Nitrogen Fluxes Research Articles

Restructuring of soil food webs reduces carbon storage potential in boreal peatlands

Microbial and faunal decomposers regulate the flux of carbon and nitrogen belowground, thus controlling the storage/release of carbon and nitrogen in soil systems. Warming is anticipated to alter decomposer biomass, and accelerate organismal metabolism and soil carbon release. We parameterized six soil food webs using empirical data for 18 trophic nodes at two boreal peatland sites under three climate scenarios (control, +2 °C, +4 °C), and model carbon and nitrogen flux, loss and retention using an energetic ecostoichiometric food web model. Differences in microbial biomass between sites dictated flux under warming. The community biomass of the fungal-dominated site was more impacted by warming, but fluxes were more responsive to warming at the bacterial-dominated site. Decreased metabolic efficiency of the soil food web at both sites in response to warming led to greater per capita carbon losses, indicating the long-term carbon storage potential of both systems is diminished.

The effect of solid nitrogen on nucleate boiling heat transfer in a slush nitrogen pool

The nucleate boiling heat transfer in a slush nitrogen pool has been studied across a heat-flux range from 5 to 400 kW/m2 and a system pressure range from 12.5 kPa (triple point pressure) to 101 kPa (atmospheric pressure). The effect of solid nitrogen on the heat transfer process was found to vary with the heat flux and the pressure. Experimental results reveal that the nucleate boiling heat transfer coefficient (HTC) of slush nitrogen is higher than that of subcooled liquid nitrogen at the low heat flux, which can be attributed to the fact that the solid nitrogen particles (with their latent heat of melting) absorb heat in the thermal boundary layer and form a sharper temperature gradient near the boiling surface. However, at the high heat flux, the HTC of slush nitrogen is lower than that of subcooled liquid nitrogen, which can be attributed to the invasion and inhibition occurring between the solid nitrogen particles and the bubbles during boiling, leading to the accumulation of vapor phase with low thermal conductivity on the boiling surface, thereby the heat transfer will be suppressed. As a result, a dual effect for the nucleate boiling heat transfer in slush nitrogen can be observed. Furthermore, the critical heat flux (CHF) of slush nitrogen is obtained and compared with the CHF of subcooled liquid nitrogen. According to the CHF model, the bubble period is the dominant parameter in determining the CHF. A calibrated CHF correlation for slush nitrogen is developed based on the hypothesis that the bubble period is prolonged as the effective viscosity of slush nitrogen increases with the solid volume fraction. The CHF values of slush nitrogen with the solid volume fraction of 10%, 20% and 30% are 0.96, 0.91 and 0.84 times those of subcooled liquid nitrogen, respectively, at the pressures higher than 50 kPa. Consequently, the solid volume fraction of slush nitrogen and the degree of subcooling of subcooled liquid nitrogen are found to have opposite effects on CHF.

Low temperature sputtering deposition of Al1−xScxN thin films: Physical, chemical, and piezoelectric properties evolution by tuning the nitrogen flux in (Ar + N2) reactive atmosphere

This work investigates the physical properties of Al1−xScxN thin films sputtered at low temperatures by varying the process conditions. Specifically, the films were deposited at room temperature by applying a radio frequency power equal to 150 W to an AlSc alloy (60:40) target, varying the nitrogen flux percentage in the (Ar + N2) sputtering atmosphere (30%, 40%, 50%, and 60%) and keeping constant the working pressure at 5 × 10−3 mbar. The structural and chemical properties of the Al1−xScxN films were studied by x-ray diffraction and Rutherford backscattering spectrometry techniques, respectively. The piezoelectric response was investigated by piezoresponse force microscopy. In addition, the surface potential was evaluated for the first time for Sc-doped AlN thin films by Kelvin probe force microscopy, providing piezoelectric coefficients free from the no-piezoelectric additional effect to the mechanical deformation, i.e., the electrostatic force. By alloying AlN with scandium, the piezoelectric response was strongly enhanced (up to 200% compared to undoped AlN), despite the low deposition temperature and the absence of any other additional energy source supplied to the adatoms during thin film growth, which generally promotes a better structural arrangement of polycrystalline film. This is a strategic result in the field of microelectromechanical systems completely fabricated at low temperatures.

