm, respectively. Therefore, the multivariable LSTM model method has higher accuracy and better application prospects in the displacement prediction of the Baijiabao landslide, which can provide a certain reference for the displacement prediction of the same type of landslide.

[Severity Differences and Mechanisms of Algal Blooms Among Sections in Pengxi River of the Three Gorges Reservoir]

Since the impoundment of the Three Gorges Dam, 50% of the first-order tributaries in the reservoir area have had frequent algal blooms but with variations regarding the geographical locations of the seriously bloomed sections and the scope of the latter being influenced by the mainstream. This study took the Pengxi River, a first-order tributary of the reservoir area, as an example in order to explore the difference in eutrophication among the river sections and the influence of the Yangtze River on its tributaries. During the spring bloom season of 2019, sampling was carried out in one-week intervals for a total duration of one month. Seven sampling sections (PX1-PX7) were set up from the confluence to upstream. According to the profiles of vertical water temperature and conductivity of each section, the influence scope and form of the backwater of the Yangtze River were inferred; in addition, severity differences and mechanisms of algal blooms among sections were explored through the comparison of the hydrology, water quality, and sediment nutrients among Gaoyang Lake (PX5), which has had serious algal blooms, and the upstream (PX6) and downstream (PX4) sections of PX5, which are both 4 km away from PX5. The results showed that during the sampling month, the average ρ(Chl-a) in the confluence area of the Pengxi River (PX1-PX4) and in the upstream (PX5-PX7) were in the range of 14.55-44.00 μg·L^-1 and 42.66-175.40 μg·L^-1, respectively. The ρ(Chl-a) of PX5 was up to 413.00 μg·L^-1, which was significantly higher than that of other sections (P<0.05). Temperature and conductivity results showed that the backwater from Yangtze River flowed into the Pengxi River from the middle and bottom layers during the period from April to May. The confluence (PX1-PX4) sections were in the intersection area of the backwater from Yangtze River and the upstream of the Pengxi River; thus, the waterbody was unstable, which was not conducive to the formation of algal blooms. However, the upstream (PX5-PX7) sections were not directly affected by the backwater from Yangtze River, leading the nutrient exchange mainly vertically. Most averages of n(TN)/n(TP) and n(DTN)/n(DTP) of PX4-PX6 were all greater than 16, indicating a phosphorus-limited state. During sampling, the average sediment total phosphorus of PX5 was 91% of that in upstream PX6, which was only 4 km away, whereas the surface water total phosphorus of PX5 was 180% of that in PX6. The important reason for this phenomenon is that the water surface width of PX5 was 3.6-4.7 times that of PX6, indicating longer wind fetch in the former section. Owing to the mountainous landscape in the Three Gorges Reservoir (TGR) region where windy weather is rare, the disturbance effect of wind and waves on PX5 was stronger than that of PX6, and the nutrients released from the sediment at the PX5 section caused by wind and waves resupplied the surface water more easily, causing more serious algal blooms at PX5 than those at the remaining sections in the Pengxi River. The main causes of the algal blooms in the tributaries of the TGR area lied in the stability of water stratification and the supply of internal phosphorus. The stability of water stratification was mainly affected by the backwater from Yangtze River, and the supply of internal phosphorus in the algal bloom season was affected by the special water stratification phenomenon of the tributaries of TGR-the "surface density layers." The duration and degree of weather disturbance to the surface density layers can be used to predict the time and scale of algal blooms.

[Pollution Characteristics and Ecological Risk Assessment of Antibiotics in Vegetable Field in Kaizhou, Chongqing]

