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

Interactions between N, P in the overlying water and flooding-induced decomposition of Cynodon dactylon in the water-level fluctuation zone

During flooding in the Water Level Fluctuation Zone (WLFZ), nutrient levels of nitrogen (N) and phosphorus (P) in the overlying water fluctuate due to soil nutrient release, impacting the decomposition of plants like Cynodon dactylon. However, limited research on the effects of these nutrient changes on plant nutrient release and water dynamics complicates accurate assessments of water quality impacts. This study used 8 water samples with varying initial nutrient levels to simulate N and P changes induced by WLFZ soil nutrients and examined the decomposition and nutrient dynamics of Cynodon dactylon. Results showed that flooding significantly increased initial levels of N and P, especially as particulate nitrogen (PN) and particulate phosphorus (PP), affecting both plant decomposition and nutrient dynamics in the water. After 60 days, Cynodon dactylon lost 47.97%-56.01% dry matter, 43.58%-54.48% total nitrogen (TN), and 14.28%-20.50% total phosphorus (TP). Initial PN and total dissolved nitrogen (TDN) promoted dry matter loss, PN and PP promoted TP loss, while PN and TDN inhibited TN loss. By day 60, no positive correlation was found between plant-released N and P and TN or TP in the overlying water. However, initial PP and PN levels were negatively correlated with TN and TP, indicating an inhibitory effect. Further analysis indicates that PN and PP released from the soil supported the formation of microbial aggregates, enhancing denitrification and phosphorus removal and thus improving water purification over time.

Nitrogen fate in riparian zones: Insights from experiments and analysis of sediment porosity and surface water-groundwater exchange

Riparian zones play a vital role in the river ecosystem. Solutes in vertical riparian zones are transported being by alternating hydraulic gradients between river water and groundwater, due to natural or human activities. This study investigates the impacts of porous sediments and alternating rate of surface water-groundwater on nitrogen removal in the riparian zone through experiments based on the field sampled. The experimental results, combined with dimensionless numbers (Péclet and Damköhler) and Partial Least Squares-Path Modeling, analyze the nitrogen fate responding to hydrodynamics changes. The results show that increased sediment porosity contributes to the ammonium removal, particularly when the oxygen content of river water is low, with the removal rate up to 72.57%. High ammonium content and dissolved organic carbon (DOC) in rural rivers lead to a constant low-oxygen condition (4 mg/L) during surface water-groundwater alternation, and promote denitrification. This threatens groundwater with ammonium pollution and causes accumulation at the top of vertical riparian zones during upwelling, potentially causing secondary river pollution. However, increasing the alternating rate hinders the nitrate denitrification and drastically changes in the redox environment of the riparian zone, despite contributing to ammonium removal. Rapid oxygen consumption during aerobic metabolism and nitrification in groundwater-surface water exchange created favorable conditions for denitrification. Floodplains sediment porosity is unfavorable for nitrification. This study improves understanding of coupled hydrologic and solute processes in vertical riparian zones, informing strategies for optimizing nitrogen attenuation and riparian zone construction.

Genome-wide analysis of soybean hypoxia inducible gene domain containing genes: a functional investigation of GmHIGD3

The response of Hypoxia Inducible Gene Domain (HIGD) proteins to hypoxia plays a crucial role in plant development. However, the research on this gene family in soybean has been lacking. In this study, we aimed to identify and comprehensively analyze soybean HIGD genes using the Glycine max genome database. As a result, six GmHIGD genes were successfully identified, and their phylogeny, gene structures, and putative conserved motifs were analyzed in comparison to Arabidopsis and rice. Collinearity analysis indicated that the HIGD gene family in soybean has expanded to some extent when compared to Arabidopsis. Additionally, the cis-elements in the promoter regions of GmHIGD and the transcription factors potentially binding to these regions were identified. All GmHIGD genes showed specific responsiveness to submergence and hypoxic stresses. Expression profiling through quantitative real-time PCR revealed that these genes were significantly induced by PEG treatment in root tissue. Co-expressed genes of GmHIGD were primarily associated with oxidoreductase and dioxygenase activities, as well as peroxisome function. Notably, one of GmHIGD genes, GmHIGD3 was found to be predominantly localized in mitochondria, and its overexpression in Arabidopsis led to a significantly reduction in catalase activity compared to wild-type plants. These results bring new insights into the functional role of GmHIGD in terms of subcellular localization and the regulation of oxidoreductase activity.

