Local-Scale Damming Impact on the Planktonic Bacterial and Eukaryotic Assemblages in the upper Yangtze River.

Dam construction and impoundment cause discontinuities in the natural biophysical gradients in rivers. These discontinuities may alter distinctive habitats and different microbial community assembly mechanisms upstream and downstream of dams, which reflect the potential impacts of damming on riverine aquatic ecosystems. In this study, we investigated the planktonic microbial assemblages of three large dams in the upper Yangtze River by using high-throughput sequencing. The results revealed that the alpha diversity indexes increased downstream of the dams. In addition, more eukaryotic ASVs solely occurred downstream of the dams, which indicated that a large proportion of eukaryotes appeared downstream of the dams. The nonmetric multidimensional scaling analysis indicated that there was no obvious geographic clustering of the planktonic microbial assemblages among the different locations or among the different dams. However, the dam barriers changed dam-related variables (maximum dam height and water level) and local environmental variables (water temperature, DOC, etc.) that could possibly affect the assembly of the planktonic microbial communities that are closest to the dams. A co-occurrence network analysis demonstrated that the keystone taxa of the planktonic bacteria and eukaryotes decreased downstream of the dams. In particular, the keystone taxa of the eukaryotes disappeared downstream of the dams. The robustness analysis indicated that the natural connectivity of the microbial networks decreased more rapidly upstream of the dams, and the downstream eukaryotic network was more stable. In conclusion, damming has a greater impact on planktonic eukaryotes than on bacteria in near-dam areas, and planktonic microbial assemblages were more susceptible to the environmental changes. Our study provides a better understanding of the ecological effects of river damming.

Damming river shapes distinct patterns and processes of planktonic bacterial and microeukaryotic communities.

Planktonic bacterial and microeukaryotic communities play important roles in biogeochemical cycles, but their biogeographic patterns and community assembly processes in large damming rivers still remain unclear. In this study, 16S rRNA and 18S rRNA coding genes were used for sample sequencing analysis of planktonic bacterial and microeukaryotic communities in the upper Yangtze River. The upper Yangtze River was divided into dam-affected zones and river zones based on the influence of dams. The results showed that there were significant differences in the bacterial and microeukaryotic communities between the two zones and that dams significantly reduced the α-diversity of the bacterial communities. Co-occurrence network analysis indicated that networks in the river zone were denser than those in the dam-affected zone. The relationships among species in bacterial networks were more complex than those in microeukaryotic networks. Dispersal limitation and ecological drift were the main processes influencing planktonic bacterial and microeukaryotic communities in the dam-affected zone respectively, whereas the role of deterministic processes increased in the river zone. Anthropogenic activities and hydraulic conditions affected suspended sediment and controlled microbial diversity in the river zone. These results suggest that dams impact planktonic bacteria more strongly than planktonic microeukaryotes, indicating that the distribution patterns and processes of the bacterial and microeukaryotic communities in large rivers are significantly different.

Changes in planktonic and sediment bacterial communities under the highly regulated dam in the mid-part of the Three Gorges Reservoir.

Bacterial communities play an important role in the biogeochemical cycle in reservoir ecosystems. However, the dynamic changes in both planktonic and sediment bacterial communities in a highly regulated dam reservoir remain unclear. This study investigated the temporal distribution patterns of bacterial communities in a transition section of the Three Gorges Reservoir (TGR) using Illumina MiSeq sequencing. Results suggested that in comparison to the planktonic bacteria, sediment bacteria contributed more to the reservoir microbial communities, accounting for 97% of the 7434 OTUs. The Shannon diversity index in the water (3.22~5.68) was generally lower than that in the sediment (6.72~7.56). In the high water level period (January and March), Proteobacteria, Actinobacteria, Cyanobacteria, and Firmicutes were the most abundant phyla, whereas in the low water level period (May, July, and September), the dominant phyla were Proteobacteria, Actinobacteria, and Bacteroidetes. Sediment samples were dominated by Proteobacteria, Chloroflexi, and Acidobacteria. Principal coordinate analysis of the bacterioplankton communities showed greater sensitivity to monthly changes than that of the sediment bacterial communities. Network analysis suggested that in comparison to planktonic bacterial communities, sediment bacterial communities were more complex and stable. The linear relationship between the CH4/CO2 ratio, water level, and relative abundance of methanotrophs highlighted the potential methane-oxidizing process in the mid-part of the TGR. Moreover, the potential impact of dam regulation on the bacterial communities was revealed by the significant relationship between abundant phyla and the inflow of the TGR. KEY POINTS: • Bacterioplankton communities showed great sensitivity to monthly changes. • Potential methane-oxidizing process was revealed in this representative area. • Water inflow regulated by dam has significant effects on dominant bacterioplankton.

