Mechanisms responsible for N2O emissions from intertidal soils of the Yangtze Estuary

Aquacultural source of nitrous oxide revealed by nitrogen isotopes

The rapid expansion of coastal aquaculture has led to an increase in the coverage of aquaculture ponds, where intense feed-derived nitrogen is causing significant emissions of nitrous oxide (N2O). Multiple N2O production pathways and the relative importance of water column vs. sedimentary production in aquaculture ponds remain uncertain. Clarifying these pathways is vital for sustainable aquaculture development. Using 15N-labeled dissolved inorganic nitrogen, the pathways and rates of N2O production in subtropical aquaculture ponds located in south China, cultivating whiteleg shrimp, Japanese seabass, and giant river prawn, were successfully characterized. Total N2O production rates ranged from 6 to 70 µmol-N m-2 d-1, with the shrimp pond exhibiting the highest total N2O production rates, followed by ponds for seabass and prawn. These differences are primarily due to varying feed amounts causing differences in dissolved nutrients in water column and sediment. Particularly, nutrient and organic matter accumulation at the surface sediment stimulated N2O production. The oxygenated sediment on a centimeter scale could produce substantially more N2O compared to the water column above on a meter scale. Partial denitrification, i.e., nitrate and nitrite reduction to N2O, was more important (> 60 %) for N2O production in aquaculture ponds. The availability of nitrite is likely a major factor driving partial denitrification for both sedimentary and water column N2O production.

Seasonal isotopic and isotopomeric signatures of nitrous oxide produced microbially in a eutrophic estuary

Anthropogenic input of excess nutrients stimulates massive nitrous oxide (N2O) production in estuaries with distinct seasonal variations. Here, nitrogen isotopic and isotopomeric signatures were utilized to investigate the seasonal dynamics of N2O production and nitrification at the middle reach of the eutrophic Pearl River Estuary in the south of China. Elevated N2O production primarily via ammonia oxidation (> 1 nM-N d-1) occurred from April to November, along with increased temperature and decreased dissolved oxygen concentration. This consistently oxygenated water column showed active denitrification, contributing 20-40 % to N2O production. The water column microbial N2O production generally constituted a minor fraction (10-15 %) of the estuarine water-air interface efflux, suggesting that upstream transport and tidal dilution regulated the dissolved N2O inventory in the middle reach of the estuary. Nitrification (up to 3000 nM-N d-1) played a critical role in bioavailable nitrogen conversion and N2O production, albeit with N2O yields below 0.05 %.

N2O emission in temperate seagrass meadows: Fluxes, pathway and molecular mechanism

Seagrass meadows act as filters for nitrogen in coastal areas, but whether they are a source or sink for N2O has been still controversy. Additionally, the production pathways of N2O as well as the microbial driving mechanism in seagrass meadows are seldom reported. In this study, the air-sea fluxes, sediment release potential, and production pathway of N2O in a temperate Zostera marina and Z. japonica mixed meadow were investigated by using gas chromatography and 15N isotopic tracing methods. The qPCR and metagenome sequencing were used to compare the difference in functional gene abundance and expression between seagrass vegetated and non-grass sediments. The results showed that the N2O air-sea fluxes in the meadow ranged from -1.97 to -1.77 nmol m⁻2 h⁻1, which was slightly lower in the seagrass region than in the adjacent bare region. Seagrass sediment N2O release potential dramatically increased after warming and nitrogen enrichment treatments. Heterotrophic nitrification was firstly investigated in seagrass meadows, and the process (26.80%-62.41%) and denitrification (37.55%-72.83%) contributed significantly to N2O production in the meadow, affected deeply by sediment organic content, while the contribution of autotrophic nitrification can be neglected. Compared with the bare sediments, the ammonia monooxygenase genes amoA, amoB and amoC, and nitrite oxidoreductase genes nxrA and nxrB, as well as nitrite reductase gene nirS and nitric oxide reductase gene norB were down-regulated, while the nitrous oxide reductase gene nosZ was up-regulated in the seagrass sediments, explaining less N2O emission in seagrass regions from the perspective of molecular. The nosZII-bearing bacteria like Bacteroidia, Polyangia, Anaerolineae, and Verrucomicrobiae could play important roles in N2O reduction in the seagrass meadow. The result is of great significance for highlighting the ability of seagrass meadows to mitigate climate changes.

