Evaluating the role of high N2O affinity complete denitrifiers and non-denitrifying N2O reducing bacteria in reducing N2O emissions in river.

Freshwater rivers are hotspots of N2O greenhouse gas emissions. Dissolved organic carbon (DOC) is the dominant electron donor for microbial N2O reduction, which can reduce N2O emission through enriching high N2O affinity denitrifiers or enriching non-denitrifying N2O-reducing bacteria (N2ORB), but the primary regulatory pathway remains unclear. Here, field study indicated that high DOC concentration in rivers enhanced denitrification rate but reduced N2O flux by improving nosZ gene abundance. Then, four N2O-fed membrane aeration biofilm reactors inoculated with river sediments from river channel, estuary, adjacent lake, and a mixture were continuously performed for 360 days, including low, high, and mixed DOC stages. During enrichment stages, the (nirS+nirK)/nosZ ratio showed no significant difference, but the community structure of denitrifiers and N2ORB changed significantly (p < 0.05). In addition, N2ORB strains isolated from different enrichment stages positioned in different branches of the phylogenetic tree. N2ORB strains isolated during high DOC stage showed significant higher maximum N2O-reducing capability (Vmax: 0.6 ± 0.4 ×10-4 pmol h-1 cell-1) and N2O affinity (a0: 7.8 ± 7.7 ×10-12 L cell-1 h-1) than strains isolated during low (Vmax: 0.1 ± 0.1 ×10-4 pmol h-1 cell-1, a0: 0.7 ± 0.4 ×10-12 L cell-1 h-1) and mixed DOC stages (Vmax: 0.1 ± 0.1 ×10-4 pmol h-1 cell-1, a0: 0.9 ± 0.9 ×10-12 L cell-1 h-1) (p < 0.05). Hence, under high DOC concentration conditions, the primary factor in reducing N2O emissions in rivers is the enrichment of complete denitrifiers with high N2O affinity, rather than non-denitrifying N2ORB.

Greenhouse gases emissions from aquaculture ponds: Different emission patterns and key microbial processes affected by increased nitrogen loading.

Global aquaculture production is expected to rise to meet the growing demand for food worldwide, potentially leading to increased anthropogenic greenhouse gases (GHG) emissions. As the demand for fish protein increases, so will stocking density, feeding amounts, and nitrogen loading in aquaculture ponds. However, the impact of GHG emissions and the underlying microbial processes remain poorly understood. This study investigated the GHG emission characteristics, key microbial processes, and environmental drivers underlying GHG emissions in low and high nitrogen loading aquaculture ponds (LNP and HNP). The N2O flux in HNP (43.1 ± 11.3 µmol m-2 d-1) was significantly higher than in LNP (-11.3 ± 25.1 µmol m-2 d-1), while the dissolved N2O concentration in HNP (52.8 ± 7.1 nmol L-1) was 150 % higher than in LNP (p < 0.01). However, the methane (CH4) and carbon dioxide (CO2) fluxes and concentrations showed no significant differences (p > 0.05). N2O replaced CH4 as the main source of Global Warming Potential in HNP. Pond sediments acted as a sink for N2O but a source for CH4 and CO2. The △N2O/(△N2O + â–³N2) in HNP (0.015 ± 0.007 %) was 7.7-fold higher than in LNP (0.002 ± 0.001 %) (p < 0.05). The chemical oxygen demand to NO2-N ratio was the most important environmental factor explaining the variability of N2O fluxes. Ammonia-oxidizing bacteria driven nitrification in water was the predominant N2O source, while comammox-driven nitrification and nosZII-driven N2O reduction in water were key processes for reducing N2O emission in LNP but decreased in HNP. The strong CH4 oxidization by Methylocystis and CO2 assimilation by algae resulted in low CH4 emissions and CO2 sink in the aquaculture pond. The Mantel test indicated that HNP increased N2O fluxes mainly through altering functional genes composition in water and sediment. Our findings suggest that there is a significant underestimation of N2O emissions without considering the significantly increased △N2O/(△N2O + â–³N2) caused by increased nitrogen loading.

Effect and microbial mechanism of suspended sediments particle size on nitrous oxide emission in eutrophic lakes.