Typhoon-induced stormwater drives nutrient dynamics and triggers phytoplankton blooms in Laizhou Bay, China

In this study, we investigated the hydrological and ecological impacts of heavy rainfall caused by the storm Rumbia and Typhoon Lekima on Laizhou Bay (LZB) through land‒sea synchronous field surveys, online remote sensors, and simulated enclosure experiments. Within two weeks of Rumbia, approximately 9% and 16% of the annual riverine total nitrogen (TN) and total phosphorus (TP) fluxes, respectively, were transported to the LZB and the proportions were 17% and 35%, respectively, for Lekima. The land use on the watersheds increased the rates of land-derived nutrient loading and altered their biogeochemical forms. Consequently, the average concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the LZB increased by 2.6 and 1.0 times post-Rumbia and by 3.5 and 1.3 times post-Lekima, respectively. Relatively lower salinity and temperature, sudden increases in DIN, and strengthened coastal currents stimulated the growth of highly adaptable and small diatoms, resulting in the first diatom blooms. Subsequently, a bloom of Noctiluca scintillans formed.

Spatiotemporal variation, composition, and implications for transport flux of nitrogen in Leizhou Peninsula coastal water, China

Nitrogen, as the main bioactive element, plays an important role in biological productivity, ecosystem function, and biogeochemical processes in marine environment. In this study, seawater samples collected from China’s Leizhou Peninsula coastal water (LZPCW) during dry, normal, and wet seasons in 2018 were explored to reveal the spatiotemporal variation, composition, and transport flux of dissolved inorganic nitrogen (DIN) linked to hydrographic condition. DIN concentration and speciation showed significantly seasonal variation (P < 0.01), and the concentration of DIN species was significantly higher in dry season than other seasons. The annual mean DIN concentration was 8.01 ± 5.79 μmol L−1. In addition, the N–NO2−, N–NO3− and N–NH4+ were significantly different in the DIN bulk in different seasons. The largest fraction of DIN was N–NO3−, followed by N–NH4+, the lowest was N–NO2−, comprising up to 67.92 ± 23.20%, 23.90 ± 23.19% and 8.18 ± 8.19%, respectively. Besides, the Beibu Gulf was subjected to 7.28 × 1010 mol DIN annual flux through the Qiongzhou Strait. The annual transport fluxes of N–NO2−, N–NO3−, and N–NH4+ accounted for 6.43%, 77.20% and 16.36%, respectively. The DIN concentration and coastal water flow led to the largest DIN flux transport in dry season. This study revealed that the coastal ocean currents, river plumes and human activities jointly drove the dynamic variations of N species in LZPCW. It provides a baseline data for studying the spatiotemporal effects of hydrographic condition on nitrogen distribution and transport flux in the LZPCW, which is implications for understanding nutrients dynamics and coastal water quality management in future.

Life cycle assessment of ammonium sulfate recovery from urban wastewater

ABSTRACT Anthropogenic nitrogen fluxes are profoundly altering the global biogeochemical nitrogen cycle. Better management of these nitrogen fluxes is essential. Recovering nitrogen from urban wastewater reduces both the energy and resources required to produce nitrogen-based fertilizer and to remove nitrogen from wastewater collected. Nitrogen can be recovered from wastewater in the form of ammonium sulfate, a common nitrogen-based fertilizer. In the urban setting, the technology can be applied to target source-separated urine or municipal wastewater. To assess the environmental sustainability of this approach, this study compared the life cycle environmental impacts of ammonium sulfate recovered from urban wastewater (through eight different recovery technology trains) and ammonium sulfate produced by six different industrial processes. The results show that wastewater-derived ammonium sulfate generally has lower potential environmental impacts than industrially produced ammonium sulfate in most of the impact categories assessed. The impact for the source-separated urine centralized recovery train is the smallest. The contribution analysis shows that energy, sulfuric acid and sodium hydroxide use are the major contributors, while the background inventory analysis shows that the results can be sensitive to the choice of region-specific background inventory. In the future, nitrogen recovery from urban wastewater is promising for the circular economy in cities.