Exploring the feasibility of sewage treatment by algal-bacterial consortia.

Current biological wastewater treatment is energy intensive. The application of algal-bacterial consortia to treat wastewater has recently attracted considerable attention because mechanical aeration is unnecessary. Therefore, algal-bacterial bioreactors are emerging as alternatives to activated sludge-based bioprocesses. Most studies have used a plate substratum to support the growth of algal-bacterial biofilms, which results in low reactor efficiencies. Usually, 2-10 days are required for targeted pollutant removal effects. Substratum structures can significantly influence reactor efficiencies. Indeed, substratum-free biofilms (granules) generally achieve high reactor efficiencies that rapidly form. 7-12 h are sufficient for a high-level pollutant removal efficiency. However, granule stability must be validated during long-term experiments (>1 year) involving real wastewater. In addition, the application of algal-bacterial membrane bioreactors represents a novel treatment approach. In membrane bioreactors, good reactor efficiencies and stabilities can be achieved. However, the maximum capacity of algal-bacterial membrane bioreactors requires further investigation. In addition, an accurate model for pollutant removal kinetics in algal-bacterial reactors is not yet available but is necessary for reactor control and up-scaling. The microbial and physical structures of algal-bacterial biofilms require more studies to clarify the system. Finally, the operational costs of algal-bacterial systems must be kept low in order to enhance their potential for sewage treatment at large scales. Good illumination control and recycling biomass for biodiesel or methane production could be applied to reducing the operation cost.

Divergent responses of taxonomic and predicted functional profiles of bacterioplankton to reservoir impoundment.

Freshwater ecosystems are undergoing extensive human disturbance of dam construction which form large amounts of reservoirs and lead to dramatic changes in hydraulic conditions. Bacterioplankton are key component of aquatic ecosystems. Investigation on their taxonomic compositions and associated functions responded to reservoir operation is essential to understand the ecological consequence of dam construction. In this study, we use the Three Gorges Reservoir as a model system. High-throughput sequencing is used to investigate the bacterioplankton community composition, and the bioinformatic tool of Tax4Fun is applied to predict the potential metabolic functions responded to reservoir impoundment. Results show that the taxonomic communities of bacterioplankton are significantly impacted by impoundment. The dominant group of Actinobacteria which accounts for 17.0%-58.1% of the retrieved sequences significantly increases after impoundment on phylum level. The influences of impoundment appear to be more apparent on order level that the relative abundances of four groups including Frankiales, Sphingomonadales, Sphingobacteriales and SubsectionI of class Cyanobacteria significantly vary after impoundment. In contrast, the predicted functional communities of bacterioplankton remain relatively stable that most of predicted functional categories including methane and nitrogen metabolisms have no significant variation after impoundment. Besides, significant distance decay patterns appear on the taxonomic communities after impoundment rather than the predicted functional communities. The environmental variables show significant impacts on the taxonomic community rather than predicted functional community, whereas the spatial variables have no effect on both taxonomic and predicted functional communities. In general, the taxonomic and predicted functional communities of bacterioplankton exhibit divergent responses to the impoundment in reservoir.

The carbon footprint of large- and mid-scale hydropower in China: Synthesis from five China's largest hydro-project.