Suspended sediment (SPS) is an important environmental factor in eutrophic lakes, where they may play a significant role in the microbial nitrogen cycle and thus affect the N2O source and sink function. This study investigated the correlation and corresponding microbial mechanisms between N2O emission fluxes and SPS particle sizes. N2O emission characteristics were investigated in four parallel operated lab-scale microcosmic systems, in which different sizes of SPS particles were inoculated (i.e., <75, 75-150, 150-300, and >300 µm). The results show that, N2O emission fluxes in the eutrophic lakes were exponentially correlated with the lake trophic level index (TLI) (R2 = 0.94, p < 0.01) and the specific surface area of the SPS (R2 = 0.38, p < 0.05). In the microcosmic systems, SPS with 75-150 µm particles had the highest N2O emission rate of 5.94 ± 0.007 µg N/L/d, which was 2.6 times that of the <75 µm particle size system. The microcosmic system with particle size >300 µm had the highest N2O reduction rate (Vmax) of 6.776 µmol/L/h, which was 16-50 times that of the other three groups. Larger particle size SPS have a smaller specific surface area, which could affect the microenvironment on SPS surface and thus affect the microbe functions. The microbial community structure results indicated that the dominant microorganisms on the SPS surface were denitrifying bacteria. The maximum (nirS + nirK)/nosZ ratio was 30.2 for the 75-150 µm system, which was nearly 2 times higher than the other systems. The >300 µm system had the highest nosZ abundance, indicating a strong ability to reduce N2O. The co-occurrence networks analysis indicated that the cooperation and competition among nitrifiers and denitrifiers determined N2O emissions. These results provide fundamental insights into the influence of SPS size on N2O emissions in eutrophic lakes.

Nonlinear pattern and algal dual-impact in N2O emission with increasing trophic levels in shallow lakes.

Shallow lakes are considered important contributors to emissions of nitrous oxide (N2O), a powerful greenhouse gas, in aquatic ecosystems. There is a large degree of uncertainty regarding the relationship between N2O emissions and the progress of lake eutrophication, and the mechanisms underlying N2O emissions are poorly understood. Here, N2O emission fluxes and environmental variables in different lakes along a trophic state gradient in the Yangtze River basin were studied. N2O emission fluxes were -1.0-53.0 µg m-2 h-1 and 0.4-102.9 µg m-2 h-1 in summer and winter, respectively, indicating that there was marked variation in N2O emissions among lakes of different trophic state. The non-linear exponential model explained differences in N2O emission fluxes by the degree of eutrophication (p < 0.01). TN and chl-a both predicted 86% of the N2O emission fluxes in shallow lakes. The predicted N2O emission fluxes based on the IPCC EF5r overestimated the observed fluxes, particularly those in hyper-eutrophic lakes. These findings demonstrated that nutrient-rich conditions and algal accumulation are key factors determining N2O emission fluxes in shallow lakes. Furthermore, this study also revealed that temperature and algae accumulation-decomposition determine an N2O emission flux in an intricate manner. A low temperature, i.e., winter, limits algae growth and low oxygen consumption for algae decomposition. The environment leaves a high dissolved oxygen concentration, slowing down N2O consumption as the final step of denitrification. In summer, with the oxygen consumed by excess algal decomposition, the N2O production is limited by the complete denitrification as well as the limited substrate supply of nitrate by nitrification in hypoxic or anoxic conditions. Such cascading events explained the higher N2O emission fluxes from shallow lakes in winter compared with summer. This trend was amplified in hyper-eutrophic shallow lakes after algal disappearance. Collectively, algal accumulation played a dual role in stimulating and impeding N2O emissions, especially in hyper-eutrophic lakes. This study expands our knowledge of N2O emissions from shallow lakes in which eutrophication is underway.

Nonlinear response of methane release to increased trophic state levels coupled with microbial processes in shallow lakes.

