Integrating experiments with system-level biogeochemical modeling to understand nitrogen cycling of reservoir sediments at elevated hydrostatic pressure

Unveiling the dynamic of nitrogen through migration and transformation patterns in the groundwater level fluctuation zone of a different hyporheic zone sediment

This study investigates the impact of water levels and soil texture on the migration and transformation of nitrate (NO3--N) and ammonium (NH4+-N) within a soil column. The concentrations of NO3--N gradually decreased from an initial concentration of 34.19 ± 0.86 mg/L to 14.33 ± 0.77 mg/L on day 70, exhibiting fluctuations and migration influenced by water levels and soil texture. Higher water levels were associated with decreased NO3--N concentrations, while lower water levels resulted in increased concentrations. The retention and absorption capacity for NO3--N were highest in fine sand soil, followed by medium sand and coarse sand, highlighting the significance of soil texture in nitrate movement and retention. The analysis of variance (ANOVA) confirmed statistically significant variations in pH, dissolve oxygen and oxidation-reduction potential across the soil columns (p < 0.05). Fluctuating water levels influenced the migration and transformation of NO3--N, with distinct patterns observed in different soil textures. Water level fluctuations also impacted the migration and transformation of NH4+-N, with higher water levels associated with increased concentrations and lower water levels resulting in decreased concentrations. Among the soil types considered, medium sand exhibited the highest absorption capacity for NH4+-N. These findings underscore the significant roles of water levels, soil texture, and soil type in the migration, transformation, and absorption of nitrogen compounds within soil columns. The results contribute to a better understanding of nitrogen dynamics under varying water levels and environmental conditions, providing valuable insights into the patterns of nitrogen migration and transformation in small-scale soil column experiments.

Micro-pressure promotes endogenous phosphorus release in a deep reservoir by favouring microbial phosphate mineralisation and solubilisation coupled with sulphate reduction

Deep reservoirs vary in their hydrostatic pressure owing to artificial water level control. The potential migration of phosphorus (P) in reservoir sediments raises the risk of harmful algal blooms. To ascertain the mechanisms of endogenous P release in reservoirs, we characterised aquatic microbial communities associated with coupled iron (Fe), P and sulphur (S) cycling at the sediment-water interface. The responses of microbial communities to hydrostatic pressures of 0.2-0.7 mega pascals (MPa; that is, micro-pressures) were investigated through a 30-day simulation experiment. Our findings unravelled a potential mechanism that micro-pressure enhanced the solubilisation of Fe/aluminium (Al)-bound P caused by microbially-driven sulphate reduction, leading to endogenous P release in the deep reservoir. Although the vertical distribution of labile Fe was not affected by pressure changes, we did observe Fe resupply at sediment depths of 2-5 cm. Metagenomic analysis revealed increased abundances of functional genes for P mineralisation (phoD, phoA), P solubilisation (pqqC, ppx-gppA) and sulphate reduction (cysD, cysC) in sediments subjected to micro-pressure, which contrasted with the pattern of S oxidation gene (soxB). There was a tight connection between P and S cycling-related microbial communities, based on significant positive correlations between labile element (P and S) concentrations and functional gene (phoD, cysD) abundances. This provided strong support that Fe-P-S coupling processes were governed by micro-pressure through modulation of P and S cycling-related microbial functions. Key taxa involved in P and S cycling (for example, Bradyrhizobium, Methyloceanibacter) positively responded to micro-pressure and as such, indirectly drove P release from sediments by facilitating P mineralisation and solubilisation coupled with sulphate reduction.

Relation disentanglement, the potential risk assessment, and source identification of heavy metals in the sediment of the Changzhao Reservoir, Zhejiang Province

Heavy metal contamination in the water body is a distinctly important issue for the water security of the reservoir. 114 sediment samples of Changzhao Reservoir were collected to investigate the spatial (horizontal and vertical) distribution characteristics, risk assessment, and source identification of heavy metals. The concentrations of heavy metals at the surface layer of sediment were slightly higher compared with that at the middle and bottom layer sediment in the most sampling sites. The concentration of Zn and Cd was significantly different in the different depths of sediment (P ≤ 0.01, Tukey HSD test). pH and Cd were identified as the key factors for TOC in the sediment by the Boruta algorithm. The proportion of "uncontaminated to moderately contaminated" for Cd, Zn, and As in the surface layer was 84.21%, 47.37%, and 34.21%, which indicated that the quality of sediment was mostly impacted by Cd, Zn, and As. The agricultural non-point source pollution is dominant according to the source identification method of APCS-MLR. Overall, this paper presents the distribution and conversion trends of heavy metals and provides the insights of the reservoir protection in the future work.