The past two decades have witnessed growing global concern about excessive greenhouse gas (GHG) emissions by reservoirs and the development of hydropower. Literature review showed that life cycle GHG emissions per energy production of collected global dataset ranged from 0.04 to 237.0 gCO2eq/kW⋅h, with a mean of 25.8 ±â€¯3.0 gCO2eq/kW⋅h. Synthesis from the China's five largest hydro-projects and other publications estimated that the large- and mid-scale hydro-projects in China had a carbon footprint between 6.2 gCO2eq/kWh and 34.6 gCO2eq/kWh, with a mean value of 19.2 ±â€¯6.8 gCO2eq/kWh (mean ±â€¯sd.). Over 80% of the carbon footprint of the hydro-projects could be conservatively allocated to hydroelectricity generation, while the rest could then be allocated to flood control services. In the Three Gorges Dam Project, the allocated life cycle GHG emissions per energy production of its hydroelectricity production was estimated to be 17.8 gCO2eq/kW⋅h. GHG emissions from reservoir sediments and in the phase of operation and maintenance were still uncertain. There is still a need of in-depth research on reservoir carbon cycling to quantify net reservoir GHG emissions.

The interactions of composting and biochar and their implications for soil amendment and pollution remediation: a review.

Compost and biochar, used for the remediation of soil, are seen as attractive waste management options for the increasing volume of organic wastes being produced. This paper reviews the interaction of biochar and composting and its implication for soil amendment and pollution remediation. The interaction of biochar and composting affect each other's properties. Biochar could change the physico-chemical properties, microorganisms, degradation, humification and gas emission of composting, such as the increase of nutrients, cation exchange capacity (CEC), organic matter and microbial activities. The composting could also change the physico-chemical properties and facial functional groups of biochar, such as the improvement of nutrients, CEC, functional groups and organic matter. These changes would potentially improve the efficiency of the biochar and composting for soil amendment and pollution remediation. Based on the above review, this paper also discusses the future research required in this field.

Performance of an anaerobic membrane bioreactor in which granular sludge and dynamic filtration are integrated.

To alleviate the fouling of a filter, simple substrates, dynamic filtration, and granular sludge were applied in an anaerobic membrane bioreactor (AnMBR). The results showed that under a transmembrane pressure < 20 kPa, the filter flux ranged between 15 and 20 l (m-2 h)-1 for a period of 30 days. The flux was higher than the typical flux of AnMBRs with conventional membranes and most current dynamic filters. In addition, the low cost of the filter avoided the need for a higher flux. Moreover, a stable granular sludge bed, which consumed all volatile fatty acids, was maintained. A compact fouling/filtration layer formed on the filter, which contributed to low effluent chemical oxygen demand concentrations and turbidity. In addition, substrate scarcity in the filtration zone resulted in the evolution of diverse bacteria on the filter.

Response of rhizosphere microbial community structure and diversity to heavy metal co-pollution in arable soil.

Due to the emerging environmental issues related to heavy metals, concern about the soil quality of farming lands near manufacturing district is increasing. Investigating the function of soil microorganisms exposed to long-term heavy metal contamination is meaningful and important for agricultural soil utilization. This article studied the potential influence of several heavy metals on microbial biomass, activity, abundance, and community composition in arable soil near industrial estate in Zhuzhou, Hunan province, China. The results showed that soil organic contents (SOC) were significantly positive correlated with heavy metals, whereas dehydrogenase activity (DHA) was greatly depressed by the heavy metal stress. Negative correlation was found between heavy metals and basal soil respiration (BSR), and no correlation was found between heavy metals and microbial biomass content (MBC). The quantitative PCR (QPCR) and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis could suggest that heavy metal pollution has significantly decreased abundance of bacteria and fungi and also changed their community structure. The results could contribute to evaluate heavy metal pollution level in soil. By combining different environmental parameters, it would promote the better understanding of heavy metal effect on the size, structure, and activity of microbial community in arable soil.

Response of denitrifying genes coding for nitrite (nirK or nirS) and nitrous oxide (nosZ) reductases to different physico-chemical parameters during agricultural waste composting.