Natural chalcopyrite mitigates nitrous oxide emissions in sediment from coastal wetlands

Coastal wetlands are one of the most important natural sources of nitrous oxide (N2O). Previous studies have shown that copper-containing chemicals are able to reduce N2O emissions from these ecosystems. However, these chemicals may harm organisms present in coastal waters and sediment, and disturb the ecological balance of these areas. Here, we first investigated the physiological characteristics and genetic potential of denitrifying bacteria isolated from coastal wetlands. Based on an isolated denitrifier carrying a complete denitrification pathway, we tested the effect of the natural mineral chalcopyrite on N2O production by the bacteria. The results demonstrated that chalcopyrite addition lowers N2O emissions from the bacteria while increasing its N2 production rate. Among the four denitrification genes of the isolate, only nosZ gene expression was significantly upregulated following the addition of 2 mg L-1 chalcopyrite. Furthermore, chalcopyrite was applied to coastal wetland sediments. The N2O flux was significantly reduced in 50-100 mg L-1 chalcopyrite-amended sets relative to the controls. Notably, the dissolved Cu concentration in chalcopyrite-amended sediment remained within the limit set by the National Sewage Treatment Discharge Standard. qPCR and metagenomic analysis revealed that the abundance of N2O-reducing bacteria with the nosZ or nirK + nosZ genotype increased significantly in the chalcopyrite-amended groups relative to the controls, suggesting their active involvement in the reduction of N2O emissions. Our findings offer valuable insights for the use of natural chalcopyrite in large-scale field applications to reduce N2O emissions.

Driving force of tidal pulses on denitrifiers-dominated nitrogen oxide emissions from intertidal wetland sediments

Intertidal wetland sediments are an important source of atmospheric nitrogen oxides (NOx), but their contribution to the global NOx budget remains unclear. In this work, we conducted year-round and diurnal observations in the intertidal wetland of Jiaozhou Bay to explore their regional source-sink patterns and influence factors on NOx emissions (initially in the form of nitric oxide) and used a dynamic soil reactor to further extend the mechanisms underlying the tidal pulse of nitric oxide (NO) observed in our investigations. The annual fluxes of NOx in the vegetated wetland were significantly higher than those in the wetland without vegetation. Their annual variations could be attributed to changes in temperature and the amount of organic carbon in the sediment, which was derived from vegetated plants and promoted the carbon-nitrogen cycle. Anaerobic denitrifiers had advantages in the intertidal wetland sediment and accounted for the major NO production (63.8 %) but were still limited by nitrite and nitrate concentrations in the sediment. Moreover, the tidal pulse was likely a primary driver of NOx emissions from intertidal wetlands over short periods, which was not considered in previous investigations. The annual NO exchange flux considering the tide pulse contribution (8.93 ± 1.72 × 10-2 kg N ha-1 yr-1) was significantly higher than that of the non-pulse period (4.14 ± 1.13 × 10-2 kg N ha-1 yr-1) in our modeling result for the fluxes over the last decade. Therefore, the current measurement of NOx fluxes underestimated the actual gas emission without considering the tidal pulse.

Effects of microplastics on denitrification and associated N2O emission in estuarine and coastal sediments: insights from interactions between sulfate reducers and denitrifiers

Global estuarine and coastal zones are facing severe microplastics (MPs) pollution. Sulfate reducers (SRB) and denitrifiers (DNB) are two key functional microorganisms in these zones, exhibiting intricate interactions. However, whether and how MPs modulate the interactions between SRB and DNB, with implications for denitrification and associated N2O emissions, remains poorly understood. Here, we simultaneously investigated the spatial response patterns of SRB-DNB interactions and denitrification and associated N2O emissions to different MPs exposure along an estuarine gradient in the Yangtze Estuary. Spatial responses of denitrification to polyvinyl chloride (PVC) and polyadipate/butylene terephthalate (PBAT) MPs exposure were heterogeneous, while those of N2O emissions were not. Gradient-boosted regression tree and multiple regression model analyses showed that sulfide, followed by nitrate (NO3-), controlled the response patterns of denitrification to MPs exposure. Further mechanistic investigation revealed that exposure to MPs resulted in a competitive and toxic (sulfide accumulation) inhibition of SRB on DNB, ultimately inhibiting denitrification at upstream zones with high sulfide but low NO3- levels. Conversely, MPs exposure induced a competitive inhibition of DNB on SRB, generally promoting denitrification at downstream zones with low sulfide but high NO3- levels. These findings advance the current understanding of the impacts of MPs on nitrogen cycle in estuarine and coastal zones, and provide a novel insight for future studies exploring the response of biogeochemical cycles to MPs in various ecosystems.

