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1
Ash, C. "Cryptic N2 Fixation." Science 337, no. 6093 (July 26, 2012): 391. http://dx.doi.org/10.1126/science.337.6093.391-a.
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Rao, V. M., Jayshree, Banwari Lal, P. Shailaja, and H. S. Narayana. "N2-fixation (N2-ase activity) inAzospirillum strains." Proceedings / Indian Academy of Sciences 98, no. 6 (December 1988): 483–88. http://dx.doi.org/10.1007/bf03053405.
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Patil, B. S., Q. Wang, V. Hessel, and J. Lang. "Plasma N2-fixation: 1900–2014." Catalysis Today 256 (November 2015): 49–66. http://dx.doi.org/10.1016/j.cattod.2015.05.005.
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Jena, P. K., and V. Rajaramamohan Rao. "Nitrogen fixation as influenced by pesticides and rice straw in paddy soils." Journal of Agricultural Science 108, no. 3 (June 1987): 635–38. http://dx.doi.org/10.1017/s0021859600080059.
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SummaryThe effect of three herbicides and an insecticide combination on nitrogen fixation in three paddy soils was investigated in a laboratory incubation study. The influence of pesticide combination on N2 fixation was evaluated in the presence and absence of rice straw under flooded and non-flooded conditions. In a non-flooded alluvial soil single or combined application of butachlor and carbofuran significantly stimulated N2 fixation. There was no effect on N2 fixation when thiobencarb was applied alone; but with thiobencarb in combination with carbofuran higher N2 fixation occurred. Both oxadiazone and thiobencarb had no effect on N2 fixation under flooded conditions, but in combination with carbofuran the N2 fixation was high. In acid sulphate saline Pokkali soil, rice straw application stimulated N2 fixation; the stimulatory effect of carbofuran either alone or in combination became more pronounced under flooded conditions. N2 fixation was low in a laterite Sukinda soil and the effect of pesticides was not changed by rice straw amendment. A uniform stimulation of N2 fixation occurred in soils when carbofuran was applied either singly or in combination with other herbicides tested. Results indicate that the effect of pesticides on N2 fixation varied with the rice straw application and water regime.
5
Zilius, Mindaugas, Irma Vybernaite-Lubiene, Diana Vaiciute, Donata Overlingė, Evelina Grinienė, Anastasija Zaiko, Stefano Bonaglia, et al. "Spatiotemporal patterns of N<sub>2</sub> fixation in coastal waters derived from rate measurements and remote sensing." Biogeosciences 18, no. 5 (March 18, 2021): 1857–71. http://dx.doi.org/10.5194/bg-18-1857-2021.
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Abstract. Coastal lagoons are important sites for nitrogen (N) removal via sediment burial and denitrification. Blooms of heterocystous cyanobacteria may diminish N retention as dinitrogen (N2) fixation offsets atmospheric losses via denitrification. We measured N2 fixation in the Curonian Lagoon, Europe's largest coastal lagoon, to better understand the factors controlling N2 fixation in the context of seasonal changes in phytoplankton community composition and external N inputs. Temporal patterns in N2 fixation were primarily determined by the abundance of heterocystous cyanobacteria, mainly Aphanizomenon flos-aquae, which became abundant after the decline in riverine nitrate inputs associated with snowmelt. Heterocystous cyanobacteria dominated the summer phytoplankton community resulting in strong correlations between chlorophyll a (Chl a) and N2 fixation. We used regression models relating N2 fixation to Chl a, along with remote-sensing-based estimates of Chl a to derive lagoon-scale estimates of N2 fixation. N2 fixation by pelagic cyanobacteria was found to be a significant component of the lagoon's N budget based on comparisons to previously derived fluxes associated with riverine inputs, sediment–water exchange, and losses via denitrification. To our knowledge, this is the first study to derive ecosystem-scale estimates of N2 fixation by combining remote sensing of Chl a with empirical models relating N2 fixation rates to Chl a.