The accumulation of antibiotics in farmland and its ecological risk have become a research hotspot at home and abroad. The objective of this study was to investigate the occurrence and accumulation of antibiotics and their potential environmental and ecological risks in vegetable fields in Kaizhou district of Chongqing country. The occurrence characteristics of antibiotics including tetracyclines, sulfonamides, quinolones, macrolides, and chloramphenicols were detected using experimental analysis. The results showed that there was an accumulation of antibiotics in the vegetable soil, and 18 antibiotics in five categories were detected (0-42.88 μg·kg^-1), mainly for tetracyclines and quinolones. The detection rate of quinolone antibiotics was the highest (15.38%-100%), especially for norfloxacin and ofloxacin (100%), whereas the tetracyclines presented the highest concentration (0-42.88 μg·kg^-1). The amount of total antibiotics in the vegetable soil was 1.64-233.11 μg·kg^-1, whereas different vegetable soils showed the following trend:water spinach soil (89.73 μg·kg^-1)>cabbage soil (32.53 μg·kg^-1)>pepper soil (32.16 μg·kg^-1)>tomato soil (32.13 μg·kg^-1)>cucumber soil (26.46 μg·kg^-1)>grassland (7.32 μg·kg^-1). The correlation results showed that there was a significantly positive correlation between total antibiotic residues and organic fertilizer application (P<0.05) but a significantly negative correlation with soil pH (P<0.05). Quinolones and sulfonamides were negatively correlated with soil water content (P<0.05), whereas quinolones positively correlated with soil available phosphorus and organic matter content (P<0.05). The potential eco-environmental risk assessment results showed that tetracyclines and quinolones in vegetable soil in Kaizhou district had certain ecological risks, of which 62%-92% and 62%-100% of soil samples with quinolones had potential toxicity to soil animals and microorganisms.

[Influence of Optimal Land Use Allocation on Phosphorus Loss in the Process of Rainfall and Runoff]

In order to explore the impact of optimized land use allocation on phosphorus loss in the runoff process of a small watershed in the Three Gorges Reservoir area, a traditional agricultural model catchment area (CG) and a catchment area after optimized land use allocation in the Shipanqiu watershed in Zhong County, Chongqing (EG) were selected as the research object, sampling at the outlets of the two catchment areas, respectively, to monitor the runoff process and different forms of phosphorus in rainfall events and to analyze the influence of land use configuration on the law of phosphorus loss. The results showed that:① in the 10 monitored rainfall and runoff processes, the ρ[total phosphorus (TP)] of EG was lower than that of CG, and the ranges of the two were 0.09-0.75 mg·L^-1 and 0.13-2.82 mg·L^-1, respectively. Compared with that of CG, EG significantly reduced the peak value of TP. ② The average EMC of TP, total dissolved phosphorus (TDP), dissolved inorganic phosphorus (DIP), and particulate phosphorus (PP) in the process of rainfall and runoff was lower than that of CG, and EG and CG showed significant differences in EMC of TP, TDP, and DIP (P<0.05); the main form of phosphorus loss in the two catchment areas in the process of rainfall and runoff was TDP, but the average TDP/TP of EG was larger. ③ The output load of EG was 45%, 43%, 57%, and 47% lower than that of the TP, TDP, DIP, and PP in CG. The output load of EG and CG of various forms of phosphorus was significantly correlated with the total runoff (P<0.01). In addition, the slope of the linear fitting of various phosphorus forms in the CG catchment area to the total amount of runoff was 1.66 to 1.75 times that of EG. The output load of various phosphorus forms of CG was more sensitive to runoff than that of EG, and the output per unit area in the process of rainfall and runoff was more sensitive than that of EG. The amount of sand could have caused the difference in phosphorus concentration and output load more than the output per unit area. Optimizing land use configuration can effectively reduce the loss of various forms of phosphorus and provide a reference for the prevention and control of phosphorus loss in small watersheds in the Three Gorges Reservoir area.

Temporal and spatial analysis of the ecosystem service values in the Three Gorges Reservoir area of China based on land use change

The Three Gorges Reservoir area (TGRA) has complex geological conditions and a fragile ecological environment. The construction of the Three Gorges Project triggered ecological and environmental issues and social disputes, which have attracted considerable attention in recent years. However, how the temporal and spatial characteristics of ecosystem service value (ESV) in the TGRA changed in each stage of the Three Gorges Project with the implementation of ecological restoration plans remains ambiguous. Based on four periods of land use data from 2000 to 2018, the changes in land use were investigated, and the ESVs were estimated. Then, the spatial distribution and dynamic changes in ecosystem services were analysed. The results showed that grassland and construction land were the land use types that had the greatest reductions and increases in area over time, respectively. The conversion of cropland to forestland, grassland and construction land represented the most important land type changes. In the past 18 years, because of an increase in forestland and water area, the ESVs increased by 2.7 billion yuan, with a growth rate of 3.46%. The conversion of cropland to forestland had the largest contribution rate to the increase in ESV. The ESV was higher in the northeast and lower in the southwest, and its changes had a significant positive autocorrelation in terms of the spatial distribution. The hot spots of ESV change were mainly distributed in the main stream of the Yangtze River and the reservoir area. This research provides a reference for land resource allocation and experience for the ecological environment protection and sustainable development of the Yangtze River Basin.

