Vegetated urban streams have sufficient purification ability but high internal nutrient loadings: Microbial communities and nutrient release dynamics

Effects of pH, temperature and hydraulic disturbance on nitrogen release from sediments in the Sunxi River, Three Gorges Reservoir Area, China

To clarify the influence of changes in the overlying water environment on internal nitrogen release from reservoir sediments, we collected surface sediments at a depth of approximately 10 cm from the Sunxi River in the tail area of the Three Gorges Reservoir area for simulation experiments. By using orthogonal simulation experiments in the laboratory, we studied the effects of water pH, temperature and hydraulic disturbance on nitrogen release in the sediment and established a quantitative linear relationship between the nitrogen release rate from the sediment and the environmental factors of the overlying water. The results indicated that the average concentrations of total nitrogen (TN) and total phosphorus (TP) in the sediment were 430 mg/kg and 200 mg/kg, respectively. The sediment TN concentration had a very significant positive correlation with the sediment organic matter content (P < 0.001). The sediment TN, NO3-N and NH4-N release intensities gradually increased with increasing incubation time, with maximum release rates of 29.24 mg/((m2⋅d), 23.11 mg/(m2⋅d) and 4.32 mg/((m2⋅d), respectively. Range analysis revealed that the significance of the effects of environmental factors on sediment TN and NH4-N release were ranked as follows: temperature > pH > disturbance, and that of NO3-N release was ranked as pH > temperature > disturbance. Temperature plays the most important role in the behavior of different forms of nitrogen release from sediments. The capacity and potential for nitrogen release from sediments offer crucial insights for assessing the risks posed to the overlying water and highlighting the importance of these factors in water quality management and prediction in the reservoir area.

Dissolved phosphorous through dry-wet-dry transitions in a small-dammed river basin: integrated understanding on transport patterns, export controls, and fate

An integrated understanding of dissolved phosphorous (DP) export mechanism and controls on export over dry and wet periods is crucial for riverine ecological restorations in dammed river basins considering its high bioavailability and retention rates at dams. Riverine DP transport patterns (composition, sources, and transport pathways), export controls, and fate were investigated over the 2020 wet season (5 events) and dry seasons before and after it (2 events: dry(before) and dry(after)) in a semi-arid, small-dammed watershed to comprehend the links between terrestrial DP sources and aquatic DP sinks. Close spatiotemporal monitoring of the full range of phosphorous and total suspended solids (TSSs) and subsequent analyses (hysteresis, hierarchical partitioning, and coefficient of variation) provided the basis for the study. Total-DP (TDP) shared 13-39% (25%) of total-P (TP) through storms, dissolved organic-P (DOP) shared 6-21% (12%), and phosphate-P shared (PO4-P) 7-22% (13%). DP forms displayed strong connections with discharge trends across the wet season, and marked changes in the shares were reported over dry-wet and wet-dry transitions. The DOP fraction of TDP increased from 4% in dry(before) baseflow to 64% at the end of the wet season. The DOP flux increment in stormflow was 20 folds compared to dry(before) baseflow, while that of PO4-P was 2 folds. DOP displayed the least spatial source heterogeneity with minimum anthropogenic pressure on inherent fluxes. DOP originated from overland and near-stream soil sources and was transported via surface runoff and soil water runoff, respectively. Across the wet season, the attrition of overland DOP sources and activation of near-stream soil DOP sources through strengthened hydrological connectivity governed the seasonal DOP trends. Surface and groundwater runoff pathways were important for PO4-P delivery during stormflow; nonetheless, wastewater treatment plant (WWTP) effluent was the main PO4-P source under both baseflow and stormflow regimes, followed by near-stream traditional agriculture lands. The interaction patterns of small dam systems with DP inputs through dry-wet periods were explained. The riverine PO4-P fluxes were profoundly impacted by in-stream biogeochemical and physical processes and small dam systems, while riverine DOP fluxes were relatively less influenced.

Amplified impacts of human activities: Non-linear responses of riverine microbial communities to distribution of land use

Rapid global urbanization poses considerable ecological risks to freshwater systems, notably leading to substantial reductions in microbial communities. To assess the impacts of human activities on these communities, we applied the high-throughput amplicon DNA sequencing to examine spatial variations in riverine microbial communities within an urbanized watershed. Coupled with the Geographical Detector Model, the effects of the land use were identified across the watershed. Results show that microbial communities were closely linked to the human-impacted land use patterns. The upstream region, dominated by forest cover (71%), exhibited the highest microbial population (3384 OTUs), whereas the urbanized downstream outlet (91% urban land) showed the lowest microbial population (471 OTUs). Additionally, the spatial distribution of the human-impacted land use appears to abruptly alter microbial pathways along the river. The spatial threshold effect of human-impacted land use is indicated by a Moran's I value exceeding 0.80. Notably, a 300-m buffer zone around different land uses seems to significantly influenced sediment microbial communities. Besides, the influence of land use on microbial communities is intensified by spatial drivers. For instance, agricultural land use was found to impact riverine Parcubacteria communities, with factor detector values increasing by over 30% in 400-500 m buffer zones. These findings provide new insights into the complex relationship between human activity and riverine microbial communities, highlighting important implications for ecosystem management in rapidly urbanization regions.