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Knapp, Angela N., Karen L. Casciotti, William M. Berelson, Maria G. Prokopenko, and Douglas G. Capone. "Low rates of nitrogen fixation in eastern tropical South Pacific surface waters." Proceedings of the National Academy of Sciences 113, no. 16 (March 14, 2016): 4398–403. http://dx.doi.org/10.1073/pnas.1515641113.
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An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N2 fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N2 fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N2 fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 µmol N m−2 d−1 based on sediment trap fluxes. Whereas the fractional importance of N2 fixation did not change, the N2-fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N2-fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N2 fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N2-fixation rates were higher, up to 100 µmol N m−2 d−1, and detected N2 fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N2 fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N2 fixation in the ETSP.
7
Chen, Hui, Rong Cai, Janki Patel, Fangyuan Dong, Hsiaonung Chen, and Shelley D. Minteer. "Upgraded Bioelectrocatalytic N2 Fixation: From N2 to Chiral Amine Intermediates." Journal of the American Chemical Society 141, no. 12 (March 5, 2019): 4963–71. http://dx.doi.org/10.1021/jacs.9b00147.
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Konno, U., U. Tsunogai, D. D. Komatsu, S. Daita, F. Nakagawa, A. Tsuda, T. Matsui, Y. J. Eum, and K. Suzuki. "Significance of N<sub>2</sub> fixation in dissolved fractions of organic nitrogen." Biogeosciences Discussions 7, no. 1 (February 1, 2010): 765–86. http://dx.doi.org/10.5194/bgd-7-765-2010.
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Abstract. Using the 15N2 tracer method and high-sensitivity δ15N analytical systems, we determined N2 fixation rates by dividing fractions into particulate organic nitrogen (PON: >0.7 μm) and dissolved organic nitrogen (DON: <0.7 μm). While N2 fixation in the DON fraction had been ignored in previous studies, we found significant N2 fixation signal in the DON fraction in our study. The areal N2 fixation rates estimated from the PON fractions varied from <1–160 μmol N m−2 d−1, and those estimated from the DON fractions ranged from <0.5–54 μmol N m−2 d−1. Thus, N2 fixation in the DON fractions accounted for 50% (ranging from <10% to 84%) of the total N2 fixation rates on an average. The new total N2 fixation flux, which includes fixation in DON fractions, has possibility to double the original estimates; therefore, the revised influx may reduce the imbalance in the global oceanic fixed nitrogen budget.
9
Doughton, JA, PG Saffigna, I. Vallis, and RJ Mayer. "Nitrogen fixation in chickpea. II. Comparison of 15N enrichment and 15N natural abundance methods for estimating nitrogen fixation." Australian Journal of Agricultural Research 46, no. 1 (1995): 225. http://dx.doi.org/10.1071/ar9950225.
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The 15N enrichment and 15N natural abundance methods for estimating N2 fixation in chickpea were compared over a range of soil NO3-N levels at crop establishment varying from 10 to 326 kg N/ha (0-120 cm depth). Barley was used as a non-N2 fixing control crop. Both methods estimated reduced N2 fixation as soil NO3-N levels at crop establishment increased. Similar estimates of % N2 fixation were obtained at high values, but at low values the enrichment method gave lower estimates, some of which were negative. The 15N natural abundance method provided realistic estimates of % N2 fixation across all soil N03-N levels at crop establishment. An asymptotic curve described a close ( R2 = 0.95) relationship between these factors. Standard errors of estimates of means for the 15N natural abundance method remained acceptable and relatively stable over the full range of measurements; however, with the 15N enrichment method they became unacceptably large at low values of % N2 fixation. These large errors may have been partly due to legume and control plants assimilating mineral N of differing 15N enrichment. High mineral N levels associated with low values of % N2 fixation were also shown to reduce reliability of N2 fixation values estimated by the 15N enrichment method. These errors caused potentially greater inaccuracy at low values of % N2 fixation than at high values. To compare N2 fixation means statistically, transformations were necessary to stabilize variance and to impart lower weightings to plots with low values of % N2 fixation.
10
Bonnet, S., O. Grosso, and T. Moutin. "Planktonic dinitrogen fixation along a longitudinal gradient across the Mediterranean Sea during the stratified period (BOUM cruise)." Biogeosciences 8, no. 8 (August 19, 2011): 2257–67. http://dx.doi.org/10.5194/bg-8-2257-2011.