Reclamation of tidal flats to paddy soils reshuffles the soil microbiomes along a 53-year reclamation chronosequence: Evidence from assembly processes, co-occurrence patterns and multifunctionality

Coastal soil microbiomes play a key role in coastal ecosystem functioning and are intensely threatened by land reclamation. However, the impacts of coastal reclamation on soil microbial communities, particularly on their assembly processes, co-occurrence patterns, and the multiple soil functions they support, remain poorly understood. This impedes our capability to comprehensively evaluate the impacts of coastal reclamation on soil microbiomes and to restore coastal ecosystem functions degraded by reclamation. Here, we investigated the temporal dynamics of bacterial and fungal communities, community assembly processes, co-occurrence patterns, and ecosystem multifunctionality along a 53-year chronosequence of paddy soil following reclamation from tidal flats. Reclamation of tidal flats to paddy soils resulted in decreased β-diversity, increased homogeneous selection, and decreased network complexity and robustness of both bacterial and fungal communities, but caused contrasting α-diversity response patterns of them. Reclamation of tidal flats to paddy soils also decreased the multifunctionality of coastal ecosystems, which was largely associated with the fungal network complexity and α-diversity. Collectively, this work demonstrates that coastal reclamation strongly reshaped the soil microbiomes at the level of assembly mechanisms, interaction patterns, and functionality level, and highlights that soil fungal community complexity should be considered as a key factor in restoring coastal ecosystem functions deteriorated by land reclamation.

Quantitative comparison of clamworm (Perinereis aibuhitensis) and crab (Macrophthalmus japonicus) burrowing effects on nitrogen and phosphorus dynamics at the sediment-water interface

Exploring nitrogen and phosphorus dynamics in estuarine ecosystems is of great significance for understanding the nutrient cycle in terrestrial and marine ecosystems. However, the ecological functions of common species remain unclear because there is a dearth of observational data. To quantitatively compare the bioturbation effects of the clamworm Perinereis aibuhitensis and crab Macrophthalmus japonicus on the migration of nitrogen and phosphorus in the sediment-pore water-overlying water interfaces, an indoor experiment was carried out using microcosms containing substrate, water, and living specimens collected from the Dagu Estuary, China. The physical structure of the sediments was greatly altered by both clamworms and crabs, and the amount of suspended particulate matter in the overlying water was in the order: crab > clamworm > control groups. Sediments, especially the 0-2 cm layer, were continuously mineralized under bioturbation, releasing NH₃-N, NO₂^--N, NO₃^--N, and dissolved total phosphorus. During the experiment, the release rate of NH₃-N (2.45 mg L^-1 to 10.23 mg L^-1), NO₂^--N (0.25 mg L^-1 to 7.34 mg L^-1), NO₃^--N (0.02 mg L^-1 to 6.03 mg L^-1), and dissolved total phosphorus (0.07 mg L^-1 to 0.16 mg L^-1) in the crab group was higher than that in the clamworm group (NH₃-N, 0.93 mg L^-1 to 6.11 mg L^-1; NO₂^--N, 0.51 mg L^-1 to 6.18 mg L^-1; NO₃^--N, 0.01 mg L^-1 to 5.24 mg L^-1, and dissolved total P, 0.04 mg L^-1 to 0.08 mg L^-1). Our findings indicate that crabs might play a more important role in promoting nitrogen and phosphorus migration than clamworms because of their stronger bioturbation strategies in sediments. Bioturbation by both clamworms and crabs reduced nitrogen and phosphorus accumulation in surface sediments and accelerated organic matter mineralization in the sediments, which may promote nutrient recycling in estuarine ecosystems.

