Abiotic Nitrogen Research Articles

Copper sulfide mineral performs non-enzymatic anaerobic ammonium oxidation through a hydrazine intermediate.

Anaerobic ammonium oxidation (anammox)-the biological process that activates ammonium with nitrite-is responsible for a significant fraction of N2 production in marine environments. Despite decades of biochemical research, however, no synthetic models capable of anammox have been identified. Here we report that a copper sulfide mineral replicates the entire biological anammox pathway catalysed by three metalloenzymes. We identified a copper-nitrosonium {CuNO}10 complex, formed by nitrite reduction, as the oxidant for ammonium oxidation that leads to heterolytic N-N bond formation from nitrite and ammonium. Similar to the biological process, N2 production was mediated by the highly reactive intermediate hydrazine, one of the most potent reductants in nature. We also found another pathway involving N-N bond heterocoupling for the formation of hybrid N2O, a potent greenhouse gas with a unique isotope composition. Our study represents a rare example of non-enzymatic anammox reaction that interconnects six redox states in the abiotic nitrogen cycle.

Short Review of Self-Powered Nitrogen Removal via Abiotic Electrochemical Catalysis

Microbial nitrification and denitrification are efficient technologies for the treatment of nitrogen-containing wastewater. However, these biotic technologies are inapplicable for the treatment of toxic substances such as heavy metals, polyaromatic hydrocarbons, adsorbable organic halogens, and polychlorinated biphenyls, which have an inhibitory effect on microbial metabolism. It is therefore necessary to develop abiotic nitrogen removal technology with comparable cost efficiency. Nitrogen contaminants are promising indirect fuel sources. The integration of electrocatalysis energy conversion with nitrogen contaminants could drive an entire electrochemical system to obtain nitrogen removal in a self-powered fashion. Research advances in the development of fuel cells have corroborated their promising application for nitrogen removal. This work aims to review the most recent advances in the utilization of ammonia and nitrate as fuels for self-powered nitrogen removal and demonstrate how close this technology is to integration with future applications. The mechanism of ammonia–oxygen fuel cells is first summarized, followed by an overview of recent research on self-powered systems based on various noble-metal-free catalysts. We then introduce different harvesting and conversion methods using nitrate with a desired power output and nitrogen removal efficiency. The final section demonstrates the shortcomings of research and future innovative perspectives for self-powered wastewater treatment.

Zero-valent iron boosts nitrate-to-ammonia bioconversion via extracellular electron donation and reduction pathway complementation

Microbial nitrate reduction to ammonia (NRA) presents a promising route to recover wastewater nitrogen resources, but its practical application is currently challenged by limited bacterial activity and reaction selectivity. Here, we propose a facile strategy to boost microbial NRA by using zero-valent iron (ZVI) as an environmentally-benign augment. Unlike the previously reported hybrid systems that rely mainly on mediated electron transfer between ZVI and bacteria for enhanced denitrification, we revealed a combined pathway of direct and mediated electron transfer from ZVI to bacteria, along with a complementation between biological and abiotic nitrogen conversion processes, to promote the NRA process. The bio-hybrid exhibited over 13-fold higher NO3− reduction activity than the individual bacteria or ZVI groups, nearly 100% NRA selectivity, and good stability for treating real wastewater. Our work provides an efficient and scalable route to combine ammonia production with wastewater valorization, which may be readily incorporated into various wastewater treatment processes to maximize resource recovery.

Synchrotron X-Ray-Driven Nitrogen Reduction on an AgCu Thin Film.

Nitrogen (N2 ) is an essential element for life, but kinetically stable N2 in the atmosphere needs to be reduced to biologically available forms as a nutrient for organisms. Abiotic nitrogen fixation is critical to the origin of life on the early Earth, which is due to lightning or mineral-based reduction. Here, synchrotron X-ray-induced silver nitrate formation on a silver copper (AgCu) thin-film is reported. Time-resolved X-ray diffraction measurements show that under intense X-ray exposure, initially formed silver oxides (AgOx) are quickly converted to silver nitrate (AgNO3 ). Interestingly, AgNO3 is first formed in its high-temperature phase with a space group of R3cH, which gradually transforms to the room temperature phase with a space group of Pbca under continuous X-ray irradiation. The result not only provides a new clue about the abiotic nitrogen reduction prior to life but also demonstrates a novel strategy of materials synthesis using synchrotron X-rays.