The present research was performed to clarify the changes of denitrifying genes (nirK, nirS, and nosZ) abundances under different physico-chemical parameters through evaluating the relationships between the genes abundances and parameters during agricultural waste composting. The genes abundances were determined by real-time quantitative PCR (qPCR). The correlations between physico-chemical parameters and denitrifying genes abundances were analysed by regression analysis. qPCR results showed that the nosZ gene abundance was higher than that of nirK and nirS genes. The nirK gene abundance was higher than nirS gene indicating that nitrite reducers with Cu-containing enzyme encoded by nirK gene were more of importance than those with cytochrome cd1 nitrite reductase encoded by nirS gene in the nitrite reduction step. Regression analysis suggested that (1) nirK gene abundance was correlated with pile temperature following quadratic model; (2) nirS gene abundance was linearly correlated with pile temperature and concentration of NH4 (+), while correlated with concentration of NO3 (-) and pH following inverse and quadratic model respectively; (3) nosZ gene abundance was quadratically correlated with pH and linearly correlated with water soluble carbon (WSC).

Environmental factors shaping the abundance and distribution of laccase-encoding bacterial community with potential phenolic oxidase capacity during composting.

Increasing molecular evidence points to a wide occurrence of laccase-like multicopper oxidase (LMCO)-encoding genes in bacteria. Most researches mainly focused on the bacterial LMCO diversity, whereas the processes and the environmental factors responsible for structuring bacterial LMCO communities remain relatively unknown in a composting system. Six gene libraries were constructed from samples in representative stages during composting. A total of 185 sequences obtained from sample DNA extracts were classified to 59 operational taxonomic units (OTUs) based on 10 % cutoff. The distribution profile of bacterial LMCO genes showed that proteobacterial- and actinobacterial-associated species were the dominant communities during composting. Pearson correlation analysis indicated that the pile temperature and water-soluble carbon (WSC) content were significantly positively correlated with bacterial LMCO gene OTU numbers, Chao1 and Shannon index, whereas the humic acid (HA)-like carbon content had the most significant effect on the distribution of the bacterial LMCO genes during composting by redundancy analysis. These findings will improve the understanding of the mutual relationship between environmental factors and bacterial LMCO community compositions in composting.

Diversity of two-domain laccase-like multicopper oxidase genes in Streptomyces spp.: identification of genes potentially involved in extracellular activities and lignocellulose degradation during composting of agricultural waste.

Traditional three-domain fungal and bacterial laccases have been extensively studied for their significance in various biotechnological applications. Growing molecular evidence points to a wide occurrence of more recently recognized two-domain laccase-like multicopper oxidase (LMCO) genes in Streptomyces spp. However, the current knowledge about their ecological role and distribution in natural or artificial ecosystems is insufficient. The aim of this study was to investigate the diversity and composition of Streptomyces two-domain LMCO genes in agricultural waste composting, which will contribute to the understanding of the ecological function of Streptomyces two-domain LMCOs with potential extracellular activity and ligninolytic capacity. A new specific PCR primer pair was designed to target the two conserved copper binding regions of Streptomyces two-domain LMCO genes. The obtained sequences mainly clustered with Streptomyces coelicolor, Streptomyces violaceusniger, and Streptomyces griseus. Gene libraries retrieved from six composting samples revealed high diversity and a rapid succession of Streptomyces two-domain LMCO genes during composting. The obtained sequence types cluster in 8 distinct clades, most of which are homologous with Streptomyces two-domain LMCO genes, but the sequences of clades III and VIII do not match with any reference sequence of known streptomycetes. Both lignocellulose degradation rates and phenol oxidase activity at pH 8.0 in the composting process were found to be positively associated with the abundance of Streptomyces two-domain LMCO genes. These observations provide important clues that Streptomyces two-domain LMCOs are potentially involved in bacterial extracellular phenol oxidase activities and lignocellulose breakdown during agricultural waste composting.

Shifts in composition and function of bacterial communities reveal the effect of small barriers on nitrous oxide and methane accumulation in fragmented rivers.

