Increased risk of heavy metal accumulation in mangrove seedlings in coastal wetland environments due to microplastic inflow.

The recovery phase of mangrove seedlings in coastal wetland ecosystems can be negatively affected by exposure to external pollutants. This study aimed to investigate the impact of microplastics (MPs) influx, specifically polystyrene (PS) and polymethyl methacrylate (PMMA), on the growth of Aegiceras corniculatum seedlings and their accumulation of heavy metals (HMs). PS and PMMA significantly increased HMs accumulation (up to 21.0-548%), particularly in the roots of seedlings, compared to the control treatment (CK). Additionally, elevated activities of malondialdehyde and catalase enzymes were observed in the leaves of seedlings, while peroxidase enzyme activity decreased. Topological analysis of the root sediment microbiota coexistence network revealed that the modularization data increased from 0.69 (CK treatment) to 1.07 (PS treatment) and 5.11 (PMMA treatment) under the combined stress of MPs and HMs. This suggests that the introduction of MPs intensifies microbial modularization. The primary cause of increased HMs accumulation in plants is the MPs input, which influences the secretion of organic acids by plants and facilitates the shift of HMs in sediment to bioavailable states. Furthermore, changes in microbial clustering may also contribute to the elevated HMs accumulation in plants. This study provides valuable insights into the effects of external pollutants on mangrove seedlings and offers new perspectives for the preservation and restoration of mangrove coastal wetlands.

Differential Nitrous oxide emission and microbiota succession in constructed wetlands induced by nitrogen forms.

Nitrous oxide (N2O) emission during the sewage treatment process is a serious environmental issue that requires attention. However, the N2O emission in constructed wetlands (CWs) as affected by different nitrogen forms in influents remain largely unknown. This study investigated the N2O emission profiles driven by microorganisms in CWs when exposed to two typical nitrogen sources (NH4+-N or NO3--N) along with different carbon source supply (COD/N ratios: 3, 6, and 9). The results showed that CWs receiving NO3--N caused a slight increase in total nitrogen removal (by up to 11.8 %). This increase was accomplished by an enrichment of key bacteria groups, including denitrifiers, dissimilatory nitrate reducers, and assimilatory nitrate reducers, which enhanced the stability of microbial interaction. Additionally, it led to a greater abundance of denitrification genes (e.g., nirK, norB, norC, and nosZ) as inferred from the database. Consequently, this led to a gradual increase in N2O emission from 66.51 to 486.77 ug-N/(m2·h) as the COD/N ratio increased in CWs. Conversely, in CWs receiving NH4+-N, an increasing influent COD/N ratio had a negative impact on nitrogen biotransformation. This resulted in fluctuating trend of N2O emissions, which decreased initially, followed by an increase at later stage (with values of 122.87, 44.00, and 148.59 ug-N/(m2·h)). Furthermore, NH4+-N in the aquatic improved the nitrogen uptake by plants and promoted the production of more root exudates. As a result, it adjusted the nitrogen-transforming function, ultimately reducing N2O emissions in CWs. This study highlights the divergence in microbiota succession and nitrogen transformation in CWs induced by nitrogen form and COD/N ratio, contributing to a better understanding of the microbial mechanisms of N2O emission in CWs with NH4+-N or NO3--N at different COD/N ratios.

Arsenic pollution, geochemical fractions, and leaching characteristics in soil samples from four contaminated sites in the Beijiang River Basin, South China.

To remediate historically polluted sites before their land-use changes, it is essential to understand the concentration distribution, geochemical fraction, and migratory behavior of As in soil with varied particle sizes for the use of a sieving procedure. This study investigated the amount and percentage of As in soil with different particle sizes (0.25, 0.25-1, and 1-2 mm) as well as its toxicity characteristic in leaching procedure at four previously contaminated sites in the Beijiang River Basin, South China. The results showed that the total As concentration in the collected soils ranged from 70.1 to 402.8 mg/kg, and only a few percent of soil particle samples had As contents below the local risk threshold value of 60 mg/kg. The amorphous hydrous oxide bound, crystalline hydrous oxide bound, and residual fractions (F3-F5) of the geochemical fraction of As in soil of polluted sites accounted for 82.2-95.7% of the total As distribution. However, the concentration of As in non-specifically bound fractions increased with the mass ratio of soil with coarse particle sizes due to the negative correlation of Fe-bearing minerals concentration with the mass ratio of soil with coarse particle size. According to redundancy analysis, soil with coarse particle sizes and non-specifically bound As were mostly responsible for the As concentration in the leachate. These findings confirmed that a sieving process was not suitable for the remediation of soil As at four historically contaminated sites in the Beijiang River Basin due to the high concentration of As in soil and non-negligible environmental risk of labile extractable As in soil with coarse particle size.

Distribution fractions and potential ecological risk assessment of heavy metals in mangrove sediments of the Greater Bay Area.