Nitrification Regulates the Spatiotemporal Variability of N2O Emissions in a Eutrophic Lake

Nitrous oxide (N₂O) emissions from lakes exhibit significant spatiotemporal heterogeneity, and quantitative identification of the different N₂O production processes is greatly limited, causing the role of nitrification to be undervalued or ignored in models of a lake's N₂O emissions. Here, the contributions of nitrification and denitrification to N₂O production were quantitatively assessed in the eutrophic Lake Taihu using molecular biology and isotope mapping techniques. The N₂O fluxes ranged from -41.48 to 28.84 μmol m^-2 d^-1 in the lake, with lower N₂O concentrations being observed in spring and summer and significantly higher N₂O emissions being observed in autumn and winter. The ¹⁵N site preference and relevant isotopic evidence demonstrated that denitrification contributed approximately 90% of the lake's gross N₂O production during summer and autumn, 27-83% of which was simultaneously eliminated via N₂O reduction. Surprisingly, nitrification seemed to act as a key process promoting N₂O production and contributing to the lake as a source of N₂O emissions. A combination of N₂O isotopocule-based approaches and molecular techniques can be used to determine the precise characteristics of microbial N₂O production and consumption in eutrophic lakes. The results of this study provide a basis for accurately assessing N₂O emissions from lakes at the regional and global scales.

Crab bioturbation affects competition between microbial nitrogen removal and retention in estuarine and coastal wetlands

As the important benthic animal in coastal wetlands, crab bioturbation may significantly affect the nitrogen (N) budgets by regulating microbial N transformation processes. However, the response of interaction between different microbial N processes to crab bioturbation remains poorly understood. Here, a 30-day microcosmic experiment was conducted using sediment collected from the Yangtze Estuary wetland, followed by the determination of temporal variations of physicochemical parameters, N removal (denitrification plus anammox, which is defined as N₂ production) and retention rates (nitrate dissimilatory reduction to ammonium, DNRA) as well as relevant gene abundances in response to different crabs densities. The results showed that crab bioturbation simultaneously promoted the rates of N₂ production and DNRA processes. These two process rates were positively associated with the intensity of crab bioturbation, which was supported by molecular analysis of relevant functional gene abundance. Crab bioturbation was more beneficial to DNRA than N₂ production. Due to this disproportionate stimulation, crab bioturbation increased the importance of DNRA, indicating that N retention was becoming more significant under crab bioturbation in estuarine and coastal wetlands. The variations of sediment total organic carbon and oxygen availability driven by crab bioturbation were the critical factors mediating the changes in microbial N removal and retention. Overall, our findings highlighted that crab bioturbation can affect the N budgets in estuarine and coastal wetlands by altering the competition between microbial N removal and retention.

High importance of coupled nitrification-denitrification for nitrogen removal in a large periodically low-oxygen estuary

The coupling between nitrification and denitrification/anammox (nitrate/nitrite used in denitrification/anammox derives from nitrification) is a significant process of reactive nitrogen (N) removal that has attracted much attention. However, the dynamics of coupled nitrification-denitrification/anammox in the periodically low-oxygen estuaries and coasts remain unclear. Here, continuous-flow experiments combined with isotope tracing techniques were conducted in periodically low-oxygen areas of the Yangtze Estuary to reveal the changes in benthic sediment denitrification and anammox as well as their coupling with nitrification. Our results showed that denitrification increased but anammox decreased during low-oxygen summer. The occurrence of low oxygen also promoted coupled nitrification-denitrification but decreased coupled nitrification-anammox. These results implied that decreased dissolved oxygen in summer did not largely restrict nitrification activity, and anaerobic denitrification/anammox regulated the magnitude of coupled nitrification-denitrification/anammox rates. Denitrification (74.95-100 %) was the dominant process in total N removal, while coupled nitrification-denitrification accounted for a higher proportion (45.68-97.05 %) of denitrification, indicating that coupling between nitrification and denitrification played a dominant role in N removal. In addition to dissolved oxygen levels, carbon and N substrate availabilities were also important variables to regulate N transformations. Overall, this study advanced our knowledge of the distribution patterns and controlling factors of N removal processes and highlighted that coupled nitrification-denitrification might have a significant but neglected role in N removal from periodically low-oxygen estuaries.

