Are nitrous oxide emissions indirectly fueled by input of terrestrial dissolved organic nitrogen in a large eutrophic Lake Taihu, China?

Brownification increases the abundance of microorganisms related to carbon and nitrogen cycling in shallow lakes

Brownification in aquatic ecosystems under global change has attracted attention. The composition and quantity of dissolved organic matter transported from various land use types to lakes differ significantly, causing varying ecological effects of lake brownification by region. Bacterial communities make a significant contribution to the material cycle of ecosystems and are sensitive to environmental changes. In this study, a series of mesocosm systems were used to simulate forest lakes and urban lakes with different degrees of brownification, and a high-throughput amplicon sequencing technique was used to explore the changes in the composition, structure, and function of bacterial communities in shallow lakes undergoing brownification. Principal coordinate analysis (PCoA) and Jensen‒Shannon distance typing analysis both indicated significant differences in bacterial communities between forest lakes and urban lakes. The α diversity of bacterial communities in urban lakes increased with the degree of brownification. However, whether forest lakes or urban lakes, brownification increased the abundance of carbon cycling-related bacterial phyla (Proteobacteria, Poribacteria, and Chloroflexi) and nitrogen cycling-related bacterial genera (Microbacteriaceae, Limnohabitans, Comamonadaceae, Bacillus, and Rhizobiales_Incertae_Sedis). Additionally, the carbon and nitrogen cycling functions of bacterial communities in forest lakes are dominant, while those in urban lakes are dominated by functions related to light. Our study has preliminarily revealed that lake brownification promotes the growth of carbon and nitrogen cycling microorganisms, providing a new paradigm for understanding the response of lake ecosystems in different catchment areas to environmental changes and the carbon and nitrogen cycling processes in shallow lake ecosystems.

An indispensable role of overlying water in nitrogen removal in shallow lakes

The nitrogen (N) removal characteristics in water columns and sediments of shallow lakes, influenced by various factors, may exhibit spatial variations in lakes with algal-macrophyte dominance. The N removal rates in water columns and sediments of Lake Taihu were investigated. Our findings indicated that the total N removal rates in Lake Taihu followed the order of algae-dominance > macrophyte-dominance > pelagic lake (without the presence of algae and macrophytes). Correlation analysis revealed that the key environmental factors affecting denitrification and anammox in sediments of algae/macrophyte-type lakes were nitrate nitrogen (NO3--N), nitrite nitrogen (NO2--N), ammonia nitrogen (NH4+-N), and chlorophyll a (Chl-a). The linear regression demonstrated that a significant correlation between the denitrification and the anammox in sediments, with a correlation coefficient of 0.81 (p < 0.01). The contributions to N removal from the water columns and sediments in Lake Taihu were 53 % and 47 %, respectively. Denitrification predominantly drove N removal from sediments, whereas anammox dominated the N removal in water columns. Thus, N removal from the water columns is nonnegligible in shallow eutrophic lakes. This study enhances our understanding of N biogeochemical cycling dynamics in sediment-water and algae/macrophyte ecosystems across various shallow eutrophic lake regions.

Increased nitrous oxide emissions from global lakes and reservoirs since the pre-industrial era

Lentic systems (lakes and reservoirs) are emission hotpots of nitrous oxide (N2O), a potent greenhouse gas; however, this has not been well quantified yet. Here we examine how multiple environmental forcings have affected N2O emissions from global lentic systems since the pre-industrial period. Our results show that global lentic systems emitted 64.6 ± 12.1 Gg N2O-N yr-1 in the 2010s, increased by 126% since the 1850s. The significance of small lentic systems on mitigating N2O emissions is highlighted due to their substantial emission rates and response to terrestrial environmental changes. Incorporated with riverine emissions, this study indicates that N2O emissions from global inland waters in the 2010s was 319.6 ± 58.2 Gg N yr-1. This suggests a global emission factor of 0.051% for inland water N2O emissions relative to agricultural nitrogen applications and provides the country-level emission factors (ranging from 0 to 0.341%) for improving the methodology for national greenhouse gas emission inventories.