Eutrophication hotspots, nitrogen fluxes and climate impacts in estuarine ecosystems: A model study of the Odra estuary system

The Odra estuary in the southern Baltic Sea comprises the Odra (Szczecin) Lagoon, the Pomeranian Bay and a number of other shallow water areas and channels. Known for its abundance of fish, eutrophication in the Odra Lagoon is a pressing issue for science and environmental management representing a global problem: What determines the seasonal variability of nitrogen and nitrogen turnover in shallow water areas, and how does seasonal variability change due to climatic changes such as warming and sea level rise? How do such changes affect nutrient exports to the regional ocean? This study employs a high-resolution unstructured model system to investigate physical-biogeochemical interactions, nitrogen turnover, and conditions leading to nitrogen export to the Baltic Sea within the Odra estuary. The research comprises hindcast and a climatic experiment with modified water level and temperature inputs. The model reproduces the thermohaline dynamics of brackish shallow water areas, phytoplankton blooms and the variability of inorganic nitrogen. The simulations identify the dynamic partitioning of the Odra Lagoon into the highly eutrophic, lake-like Small Lagoon and more frequently flushed, zooplankton-rich Great Lagoon. Although the two years of the hindcast simulation feature very different boundary conditions in terms of river forcing, comparable patterns of seasonal nitrogen export emerge. In a climate change experiment with increased sea levels and global temperatures, the system appears sensitive, but remains stable with regard to nutrient transport and is therefore predictable. The climate change experiment reveals enhanced primary producer biomass concentrations, suggesting heightened eutrophication. While in the shallow waters of Odra Lagoon oxygen concentration remains relatively stable, oxygen depletion intensifies as the lagoon outflows enter the Pomeranian Bay. This phenomenon is linked to increased denitrification within the stratified Odra plume. Deeper, meandering channels, such as Swina, demonstrate resilience to oxygen reduction, influenced by sea level rise and enhanced currents. Based on the temporal-spatial high-resolution coupled, validated simulations, it is possible to develop tailor-made management solutions without having to run expensive and complicated observation campaigns in the shallow waters with complex topography.

Spatial and seasonal pattern of microbial nitrate reduction in coastal sediments in the Vistula River plume area, Gulf of Gdańsk

Estuaries can remove and/or retain land-derived nitrogen (N) and act as filters buffering N loads to the open sea. The N coastal filter can be seasonally variable depending on water temperature and transported loads, two factors acting in synergy and strongly influenced by climate change. The capacity of sediments to mitigate riverine N loads was investigated at four sites in the Vistula River plume area (Gulf of Gdańsk, Southern Baltic Sea). Samplings were carried out in two contrasting seasons: spring and summer, characterized by different water temperatures and nitrate (NO3-) levels. Inorganic N fluxes, and rates of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were measured in intact sediment cores by means of dark incubations and 15N-nitrate concentration-series experiments. Sampling sites were selected along a gradient of depth (5 to 24 m), that was also a gradient of sediment organic matter content. In both seasons, denitrification rates increased along with depth and from spring (6.5 ± 7.0 µmol m-2 h-1) to summer (20.4 ± 15.4 µmol m-2 h-1), despite lower NO3- concentrations in summer. In spring, at higher NO3- loading, denitrification was likely limited by low water temperature, and elevated sediment oxygen penetration. Coupled denitrification-nitrification prevailed over denitrification of water column NO3- across all sites and seasons, contributing to over 80% of the total denitrification. Notably, no anammox was detected at the sampling sites. DNRA exhibited low to undetectable rates in spring, especially at the shallowest sites. However, during summer, N recycling via DNRA increased and ranged from 0.7 to 14.9 µmol m-2 h-1. The denitrification efficiency (DE), calculated as the ratio between molecular nitrogen (N2) flux and dissolved inorganic N effluxes from sediments, ranged from 0 to 37% in spring, whereas in summer DE did not exceed 16%. Despite the dominance of denitrification over DNRA, the analyzed sediments acted as weak N buffers under in situ dark conditions. However, concentration-series experiments suggested high potential denitrification capacity, exceeding 400 µmol m-2 h-1, in response to short-term, large riverine inputs of NO3-.

Comparison of carbon and nitrogen content in natural and Aulacomya atra (MOLINA) biodeposits.

Comparison of carbon and nitrogen content in natural and Aulacomya atra (MOLINA) biodeposits.