Rivers are often blocked by barriers to form different habitats, but it is not clear whether this change will affect the accumulation of N2O and CH4 in rivers. Here, low barriers (less than 2 m, LB) increased N2O concentration by 1.13 times and CH4 decreased by 0.118 times, while high barriers (higher than 2 m, less than 5 m high, HB) increased N2O concentration by 1.19 times and CH4 by 2.76 times. Co-occurrence network analysis indicated LB and HB can promote the enrichment of Cyanobium and Chloroflexi, further limiting complete denitrification and increasing N2O accumulation. The LB promotes methanotrophs (Methylocystis, Methylophilus, and Methylotenera) to compete with denitrifiers (Pseudomonas) in water, and reduce CH4 accumulation. While the HB can promote the methanotrophs to compete with nitrifiers (Nitrosospira) in sediment, thus reducing the consumption of CH4. LB and HB reduce river velocity, increase water depth, and reduce dissolved oxygen (DO), leading to enrichment of nirS-type denitrifiers and the increase of N2O concentration in water. Moreover, the HB reduces DO concentration and pmoA gene abundance in water, which can increase the accumulation of CH4. In light of the changes in the microbial community and variation in N2O and CH4 accumulation, the impact of fragmented rivers on global greenhouse gas emissions merits further study.

Microbial community and soil enzyme activities driving microbial metabolic efficiency patterns in riparian soils of the Three Gorges Reservoir.

Riparian zones represent important transitional areas between aquatic and terrestrial ecosystems. Microbial metabolic efficiency and soil enzyme activities are important indicators of carbon cycling in the riparian zones. However, how soil properties and microbial communities regulate the microbial metabolic efficiency in these critical zones remains unclear. Thus, microbial taxa, enzyme activities, and metabolic efficiency were conducted in the riparian zones of the Three Gorges Reservoir (TGR). Microbial carbon use efficiency and microbial biomass carbon had a significant increasing trend along the TGR (from upstream to downstream); indicating higher carbon stock in the downstream, microbial metabolic quotient (qCO2) showed the opposite trend. Microbial community and co-occurrence network analysis revealed that although bacterial and fungal communities showed significant differences in composition, this phenomenon was not found in the number of major modules. Soil enzyme activities were significant predictors of microbial metabolic efficiency along the different riparian zones of the TGR and were significantly influenced by microbial α-diversity. The bacterial taxa Desulfobacterota, Nitrospirota and the fungal taxa Calcarisporiellomycota, Rozellomycota showed a significant positive correlation with qCO2. The shifts in key microbial taxa unclassified_k_Fungi in the fungi module #3 are highlighted as essential factors regulating the microbial metabolic efficiency. Structural equation modeling results also revealed that soil enzyme activities had a highly significant negative effect on microbial metabolism efficiency (bacteria, path coefficient = -0.63; fungi, path coefficient = -0.67).This work has an important impact on the prediction of carbon cycling in aquatic-terrestrial ecotones. Graphical abstract.

Study on binding modes between cellobiose and ß-glucosidases from glycoside hydrolase family 1.

The hydrolysis of cellobiose by ß-glucodisases is an important step of cellulose biodegradation. However, the interactive mechanism between cellobiose and ß-glucosidases is still unclear until now. Thus, in this study, we explored the binding modes between cellobiose and three ß-glucosidases from glycoside hydrolase family 1 by means of molecular docking. The three ß-glucosidases were named as TmGH1 (from bacterium Thermotoga), SsGH1 (from archaea Sulfolobus solfataricus) and TrGH1 (from fungus Trichoderma reesei) respectively, according to the monophyletic groups they belong to. Molecular dockings were performed between cellobiose and the three ß-glucosidases, resulting in three optimum docking complexes, that is TmGH1-cellobiose, SsGH1-cellobiose and TrGh1-cellobiose complexes. Our docking results indicated that there were non-bonded interactions between cellobiose and the three ß-glucosidases. The binding affinities of the three complexes were -13.6669kJ/mol, -13.2973kJ/mol and -18.6492kJ/mol, respectively. Then the detailed interactions were investigated, which revealed the key amino acid residues interacted with cellobiose by hydrogen bonds (H-bonds) or hydrophobic interactions. It was observed that most of the key residues involved in the non-bonded interactions were equivalent and conserved for the three complexes, and these residues were a glutamine, a histidine, a tyrosine, a phenylalanine, three glutamics, and four tryptophans. This information is of great importance for designing ß-glucosidase with higher cellobiose-hydrolyzing efficiency.

Responses of Phanerochaete chrysosporium to toxic pollutants: physiological flux, oxidative stress, and detoxification.