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Abstract. This study provides extensive data on planktonic N2 fixation rates across the whole Mediterranean Sea. They show that N2 fixation occurs in Mediterranean waters during the stratification period, with a clear decreasing trend from the oligotrophic western basin (10–76 μmol m−2 d−1) to the ultra oligotrophic eastern basin (0–0.4 μmol m−2 d−1). Highest rates are measured in the less oligotrophic western basin, between the surface and 75 m-depth, where 45 to 75 % of N2 fixation are found within the picoplanktonic fraction (<3 μm). While the biogeochemical impact of N2 fixation in the eastern basin seems negligible, N2 fixation is able to sustain up to 35 % of new primary production during the stratified period in the western basin. These data disagree with indirect estimates of N2 fixation based on geochemical tracers and nutrient budgets, which indicates that N2 fixation increases with increasing N:P ratios and decreasing stable N isotopic signature of particulate organic nitrogen and NO3− from west to east. These results finally point out the need to assess N2 fixation at a higher temporal resolution in order to better understand the diazotrophs' dynamic under contrasted biogeochemical conditions.
11
Mahaffey, C. "The conundrum of marine N2 fixation." American Journal of Science 305, no. 6-8 (June 1, 2005): 546–95. http://dx.doi.org/10.2475/ajs.305.6-8.546.
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Gibson, Alan H. "Symbiotic N2 fixation of crop legumes." Field Crops Research 41, no. 3 (June 1995): 192–93. http://dx.doi.org/10.1016/s0378-4290(95)90029-2.
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Bergman, B. "N2 Fixation by non-heterocystous cyanobacteria." FEMS Microbiology Reviews 19, no. 3 (February 1997): 139–85. http://dx.doi.org/10.1016/s0168-6445(96)00028-9.
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14
Field, Leslie D. "An iron step towards N2 fixation." Nature Chemistry 2, no. 7 (May 16, 2010): 520–21. http://dx.doi.org/10.1038/nchem.690.
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Bergman, B., J. R. Gallon, A. N. Rai, and L. J. Stal. "N2 Fixation by non-heterocystous cyanobacteria1." FEMS Microbiology Reviews 19, no. 3 (January 17, 2006): 139–85. http://dx.doi.org/10.1111/j.1574-6976.1997.tb00296.x.
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Serraj, R., T. R. Sinclair, and L. C. Purcell. "Symbiotic N2 fixation response to drought." Journal of Experimental Botany 50, no. 331 (February 1, 1999): 143–55. http://dx.doi.org/10.1093/jxb/50.331.143.
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Konno, U., U. Tsunogai, D. D. Komatsu, S. Daita, F. Nakagawa, A. Tsuda, T. Matsui, Y. J. Eum, and K. Suzuki. "Determination of total N <sub>2</sub> fixation rates in the ocean taking into account both the particulate and filtrate fractions." Biogeosciences 7, no. 8 (August 12, 2010): 2369–77. http://dx.doi.org/10.5194/bg-7-2369-2010.
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Abstract. Using the 15N2 tracer method and high-sensitivity δ15N analytical systems, we determined N2 fixation rates for ocean samples by dividing them into particulate (>0.7 μm) and filtrate (<0.7 μm) fractions. While N2 fixation in the filtrate fraction had been ignored in previous studies, we found a significant N2 fixation rates in the filtrate fraction in our study. The areal N2 fixation rates in the western North Pacific Ocean estimated from the particulate fraction varied from <1 to 160 μmol N m-2 d−1, and those rates estimated from the filtrate fraction ranged from <0.5 to 54 μmol N m-2 d−1. Thus, N2 fixation in the filtrate fraction accounts for on average 50% (ranging from <10% to 84%) of the total N2 fixation rates. If these results are confirmed generally in the ocean, the new total N2 fixation flux, which includes fixation in the filtrate fraction, possibly doubles the original estimates; therefore, the revised influx may reduce the imbalance in the global oceanic fixed nitrogen budget.