Antibiotics sulfamethoxazole alter nitrous oxide production and pathways in estuarine sediments: Evidenced by the N15-O18 isotopes tracing

Estuarine antibiotic residues are profoundly impacting microbial nitrogen (N) cycling and associated N₂O production, but the response of N₂O production pathways to antibiotics remains poorly understood. Here, ¹⁵N-¹⁸O labeling technique combined with molecular methods were used to investigate the impacts of sulfamethoxazole on the contribution of ammonia oxidation (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N₂O production in estuarine sediments. Results showed that environmental concentration of sulfamethoxazole (4 ng/g) promoted the total N₂O production by 17.1% through nitrifier denitrification. Environmentally relevant (40-4000 ng/g) and irrelevant (40,000 ng/g) concentration of sulfamethoxazole drove nitrification denitrification to gradually lose the dominant role in total N₂O production and ammonia oxidation-derived N₂O, replaced by HD and nitrifier nitrification, while total N₂O production were inhibited. Furthermore, when HD dominated the total N₂O production, the HD-derived N₂O increased by 63.6% with sulfamethoxazole concentration reaching 40,000 ng/g. The mechanistic investigation further showed that nitrifying bacteria were more susceptible to sulfamethoxazole than nitrifying archaea and denitrifiers. The increased expression of nirS gene carried by non-dominant denitrifiers improved the ratio of nirS:nosZ and hence increased HD-derived N₂O under high sulfamethoxazole stresses. Overall, our results provide a comprehensive view into how antibiotics regulate N₂O production and its pathways in estuarine sediments.

Microplastics alter nitrous oxide production and pathways through affecting microbiome in estuarine sediments

Increasing microplastics (MPs) pollution in estuaries profoundly impacts microbial ecosystems and biogeochemical processes. Nitrous oxide (N₂O), a powerful greenhouse gas, is an important intermediate product of microbial nitrogen cycling. However, how MPs regulate N₂O production and its pathways remain poorly understood. Here, impacts of traditional petroleum-based and emerging biodegradable MPs on microbial N₂O production and its pathways were studied through dual-isotope (¹⁵N-¹⁸O) labeling technique and molecular methods. Results indicated that both traditional petroleum-based and emerging biodegradable MPs promoted sedimentary N₂O production, whereas pathways varied. Biodegradable polylactic acid (PLA) MPs displayed greater promotion of N₂O production than petroleum-based MPs, polyvinyl chloride (PVC) and polyethylene (PE), of which PLA promoted through nitrifier nitrification (NN) and heterotrophic denitrification (HD), PE through nitrifier denitrification and HD, and PVC through NN. By combining the analysis of N₂O production rates with sediment chemical and microbiological properties, we demonstrated that the enrichment of nitrifying and denitrifying bacteria, as well as related functional genes directly and/or indirectly increased N₂O production primarily by interacting with carbon and nitrogen substrates. Different response of nitrogen cycling microbes to MPs led to the difference in N₂O increase pathways, of which nitrifying bacteria significantly enriched in all MPs treatments due to the niches provided by MPs. However, part of denitrifying bacteria significantly enriched in treatments containing PLA and PE MPs, which may serve as organic carbon substrates. This work highlights that the presence of MPs can promote sedimentary N₂O production, and the emerging biodegradable MPs represented by PLA may have a greater potential to enhance estuarine N₂O emissions and accelerate global climate change.

Sediment nitrogen mineralization and immobilization affected by non-native Sonneratia apetala plantation in an intertidal wetland of South China

The mineralization and immobilization of nitrogen (N) are critical biogeochemical transformations in estuarine and coastal sediments. However, the biotic and abiotic mechanisms that regulate the two processes in different aged mangrove sediments remain poorly understood. Here, we used ¹⁵N isotope dilution method to investigate the changes in sediment N mineralization (GNM) and NH₄⁺ immobilization (GAI) of different aged mangrove habitats (including 0, 10, and 20 years Sonneratia apetala, as well as >40 years mature native Kandelia obovata) in Qi'ao Island, Guangdong Province, China. Measured GNM and GAI rates ranged from 2.69 to 17.53 μg N g^-1 d^-1 and 2.29-21.38 μg N g^-1 d^-1, respectively, which varied both spatially and seasonally. The ratio of GNM to total N (PAM%, 0.24-0.86%) also varied spatially and seasonally, but the ratio of GAI to GNM (RAI, 0.79-1.54) only varied spatially. Mangrove restoration significantly increased the N mineralization and immobilization rates, but remained lower than those of mature native Kandelia obovata habitat. The sediment bacterial abundance, labile organic matter and temperature are the dominant factors in controlling N mineralization and immobilization. Our findings suggested that exotic mangrove Sonneratia aperale plantation can enhance sediment N mineralization and immobilization rates and improve N stability through accumulated biomass rapidly. Overall, these results provide new insights into sediment N transformation processes and associated influencing mechanisms in such intertidal wetlands profoundly influenced by human activities.