Ammonia concentration and recovery in an up-scaled electrochemical cell through screening of cation exchange membrane

An intensifying global alarm over excessive total ammonia nitrogen (TAN) calls for innovative recovery strategies. Although electrochemically-driven TAN concentration and recovery have been explored, limited research on upscaling lab-scale advancements with careful system engineering leaves a notable gap in practical application. Here, in the context of implementing an upscaled electrochemical system, we carefully chose a suitable cation exchange membrane to ensure the best ammonium migration, energy efficiency, and stability. Systematically examining the effects of applied current density and loading rate using Neosepta CSE, we achieved final concentrated TAN levels in the receiving catholyte, reaching 836.7 mM (4.4-fold), 778.8 mM (3.5-fold), and 980.4 mM (2.8-fold), with nitrogen flux values of 801, 817, and 955 g-N m−2 d−1 for synthetic, food, and livestock wastewater, respectively, at a current density of 25 mA cm−2 and a loading rate of 2.5 mL cm−2 h−1. Successful upscaling to an 8-cell stack, capable of treating 100 L every 20 h (recovering 207 g-N d−1) of synthetic wastewater, showcases the feasibility of upscaled electrochemical systems for TAN recovery.

Trophic cascades within and across ecosystems: The role of anti-predatory defences, predator type and detritus quality.

Species in one ecosystem can indirectly affect multiple biodiversity components and ecosystem functions of adjacent ecosystems. The magnitude of these cross-ecosystem effects depends on the attributes of the organisms involved in the interactions, including traits of the predator, prey and basal resource. However, it is unclear how predators with cross-ecosystem habitat interact with predators with single-ecosystem habitat to affect their shared ecosystem. Also, unknown is how such complex top-down effects may be mediated by the anti-predatory traits of prey and quality of the basal resource. We used the aquatic invertebrate food webs in tank bromeliads as a model system to investigate these questions. We manipulated the presence of a strictly aquatic predator (damselfly larvae) and a predator with both terrestrial and aquatic habitats (spider), and examined effects on survival of prey (detritivores grouped by anti-predator defence), detrital decomposition (of two plant species differing in litter quality), nitrogen flux and host plant growth. To evaluate the direct and indirect effects each predator type on multiple detritivore groups and ultimately on multiple ecosystem processes, we used piecewise structural equation models. For each response variable, we isolated the contribution of different detritivore groups to overall effects by comparing alternate model formulations. Alone, damselfly larvae and spiders each directly decreased survival of detritivores and caused multiple indirect negative effects on detritus decomposition, nutrient cycling and host plant growth. However, when predators co-occurred, the spider caused a negative non-consumptive effect on the damselfly larva, diminishing the net direct and indirect top-down effects on the aquatic detritivore community and ecosystem functioning. Both detritivore traits and detritus quality modulated the strength and mechanism of these trophic cascades. Predator interference was mediated by undefended or partially defended detritivores as detritivores with anti-predatory defences evaded consumption by damselfly larvae but not spiders. Predators and detritivores affected ecosystem decomposition and nutrient cycling only in the presence of high-quality detritus, as the low-quality detritus was consumed more by microbes than invertebrates. The complex responses of this system to predators from both recipient and adjacent ecosystems highlight the critical role of maintaining biodiversity components across multiple ecosystems.

A comprehensive observation of organic and inorganic nitrogen in gases, particulate matter and precipitation in the northern suburb of Nanjing, East China, with an emphasis on size-resolved particulate nitrogen

China is considered one of the nitrogen deposition hotspots in the world. Measurements to date have focused mainly on inorganic nitrogen, organic nitrogen and the size distribution of particulate nitrogen has yet to be studied. Lack of dry deposition observations imposes a reliance on models, resulting in a much larger uncertainty relative to wet deposition which is routinely measured. In this study, dry and wet organic and inorganic nitrogen deposition were comprehensively measured in Nanjing in 2019, and nitrogen species in size-segregated particles were investigated. Total annual nitrogen deposition flux was up to 29.96 kg N ha−1 yr−1 in Nanjing, with wet deposition contributed slightly more (51.90%) to total deposition than dry deposition (48.10%). Observations revealed that reduced nitrogen (NHx = NH3 + NH4+) contributed 65.49% of the total inorganic nitrogen deposition budget, which implied that the control of reduced N in urban environments is needed to improve local air quality. Dry deposition of ammonia played an especially key role in dry deposition, contributing from 18.17% to 55.91% in different seasons. The deposition flux of water-soluble organic nitrogen (WSON) dominated by wet deposition, with the wet WSON about 4.6 times higher than that of dry deposition and contributed 41.18% of the wet nitrogen deposition. The wet N deposition in summer was significantly higher than in the other three seasons. The seasonal patterns in dry deposition were driven mostly by seasonal concentration patterns. The highest deposition of ammonia occurred in summer, while the deposition of nitrogen dioxide, particulate ammonium and nitrate peaked in winter. The maximum dry and wet WSON deposition appeared in autumn. Higher NH4+ concentration almost distributed in each size bin and centered in ranges of 0.18–1.0 μm, while higher NO3− concentration scattered in particle size ranges of 1.8–5.6 μm and 0.18–1.0 μm.