[Comparison Between Tributary and Main Stream and Preliminary Influence Mechanism of CO2 Flux Across Water-air Interface in Wanzhou in the Three Gorges Reservoir Area]

The main stream of the Three Gorges Reservoir area in Wanzhou and its tributary (the Pengxi River) were selected as a survey area to monitor the CO₂ concentration. Twelve related indicators were selected during the blooming period from April to September 2019, which were divided into Climate factors, Water environment factors, Carbon source factors, Nutrient factors, and Sediment factors. These factors were considered for further discussion of the impact pathways and contribution to CO₂ flux. The average CO₂ fluxes of Gaoyang (the Pengxi River), Huangshi (the Pengxi River), and Wanzhou (the main stream) were (1.445±1.739), (3.118±2.963), and (2.899±1.144) mmol·(m²·h)^-1, respectively, showing that Gaoyang < Wanzhou < Huangshi. The CO₂ flux of tributary showed a large variation, while the main stream had a relatively small variation, which is a stable "source" of CO₂. The main stream of the Yangtze River, as a hub for the transportation of biomass from land to sea, has higher carbon concentration and higher flow rate than its tributaries, which makes the CO₂ flux of the main stream usually larger than that of the tributary. However, the difference in hydrological conditions result in spatial differences in CO₂ flux at different points of the same tributary. Gaoyang is located in the reservoir bay, which is conducive to the growth of phytoplankton and the CO₂ flux is lower; Huangshi is located in a river with a faster flow rate. The backwater support and backflow of the main stream make the CO₂ flux significantly greater than that at Wanzhou. The effects of various indicators on the CO₂ flux are also markedly different in the tributary and main stream. Temperature (T), DO, dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) have significant effects on CO₂ fluxes in the tributary, while NH₄⁺-N has a significant impact on CO₂ fluxes in the main stream. Nutrient factors and carbon source factors contribute 32.37% and 27.25%, respectively, to CO₂ flux, accounting for more than half of the total, followed by climate factors, water environment factors, and sediment factors, which contribute 18.81%, 13.49%, and 8.08%, respectively. Reservoir CO₂ emission control can focus on controlling the eutrophication and carbon sources; phenomena such as global warming and sedimentation will also have a certain impact on the CO₂ emission of reservoirs.

[Effects of Different Land Use Practices on Nitrogen Loss from Runoff During Rainfall Events]

To understand the effect of nitrogen from runoff during rainfall events for different land uses, sub-catchments A and B in the small Shipanqiu watershed in Zhong County, Chongqing-which were managed using different land use practices-were taken as research objects. Runoff flow and nitrogen levels at the outlet of the catchment were monitored. Sub-catchment A is an agroforestry-water complex and sub-catchment B is the site of traditional agriculture. EMC was used to evaluate the average concentration of runoff nitrogen during rainfall events, and the effect of this runoff nitrogen on the small watershed with different land use systems was analyzed. The results showed that the TN concentration in catchment B (1.37-15.17 mg·L^-1) > catchment A (0.84-9.28 mg·L^-1); the ratio of the first peak to the second peak in catchment A was 62%, which was far less than the 97% in catchment B; the average DN/TN values were 69% and 75% in catchments A and B, respectively; and the average NN/DN values were 67% and 80% in catchments A and B, respectively. The different land use practices have significant impacts on nitrogen loss. Compared with the catchment where traditional agricultural practices were followed, the agroforestry-water complex catchment effectively reduced the loss of nitrogen and decreased the first TN peak value and DN/TN and NN/DN values. This study provides a scientific basis for the prevention and control of non-point source pollution in small watersheds in the area of the Three Gorges Reservoir.