Long-Term Chili Monoculture Alters Environmental Variables Affecting the Dominant Microbial Community in Rhizosphere Soil.

Continuous cropping negatively affects soil fertility, physicochemical properties and the microbial community structure. However, the effects of long-term chili monoculture on the dominant microbial community assembly are not known. In this study, the impact of long-term chili monoculture on the correlation between the dominant microbial community and soil environmental variables was assessed. The results indicated that increasing duration of chili monoculture generated significant changes in soil nutrients, soil aggregates and soil enzymes: nutrient contents increased overall, mechanically stable macroaggregates increased and microaggregates decreased, water-stable macroaggregates and microaggregates decreased, β-glucosidase decreased nonlinearly, and nitrate reductase and alkaline phosphatase activities showed a nonlinear increase. Moreover, an increasing number of years of chili monoculture also affected the structure of the dominant microbiota, with substantial changes in the relative abundances of 11 bacterial and fungal genera. The drivers of the dominant microbial community assembly in rhizosphere soil were soil moisture, abiotic nitrogen, pH and salt.

Compact wastewater treatment process based on abiotic nitrogen management achieved high-rate and facile pollutants removal

Chemically enhanced primary treatment (CEPT), ammonium ion exchange and regeneration (AIR) and membrane bioreactor (MBR) were coupled as CAIRM to treat domestic wastewater compactly and efficiently. CAIRM achieved efficient removal of chemical oxygen demand, ammonia nitrogen, total nitrogen (TN) and total phosphorus with total hydraulic retention time of 4.6 h, and obtained 2.3 ± 0.9 mg/L TN in the effluent. CEPT removed phosphate and impurities and prevented AIR from pollution. AIR maintained excellent nitrogen removal with a slight decrease in the exchange capacity of ion exchangers. MBR polished the effluent from AIR, and the larger particle size and better dewaterability of sludge mitigated the membrane fouling. Many heterotrophic genera, such as Rhodobacter and Defluviimonas, were enriched in the oligotrophic MBR. This study demonstrates the viability and stability of CAIRM in efficient wastewater treatment, which will address critical challenges in insufficient nitrogen removal and high land occupancy of current processes.

Trade-offs between effluent quality and ammonia volatilisation with CO2 augmented microalgal treatment of anaerobically digested food-waste centrate

Diversion of food waste from landfill disposal to waste-to-energy facilities has become both an environmentally and economically viable option to support the circular bioeconomy. However, the liquid centrate produced during anaerobic digestion is high in total ammonia, with concentrations ~2000 g m−3, and can release gaseous emissions, including ammonia, methane, CO2 and nitrous oxide, to the atmosphere. Further treatment is required before discharge to sewer, or to the environment. Microalgal wastewater treatment systems augmented with CO2 offer a promising and cost-effective treatment solution for reducing both total ammonia concentrations and ammonia volatilisation. In this study, we investigate the effects of augmenting CO2 on nutrient removal and specifically nitrogen losses, as well as biomass productivity under two difference hydraulic retention times (HRT). Both CO2 addition and HRT affect nitrogen losses, with the percentage removal of total ammonia significantly lower (p < 0.01) when CO2 was added to the treatments, while increased HRT significantly increased (p < 0.05) total ammonia percentage removal. Total nitrogen budgets showed significantly lower (p < 0.01) abiotic nitrogen losses from the system when CO2 was added to the culture but at the expense of effluent quality. Both total suspended solids and volatile suspended solids significantly increased (p < 0.01) under longer HRT (8 days), with CO2 addition, while chlorophyll-a biomass significantly increased (p < 0.01) on longer HRT, regardless of CO2 addition. These results demonstrate that, while CO2 augmentation helped to mitigate ammonia losses to atmosphere, the trade-off was poorer effluent quality. Coupling CO2 augmentation with longer HRT increased biomass production and nutrient removal efficiency. This study provides an insight into how simple operational changes can alleviate some of the trade-offs between atmospheric losses and effluent quality. However, in order to manage the trade-off between reduced atmospheric losses and poorer effluent quality, further optimisation of the operation of the microalgal system treating food-waste centrate is required.