Elucidation of the dominant factors influencing N2O emission in water-level fluctuation zones in a karst canyon reservoir, southwest China

The water-level fluctuations zones (WLFZs) are crucial transitional interfaces within river-reservoir systems, serving as hotspots for N2O emission. However, the comprehension of response patterns and mechanisms governing N2O emission under hydrological fluctuation remains limited, especially in karstic canyon reservoirs, which introduces significant uncertainty to N2O flux assessments. Soil samples were collected from the WLFZs of the Hongjiadu (HJD) Reservoir along the water flow direction from transition zone (T1 and T2) to lacustrine zone (T3, T4 and T5) at three elevations for each site. These soil columns were used to conduct simulation experiments under various water-filled pore space gradients (WFPSs) to investigate the potential N2O flux pattern and elucidate the underlying mechanism. Our results showed that nutrient distribution and N2O flux pattern differed significantly between two zones, with the highest N2O fluxes in the transition zone sites and lacustrine zone sites were found at 75 % and 95 % WFPS, respectively. Soil nutrient loss in lower elevation areas is influenced by prolonged impoundment durations. The higher N2O fluxes in the lacustrine zone can be attributed to increased nutrient levels resulting from anthropogenic activities. Furthermore, correlation analysis revealed that soil bulk density significantly impacted N2O fluxes across all sites, while NO3-and SOC facilitated N2O emissions in T1-T2 and T4-T5, respectively. It was evident that N2O production primarily contributed to nitrification in the transition zone and was constrained by the mineralization process, whereas denitrification dominated in the lacustrine zone. Notably, the annual N2O efflux from WLFZs accounted for 27 % of that from the water-air interface in HJD Reservoir, indicating a considerably lower contribution than anticipated. Nevertheless, this study highlights the significance of WLFZs as a vital potential source of N2O emission, particularly under the influence of anthropogenic activities and high WFPS gradient.

Diazotrophic community in the sediments of Poyang Lake in response to water level fluctuations

Water level fluctuations (WLFs) are typical characteristic of floodplain lakes and dominant forces regulating the structure and function of lacustrine ecosystems. The sediment diazotrophs play important roles in contributing bioavailable nitrogen to the aquatic environment. However, the relationship between the diazotrophic community and WLFs in floodplain lakes is unknown. In this paper, we carried out a comprehensive investigation on the alpha diversity, abundance, composition and co-occurrence network of the sediment diazotrophs during different water level phases in Poyang Lake. There were no regular variation patterns in the alpha diversity and abundance of the sediment diazotrophs with the water level phase transitions. The relative abundance of some diazotrophic phyla (including Alphaproteobacteria, Deltaproteobacteri, Euryarchaeota, and Firmicutes) and genera (including Geobacter, Deferrisoma, Desulfuromonas, Rivicola, Paraburkholderia, Methylophilus, Methanothrix, Methanobacterium, and Clostridium) was found to change with the water level phase transitions. The results of ANOSIM, PerMANOVA, and DCA at the OTU level showed that the diazotrophic community structure in the low water level phase was significantly different from that in the two high water level phases, while there was no significant difference between the two high water level phases. These results indicated that the diazotrophic community was affected by the declining water level in terms of the composition, while the rising water level contributed to the recoveries of the diazotrophic community. The diazotrophs co-occurrence network was disrupted by the declining water level, but it was strengthened by the rising water level. Moreover, redundancy analysis showed that the variation of the diazotrophic community composition was mostly related to sediment total nitrogen (TN) and total phosphorous (TP). Interestingly, the levels of sediment TN and TP were also found to vary with the water level phase transitions. Therefore, it might be speculated that the WLFs may influence the sediment TN and TP, and in turn influence the diazotrophic community composition. These data can contribute to broadening our understanding of the ecological impacts of WLFs and the nitrogen fixation process in floodplain lakes.

Effects of flow rate and wastewater concentration on the transformation of nitrogen in sediment-water system of sewage pipelines

In this work, the transformation law of nitrogen in sediment-water system under different flow rates and wastewater concentrations were investigated in a simulated sewage pipeline system. Results showed that the different flow rates and wastewater concentrations in the pipeline caused differences in microbial community in sediments and nitrogen transformation. When the flow rate increased from 0.05 to 0.2 m/s, the scouring effect was enhanced, resulting in higher concentrations of NH4 + -N and NO3 - -N in the overlying water. At 0.2 m/s, the relative abundance of Clostridium_sensu_stricto_1 in sediments was higher, resulting in a greater conversion of amino acid nitrogen (AAN) to NH4 + -N. Meanwhile, many denitrifying bacteria (Trichococcus, Dechloromonas, norank_f__norank_o__Gaiellales, Thiobacillus) had high relative abundance in the sediments, and the denitrification process was common. When the wastewater concentration was high, the nitrification reaction was great in overlying and interstitial water. Moreover, the ammoniation process was great in the sediments, and the variation flux of AAN was large (remarkably reduced). PRACTITIONER POINTS: AAN transformed to NH4 + -N in sediment under different flow rate and concentration. Scouring was enhanced at 0.2 m/s, increasing nitrogen contents in overlying water. Difference in microbial community led to more AAN conversion to NH4 + -N at 0.2 m/s. The ammoniation process was greater in sediment at a high concentration of sewage. NH4 + -N migrated from overlying water to sediment at a high concentration of sewage.