Spatial distribution characteristics of denitrification functional genes and the environmental drivers in Liaohe estuary wetland

Genes nirS, nirK, and nosZ are specific for the denitrification process, which is associated with greenhouse gas N2O emission. The abundances and diversities of community containing these three genes are usually used as a common index to reflect the denitrification process, and they would be affected by differences in environmental factors caused by changes from warm to cold conditions. The quantification of denitrification in natural wetlands is complex, and straightforward identification of spatial distribution and drivers affecting the process is still developing. In this study, the bacterial communities, gene diversities, and relative abundances involved in denitrification were investigated in Liaohe Estuary Wetland. We analyzed the relative abundances, diversities, and communities of bacteria containing the three genes at warm and cold conditions using Illumina MiSeq sequencing and detected the potential environmental factors influencing their distribution by using a random forest algorithm. There are great differences in the community composition of the bacteria containing genes nirS, nirK, and nosZ. All the abundant taxa of nirS and nirK communities belonged to phylum Proteobacteria. Compared with the community composition of bacteria containing nirS and nirK, the community of bacteria containing nosZ is more diverse, and the subdivision taxa of phylum Euryarchaeota was also abundant in the community containing nosZ. The distribution characteristics of the relative abundance of nirS and nirK showed obvious differences both at warm and cold climate conditions. The oxidation-reduction potential, nitrite nitrogen, and salinity were detected as potential variables that might explain the diversity of nirS. The total nitrogen and nitrite nitrogen were the important variables for predicting the relative abundance of nirS at warm climate condition, while oxidation-reduction potential and pH contributed to the diversity of nirS at cold condition. The bulk density of sediment was detected as a potential variable affecting the relative abundance of nirK at warm and cold conditions, and diversity of nirK at warm condition, while nitrite nitrogen was detected as an important environmental factor for predicting the diversity of nirK at cold condition. Overall, our results show that the key environmental factors, which affect the relative abundance, diversity, and community of bacteria containing the functional denitrification genes, are not exactly the same, and the diversities of nirS, nirK, and nosZ have a higher environmental sensitivity than their relative abundances.

Iron scraps packing rapidly enhances nitrogen removal in an aerobic sludge system and the mechanism

Simultaneous nitrification and denitrification (SND) has the advantage of energy saving and carbon demand reduction. Here, readily available low-cost iron scraps packing was added to an aerobic sludge system. This successfully enhanced the efficiency of total nitrogen removal from 37.7 ± 13.2 % to 62.7 ± 7.9 % over 2 days. While electrons from iron biocorrosion did not contribute to nitrate reduction, iron promoted heterotrophic denitrification. The iron scraps changed the spatial distribution of the microbial community, where more denitrification bacteria accumulated around the packing and higher denitrification capacity was noted. Metagenomic analysis of the sludge cultured in the presence of iron scraps for 2 days revealed that, with the exception of the enriched amoA/B/C gene expression, the abundance of other key nitrogen removal genes showed little variation. Furthermore, the structure of the microbial community was unchanged probably due to the relatively short culturing period. However, metatranscriptomic analysis indicated that iron increased the abundance of nitrifying bacteria (i.e. unclassified Nitrosomonas, Nitrosomonas sp. Is79A3 and Nitrospira defluvii) and promoted higher expression of nitrification genes. Notably, iron scraps packing decreased the abundance of the key denitrification bacteria Thauera sp. MZ1T from 52.92 to 7.58 %. The expression of napA/B also decreased, while expression of narG/H/I increased by 9 to 23 fold and a 2 to 3 fold over expression was noted for nirS, norB/C and nosZ in the presence of iron scraps. This suggested that aerobic denitrification was inhibited and anaerobic denitrification was promoted. This study has provided in-depth understanding of the influence of iron on SND to improve the application of iron-supported biological processes.

Comprehensive analysis of the migration and transformation of nutrients between sediment and overlying water in complex habitat systems