Effects of Ecological Restoration Measures on Soil Erosion Risk in the Three Gorges Reservoir Area Since the 1980s

Ecosystem degradation accompanied by soil erosion risk is caused by the interaction of many factors, including climate change and human activities. Therefore, before attempting the optimal form of ecological restoration, we must know the key factors responsible for soil erosion risk and determine their impacts on the ecosystem health. To test this approach, we conducted a case study in the Three Gorges Reservoir Area from 1980 to 2015, where extensive restoration (primarily afforestation) has been conducted. The results showed that climate was most important during Period I (1980 to 1984), and explained 84% of the variation in erosion. However, vegetation became equally important during Period II (1985 to 2006), when it accounted for 51% of the variation. Climate became as important as vegetation during Period III (2007 to 2015), when it accounted for 51% of the variation. The temporal variation in the dominant factors that controlled soil erosion risk suggests that the ecological effect of vegetation improvement resulting from ecological restoration in Three Gorges Reservoir Area has been gradually enhanced since the 1980s.

[Emission Characteristics of Nitrogen and Phosphorus in a Typical Agricultural Small Watershed in Tuojiang River Basin]

The emission of nitrogen and phosphorus via non-point source pollution from a small watershed has become the main pollution source of river waters, while climatic conditions and human activities directly affect the changes in rainfall-runoff and types of land use that are closely related to nitrogen and phosphorus pollution. In this study, we explore the runoff loss, nitrogen and phosphors concentration, and nitrogen and phosphorus emission in Huajiaogou small watershed on the upper reaches of Yangtze River. The rainfall, runoff, and temporal changes of nitrogen and phosphorus were analyzed using the continuous position monitoring data. The results showed that:① the runoff volumes were 10.05×10⁵ m³ and 3.34×10⁵ m³ from July 1^st to September 30^th, accounting for 76.58% and 56.51% in 2012 and 2013, respectively, and they were positively correlated to rainfall. The peak concentrations of ammonia nitrogen (NH₄⁺-N) from April 1^st to June 30^th were 11.51 mg ·L^-1 and 4.44 mg ·L^-1in 2012 and 2013, respectively. ② The NH₄⁺-N emission risk period was from July 1^st to September 30^th, accounting for 78.45% and 62.24% in 2012 and 2013, respectively. The peak concentration and emission risk period of total nitrogen (TN) and nitrate nitrogen (NO₃^--N) were from July 1^st to September 30^th, and NO₃^--N was the main form of the total nitrogen emission. The peak concentration of NO₃^--N was 6.06 mg ·L^-1 and 11.43 mg ·L^-1in 2012 and 2013, respectively, and the loss in NO₃^--N from July 1^st to September 30^th accounted for 88.74% and 65.55% in 2012 and 2013, respectively. ③The emission risk period of total phosphorus (TP), dissolved total phosphorus (DTP), and particulate phosphorus (PP) was also from July 1^st to September 30^th, and the particulate phosphorus was the main form of the total phosphorus emission. The particulate phosphorus emission from July 1^st to September 30^th accounted for 36% and 68% in 2012 and 2013, respectively, and the ration of particle phosphorus to total phosphorus was easily affected by rainfall.

[Characteristics of Nitrogen and Phosphorus Output and Loss Flux in the Shipanqiu Watershed, Three Gorges Reservoir Area]

As the source of non-point pollution in the Three Gorges Reservoir Area, small watershed is a key control object in alleviating deterioration of water quality. In the Three Gorges Reservoir Area, the Shipanqiu small watershed with various land-use types was selected as the research object, and the water quantity and quality of the outlet section of the watershed were continuously monitored. We carried out analysis of the small watershed runoff loss and nitrogen and phosphorus pollutants with concentration, analyzed the morphology change characteristics of runoff erosion, calculated the small watershed of pollutant emission flux, and analyzed the nitrogen and phosphorus nutrient loss and main human and natural factors, especially in the Three Gorges Reservoir Area of agriculture where nonpoint source pollution research has important practical significance. The results showed that the rainfall in the watershed varied significantly with the seasons, and the rainfall was mainly distributed from April to June, which was the main output period of nitrogen and phosphorus loss in the small watershed, accounting for 58.94% and 67.60% of the total nitrogen and phosphorus load, respectively, in the whole year. The total annual runoff in the Shipanqiu small watershed was 8.02×10⁴ m³, and the annual total nitrogen loss flux was 5.04 kg·hm^-2, of which nitrate nitrogen (2.54 kg·hm^-2) was the main part. The total phosphorus output was 0.534 kg·hm^-2, and the soluble total phosphorus (0.422kg·hm^-2) accounted for 79.00% of the total phosphorus flux. The loss flux of total nitrogen was 9.51 times that of total phosphorus, and the non-point source pollution risk of nitrogen was much greater than that of phosphorus. Therefore, for the Shipanqiu small watershed, it is especially important to prevent nitrogen loss in paddy fields when fertilization and rainfall coincide.