18
Ridame, C., M. Le Moal, C. Guieu, E. Ternon, I. C. Biegala, S. L'Helguen, and M. Pujo-Pay. "Nutrient control of N<sub>2</sub> fixation in the oligotrophic Mediterranean Sea and the impact of Saharan dust events." Biogeosciences Discussions 8, no. 2 (March 15, 2011): 2629–57. http://dx.doi.org/10.5194/bgd-8-2629-2011.
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Abstract. A better understanding of the factors controlling N2 fixation is a pre-requisite for improving our knowledge on the contribution of N2 fixation in the nitrogen cycling in the Mediterranean Sea. Trace-metal clean nutrient/dust additions bioassays (+P, +PFe, +dust) were performed at three stations located in the western, central and eastern Mediterranean Sea, in summer 2008 as part of the BOUM cruise. The main goals were to investigate the nutrient factor(s) limiting N2 fixation (uptake of 15N2) and to evaluate the potential impact of a Saharan dust event on this biological process during the stratification period. Initially, surface waters at the three stations were DIP-depleted (<10 nM) while the DFe concentrations were relatively high (from 1.2 to 2.3 nM) most likely due to atmospheric iron accumulation in the surface mixed layer. At all stations, Saharan dust input relieved the ambient nutrient limitation of diazotrophic activity as demonstrated by the strong stimulation of N2 fixation (from x2.3 to x5.3). The highest dust stimulation of N2 fixation was recorded at the station located in the eastern basin (x5.3). The responses of diazotrophic activity to nutrients addition were contrasted at the sampled stations suggesting a spatial variability of the factor controlling N2 fixation over the whole basin. At all stations, N2 fixation was not limited by Fe nor co-limited by P and Fe. At the western station, N2 fixation was DIP limited while at the eastern one, N2 fixation was first DIP limited then was limited by one or several chemical element(s) released by dust. Our results demonstrated that a Saharan dust input was able to relieve the successive on-going N2 fixation limitations. Very interestingly, at the station located in the central basin, N2 fixation was not limited by the availability of P yet it was strongly stimulated by dust additions (up to x3.1). A chemical element or a combination of several, released by the added dust may have been responsible for the observed stimulations of N2 fixation. These results indicated that Saharan dust pulses to the surface Mediterranean waters, in addition to P and Fe, could be a source of chemical(s) element(s) that are necessary for metabolic processes and therefore influence rates of N2 fixation.
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Ridame, C., C. Guieu, and S. L'Helguen. "Strong stimulation of N<sub>2</sub> fixation in oligotrophic Mediterranean Sea: results from dust addition in large in situ mesocosms." Biogeosciences 10, no. 11 (November 15, 2013): 7333–46. http://dx.doi.org/10.5194/bg-10-7333-2013.
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Abstract. The response of N2 (dinitrogen) fixation to contrasted (wet and dry) Saharan dust deposition was studied in the framework of the DUNE project (a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem) during which realistic simulations of dust deposition (10 g m−2) into large mesocosms (52 m3) were performed. Three distinct experimental dust additions were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition, DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R: simulation of 2 successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. Here we show that wet and dry dust deposition induced a rapid (24 h or 48 h after dust additions), strong (from 2- to 5.3-fold) and long (at least 4–6 days duration) increase in N2 fixation, indicating that both wet and dry Saharan dust deposition was able to relieve efficiently the nutrient limitation(s) of N2 fixation. This means in particular that N2 fixation activity was not inhibited by the significant input of nitrate associated with the simulated wet deposition (~ 9 mmol NO3− m−2). The input of new nitrogen associated with N2 fixation was negligible relative to the atmospheric NO3− input associated with the dust. The contribution of N2 fixation to primary production was negligible (≤ 1%) before and after dust addition in all experiments, indicating that N2 fixation was a poor contributor to the nitrogen demand for primary production. Despite the stimulation of N2 fixation by dust addition, the rates remained low, and did not significantly change the contribution of N2 fixation to new production since only a maximum contribution of 10% was observed. The response of N2 fixation by diazotrophs and CO2 fixation by the whole phytoplankton community suggests that these metabolic processes were limited or co-limited by different nutrients. With this novel approach, which allows us to study processes as a function of time while atmospheric particles are sinking, we show that new atmospheric nutrients associated with Saharan dust pulses do significantly stimulate N2 fixation in the Mediterranean Sea and that N2 fixation is not a key process in the carbon cycle in such oligotrophic environments.