The white-rot fungus Phanerochaete chrysosporium has been widely used for the treatment of waste streams containing heavy metals and toxic organic pollutants. The development of fungal-based treatment technologies requires detailed knowledge of the relationship between bulk water quality and the physiological responses of fungi. A noninvasive microtest technique was used to quantify real-time changes in proton, oxygen, and cadmium ion fluxes following the exposure of P. chrysosporium to environmental toxic (2,4-dichlorophenol and cadmium). Significant changes in H(+) and O(2) flux occurred after exposure to 10 mg/L 2,4-dichlorophenol and 0.1 mM cadmium. Cd(2+) flux decreased with time. Reactive oxygen species formation and antioxidant levels increased after cadmium treatment. Superoxide dismutase activity correlated well with malondialdehyde levels (r(2) = 0.964) at low cadmium concentrations. However, this correlation diminished and malondialdehyde levels significantly increased at the highest cadmium concentration tested. Real-time microscale signatures of H(+), O(2), and Cd(2+) fluxes coupled with oxidative stress analysis can improve our understanding of the physiological responses of P. chrysosporium to toxic pollutants and provide useful information for the development of fungal-based technologies to improve the treatment of wastes cocontaminated with heavy metals and organic pollutants.

Terrigenous organic carbon drives methane dynamics in cascade reservoirs in the upper Yangtze China.

Methane (CH4) emissions from freshwaters to the atmosphere have a profound impact on global atmospheric greenhouse gas (GHG) concentrations. Anthropogenic footprints such as dam construction and reservoir operation significantly changed the fate and transport of CH4 in freshwaters. The source of particulate organic carbon (POC) in reservoirs is a critical factor controlling CH4 production and emissions. However, little is known of how reservoir operation mediates the transport of POC and regulates CH4 accumulation in cascade hydroelectric reservoirs. Here, spatial and temporal variations in POC and CH4 were explored in the Xiluodu (XLD) and Xiangjiaba (XJB) reservoirs which are deep valley cascade reservoirs located in the main channel of the upper Yangtze River. Based on the δ13C-POC and N/C mole ratio of particulate organic matter, the results of multi-endmember stable isotope mixing models by a Bayesian model showed that terrigenous POC and autochthonous POC accounted for approximately 55% ± 18% and 43% ± 19% (SD, n = 179) of POC, respectively. Together with other hydrological and environmental parameters, we found that the input of terrigenous POC was dominantly influenced by water level variations and flow regulation due to reservoir operation. The cumulative effect of POC caused by cascade dams was not apparent. Terrigenous POC were more likely to drive CH4 accumulation in our study. Evident low level of CH4 in both reservoirs were likely affected by low sedimentation of POC and microbial CH4 oxidation. We hope our study could provide a conceptual framework for further modeling of CH4 dynamics in cascade reservoirs.

Sedimentation supports life-cycle CH4 production and accumulation in a river valley reservoir: A hierarchical Bayesian modeling approach.

Reservoirs have been recognized as a source of methane (CH4). With the gradual increase in the number of the world's reservoirs, predicting the long-term variation of reservoir CH4 emissions is important to understand the global change in carbon cycling due to reservoir creation and operation. Here, we first categorized the origins and transport of organic carbon (OC) by reservoir creation and operation into the following four aspects: a) the decomposition of flooded organic matter, b) the sedimentation of OC from upstream sediment inputs, c) the transition of the aquatic ecosystem from lotic to lentic type, stimulating the production of autochthonous OC; and d) reservoir as the collector of anthropogenic OC inputs from surrounding communities. It was assumed that OC from the four aspects jointly determined the production and accumulation of reservoir CH4 concentration, supporting life-cycle reservoir CH4 emissions. A hierarchical Bayesian model of reservoir CH4 concentration was established and calibrated by observed monthly datasets in 2018 in the Xiangjiaba Reservoir (XJB), a river valley dammed reservoir in the upper Yangtze River, China. The model explained the relative contributions of the four aspects to reservoir CH4 production and accumulation. Approximately 78% of the CH4 concentration was contributed by the decomposition of flooded organic matter during the first 10 years after impoundment. However, the contribution of flooding faded away after 10 years of impoundment. With the increase in reservoir age, sedimentation of OC dominantly determined the reservoir CH4 production and accumulation. Scenario analysis of the XJB's life cycle demostrated that the CH4 concentration in the XJB would reach its peak approximately 70 - 80 years after impoundment. In the cascade system, the upstream reservoir will help to reduce sediment OC input, and to mitigate downstream reservoir CH4 production and accumulation. Our effort provided a new modeling approach for long-term management strategies to reduce reservoir CH4 emissions under global change.