Biotic and abiotic nitrogen immobilization in soil incorporated with crop residue

Incorporating crop residue favors biotic and abiotic nitrogen (N) immobilization, which can effectively conserve active N in soil. Here we summarize the occurrence characteristics of biotic and abiotic N immobilization and affecting factors, and then identify a general pattern of the relative importance of the two processes in soil incorporated with crop residue. When microorganisms decompose crop residue for a source of energy and carbon (C) to support their metabolism, they need N to build cellular components, resulting in N immobilization in biomass. Alternatively, N can be incorporated into soil organic matter (SOM) through the following known mechanisms: ammonium (NH4+) fixation by clay minerals, condensation of ammonia (NH3) with phenol or quinone rings, and nitrosation of nitrite (NO2-) with phenolic compounds. The biotic and abiotic contribution to N immobilization is significantly controlled by soil properties and crop residual quality. When a large fraction of C is in the form of labile organic compounds, biotic N immobilization may be important. In contrast, high fraction of lignin-derived phenolic compounds or other recalcitrant organic compounds facilitates abiotic N immobilization. In the N-limited soil, the increase in N availability contributes to microbial activity, which would stimulate biotic processes. However, when microbial metabolism switches from N-limited to C-limited due to the increase in N availability, abiotic processes become important. Abiotic processes even predominate over biotic processes under N-enriched conditions. The relative roles of biotic or abiotic N immobilization in soil incorporated with crop residue are dependent on the availability of C relative to N and soil N availability. Carbon and N availability in turn is related to their forms. Moreover, N availability is affected by soil pH. The mechanisms by which N is immobilized determine the fate of immobilized N and are of vital importance for the management of N availability and losses through combining N fertilizer with crop residue. More investigation is necessary to quantify the contribution of biotic and abiotic processes during the decomposition of crop residue and to determine the factors affecting N immobilization in the field.

Pyrite-induced uv-photocatalytic abiotic nitrogen fixation: implications for early atmospheres and Life

The molecular form of nitrogen, N2, is universally available but is biochemically inaccessible for life due to the strength of its triple bond. Prior to the emergence of life, there must have been an abiotic process that could fix nitrogen in a biochemically usable form. The UV photo-catalytic effects of minerals such as pyrite on nitrogen fixation have to date been overlooked. Here we show experimentally, using X-ray photoemission and infrared spectroscopies that, under a standard earth atmosphere containing nitrogen and water vapour at Earth or Martian pressures, nitrogen is fixed to pyrite as ammonium iron sulfate after merely two hours of exposure to 2,3 W/m 2 of ultraviolet irradiance in the 200–400 nm range. Our experiments show that this process exists also in the absence of UV, although about 50 times slower. The experiments also show that carbonates species are fixed on pyrite surface.

(Invited) Investigating the Photoelectrocatalytic Activity of Titania for Nitrogen Fixation Using Rotating Ring-Disk Electrode Voltammetry

Reports of photocatalytic nitrogen fixation on titania have surfaced for nearly eight decades [1]. Demonstrations have occurred in both the gas and aqueous phase, and with various environmental conditions (relative humidity, temperature, pressure). Nearly all experimental observations suggest that abiotic nitrogen photofixation may be possible at ambient temperature and pressure [2,3]. This is impactful as it promotes the possibility for nutrient based fertilizer production from environmentally abundant materials (minerals) using only the sun as a source of energy. However, the reaction pathway that results in ammonia production is not well understood. Furthermore, the wide band gap mineral coupled with the location of the band edges, suggest that nitrogen reduction not likely. Here, we aim to investigate photo-catalytic nitrogen fixation by titania in an aqueous environment through a series electrocatalytic rotating ring disk voltammetry experiments. We demonstrate that this experimental approach is effective at discerning the low-level ammonium concentration intrinsic to photocatalytic nitrogen fixation experiments through monitoring the ammonia oxidation peak at the ring electrode. Results show that that no ammonium is detected without illumination with both a rutile and mix-phased titania photocatalyst. With illumination ammonium was detected with the rutile phase titania, but not with the mixed phase photocatalyst. This suggest that mineral phase plays an important role in promoting nitrogen photofixation. Rotating ring disk electrode experiments may provide a new avenue to attain a higher degree of precision for detecting ammonia at low levels from both photo and electrocatalyst. [1]N. Dhar, E. Seshacharyulu and N. Biswas, Proc. Natl. Acad. Sci., India, 1941,7, 115–131 [2]A. J. Medford and M. C. Hatzell, ACS Catal., 2017, 2624–2643. [3]G. Schrauzer and T. Guth, J. Am. Chem. Soc. , 1977, 99, 7189–7193.