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Bonnet, S., O. Grosso, and T. Moutin. "Planktonic dinitrogen fixation in the Mediterranean Sea: a major biogeochemical process during the stratified period?" Biogeosciences Discussions 8, no. 1 (February 10, 2011): 1197–225. http://dx.doi.org/10.5194/bgd-8-1197-2011.
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Abstract. This study provides extensive data on planktonic N2 fixation fluxes across the whole Mediterranean Sea, representing a variety of trophic conditions. They show that N2 fixation occurs in Mediterranean waters during the stratification period, with a clear decreasing trend from the western basin (10–76 μmol m−2 d−1) to the eastern basin (0–0.4 μmol m−2 d−1. Highest rates are measured in the less oligotrophic areas, between the surface and 75 m-depth, and 45 to 75% of N2 fixation are performed within the picoplanktonic fraction (< 3 μm). While the biogeochemical impact of N2 fixation in the eastern basin seems negligible, N2 fixation is able to sustain up to 35% of new primary production during the stratified period and accounts for up to 25% of the external "new" N supply to the western basin during that period. These data disagree with indirect estimates of N2 fixation based on geochemical tracers and nutrient budgets, who suspected N2 fixation to increase with increasing N/P ratios and decreasing stable N isotopic signature of particulate organic nitrogen and NO3− from west to east. These results finally point out the need to assess N2 fixation at other seasons characterized by less oligotrophic conditions.
21
Unkovich, Murray. "Nitrogen fixation in Australian dairy systems: review and prospect." Crop and Pasture Science 63, no. 9 (2012): 787. http://dx.doi.org/10.1071/cp12180.
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Quantitative measurement of N2 fixation has rarely been conducted in Australian dairy pastures. The available data indicate that annual N2 fixation rates in Australian dairy pastures are generally low, due to low pasture legume content. With typical legume contents of grazed pastures less than 30% of total pasture biomass production, annual N2 fixation in herbage is usually much less than 50 kg ha–1 year–1. Other factors which are likely to be able to contribute to increased N2 fixation input (rhizobia, mineral N management, soil acidity, soil water contents) will have little impact until such time as legume contents are increased. In contrast, for some hay systems, such as those using lucerne, N2 fixation input is shown to be high (200–300 kg ha–1 year–1). While pasture clover contents remain low there is little value in study or measurement of N2 fixation, nor in complex modelling, as N2 fixation will be of little quantitative importance. However, where legume contents, and thus potential N2 fixation are increased, there is scope for investigation into potential increases in N input from this source, which is invariably linked to fertiliser application, the management of grazing and the N returns in urine and dung. These are the major influences on sward N dynamics and legume N2 fixation. The inoculant rhizobia used for white clover in Australia (TA1) is likely to be suboptimal. Isolated in Tasmania in 1953 it has been shown to be inferior in N2 fixation compared with other strains on several occasions. Root pests and diseases are likely to be prevalent and impact directly on clover root growth and perhaps nodulation. Modelling is often used to describe the probable influence of management and/or climate on the operation of agricultural systems. Reliable modelling of N2 fixation requires capacity to integrate the effects of grazing and pasture composition on soil mineral N dynamics, the influence of this mineral N on nodulation and on suppression of N2 fixation, and environmental and management influences on soil rhizobial populations. Currently no models have demonstrated this capacity. At present, a suitably calibrated regression model is probably a good option for modelling N2 fixation in Australian dairy pastures. Environmental benefits ensuing from increasing N2 fixation and substituting this for fertiliser N are likely to be greater off-farm (reduced GHG emissions at site of fertiliser manufacture) than on, if current fertiliser management is optimal. Nevertheless substituting fixed N for fertiliser N would have modest environmental and feed efficiency benefits.