Response of denitrification microbiome to the nitrogen flux in three Gorges reservoir (TGR) sediments during two seasonal water fluctuation events

Three Gorges Reservoir (TGR) water fluctuation creates high water level (HWL) and low water level (LWL) condition in TGR aquatic ecosystem. HWL fluies significant nutrients, mainly introducing carbon and nitrogen into the ecosystem. The nitrogen input is a concern for water quality management of TGR since the possible eutrophication caused by nitrogen spike. Sediment denitrification is widely recognized as the dominant nitrogen removal process in freshwater ecosystem. Therefore, the response of TGR sediments microbiome to the input nitrogen flucatution is crucial for both nitrogen balance and the eutrophication status of the ecosystem. Using high throughout sequencing of 16S rRNA gene and the predicted denitrification enzyme, and qualitative PCR of denitrification functional genes, we investigated how TGR sediments denitrification microbiome respond to the input nitrogen flux during two seasonal water fluctuation events. Concomitant to expected input carbon and nitrogen, we observed distinct microbial community structure and denitrification microbiota in HWL and LWL, and also in seasonal sampling events. Sediments pH, total nitrogen and nitrate were the significant impact factors in shaping the microbial community structure. Important denitrification microbiota (e.g., Saprospiraceae, Gemmatimonadaceae, Pseudomonas) are the main taxa of the microbial community and also showed water level and seasonal variation. The relative abundance of denitrification enzyme (nar, nir, nor, nos) and function genes (nirS, nirK, nosZ) were higher in LWL than HWL. Denitrification enzyme were significantly (p < 0.05) correlated with the nitrate concentration. In addition, the relative abundance of denitrification enzyme and function genes increased during the transition from 2014 HWL to 2015 LWL. Results suggested that TGR sediments denitrification is nitrate concentration dependent. The denitrification microbiome is initially inhibited due to high nitrate input, then they developed denitrification ability in response to high nitrate concentration.

Mechanisms and effects of novel ammonifying microorganisms on nitrogen ammonification in cow manure waste composting

It is essential to reduce nitrogen losses and to improve nitrogen conversion during organic waste composting because of environmental protection and sustainable development. To reveal newly domesticated ammonifying microorganisms (AM) cultures on the ammonification and nitrogen conversion during the composting, the screened microbial agents were inoculated at 5 % concentration (in weight basis) into cow manure compost under five different treatments: sterilized distilled water (Control), Amm-1 (mesophilic fungus-F1), Amm-2 (mesophilic bacterium-Z1), Amm-3 (thermotolerant bacterium-Z2), and Amm-4 (consortium: F1, Z1, and Z2), and composted for 42 days. Compared to control, AM inoculation prolonged the thermophilic phases to 9-19 days, increased the content of NH4+-N to 1.60-1.96 g/kg in the thermophilic phase, reduced N2O and NH3 emissions by 22.85-61.13 % and 8.45-23.29 %, increased total Kjeldahl nitrogen, and improved cell count and viability by 12.09-71.33 % and 66.71-72.91 %. AM was significantly associated with different nitrogen and microbial compositions. The structural equation model (SEM) reveals NH4+-N is the preferable nitrogen for the majority of bacterial and fungal growth and that AM is closely associated with the conversion between NH3 and NH4+-N. Among the treatments, inoculation with Amm-4 was more effective, as it significantly enhanced the driving effect of the critical microbial composition on nitrogen conversion and accelerated nitrogen ammonification and sequestration. This study provided new concepts for the dynamics of microbial in the ammonification process of new AM bacterial agents in cow manure compost, and an understanding of the ecological mechanism underlying the ammonification process and its contribution to nitrogen (N) cycling from the perspective of microbial communities.