Metabolic Rates of Rainbow Trout Eggs in Reconstructed Salmonid Egg Pockets

In situ evaluations of the metabolic rates (i.e., respiration and excretion) of salmonid eggs are mostly indirect, focusing on the sampling of hyporheic water from wild or artificial nests. Comparatively, experimental studies carried out under controlled, laboratory conditions are less abundant due to methodological difficulties. This study presents a novel experimental setup aimed to address this issue and enable the measurement of oxygen and dissolved inorganic nitrogen fluxes in simulated rainbow trout (O. mykiss) egg pockets. The experimental setup consists of reconstructed egg pockets in cylindrical cores under flow-through conditions. Live and dead eyed-stage eggs were incubated in a natural, sterilised gravel substrate. Hyporheic water circulation was ensured using peristaltic pumps, with the possibility to collect and analyse inflowing and outflowing water for chemical analyses. Microcosm incubations, with closed respirometry of eggs in water alone, were also carried out in order to infer the importance of microbial respiration in the simulated egg pockets. The results show an increasing trend in oxygen demand, due to the development of biofilm in the reconstructed egg pockets and increased egg respiration rates. Moreover, egg pockets showed positive ammonium net fluxes connected with the advancing developmental egg stage, while nitrate removal peaked during the last phase of the experiment, mainly due to the formation of oxic-hypoxic interfaces, leading to couple nitrification–denitrification processes. The suggested approach enables to test a number of in situ situations, including the effects of extreme hydrological conditions, sediment clogging and sudden changes in water chemistry or temperature on the survival and metabolic performances of nests, at different egg development stages.

Sponges on shifting reefs: holobionts show similar molecular and physiological responses to coral versus macroalgal food

IntroductionMany coral reefs witness an ongoing coral-to-algae phase shift. Corals and algae release large quantities of (in)organic nutrients daily, of which a large part is utilized by sponges. In turn, sponges are important cyclers of precious resources to other inhabitants on reefs residing in oligotrophic waters. Here, we investigated whether sponge holobionts (i.e., host and prokaryotic symbionts) adapt their physiology to food released by coral- versus macroalgae.MethodsThereto, two sponge species, Plakortis angulospiculatus and Halisarca caerulea (high and low microbial abundance, respectively), were continuously exposed for 12 days to coral and macroalgal exudates in running seawater aquaria. Transcript expression of host and prokaryotic symbionts, changes in prokaryotic community composition, and holobiont physiological responses (i.e., respiratory demand, fluxes of carbon and nitrogen) were investigated after coral- versus macroalgae dominated treatments and compared to a seawater only control treatment.ResultsIn both sponge holobionts differential transcript expression between the coral and macroalgae treatments was very low (&amp;lt;0.01% of total transcripts). Differential expression was found in genes targeting cellular signaling pathways, e.g., cell proliferation (upregulated in coral treatment), and immune response (upregulated in macroalgal treatment). The sponge-associated prokaryotic community composition and sponge physiological responses were similar in all treatments, yet differed significantly between the two species.DiscussionAfter 12 days of exposure sponges appear to opportunistically feed on different food sources without having to adjust their metabolic pathways or associated prokaryotic communities. This suggests that sponges could be well-adapted to predicted changes in food source availability due to coral-to-algal phase shifts on many coral reefs.