Shallow lakes are a crucial source of methane (CH4), a potent greenhouse gas, to the atmosphere. However, large uncertainties still exist regarding the response of CH4 emissions to the increasing trophic levels of lakes as well as the underlying mechanisms. Here, we investigate the CH4 emission flux from lakes with different trophic states in the middle and lower reaches of the Yangtze River basin, China to evaluate the effect of the trophic lake index (TLI) on CH4 emissions. The mean CH4 emission fluxes from mesotrophic, eutrophic, middle-eutrophic, and hyper-eutrophic lakes were 0.1, 4.4, 12.0, and 130.4 mg m-2 h-1, respectively. Thus, the CH4 emission flux ranged widely and was positively correlated with the degree of eutrophication. The relative abundance of methanogens with respect to the total population for the mesotrophic, eutrophic, mid-eutrophic, and hyper-eutrophic states was 0.03%, 0.35%, 0.94%, and 1.17%, respectively. The biogeographic-scale pattern of lakes classified as each of these four trophic states indicated that CH4 emissions could be well-predicted by the NH4+-N concentration in the water column, as both NH4+-N and CH4 were produced during mineralisation of labile organic matter in lake sediment. In addition, the shift from clear to turbid water, which is an unhealthy evolution for lakes, was associated with a nonlinear increase in the CH4 emissions from the studied lakes. In particular, the hypereutrophic lakes functioned as CH4 emission hotspots. Our findings highlight that nutrient levels, as a potential facilitator of CH4 emissions, should be considered in future research to accurately evaluate the greenhouse gas emissions from shallow lakes.

Effect evaluation of microplastics on activated sludge nitrification and denitrification.

A large amount of microplastics have entered conventional wastewater treatment plants, and their effects on activated sludge nitrification and denitrification are rarely reported. This study investigated the effects of microplastics on activated sludge nitrification and denitrification using five typical microplastics, namely, polyvinyl chloride (PVC), polypropylene, polyethylene, polystyrene, and polyester (PES) with concentrations of 0, 1000, 5000, and 10,000 particles/L. Results indicated that microplastics had negative effects on ammonia oxidation rate and low effect on nitrite oxidation rate during nitrification. The total inorganic nitrogen did not have much difference during 3 h nitrification under all the tested conditions. The addition of microplastics showed positive effects on denitrification, especially for PVC and PES at microplastic concentration of 5000 particles/L. Nitrification and denitrification did not evidently stop under all the tested conditions, indicating that the selected microplastic types and concentrations were not toxic to nitrification and denitrification within 3 h. The high abundance of PVC microplastics remarkably increased the nitrous oxide (N2O) emission during denitrification. The N2O emission in the test with 10,000 particle/L of PVC was 4.6times higher than the blank control. This study indicated that microplastics with <10,000 particle/L concentration in wastewater had low effects on nitrification and denitrification, whereas they had high effects on the N2O emission during denitrification.

Suspended particles potentially enhance nitrous oxide (N2O) emissions in the oxic estuarine waters of eutrophic lakes: Field and experimental evidence.

Estuaries are considered hot spots for the production and emissions of nitrous oxide (N2O) and easily occur suspended particles (SPS), however, current understanding about the role of SPS in the N2O emissions from the oxic estuarine waters of lacustrine ecosystems is still limited. In this study, field investigations were performed in the estuaries of hypereutrophic Taihu Lake, and laboratory simulations were simultaneously conducted to ascertain the characteristics of N2O emissions with different SPS concentrations. The results showed that the N2O emission fluxes ranged from 9.75 to 118.38 µg m-2 h-1, indicating a high spatial heterogeneity for the N2O emissions from the estuaries of Taihu Lake. Although the dissolved oxygen (DO) concentrations were up to 7.85 mg L-1 in the estuarine waters, from where the N2O emissions fluxes were approximately three times that of the lake regions. Multiple regression model selected the total nitrogen (TN), SPS, and DO concentrations as the crucial factors influencing the N2O emission fluxes. Particularly for SPS, the simulation results showed that the N2O concentrations increased gradually with the increase in the SPS concentrations of an oxic water column containing 4 mg L-1 of NO3--N, indicating that a high SPS concentration can accelerate the N2O emissions. It was related to the change of denitrifying bacteria population in the SPS, as evidenced by its significantly positive correlation with N2O emissions (p < 0.01). Our findings will draw attentions to the role of SPS playing in the N2O productions and emissions in eutrophic lakes, and its effect on nitrogen cycle should be considered in the future study.