Photochemical Nitrogen Fixation on Titania

Photocatalysis is the ideal solution to sustainable ammonia synthesis, as it will enable the distributed production of fertilizers and fuels from reactants that are readily available at ambient conditions: air, water and photons. Given the massive amount of energy currently expended to produce and transport fertilizer, such a process would have an enormous impact on energy consumption and carbon emissions [1,2,3]. Photocatalytic nitrogen reduction has been demonstrated over titania catalysts, although the production rates are too low to be practical. Despite the enormous promise of this reaction, there is little understanding of the fundamental chemical and physical processes that enable it. In this presentation, defects, like oxygen vacancies, and dopants like iron and carbon have been demonstrated to enhance photocatalytic activity for TiO2 for nitrogen fixation. The results indicate that the defects and dopants aid in promoting nitrogen adsorption which consequently may drive photocatalytic performance. Furthermore, insight acquired through ambient pressure XPS and DRIFTs provide new insight into the role this process may play in natural environments. [1] Medford, Andrew J., and Marta C. Hatzell. "Photon-driven nitrogen fixation: current progress, thermodynamic considerations, and future outlook." ACS Catalysis 7.4 (2017): 2624-2643. [2]Schrauzer, G. N., Strampach, N., Hui, L. N., Palmer, M. R., & Salehi, J. (1983). Nitrogen photoreduction on desert sands under sterile conditions. Proceedings of the National Academy of Sciences, 80(12), 3873-3876. [3]Doane, T. A. (2017). The Abiotic Nitrogen Cycle. ACS Earth and Space Chemistry, 1(7), 411-421.

The Abiotic Nitrogen Cycle

Natural environments on Earth are amenable to a diverse array of chemical reactions that can convert one form of nitrogen into another, often with the participation of additional substances such as minerals, dissolved metals, and organic compounds. These processes collectively define a natural chemical nitrogen cycle, analogous to the familiar biologically driven cycle but even more intricate with respect to the number of pathways by which nitrogen can be transformed and transported across land, air, and water. The fully assembled abiotic nitrogen cycle manifests a landscape rich in opportunities for investigation either with or without parallel attention to biological processes.

Anomalous nitrogen isotopes in ultrahigh-pressure metamorphic rocks from the Sulu orogenic belt: Effect of abiotic nitrogen reduction during fluid–rock interaction

Modern nitrogen (N) fixation is primarily mediated by biological processes. However, in the early Earth where biological activity was absent or limited, abiotic N reduction in hydrothermal systems is thought to be a key process to transform atmospheric N2 and NOx to ammonium, an essential nutrient to support the emergence of life and also an N form that can be incorporated into rocks. Surprisingly, evidence for abiotic N reduction in the rock record has not been clearly identified. In this study, we reported anomalously low N isotope compositions (δN15 values as low as −15.8‰) of mica samples in ultrahigh-pressure metamorphic rocks from the Donghai area in the Sulu orogenic belt, eastern China. Compared with mica samples with typical crustal δN15 values (3–9‰) in similar metamorphic rocks from the western Dabie orogen, the 15N-depleted mica samples from the Sulu orogen are characterized by significant N enrichment (10 times higher) and extreme 18O depletion (δO18 values as low as −9‰). These features can be best explained by assimilation of N from a source characterized by extremely low δN15 values (less than ∼−16‰). The extremely low δN15 value would be produced by abiotic N reduction during reaction of a meteoric-hydrothermal fluid with crustal rocks before subduction. This observation provides a clue to the occurrence of abiotic N reduction in continental supracrustal rocks and infer that abiotic N reduction process could be a fundamental process driving the geological N cycling in early Earth.