Dual benefits of long-term ecological agricultural engineering: Mitigation of nutrient losses and improvement of soil quality

Soil erosion of sloped farmland in the Three Gorges Reservoir area (TGRA) has led to the serious loss of nutrients, soil quality degradation and the downstream water quality being threatened. Thus, a series of ecological agricultural engineering measures was established in 2011, as a field experiment using citrus (navel orange) plants to reduce soil erosion, which was monitored from 2011 to 2018. These ecological agricultural engineering measures included three treatments: 1) citrus intercropped with white clover (WC), 2) citrus orchard land mulched with straw (SM) and 3) citrus intercropped with hemerocallis (Hemerocallis flava) contour hedgerows (CH). The conventional citrus orchard management was regarded as control (CK). The results show, that compared with CK, nutrient loss from the experiments were reduced by the following amounts: for nitrogen - WC (35.5%), SM (44.0%) and CH (52.0%); for phosphorus - WC (40.0%), SM (51.7%) and CH (58.3%). Therefore, the ecological agricultural engineering measures effectively mitigate the nutrient loss loads of the navel orange citrus gardens. The citrus intercropped with the hemerocallis hedgerows is the most effective measure for the control of nutrient loss. After 8 years of experiment, the soil quality represented by average soil quality index (SQI) in these three treatments, are significantly higher than that of the CK (and the beginning of the experiment). This is because the application of these measures prevented the loss of: soil organic matter, bulk density and total phosphorus. It is predicted that the soil qualities of these three treatments will remain in the range of soil grade II and I for the next 5 years but the soil quality of CK will decrease to soil quality grade II and III. These results show that ecological agricultural engineering measures are a long-term promising and feasible method to reduce soil erosion and enhance soil quality.

Establishment of Landslide Groundwater Level Prediction Model Based on GA-SVM and Influencing Factor Analysis

The monitoring and prediction of the landslide groundwater level is a crucial part of landslide early warning systems. In this study, Tangjiao landslide in the Three Gorges Reservoir area (TGRA) in China was taken as a case study. Three groundwater level monitoring sensors were installed in different locations of the landslide. The monitoring data indicated that the fluctuation of groundwater level is significantly consistent with rainfall and reservoir level in time, but there is a lag. In addition, there is a spatial difference in the impact of reservoir levels on the landslide groundwater level. The data of two monitoring locations were selected for establishing the prediction model of groundwater. Combined with the qualitative and quantitative analysis, the influencing factors were selected, respectively, to establish the hybrid Genetic Algorithm-Support Vector Machine (GA-SVM) prediction model. The single-factor GA-SVM without considering influencing factors and the backpropagation neural network (BPNN) model were adopted to make comparisons. The results showed that the multi-factor GA-SVM performed the best, followed by multi-factor BPNN and single-factor GA-SVM. We found that the prediction accuracy can be improved by considering the influencing factor. The proposed GA-SVM model combines the advantages of each algorithm; it can effectively construct the response relationship between groundwater level fluctuations and influencing factors. Above all, the multi-factor GA-SVM is an effective method for the prediction of landslides groundwater in the TGRA.

[Characteristics of Nitrogen and Phosphorus Losses and Runoff in a Typical Purple Soil Watershed in the Three Gorges Reservoir Area]

To understand the runoff pollution characteristics of the typical purple soil small watershed in the Three Gorges Reservoir (TGR), the rainfall-runoff duration under typical land use types of the Xinzheng watershed and the nitrogen (N) and phosphorus (P) concentrations in the outlet runoff of each catchment were dynamically monitored, and the loss rule of N and P in the rainfall runoff in the small watershed in the TGR was investigated and analyzed. The results showed that the losses of runoff N and P were 13.69 kg·(hm²·a)^-1 and 1.50 kg·(hm²·a)^-1, respectively. N and P in agricultural fertilizers and rainfall scouring were the main causes of runoff pollution in the Xinzheng watershed. The average concentration of total nitrogen (TN) and total phosphorus (TP) reached 10.05 mg·L^-1 and 1.10 mg·L^-1, far exceeding the occurrence standard of eutrophication, which should be cause for concern. The nitrate nitrogen (NN) and particulate phosphorus (PP) in the rainfall were 69.47 kg and 6.83 kg on August 15, 2010, accounting for 53.91% and 53.78% of TN and TP, respectively. The NN and AN were 6.68 kg and 5.61 kg, respectively, in the rainfall on August 26, 2010, accounting for 37.74% and 31.69% of TN and PP was 1.36 kg, accounting for 57.63% of TP, indicating that nitrogen loss was mainly through the soluble state, while phosphorus migration was dominated by particulate matter. Heavy rainfall in the Xinzheng watershed had a significant impact on the loss of nitrogen and phosphorus. This was related to the properties of purple soil, such as thin soil layer, frequent cultivation and relatively loose soil.