The restoration of mangrove in coastal wetlands of China has been started since the 1990s. However, various pollutants, especially for heavy metals (HMs), contained in wastewater might present a significant risk to mangrove forests during the restoration. In this study, sediments of five typical mangrove wetlands with varying restoration years and management measures in the Greater Bay Area were collected to evaluate the distribution fractions and potential ecological risk of HMs. Cd (0.2-1.6 mg/kg) was found in high concentrations in the exchangeable fraction (37.8-71.5%), whereas Cu (54.2-94.8 mg/kg), Zn (157.6-332.6 mg/kg), Cr (57.7-113.6 mg/kg), Pb (36.5-89.9 mg/kg), and Ni (29.7-69.5 mg/kg) primarily presented in residual fraction (30.8-91.9%). According to the geo-accumulation index (Igeo) analysis, sediment Cd presented a high level of pollution (3 ≤ Igeo ≤ 4), while Zn and Cu were associated with moderately pollution (1 ≤ Igeo ≤ 2). Besides, high ecological risk of Cd was found in sediments of five mangroves, with risk assessment code (RAC) ranging from 45.9 to 84.2. Redundancy analysis revealed that the content of NO3--N was closely related to that of HMs in sediments and, pH value and NO3--N concentration affected the distribution of HMs geochemical fractions. High concentration of HMs in QA and NS sampling sites was caused by the formerly pollutants discharge, resulting in these sediments still with a higher HM pollution level after the plant of mangrove for a long period. Fortunately, strict drainage standards for industrial activities in Shenzhen significantly availed for decreasing HMs contents in mangrove sediments. Therefore, future works on mangrove conversion and restoration should be linked to the water purification in the GBA.

Reducing the inhibitive effect of fluorine and heavy metals on nitrate reduction by hydroxyapatite substrate in constructed wetlands.

Bio-toxic inorganic pollutants, e.g., fluorine (F) and heavy metals (HMs), in wastewaters are the potential threats to nitrate (NO3--N) reduction by microorganisms in constructed wetlands (CWs). Selection of suitable substrate with high F and HMs adsorption efficiency and capacity is a potential alternative for simultaneous removal of these pollutants in CWs. Herein, this study investigated the feasibility of applying hydroxyapatite (HA)-gravel media for F and HMs adsorption and its effect on NO3--N reduction in CWs (HA CWs) by comparing the CWs filled with gravel substrate (CK CWs). The results indicated that the removal efficiency of F, Cr, As, and NO3--N in HA CWs increased by 113.6-, 3.3-, 2.7-, and 0.6-folds, respectively, compared to CK CWs. The NO3--N reduction rate decreased by 11-46% in CK CWs after the presence of F and HMs in influent, while for HA CWs, it was only 13-22%. Excellent F and HMs adsorption capacity of HA substrate availed for wetland plants resisting F/HMs toxicity and making catalase activity lower. The HA substrate in CWs resulted in the certain succession of nitrogen-transforming bacteria, e.g., nitrifiers (Nitrospira) and denitrifiers (Thiobacillus and Desulfobacterium). More importantly, key functional genes, including nirK/nirS, korA/korB, ChrA/ChrD, arsA/arsB, catalyzing the processes of nitrogen biotransformation, energy metabolism, NO3--N and metal ions reduction were also enriched in HA CWs. This study highlights HA substrate reduce the inhibitive effect of F and HMs on NO3--N reduction, and provides new insights into how microbiota structurally and functionally respond to different substrates in CWs.

Interactive effects of aquatic nitrogen and plant biomass on nitrous oxide emission from constructed wetlands.

Understanding of mechanisms in nitrous oxide (N2O) emission from constructed wetland (CW) is particularly important for the establishment of related strategies to reduce greenhouse gas (GHG) production during its wastewater treatment. However, plant biomass accumulation, microbial communities and nitrogen transformation genes distribution and their effects on N2O emission from CW as affected by different nitrogen forms in aquatic environment have not been reported. This study investigated the interactive effects of aquatic nitrogen and plant biomass on N2O emission from subsurface CW with NH4+-N (CW-A) or NO3--N (CW-B) wastewater. The experimental results show that NH4+-N and NO3--N removal efficiencies from CW mesocosms were 49.4% and 87.6%, which indirectly lead to N2O emission fluxes of CW-A and CW-B maintained at 213 ± 67 and 462 ± 71 µg-N/(m2·h), respectively. Correlation analysis of nitrogen conversion dynamic indicated that NO2--N accumulation closely related to N2O emission from CW. Aquatic NH4+-N could up-regulate plant biomass accumulation by intensifying citric acid cycle, glycine-serine-threonine metabolism etc., resulting in more nitrogen uptake and lower N2O emission/total nitrogen (TN) removal ratio of CW-A compared to CW-B. Although the abundance of denitrifying bacteria and N2O reductase nosZ in CW-B were significantly higher than that of CW-A, after fed with mixed NH4+-N and NO3--N influent, N2O fluxes and N2O emission/TN removal ratio in CW-A were extremely close to that of CW-B, suggesting that nitrogen form rather than nitrogen transformation microbial communities and N2O reductase nosZ determines N2O emission from CW. Hence, the selection of nitrate-loving plants will play an important role in inhibiting N2O emission from CW.