Soil moisture determines nitrous oxide emission and uptake

Soils are major sources and sinks of nitrous oxide (N₂O). The main pathway of N₂O emission is performed through soil denitrification; however, the uptake phenomenon in denitrification is overlooked, leading to an underestimation of N₂O production. Soil moisture strongly influences denitrification rates, but exact quantifications coupled with nosZ, nirK, and nirS gene analysis remain inadequately unaccounted for. In this study, a ¹⁵N-N₂O pool dilution (¹⁵N₂OPD) method was used to measure N₂O production rates under different soil moisture levels. Therefore, 20%, 40%, 60%, 80% and 100% soil water holding capacity (WHC) were used. The results revealed that N₂O uptake rates increased proportionally with soil moisture content and peaked at 80% WHC with 4.17 ± 2.74 μg N kg^-1 soil h^-1. The N₂O production and net emission rates similarly peaked at 80% WHC, reading at 32.50 ± 4.86 and 27.63 ± 3.09 μg N kg^-1 soil h^-1 during the incubation period (18 days). Soil moisture content increased the gene copy number of the nosZ, NH₄⁺ content, and denitrification potential in soil. N₂O uptake at WHC 80-100% was significantly greater than that at WHC 20-60%. It was attributed to a decrease in O₂ and the high NO₃^- concentration inhibition (> 50 mg N kg^-1 of soil NO₃^--N content). Principal components analysis (PCA) indicated that the number of nosZ genes was the major driver of N₂O uptake, especially nosZ clade II. Thus, the results of this study deepen our understanding of the mechanisms underpinning N₂O sources and sinks in soils and provide a useful gene-based indicator to estimate N₂O uptake.

Effects of Aquatic Acidification on Microbially Mediated Nitrogen Removal in Estuarine and Coastal Environments

Acidification of estuarine and coastal waters is anticipated to influence nitrogen (N) removal processes, which are critical pathways for eliminating excess N from these ecosystems. We found that denitrification rates decreased significantly under acidified conditions (P < 0.05), which reduced by 41-53% in estuarine and coastal sediments under an approximately 0.3 pH reduction of the overlying water. However, the N removal rates through the anaerobic ammonium oxidation (anammox) process were concomitantly promoted under the same acidification conditions (increased by 47-109%, P < 0.05), whereas the total rates of N loss were significantly inhibited by aquatic acidification (P < 0.05), as denitrification remained the dominant N removal pathway. More importantly, the emission of nitrous oxide (N₂O) from estuarine and coastal sediments was greatly stimulated by aquatic acidification (P < 0.05). Molecular analyses further demonstrated that aquatic acidification also altered the functional microbial communities in estuarine and coastal sediments; and the abundance of denitrifiers was significantly reduced (P < 0.05), while the abundance of anammox bacteria remained relatively stable. Collectively, this study reveals the effects of acidification on N removal processes and the underlying mechanisms and suggests that the intensifying acidification in estuarine and coastal waters might reduce the N removal function of these ecosystems, exacerbate eutrophication, and accelerate global climate change.

Crab bioturbation alters nitrogen cycling and promotes nitrous oxide emission in intertidal wetlands: Influence and microbial mechanism