[Effects of Agricultural Activities on Soil Mercury Changes in the Water-Level-Fluctuating Zone of the Three Gorges Reservoir]

Farming in the water-level-fluctuating zone (WLFZ) of the Three Gorges reservoir, China, may result in a wide range of environment problems when the sediment is exposed to air. In this study, Qukou, Chongqing, was selected as the research site due to its large WLFZ area and significant agricultural activities. Four kinds of land use types, including rice, maize, vegetables, and grassland, were selected to investigate the distribution of mercury (Hg) in the surface soil. The results showed that the mean concentrations of soil total Hg (THg), bioavailable Hg (Hg-wh), and methylmercury (MeHg) in the surface soil were 25.80-68.74 ng ·g^-1, 0.44-0.88 ng ·g^-1, and 0.08-0.85 ng ·g^-1, respectively. The concentrations of THg, Hg-wh, and MeHg in non-cultivated soil were higher than those in cultivated soil, indicating that farming disturbances could accelerate soil Hg loss. The MeHg concentrations in both non-cultivated and cultivated soil increased up to the maximum value 1-2 months after drying, and then gradually decreased to a relatively stable level. The peak value was approximately four times higher than that at the end of flooding. The percentage of MeHg to THg (% MeHg) was similar to the distribution of soil MeHg, and the peak value occurred at 1 month after drying. However, when% MeHg decreased to the stable level, no significant difference was found compared to the value at the end of flooding (P>0.05). Furthermore, the soil% MeHg had a significantly positive correlation with Hg-wh (r=0.642, P<0.01), while there was no significant correlation with THg (P>0.05), suggesting that Hg methylation was mainly affected by the bioavailability of Hg in the seasonally inundated soil of the WLFZ.

[Effect of Sediments on Bioaccumulation of Mercury in Fish Body in the Water-Level-Fluctuating Zone of the Three Gorges Reservoir Area]

Mercury (Hg) in the aquatic environment is easy to accumulate in fish. In order to study the effect of the sediments on Hg accumulation in fish in the water-level-fluctuation zone of the Three Gorges Reservoir, we conducted a 90-days simulated flooding experiment by using the sediments with different concentrations of Hg. Our study showed that the concentrations of the total mercury (THg) and methyl mercury (MeHg) in the overlying water increased after flooding, and the concentrations in the muscle of fish kept increasing in the period of experiment, the concentrations in the viscera and head increased in the earlier period but seemingly decreased in the later period. The bioaccumulated Hg content in the fresh was higher than that in the viscera and head, between which there was no significant difference. Compared with the control group (no sediment), the presence of sediments obviously increased the content of Hg bioaccumulated in fish, and the bioaccumulated Hg level increased with the Hg concentrations in sediment. The THg and MeHg in different fish parts presented a similar variation trend with the BCF ranging 1.93×10⁵-8.89×10⁵ for MeHg and 1.3×10³-12.8×10³ for inorganic mercury, indicating that MeHg was more prone to accumulate in fish. The MeHg in fish was significantly related with THg, and accumulated MeHg occupied about 80.1% (muscle), 79.3% (visceral) and 66.7% (head) of increased THg. After the reflooding of the sediment in the water-level-fluctuating zone, net methylation could occur with MeHg as the product, and then MeHg would diffuse to overlay water, further increasing the Hg bioaccumulation in fish. Therefore, the potential pollution risk of Hg in the water-level-fluctuationg zone with large area of the Three Gorges Reservoir cannot be ignored.