Abiotic Nitrogen Fixation on Terrestrial Planets: Reduction of NO to Ammonia by FeS

Understanding the abiotic fixation of nitrogen and how such fixation can be a supply of prebiotic nitrogen is critical for understanding both the planetary evolution of, and the potential origin of life on, terrestrial planets. As nitrogen is a biochemically essential element, sources of biochemically accessible nitrogen, especially reduced nitrogen, are critical to prebiotic chemistry and the origin of life. Loss of atmospheric nitrogen can result in loss of the ability to sustain liquid water on a planetary surface, which would impact planetary habitability and hydrological processes that shape the surface. It is known that NO can be photochemically converted through a chain of reactions to form nitrate and nitrite, which can be subsequently reduced to ammonia. Here, we show that NO can also be directly reduced, by FeS, to ammonia. In addition to removing nitrogen from the atmosphere, this reaction is particularly important as a source of reduced nitrogen on an early terrestrial planet. By converting NO directly to ammonia in a single step, ammonia is formed with a higher product yield (~50%) than would be possible through the formation of nitrate/nitrite and subsequent conversion to ammonia. In conjunction with the reduction of NO, there is also a catalytic disproportionation at the mineral surface that converts NO to NO₂ and N₂O. The NO₂ is then converted to ammonia, while the N₂O is released back in the gas phase, which provides an abiotic source of nitrous oxide.

Soil nitrogen conservation mechanisms in a pristine south Chilean Nothofagus forest ecosystem

A 15N tracing study was carried out to identify microbial and abiotic nitrogen (N) transformations in a south Chilean Nothofagus betuloides forest soil which is characterized by low N inputs and absence of human disturbance. Gross N transformation rates were quantified with a 15N tracing model in combination with a Markov chain Monte Carlo sampling algorithm for parameter estimation. The 15N tracing model included five different N pools (ammonium (NH 4 +), nitrate (NO 3 −), labile (N lab) and recalcitrant (N rec) soil organic matter and adsorbed NH 4 +), and ten gross N transformation rates. The N dynamics in the N. betuloides ecosystem are characterized by low net but high gross mineralization rates. Mineralization in this soil was dominated by turnover of N lab, while immobilization of NH 4 + predominantly entered the N rec pool. A fast exchange between the NH 4 + and the adsorbed NH 4 + pool was observed, possibly via physical adsorption on and release from clay lattices, providing an effective buffer for NH 4 +. Moreover, high NH 4 + immobilization rates into the N rec pool ensure a sustained ecosystem productivity. Nitrate, the most mobile form of N in the system, is characterized by a slow turnover and was produced in roughly equal amounts from NH 4 + oxidation and organic N oxidation. More than 86% of the NO 3 − produced was immediately consumed again. This study showed for the first time that dissimilatory nitrate reduction to ammonium (DNRA) was almost exclusively (>99%) responsible for NO 3 − consumption. DNRA rather than NO 3 − immobilization ensures that NO 3 − is transformed into another available N form. DNRA may therefore be a widespread N retention mechanism in ecosystems that are N-limited and receive high rainfalls.

Abiotic Nitrogen Uptake in Semiarid Grassland Soils of the U.S. Great Plains

Soils represent the largest terrestrial sink for N, yet current understanding of nutrient cycling cannot account for some of the mechanisms and sinks that stabilize anthropogenic N. We assessed the influence of soil properties, particularly soil organic C, pH, and clay content on potential biotic and abiotic N assimilation in soils collected across a temperature gradient in the U.S. Great Plains. We pulse labeled HgCl2‐sterilized and unsterilized soils with 15N to examine the relative importance of abiotic and biotic N assimilation in short‐term laboratory incubations. Estimates of total N assimilation in unsterilized soils ranged from 1.21 to 2.40 mg N kg−1 soil. Soil C content accounted for 50 and 60% of the variance in estimates of biotic immobilization and total N assimilation, respectively. Estimates of abiotic N assimilation ranged from 0.089 to 0.80 mg N kg−1 soil. Abiotic N uptake represented a large proportion of total N assimilation (mean equals 20%) in short‐term laboratory incubations. In contrast to previous reports, abiotic N uptake was negatively correlated to soil clay content and pH, perhaps because of differences in mineralogy and soil organic matter composition across the gradient. These results emphasize the importance of nonbiological N uptake in semiarid soils and suggest that abiotic pools could be an important sink for N.