Intertidal wetlands provide important ecosystem functions by acting as nitrogen (N) cycling hotspots, which can reduce anthropogenic N loading from land to coastal waters. Benthic bioturbations are thought to play an important role in mediating N cycling in intertidal marshes. However, how the burrowing activity of benthos and their microbial symbionts affect N transformation and greenhouse gas nitrous oxide (N₂O) emission remains unclear in these environments. Here, we show that bioturbation of crabs reshaped the structure of intertidal microbial communities and their N cycling function. Molecular analyses suggested that the microbially-driven N cycling might be accelerated by crab bioturbation, as the abundances of most of the N related functional genes were higher on the burrow wall than those in the surrounding bulk sediments, except for genes involved in N fixation, dissimilatory nitrate reduction to ammonium (DNRA), and N₂O reduction, which were further confirmed by isotope-tracing experiments. Especially, the potential rates of the main N₂O production pathways, nitrification and denitrification, were 2-3 times higher in the burrow wall sediments. However, even higher N₂O emission rates (approximately 6 times higher) were observed in this unique microhabitat, which was due to a disproportionate increase in N₂O production over N₂O consumption driven by burrowing activity. In addition, the sources of N₂O were also significantly affected by crab bioturbation, which increased the contribution of hydroxylamine oxidation pathway. This study reveals the mechanism through which benthic bioturbations mediate N cycling and highlights the importance of considering burrowing activity when evaluating the ecological function of intertidal wetlands.

Stimulation of N2 O emission via bacterial denitrification driven by acidification in estuarine sediments

Ocean acidification in nitrogen-enriched estuaries has raised global concerns. For decades, biotic and abiotic denitrification in estuarine sediments has been regarded as the major ways to remove reactive nitrogen, but they occur at the expense of releasing greenhouse gas nitrous oxide (N₂ O). However, how these pathways respond to acidification remains poorly understood. Here we performed a N₂ O isotopocules analysis coupled with respiration inhibition and molecular approaches to investigate the impacts of acidification on bacterial, fungal, and chemo-denitrification, as well as N₂ O emission, in estuarine sediments through a series of anoxic incubations. Results showed that acidification stimulated N₂ O release from sediments, which was mainly mediated by the activity of bacterial denitrifiers, whereas in neutral environments, N₂ O production was dominated by fungi. We also found that the contribution of chemo-denitrification to N₂ O production cannot be ignored, but was not significantly affected by acidification. The mechanistic investigation further demonstrated that acidification changed the keystone taxa of sedimentary denitrifiers from N₂ O-reducing to N₂ O-producing ones and reduced microbial electron-transfer efficiency during denitrification. These findings provide novel insights into how acidification stimulates N₂ O emission and modulates its pathways in estuarine sediments, and how it may contribute to the acceleration of global climate change in the Anthropocene.

Effects of sulfamethoxazole on coupling of nitrogen removal with nitrification in Yangtze Estuary sediments

Coupling of nitrogen removal processes with nitrification (NR_n) are vital synergistic nitrogen elimination mechanisms in aquatic environments. However, the effects of antibiotics on NR_n are not well known. In the present work, 20-day continuous-flow experiments combined with ¹⁵N tracing techniques and quantitative PCR were performed to simulate the impact of sulfamethoxazole (SMX, a sulfonamide antibiotic) with near in situ concentration on NR_n processes in sediments of Yangtze Estuary. Results showed that SMX with near in situ concentration significantly decreased NR_n, NR_w (uncoupling of nitrogen removal processes with nitrification) and actual nitrogen removal rates via inhibiting nitrogen transformation functional genes (AOB, narG, nirS, nosZ) and anammox 16S rRNA gene, while the coupling links between nitrification and nitrogen removal processes were not broken by the exposure. The proportion of NR_n in total nitrogen removal processes decreased by approximately 10% with SMX addition, due to the different inhibition on AOB, denitrifying genes and anammox 16S rRNA gene. N₂O production and nitrite accumulation remarkably increased with SMX addition under simultaneous nitrification and denitrification, and they strongly correlated with each other. The more severely inhibition on nirS gene (13.6-19.8%) than Nitrospira nxrB gene (0.3-8.2%) revealed that the increased nitrite accumulation with SMX addition mainly occurred in heterotrophic denitrification, suggesting that the increased N₂O production was dominated by the heterotrophic nitrite reduction. Moreover, we estimated that the ratio of external inorganic N eliminated by actual nitrogen removal can upgrade to 6.4-7.4% under circumstances of no inhibition by SMX. This study revealed the effects of SMX with near in situ concentration on NR_n processes and illustrated the microbial mechanism on functional genes level. Our results highlighted the inhibitory effects of SMX on NR_n may contribute to reactive N retention and N₂O production in estuarine and coastal ecosystems.