Impact of climate change and anthropogenic activities on lacustrine ecosystems of the Kashmir Valley, NW Himalaya, India

AbstractDue to the significant sedimentation and anthropogenic pressures, the Kashmir Himalaya lakes (Dal, Wular, Manasbal) located within and in the vicinity of urban settlements are experiencing massive degradation. The unique climate, geographic setting, and rock degradation owing to the tectonic uplift of the area, contribute to increased sedimentation in these lakes. The sewage and other anthropogenic pressures have also increased various minerals including nitrogen influx in these lakes. Total organic carbon levels in the Dal Lake are high and range from 2.72% to 25.12%, with nitrogen levels ranging from 0.92% to 9.81%. The total organic carbon of Wular and Manasbal Lakes ranges from 0.83% to 4.52%, and 3.11% to 13.74% with nitrogen ranging from 0.06% to 0.5% and 0.20% to 1.96%, respectively. The estimated sedimentation rates for the Wular, Manasbal, and Dal Lakes are 0.125 cm/yr, 0.44 cm/yr, and 0.93 cm/yr. Given these factors and the geographic setting of Kashmir Himalaya, it is believed that under the sustained threat of climate change and anthropogenic pressure, mitigation strategies to control sedimentation and pollution are unavoidable for the long‐term survival of these valuable ecosystems. This review examines contributing factors, explores potential consequences, and proposes mitigation measures, to save these important aquatic ecosystems from further deterioration.

Prioritizing river basins for nutrient studies

Increases in fluxes of nitrogen (N) and phosphorus (P) in the environment have led to negative impacts affecting drinking water, eutrophication, harmful algal blooms, climate change, and biodiversity loss. Because of the importance, scale, and complexity of these issues, it may be useful to consider methods for prioritizing nutrient research in representative drainage basins within a regional or national context. Two systematic, quantitative approaches were developed to (1) identify basins that geospatial data suggest are most impacted by nutrients and (2) identify basins that have the most variability in factors affecting nutrient sources and transport in order to prioritize basins for studies that seek to understand the key drivers of nutrient impacts. The “impact” approach relied on geospatial variables representing surface-water and groundwater nutrient concentrations, sources of N and P, and potential impacts on receptors (i.e., ecosystems and human health). The “variability” approach relied on geospatial variables representing surface-water nutrient concentrations, factors affecting sources and transport of nutrients, model accuracy, and potential receptor impacts. One hundred and sixty-three drainage basins throughout the contiguous United States were ranked nationally and within 18 hydrologic regions. Nationally, the top-ranked basins from the impact approach were concentrated in the Midwest, while those from the variability approach were dispersed across the nation. Regionally, the top-ranked basin selected by the two approaches differed in 15 of the 18 regions, with top-ranked basins selected by the variability approach having lower minimum concentrations and larger ranges in concentrations than top-ranked basins selected by the impact approach. The highest ranked basins identified using the variability approach may have advantages for exploring how landscape factors affect surface-water quality and how surface-water quality may affect ecosystems. In contrast, the impact approach prioritized basins in terms of human development and nutrient concentrations in both surface water and groundwater, thereby targeting areas where actions to reduce nutrient concentrations could have the largest effect on improving water availability and reducing ecosystem impacts.

Nitrogen loading resulting from major floods and sediment resuspension to a large coastal embayment

Major floods pose a severe threat to coastal receiving environments, negatively impacting environmental health and ecosystem services through direct smothering with sediment and nutrient loading. This study examined the short and long-term impacts of the February 2022 major flood event on mud extent and sediment nitrogen flux in Moreton Bay (the Bay), a large, sub-tropical embayment in Southeast Queensland, Australia. Short-term impacts were assessed three days after the flood peak by sampling surface water at 47 sites in the direction of the predominant circulation pattern. Longer-term impacts were assessed by undertaking an intensive sediment survey of 223 sites and a nutrient flux experiment using sediment core incubations to simulate calm and resuspension conditions for the four key sediment classes. Short-term impacts revealed elevated turbidity levels extended across the Bay but were highest at the Brisbane River mouth, ammonium concentrations varied inversely with surface turbidity, whereas nitrate concentrates closely tracked surface turbidity. The sediment survey confirmed fine sediment deposition across 98 % of the Bay. Porewater within the upper 10 cm contained a standing pool of 280 t of ammonium, with concentrations more than three orders of magnitude higher than overlying surface waters. The nutrient flux experiment revealed an order of magnitude higher sediment ammonium flux rate in the sandy mud sediment class compared to the other sediment classes; and for simulated resuspension conditions compared to calm conditions for sand, muddy sand, and mud sediment classes. Scaling across the whole Bay, we estimated a mean annual sediment flux of 17,700 t/year ammonium, with a range of 13,500 to 21,900 t/year. Delivery of fine sediments by major floods over the last 50 years now impact >98 % of the benthic zone and provide a major loading pathway of available nitrogen to surface waters of Moreton Bay; representing a significant threat to ecosystem health.