Stability of Aquatic Nitrogen Cycle Under Dramatic Changes of Water and Sediment Inflows to the Three Gorges Reservoir

Stability of nitrogen cycle is a key indicator to aquatic health. In recent years, water and sediment inflows to the Three Gorges Reservoir (TGR) have changed significantly. To reveal the effects of such dramatic hydrological changes on aquatic nitrogen cycle, this paper at first analyzed the changing trends of water and suspended sediment discharges of TGR based on dynamic harmonic regression, and found that the intra-year distribution of water flow was significantly homogenized between flood and dry seasons, with the seasonal variations narrowed by 43.5%-69.9% during 2007-2016, while sediment concentration sharply dropped (the non-periodic term decreased by 1.48%-2.07%/month). Modified with the effects of sediment concentration variations on nitrification/denitrification rates, the proposed numerical model surprisingly showed that ammonia nitrogen and total nitrogen concentrations in TGR were insensitive to either water flow homogenization or sediment reduction, implying relative stability of microbial community related to nitrogen cycle, which is a positive sign for aquatic health. However, N2 emission varied more violently. The variation range of nitrogen gas (N2) emitted from TGR enlarged by 30% with the homogenization of water inflow from 2010 to 2016, while the annual total N2 emission decreased by 7% due to the reduction of sediment concentration, indicating quick response and strong adaption of the microbial N2 producing process to the environmental changes of TGR, which is beneficial for maintaining ecological functions related to nitrogen cycling. This work helps understanding nitrogen cycle of reservoirs experiencing dramatic changes in water and sediment inflows.

The contribution of sediment desiccation and rewetting process to eutrophication in the presence and absence of emergent macrophytes

The purpose of current study was to investigate the effects of sediment desiccation on nutrient dynamics and eutrophication in wetlands during the presence or absence of wiry and sturdy rooted emergent macrophytes, based on the hypothesis that sediment desiccation negatively correlated with plants nutrient uptake abilities and positively with nutrients fluxes at sediment-water interface. Growth of four emergent macrophytes, including two wiry rooted plants, i.e., Alocasia cucullata and Aglaonema commutatum, and two sturdy rooted plants, i.e., Cannabis indica and Acorus calamus, were grown and investigated in dried-rewetted sediments (DS) and constantly wet sediments (WS), respectively, for 6 months. The findings revealed that sediment drying and rewetting process significantly decreased the diffusion of overlying nutrient into sediment and the particle size density, porosity, and nutrients' repository ability in DS treatments, while the sediment bulk density and mineralization of organic macronutrients increased. Compared to WS treatments, the DS treatments impaired plant growth, root biomass, shoot biomass, and stimulated higher fluxes of ammonium nitrogen ([Formula: see text]-N, 0.042-0.081 mg m ^{- 2} d ^{- 1}) and phosphate (P[Formula: see text] 0.009-0.030 mg m^-2 d^-1) at sediment-water interface upon rewetting. The higher internal release of macronutrients and dissolved organic carbon (DOC) from DS led to the higher chlorophyll-a (Chl-a) concentrations (34.47-21.28 to 41.76-33.36 μg L^-1) in their water column than in the water column of WS. The wiry rooted plants with higher root biomass displayed lower internal release of [Formula: see text]-N, PO₄³-P and DOC and water column Chl-a concentrations than the sturdy rooted plants in two sediment types. Root biomass of plants correlated positively with TN (63-87%) and TP (56-78%) removal percentages from WS and DS. These results demonstrated that sediment desiccation process reduced plant growth and enhanced internal loading of nutrients and consequently accelerated eutrophication in these wetlands.

Long-term impacts of reservoir operation on the spatiotemporal variation in nitrogen forms in the post-Three Gorges Dam period (2004-2016)

Nitrogen (N) is an essential nutrient limiting life, and its biochemical cycling and distribution in rivers have been markedly affected by river engineering construction and operation. Here, we comprehensively analyzed the spatiotemporal variations and driving environmental factors of N distributions based on the long-term observations (from 2004 to 2016) of seven stations in the Three Gorges Reservoir (TGR). In the study period, several water quality indexes of the river reach improved, whereas N pollution was severe and tended to be aggravated after the TGR impoundment. The anti-seasonal reservoir operation strongly affected the variations in N forms. The total nitrogen (TN) concentration in the mainstream of the Yangtze River continuously increased, although it was still lower than that in the incoming tributaries (Wu and Jialing rivers). Further analysis showed that this increase occurred probably because of external inputs, including the upstream (76%), non-point (22%), and point source pollution inputs (2%). Additionally, different N forms showed significant seasonal variations; among them, the TN and nitrate nitrogen concentrations were the lowest in the impoundment season (October-February), and the ammonia nitrogen concentrations were the highest in the sluicing season (March-May). Redundancy analysis revealed that the water level and distance to the Three Gorges Dam were significant contributors to N forms distribution. Our findings could provide a basis for managing and predicting the water quality in the Yangtze River.