[Spectral Characteristics of Chromophoric Dissolved Organic Matter (DOM) from a Typical Reservoir Lake from Inland of Three Gorges Reservoir Areas: In the View of Riparian Ecosystem Analysis]

For further understanding the geochemical characteristics of dissolved organic matter (DOM) in "reservoir-type" lake, in this study a typical "reservoir-type" lake, Changshou Lake located in inlands of Three Gorges Reservoir areas, was selected to investigate the composition, sources and spatial distributions of chromophoric DOM (CDOM) in this lake, through UV-Vis and three-dimensional fluorescence spectroscopy combined within analysis of riparian eco-system differences. The results showed that DOM concentrations including dissolved organic carbon (DOC) and CDOM abundance in different sampling sites varied spatially to a certain degree, in contrast to fluorescence component of DOM showing a constant level. In backwater zone (also called stagnant zone alternatively), due to accumulation of DOM and limited impact of terrestrial contribution, the endogenesis (autochthonous production) was the predominant geochemical process showing an obvious "authigenic-control" characteristic, and relatively lower aromaticity and molecular weight. In comparison, sampling sites surrounding artificial forests and tourism development, the highly humic (highly aromatic) substances resulted from terrestrial inputs were the main source, but discharge due to human activities was also responsible for highly protein-like component observed in DOM. Additionally, in the entry zones of lake from upstream river, DOM was affected by fruit plantations and residences, contribution from upstream river could also be neglected. From the other aspects, some significant correlations were also observed, which were independent of riparian eco-system differences, for example, the SUVA₂₈₀ versus S_(275-295) (negative), CDOM versus FDOM (positive), and CDOM and S_(275-295) (negative), indicating the correlation was an intrinsic property of DOM that could not be affected by the surrounding environment. Meanwhile, the main chromophoric component of CDOM in Changshou Lake was high molecular weight (HMW) components within highly aromatic structures. At least, 51% of CDOM dynamic could be explained by changes of FDOM, especially in backwater zone the co-variance was more obvious. Furthermore, when the traditional fluorescence index (FI) could not comprehensively explain the differences of DOM sources due to very similar (statistically insignificant) values, integration of UV-Vis and fluorescence spectral characteristics and detailed discussion of riparian eco-system differences, might be an important way to help further resolution of DOM composition and sources in aquatic environments, such as lake, river and watershed.

Soil stability characteristics of mulberry lands at hydro-fluctuation belt in the Three Gorges Reservoir area, China

The hydro-fluctuation belt in the Three Gorges Reservoir area is a typical seasonal and artificial wetland system and ecologically fragile zone. Using the widely existing mulberry forest lands in the hydro-fluctuation belt as an example and the 180-m water-level forest land as a control, this paper analyzes the soil stability of mulberry forestlands at different water levels in the hydro-fluctuation belt by analyzing and comparing the changes between soil physical and mechanical properties. The results indicated that (1) water-level changes, such as rising, flooding, draining, and exposure, affect the soil structure in mulberry forestlands. The soil agglomeration statuses for the soil layers decreased from 180 > 175 > 170 > 165 m, and the soil agglomeration statuses at a depth of 0∼20 cm decreased by 43.79, 44.95, and 57.45% compared with the control. (2) The soil water stability index decreased as follows: 180 > 170 > 175 > 165 m, which only accounted for 50.00, 47.73, and 40.91% of the control. In addition, the soil water stability indexes for the topsoils at various water levels were 1.87 (180 m), 1.67 (175 m), 2.92 (170 m), and 1.86 (165 m) times greater than those of the subsoils; thus, the resistance to hydraulic dispersion and disintegration were greater in the topsoil than in the subsoil. (3) The soil aggregate stability index decreased from 180 > 165 > 170 > 175 m and by 22.75, 23.53, and 35.29% compared with the control. (4) The soil shear strengths (composed of the cohesive force C and the internal friction angle φ) of the topsoils at water levels of 175, 170, and 165 m were significantly lower than in the control, and the internal friction angles decreased by 10.52, 19.08, and 43.25% and the cohesive force decreased by 9.88, 16.36, and 27.51%, respectively. The stability of the soil structure was greatly influenced by the soil clay content, soil organic matter content, and waterlogging duration. The study results could provide scientific support for soil and water conservation in the hydro-fluctuation belt and for biological filter construction in the Three Gorges Reservoir area to control the transport of sediment and non-point source pollutants.