Substantial contribution of tree canopy nitrifiers to nitrogen fluxes in European forests

Human activities have greatly increased the reactive nitrogen in the biosphere, thus profoundly altering global nitrogen cycling. The large increase in nitrogen deposition over the past few decades has led to eutrophication in natural ecosystems, with negative effects on forest health and biodiversity. Recent studies, however, have reported oligotrophication in forest ecosystems, constraining their capacity as carbon sinks. Here we demonstrate the widespread biological transformation of atmospheric reactive nitrogen in the canopies of European forests by combining nitrogen deposition quantification with measurements of the stable isotopes in nitrate and molecular analyses across ten forests through August–October 2016. We estimate that up to 80% of the nitrate reaching the soil via throughfall was derived from canopy nitrification, equivalent to a flux of up to 5.76 kg N ha−1 yr−1. We also document the presence of autotrophic nitrifiers on foliar surfaces throughout European forests. Canopy nitrification thus consumes deposited ammonium and increases nitrate inputs to the soil. The results of this study highlight widespread canopy nitrification in European forests and its important contribution to forest nitrogen cycling.

Ammonia-oxidizing bacteria and archaea exhibit differential nitrogen source preferences.

Ammonia-oxidizing microorganisms (AOM) contribute to one of the largest nitrogen fluxes in the global nitrogen budget. Four distinct lineages of AOM: ammonia-oxidizing archaea (AOA), beta- and gamma-proteobacterial ammonia-oxidizing bacteria (β-AOB and γ-AOB) and complete ammonia oxidizers (comammox), are thought to compete for ammonia as their primary nitrogen substrate. In addition, many AOM species can utilize urea as an alternative energy and nitrogen source through hydrolysis to ammonia. How the coordination of ammonia and urea metabolism in AOM influences their ecology remains poorly understood. Here we use stable isotope tracing, kinetics and transcriptomics experiments to show that representatives of the AOM lineages employ distinct regulatory strategies for ammonia or urea utilization, thereby minimizing direct substrate competition. The tested AOA and comammox species preferentially used ammonia over urea, while β-AOB favoured urea utilization, repressed ammonia transport in the presence of urea and showed higher affinity for urea than for ammonia. Characterized γ-AOB co-utilized both substrates. These results reveal contrasting niche adaptation and coexistence patterns among the major AOM lineages.

Relationship between Nitrogen Dynamics and Key Microbial Nitrogen-Cycling Genes in an Intensive Freshwater Aquaculture Pond.

Intensive aquaculture in high-density hybrid snakehead [Channa maculata (♀) × Channa argus (♂)] fishponds can lead to toxic conditions for fish. This study investigated nitrogen migration and transformation in these fishponds during different cultivation periods. Using qPCR technology, we analyzed the abundance variation of nitrogen-cycling microorganisms in water and sediment to reveal the nitrogen metabolism characteristics of hybrid snakehead fishponds. The results showed that fish biomass significantly impacts suspended particulate matter (SPM) flux. At the sediment-water interface, inorganic nitrogen fluxes showed predominant NO3--N absorption by sediments and NH4+-N and NO2--N release, especially in later cultivation stages. Sediments were rich in nirS and AMX 16S rRNA genes (ranging from 4.04 × 109 to 1.01 × 1010 and 1.19 × 108 to 2.62 × 108 copies/g, respectively) with nirS-type denitrifiers potentially dominating the denitrification process. Ammonia-oxidizing bacteria (AOB) were found to dominate the ammonia oxidation process over ammonia-oxidizing archaea (AOA) in both water and sediment. Redundancy analysis revealed a positive correlation between SPM flux, Chlorophyll a (Chl-a), and denitrification genes in the water, and between nitrogen-cycling genes and NH4+/NO2- fluxes at the interface. These findings provide a scientific basis for nitrogen control in hybrid snakehead fishponds.