Effects of changes in land use structure on nitrogen input in the Pingzhai Reservoir watershed, a karst mountain region

Optimizing land use composition to control nitrogen input into water bodies is one way to address surface source pollution in karst mountain regions. In this study, changes in land use, N sources, and spatial and temporal changes of N migration in the Pingzhai Reservoir watershed were evaluated from 2015 to 2021, and the relationship between land use composition and N input was elucidated. N was the main pollution in the water of the watershed; NO₃^- was the dominant form of N, and it did not react during migration. N came from soil, livestock manure or domestic sewage, and atmospheric deposition. Isolating the fractionation effects of source nitrogen is crucial to improve the accuracy of nitrogen and oxygen isotope traceability in the Pingzhai Reservoir. From 2015 to 2021, the grassland area in the Pingzhai Reservoir increased by 5.52%, the woodland area increased by 2.01%, the water area increased by 1.44%, the cropland decreased by 5.8%, unused land decreased by 3.18%, and construction land remained unchanged. Policies and reservoir construction were the main drivers of changes in land-use type in the catchment. Changes in land use structure affected nitrogen input patterns, with unused land having a highly significant positive correlation with inputs of NH₃-N, NO₂^-, and TN, and construction land having a significant positive correlation with the input of NO₂^-. The inhibitory effect of forest and grassland on nitrogen input in the basin was offset by the promoting effect of cropland and construction land on nitrogen input, with unused land becoming a new focus area for nitrogen emissions due to a lack of environmental management. Modifying the area of different land use types in the watershed can effectively control nitrogen input to the watershed.

The mechanism of C-N-S interconnection degradation in organic-rich sediments by Ca(NO3)2 - CaO2 synergistic remediation

The rebound of black-odorous occurred in organic-rich sediments has become a critical issue due to its great harm to the ecological environment. Elements such as S, C, and N play a crucial role in the biogeochemical cycle of black-odorous rivers. As electronic acceptors, Ca(NO₃)₂ and CaO₂ can effectively remove acidified volatile sulfide (AVS) and organic matter to control the black-odorous rebound. However, the remediation mechanisms in organic-rich sediments by Ca(NO₃)₂ and CaO₂ are unclear. The present study explored the mechanism of C-N-S interconnection degradation in organic-rich urban river sediments by adding different ratios and sequences of Ca(NO₃)₂ and CaO₂. The results showed that Ca(NO₃)₂ remediation followed by CaO₂ and the accepted electron ratio 1:1 of Ca(NO₃)₂ to CaO₂ is an effective method for controlling the rebound of black-odorous and reducing the accumulation NO₂^--N. Mainly attributed to that, CaO₂ enhanced the degradation of organic matter by stimulating enzymatic activities in the sediments, which is also the main reason for controlling the rebound of black-odorous. Since CaO₂ releases O₂ and •OH, which inhibit nosZgenes, NO₂^--N accumulates when remedied simultaneously with Ca(NO₃)₂ and CaO₂. Co-occurrence network analysis illustrated that sulfur-driven autotrophic denitrification bacteria, heterotrophic denitrifying bacteria, and sulfate-reducing bacteria interact strongly inside one module, clarifying a solid interaction of C-N-S substances among these bacteria. Our results reveal the C-N-S interconnection degradation mechanism and provide a new perspective on applying biochemical remediation in organic-rich urban river sediments.

Effects of long-term exposure to silver nanoparticles on the structure and function of microplastic biofilms in eutrophic water

Microplastics are frequently detected in natural aquatic systems proximate to populated areas, such as urban rivers and lakes, and can be rapidly colonized by microbial communities. Microplastics and silver nanoparticles (AgNPs) share similar pathways into natural waters and tend to form heteroaggregations. However, very little is known about the long-term impacts on the structure and function of microplastic biofilms when chronically exposed to silver nanoparticles. Thus, the present study assessed the accumulation property of AgNPs on polymethyl methacrylate (PMMA) microplastics via adsorption tests and studied the chronic effects of AgNPs on the structure and function of microplastic biofilms via 30-day microcosmic experiments in eutrophic water. The adsorption tests showed that the biofilms-colonized PMMA microplastics presented the highest adsorption of 0.98 mg/g in the 1 mg/L AgNPs microcosms. After the 30-day exposure, lactic dehydrogenase release and reactive oxygen species generation of PMMA biofilms increased by 33.23% and 23.98% compared to the MPs-control group with no-AgNPs, indicating that the number of dead cells colonizing microplastics significantly increased. Network analysis suggested that the stabilization of the bacterial community declined with the long-term exposure to AgNPs through the reduction of the modularity and average path length of the network. Compared to the MPs-control group, long-term exposure to AgNPs caused cumulatively inhibitory effects on the nitrogen removal and the N₂O emissions in eutrophic water. The isotopomer analysis revealed that the contribution rate of NO₂^- reduction to N₂O emissions was gradually increasing with the AgNPs exposure. Real-time PCR analysis showed that denitrification genes were less sensitive to AgNPs than the nitrification genes, with gene nosZ performed the most negligible response. Overall, our results revealed that long-term exposure to AgNPs could alter biogeochemical cycling involved by microplastic biofilms and cumulatively reduce the self-recovery of the eutrophic ecosystem.