[Effects of Farming Practices on Soil Nitrogen and Phosphorus Concentrations and Its Loss in the Drawdown Area of the Tributary Embayment of the Three Gorges Reservoir].

As the Three Gorges Reservoir (TGR) periodically operates at low water levels, its drawdown area has been utilized for cultivation by local farmers due to the overlap of the non-inundated period and the crop-growth period. However, traditional agricultural planting may affect the aquatic environment of the TGR area. To explain the effects of agricultural farming and abandoned farming on the water environment, a study was conducted in the drawdown area in an embayment of the Pengxi River (a tributary of the TGR). Corn, potato, and peanut fields were investigated for nitrogen and phosphorus content in surface soil, during the farming period (March to September 2018) and the conversion period (March to September 2019). Nitrogen and phosphorus balance models were constructed for farmland and abandoned farmland, to compare and analyze the budgets and loss risk of nitrogen and phosphorus from soil in the drawdown area. The results showed that the ammonia nitrogen (NH4+-N), total phosphorus (TP), and inorganic phosphorus (IP) content of soil in the corn field varied significantly across different planting periods. The concentrations of ammonium nitrogen and nitrate nitrogen (NO3--N) were significantly higher in farmland soil than in abandoned farmland soil, and the concentrations of total phosphorus (TP), inorganic phosphorus (IP), and calcium-bound phosphorus (Ca-P) were significantly lower in farmland soil than in abandoned farmland soil. The different soils were ranked according to the intensity of nitrogen and phosphorus surplus as follows:corn field>potato field>peanut field. The apparent surplus values in the different farmland soils were 76.89 kg ·hm-2(corn field), 51.92 kg ·hm-2(potato field), and 43.74 kg ·hm-2(peanut field) for nitrogen, and 79.69 kg ·hm-2(corn field), 75.76 kg ·hm-2(potato field), and 17.78 kg ·hm-2(peanut field) for phosphorous. Overall, the surplus intensities of nitrogen and phosphorus in all three croplands were higher than the respective risk thresholds, indicating potential nitrogen and phosphorus pollution in the three farmland types. Agricultural farming in the drawdown area may therefore increase the risk of nitrogen and phosphorus loss and is not conducive to the protection of the aquatic environment.

Aerobic methanotrophs in an urban water cycle system: Community structure and network interaction pattern.

Aerobic methane-oxidizing bacteria (MOB) play an important role in reducing methane emissions in nature. Most current researches focus on the natural habitats (e.g., lakes, reservoirs, wetlands, paddy fields, etc.). However, methanotrophs and the methane-oxidizing process remain essentially unclear in artificial habitat, such as the urban water cycle systems. Here, high-throughput sequencing and qPCR were used to analyze the community structure and abundance of MOB. Six different systems were selected from Yunyang City, Chongqing, China, including the raw water system (RW), the water supply pipe network system (SP), the wastewater pipe network system (WP), the hospital wastewater treatment system (HP), the municipal wastewater treatment plant system (WT) and the downstream river system (ST) of a wastewater treatment plant. Results clearly showed that the MOB community structure and network interaction patterns of the urban water cycle system were different from those of natural water bodies. Type I MOB was the dominant clade in HP. Methylocysis in Type II was the most abundant genus among the whole urban water cycle system, indicating that this genus had a high adaptability to the environment. Temperature, dissolved oxygen, pH and concentration significantly affected the MOB communities in the urban water cycle system. The network of MOB in WT was the most complicated, and there were competitive relationships among species in WP. The structure of the network in HP was unstable, and therefore, it was vulnerable to environmental disturbances. Methylocystis (Type II) and Methylomonas (Type I) were the most important keystone species in the entire urban water cycle system. Overall, these findings broaden the understanding of the distribution and interaction patterns of MOB communities in an urban water cycle system and provide valuable clues for ecosystem restoration and environmental management.