Effects of perfluoroalkyl acids on nitrogen release, transformation and microbial community during the debris decomposition of Alisma orientale and Iris pseudacorus

The release of nutrients into water during debris decomposition is a serious concern, leading to severe environmental pollution. To understand the effects of extensively present emerging contaminants (such as perfluoroalkyl acids (PFAAs)) on the nitrogen (N) release and transformation, the concentration dynamics of different N species in surrounding water and changes in microbial communities on biofilm during the 70-days decomposition of two typical submerged macrophyte (Alisma orientale and Iris pseudacorus) debris were studied. The results showed that large amounts of N species (especially organic and ammonium N) were released during decomposition. PFAAs with a low concentration (1 μg/L) could stimulate total N (TN) release, whereas PFAAs with a high concentration (≥ 10 μg/L) might have inhibited TN release. Higher intensities of ammonification, nitrosification, and denitrification, but lower intensities of nitrification were observed in water in the presence of PFAAs. Microbiota associated with organic matter hydrolysis, nitrification and denitrification, as well as PFAA degrading/tolerant bacteria, were beneficial and might have occupied dominant states. Redundancy analysis showed that PFAAs were positively associated with the amounts of nitrate, denitrifiers, and azotobacteria but negatively correlated with the TN, ammonia, nitrite, organic N, and nitrosobacteria amounts (p = 0.0002). The complete N metabolism pathway was identified using PICRUSt and KEGG. Functional genes related to ammonification (0.76‰-2.16‰), N reduction (3.43‰-5.05‰), and assimilation (0.81‰-2.16‰) were more abundant than others in all treatments. This study provides a more comprehensive understanding of N cycling during debris decomposition under the increasingly intractable threat of emerging contaminants in aquatic ecosystems.

Role of hydrophytes in constructed wetlands for nitrogen removal and greenhouse gases reduction

With the prominence of global climate change and proposal of carbon reduction concept, how to maximize the comprehensive effect of nitrogen removal and greenhouse gases (GHGs) reduction in constructed wetlands (CWs) has become crucial. As indispensable biological component of CWs, hydrophytes have received extensive attention owing to their application potential. This review comprehensively evaluates the functions of hydrophytes in nitrogen removal and GHGs reduction in CWs in terms of plants themselves, plant-mediated microbes and plant residues (hydrophyte carbon sources and hydrophyte-derived biochars). On this basis, the strategies for constructing an ideal CW system are put forward from the perspective of full life-cycle utilization of hydrophytes. Finally, considering the variability of plant species composition in CWs, outlooks for future research are specifically proposed. This review provides guidance and novel perspectives for the full life-cycle utilization of hydrophytes in CWs, as well as for the construction of an ideal CW system.

Enhanced nutrient removal in agro-industrial wastes-amended hybrid floating treatment wetlands treating real sewage: Laboratory microcosms to field-scale studies

The use of natural agro-industrial materials as suspended fillers (SFs) in floating treatment wetlands (FTWs) to enhance nutrient removal performance has recently been gaining significant attention. However, the knowledge concerning the nutrient removal performance enhancement by different SFs (alone and in mixtures) and the major removal pathways is so far inadequate. The current research, for the first time, carried out a critical analysis using five different natural agro-industrial materials (biochar, zeolite, alum sludge, woodchip, flexible solid packing) as SFs in various FTWs of 20 L microcosm tanks, 450 L outdoor mesocosms, and a field-scale urban pond treating real wastewater over 180 d. The findings demonstrated that the incorporation of SFs in FTWs enhanced the removal performance of total nitrogen (TN) by 20-57% and total phosphorus (TP) by 23-63%. SFs further enhanced macrophyte growth and biomass production, leading to considerable increases in nutrient standing stocks. Although all the hybrid FTWs showed acceptable treatment performances, FTWs set up with mixtures of all five SFs significantly enhanced biofilm formation and enriched the abundances of the microbial community related to nitrification and denitrification processes, supporting the detected excellent N retention. N mass balance assessment demonstrated that nitrification-denitrification was the major N removal pathway in reinforced FTWs, and the high removal efficiency of TP was attributable to the incorporation of SFs into the FTWs. Nutrient removal efficiencies ranked in the following order among the various trials: microcosm scale (TN: 99.3% and TP: 98.4%) > mesocosm scale (TN: 84.0% and TP: 95.0%) > field scale (TN: -15.0-73.7% and TP: -31.5-77.1%). These findings demonstrate that hybrid FTWs could be easily scaled up for the removal of pollutants from eutrophic freshwater systems over the medium term in an environmentally-friendly way in regions with similar environmental conditions. Moreover, it demonstrates hybrid FTW as a novel way of disposing of significant quantities of wastes, showing a win-win means with a huge potential for large-scale application.