Under the influence of environmental change, disturbance and other external conditions, sediments release internal nutrients to the overlying water and become a contamination source in the lake. Complex habitat systems provide a unique opportunity for determining the influences of environmental changes in lakes. In this study, Baiyangdian Lake (BYDL) was divided into different habitat systems (connected water areas, river courses, reed fields, lotus ponds, fishponds, farmland, and thorps) based on the influence of natural and artificial activities. The physical and chemical properties of overlying water and sediment in different habitat systems were investigated. In addition, statistical analytical methods were used to analyze the relationship between sediment characteristics and overlying water parameters in different habitat systems. The results showed that nitrogen and phosphorus in the overlying water could accumulate in the sediments, while disturbance was one of the main factors affecting the release of nutrients from sediments. Disturbance promoted the suspension of sediments and increased the oxygen content, thereby facilitating the internal release of nutrients. However, there were also some differences in the process of internal release of nutrients between the habitat systems. Nitrogen in the overlying water was closely related to the source of organic matter (r > 0.950), especially in the ponds (including lotus ponds, reed fields, and fishponds), and phosphorus was mainly influenced by turbidity (r > 0.870). In the river course (p = 0.198, n = 26), the disturbance and increase in pH promoted the internal release of nutrients from the sediments (contributions of 35.2 % and 25.1 %, respectively). In the ponds, the aquatic macrophytes reduced the release of nitrogen and phosphorus in sediments. Overall, this study provides more information on the migration and transformation of nutrients between sediment and overlying water in lakes with multiple habitats.

Distinctive Microbial Processes and Controlling Factors Related to Indirect N2O Emission from Agricultural and Urban Rivers in Taihu Watershed

Inland rivers are hotspots of anthropogenic indirect nitrous oxide (N₂O) emissions, but the underlying microbial processes remain poorly understood. This study measured N₂O fluxes from agricultural and urban rivers in Taihu watershed and investigated the microbial processes driving N₂O production and consumption. The N₂O fluxes were significantly higher in agricultural rivers (140.1 ± 89.1 μmol m^-2 d^-1) than in urban rivers (25.1 ± 27.0 μmol m^-2 d^-1) (p < 0.001). All wind-based models significantly underestimated N₂O flux in urban rivers (p < 0.05) when using the Intergovernmental Panel on Climate Change method because they underestimated the N₂O emission factor (EF_5r). Wind speed and nitrate were the key factors affecting N₂O fluxes in agricultural and urban rivers, respectively. NirK-type denitrifiers produced N₂O in urban river water, while nirS-type denitrifiers consumed N₂O in the sediments of all rivers. Co-occurrence network analysis indicated organics from Microcystis served as electron donors for denitrifiers (dominated by Flavobacterium) in water, while direct interspecies electron transfer between Thiobacillus and methanogens and between Dechloromonas and sulfate-reducing bacteria enhanced N₂O reduction in sediments. This study advances our knowledge on the distinctive microbial processes that determine N₂O emissions in inland rivers and illustrates the need to revise EF_5r for N₂O estimation in urban rivers.

Linkages between greenhouse gases (CO2, CH4, and N2O) and dissolved organic matter composition in a shallow estuary

Estuarine systems receive large amounts of organic matter that enhance the production of greenhouse gases (GHGs), such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Despite considerable research on GHGs and dissolved organic matter (DOM) distribution in estuaries, little is known about the linkage between these gases and DOM composition. Here we evaluated the relationship between three GHGs (CO₂, CH₄, and N₂O) and DOM composition, determined through optical properties, in Guadalete estuary (Bay of Cadiz, Spain). The partial pressure of CO₂, and CH₄ and N₂O concentrations ranged between 332.8 and 6807.1 μatm, 19.9-6440.1 nM, and 6.8-283.9 nM, respectively. Thus, the Guadalete estuary was a source of CO₂, CH₄ and N₂O to the atmosphere. We validated three PARAFAC components related to humic-like fluorescence from terrestrial, microbial and effluent sources, and one with protein-like material. Humic-like components accounted for 86% ± 6% of the total FDOM pool, indicating a predominantly allochthonous DOM origin. The three GHGs were significantly linked to DOC concentration and DOM composition, exhibiting different patterns in these linkages. Terrestrial and microbial humic-like substances with increasing aromaticity might enhance pCO₂ in Guadalete estuary. Dissolved CH₄ concentrations showed the strongest relationship with DOM composition, indicating that humic and protein-like material are linked with their distribution. In contrast, dissolved N₂O was only related with the protein-like fraction and with humic-like material derived from anthropogenic activities (sewage and agriculture). Our results further indicate that a possible coupling between benthic fluxes of GHGs and DOM might be occurring in this shallow estuary. We conclude that it is important to account for DOM composition when studying GHGs distribution in estuarine systems to understand their roles and potential responses associated with climate change.

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 EF_5r 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.