Abiotic Nitrogen Uptake in Semiarid Grassland Soils of the U.S. Great Plains

Soils represent the largest terrestrial sink for N, yet current understanding of nutrient cycling cannot account for some of the mechanisms and sinks that stabilize anthropogenic N. We assessed the influence of soil properties, particularly soil organic C, pH, and clay content on potential biotic and abiotic N assimilation in soils collected across a temperature gradient in the U.S. Great Plains. We pulse labeled HgCl2-sterilized and unsterilized soils with 15N to examine the relative importance of abiotic and biotic N assimilation in short-term laboratory incubations. Estimates of total N assimilation in unsterilized soils ranged from 1.21 to 2.40 mg N kg−1 soil. Soil C content accounted for 50 and 60% of the variance in estimates of biotic immobilization and total N assimilation, respectively. Estimates of abiotic N assimilation ranged from 0.089 to 0.80 mg N kg−1 soil. Abiotic N uptake represented a large proportion of total N assimilation (mean equals 20%) in short-term laboratory incubations. In contrast to previous reports, abiotic N uptake was negatively correlated to soil clay content and pH, perhaps because of differences in mineralogy and soil organic matter composition across the gradient. These results emphasize the importance of nonbiological N uptake in semiarid soils and suggest that abiotic pools could be an important sink for N.

Nitrogen reduction under hydrothermal vent conditions: implications for the prebiotic synthesis of C-H-O-N compounds.

Dinitrogen is reduced in dilute hydrogen sulfide (H2S) solutions to ammonium at 120 degrees C. Experiments with dissolved dinitrogen (partial pressure 50 bar) in a 12 x 10(-3) mol/L H2S(aq) solution yield approximately 10(-5) mol/L NH4+ within 2-7 days. These yields are consistent with the equilibrium NH4+ concentration for the N-S-H system under these conditions. The formation of ammonium is catalyzed by the presence of freshly precipitated iron monosulfide. These results indicate that dinitrogen can be reduced at moderate temperatures in hydrothermal vent systems. Abiotic nitrogen reduction could have taken place within primordial hydrothermal vents, supplying some ammonia for the synthesis of C-H-O-N compounds via abiotic processes. The yield of ammonia via dinitrogen reduction by hydrogen sulfide, however, is so low that it is doubtful this process could have produced enough ammonia to sustain prebiotic hydrothermal synthesis of C-H-O-N compounds in or around vent systems.

Biotic and Abiotic Nitrogen Retention in a Variety of Forest Soils

Nitrogen (N) immobilization in sterilized (abiotic) and non‐sterilized (biotic) O and A horizons was studied to determine the relative importance of biotic and abiotic processes in N retention in forest ecosystems. We collected samples from a variety of forest locations in Washington, Nevada, California, Tennessee, and North Carolina with differing soil types, vegetation, N status, and soil acidity. Included among these sites were adjacent stands of N2‐fixing and non‐N2‐fixing species and sites of differing N status due to slope position at a given location. We treated O and A horizon samples from each site with (15NH4)2SO4; sterilization was achieved by adding HgCl2, which proved to be highly effective. We found significant levels of both abiotic and biotic N immobilization in all soils. Biotic N immobilization was much greater in the N‐poor sites in California and Nevada than in the other sites and was inversely related to N concentration overall. Biotic immobilization was directly related to pH and base saturation among all sites, but we hypothesize that these correlations resulted from a correlation between those parameters and N concentration. Abiotic N immobilization varied less than biotic N immobilization across sites and was unrelated to N concentration or pH. The percentage of total N immobilization as abiotic N immobilization varied considerably (from 6–90%), and was positively correlated with N concentration. These results suggest that abiotic N immobilization can be a significant process in a variety of soil types. Across soil types with increasing N saturation, biotic N immobilization decreases and abiotic N immobilization accounts for a greater proportion of total N immobilization.

Abiotic nitrogen reduction on the early Earth.

The production of organic precursors to life depends critically on the form of the reactants. In particular, an environment dominated by N2 is far less efficient in synthesizing nitrogen-bearing organics than a reducing environment rich in ammonia. Relatively reducing lithospheric conditions on the early Earth have been presumed to favour the generation of an ammonia-rich atmosphere, but this hypothesis has not been studied experimentally. Here we demonstrate mineral-catalysed reduction of N2, NO2- and NO3- to ammonia at temperatures between 300 and 800 degrees C and pressures of 0.1-0.4 GPa-conditions typical of crustal and oceanic hydrothermal systems. We also show that only N2 is stable above 800 degrees C, thus precluding significant atmospheric ammonia formation during hot accretion. We conclude that mineral-catalysed N2 reduction might have provided a significant source of ammonia to the Hadean ocean. These results also suggest that, whereas nitrogen in the Earth's early atmosphere was present predominantly as N2, exchange with oceanic, hydrothermally derived ammonia could have provided a significant amount of the atmospheric ammonia necessary to resolve the early-faint-Sun paradox.