The hypoxia-reoxygenation stress in plants

Plants are very plastic in adapting growth and development to changing adverse environmental conditions. This feature will be essential for plants to survive climate changes characterized by extreme temperatures and rainfall. Although plants require molecular oxygen (O2) to live, they can overcome transient low-O2 conditions (hypoxia) until return to standard 21% O2 atmospheric conditions (normoxia). After heavy rainfall, submerged plants in flooded lands undergo transient hypoxia until water recedes and normoxia is recovered. The accumulated information on the physiological and molecular events occurring during the hypoxia phase contrasts with the limited knowledge on the reoxygenation process after hypoxia, which has often been overlooked in many studies in plants. Phenotypic alterations during recovery are due to potentiated oxidative stress generated by simultaneous reoxygenation and reillumination leading to cell damage. Besides processes such as N-degron proteolytic pathway-mediated O2 sensing, or mitochondria-driven metabolic alterations, other molecular events controlling gene expression have been recently proposed as key regulators of hypoxia and reoxygenation. RNA regulatory functions, chromatin remodeling, protein synthesis, and post-translational modifications must all be studied in depth in the coming years to improve our knowledge on hypoxia-reoxygenation transition in plants, a topic with relevance in agricultural biotechnology in the context of global climate change.

Migration and transformation of nitrogen in sediment-water system within storm sewers

In the sediment-water system of storm sewers (e.g., sediments, interstitial water, and the water column), the migration of nitrogen and its biological transformation with different dissolved oxygen conditions were investigated. Results showed that in an aerobic segment, γ-proteobacteria, α-proteobacteria, and nitrospira, which are aerobic, grew actively in water column and interstitial water through ammonification and nitrification. In anoxic segment, ammonification depended mainly on clostridia, whereas nitrification was inhibited. Thus, after 20 days, the concentration of NH₄⁺-N in the aerobic segment became noticeably lower (5.97 mg/L) than that in the anoxic segment (18.09 mg/L). In sediments, the biological transformation of organic nitrogen in the anoxic environment was more complete, resulting in elevating amino acid nitrogen and NH₄⁺-N in the anoxic segment compared to the aerobic segment. Furthermore, the concentration gradient of NH₄⁺-N between interstitial water and water column in aerobic and anoxic segments, thereby causing NH₄⁺-N to migrate from interstitial water to the water column. In the sediment-water system, the different forms of nitrogen changes were the common result of biological transformation and material migration.

Limno-ecological assessment of lentic ecosystems in the western Mediterranean basin (Turkey) using phytoplankton indices

Great attention has been given to freshwater ecosystems worldwide due to the increased exploitation of water resources and the degradation of water quality. This study was aimed to demonstrate the phytoplankton-stressor interactions using multivariate approaches and assess the ecological conditions of 28 sampling stations of 12 lentic ecosystems (five lakes and seven reservoirs) in the western Mediterranean basin using phytoplankton indices in dry and rainy seasons 2018. Freshwater and brackish water systems were separately ordinated by canonical correspondence analysis. The brackish ecosystems were under pressures of Ar (arsenic), N-N[Formula: see text] (nitrite), Ca (calcium), EC (electrical conductivity), Cl (chloride), B (boron), etc., whereas shallow freshwater ecosystems were associated with total organic carbon. These factors had significant effects on phytoplankton distribution among lentic ecosystems on the basin. Ecological associations of phytoplankton assemblages varied in the western Mediterranean basin during the study. Pseudanabaena catenata and Palatinus apiculatus, considered tolerant taxa, are associated with higher Ar, EC, and V (vanadium) while Desmodesmus abundans and Microcystis flos-aquae are related to total organic carbon. The modified PTI (phytoplankton trophic index) had the highest correlation coefficient value. Scores of the PTI varied from 2.02 in Çavdır Reservoir to 2.59 in Lake Kocagöl. Results of phytoplankton indices indicated that two (Gölhisar and Yazır) lakes and two (Yapraklı and Çavdır) reservoirs were classified as Good, and three lakes and five reservoirs were classified as Moderate condition. Other predicted classifications (high, poor, and bad) by phytoplankton metrics were not represented. The modified PTI could be a suitable phytoplankton metric for assessing the ecological status of lentic ecosystems in the Mediterranean system according to its highest correlation coefficient value. Overall, the results of this limno-ecological study demonstrated that responses of phytoplankton taxa to explanatory factors provide crucial ecological information about their ecology and to estimate the ecological status of lentic ecosystems.