22
Ridame, C., M. Le Moal, C. Guieu, E. Ternon, I. C. Biegala, S. L'Helguen, and M. Pujo-Pay. "Nutrient control of N<sub>2</sub> fixation in the oligotrophic Mediterranean Sea and the impact of Saharan dust events." Biogeosciences 8, no. 9 (September 29, 2011): 2773–83. http://dx.doi.org/10.5194/bg-8-2773-2011.
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Abstract. A better understanding of the factors controlling N2 fixation is a pre-requisite for improving our knowledge on the contribution of N2 fixation process in the nitrogen cycling. Trace-metal clean nutrient/dust addition bioassays (+P, +PFe, +dust) were performed at three stations located in the western, central and eastern Mediterranean Sea, in summer 2008 as part of the BOUM cruise. The main goals were (1) to investigate the nutrient factor(s) limiting N2 fixation (uptake of 15N2) and (2) to evaluate the potential impact of a Saharan dust event on this biological process during the stratification period. Initially, surface waters at the three stations were DIP-depleted (<10 nM) while the DFe concentrations were relatively high (from 1.2 to 2.3 nM) most likely due to atmospheric iron accumulation in the surface mixed layer. At all stations, Saharan dust input relieved the ambient nutrient limitation of the diazotrophic activity as demonstrated by the strong stimulation of N2 fixation (from 130 % to 430 %). The highest dust stimulation of N2 fixation was recorded at the station located in the eastern basin. The response of diazotrophic activity to nutrient additions was variable between the sampled stations suggesting a spatial variability of the factor controlling N2 fixation over the whole basin. At all stations, N2 fixation was not limited by Fe nor co-limited by P and Fe. At the western station, N2 fixation was DIP limited while at the eastern one, N2 fixation was first DIP limited, then was limited by one or several chemical element(s) released by dust. Our results demonstrated that a Saharan dust input was able to relieve these successive on going limitations. Very interestingly, at the station located in the central basin, N2 fixation was not limited by the availability of P yet it was strongly stimulated by dust addition (x3.1). A chemical element or a combination of several, released by the added dust may have been responsible for the observed stimulations of N2 fixation. These results indicated that Saharan dust pulses to the surface Mediterranean waters, in addition to P and Fe, could be a source of chemical(s) element(s) that are necessary for metabolic processes and therefore influence rates of N2 fixation.
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Benavides, Mar, Katyanne M. Shoemaker, Pia H. Moisander, Jutta Niggemann, Thorsten Dittmar, Solange Duhamel, Olivier Grosso, et al. "Aphotic N<sub>2</sub> fixation along an oligotrophic to ultraoligotrophic transect in the western tropical South Pacific Ocean." Biogeosciences 15, no. 10 (May 22, 2018): 3107–19. http://dx.doi.org/10.5194/bg-15-3107-2018.
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Abstract. The western tropical South Pacific (WTSP) Ocean has been recognized as a global hot spot of dinitrogen (N2) fixation. Here, as in other marine environments across the oceans, N2 fixation studies have focused on the sunlit layer. However, studies have confirmed the importance of aphotic N2 fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N2 fixation in the WTSP, we measured N2 fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) with the aim of searching for relationships between the composition of DOM and non-cyanobacterial N2 fixation in the aphotic ocean. N2 fixation rates were low (average 0.63 ± 0.07 nmol N L−1 d−1) but consistently detected across all depths and stations, representing ∼ 6–88 % of photic N2 fixation. N2 fixation rates were not significantly correlated with DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, mostly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subcluster 1G dominated at 650 dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available are still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N2 fixation activity at all depths sampled during our study suggest that aphotic N2 fixation may contribute significantly to fixed nitrogen inputs in this area and/or areas downstream of water mass circulation.
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Löscher, Carolin R., Annie Bourbonnais, Julien Dekaezemacker, Chawalit N. Charoenpong, Mark A. Altabet, Hermann W. Bange, Rena Czeschel, Chris Hoffmann, and Ruth Schmitz. "N<sub>2</sub> fixation in eddies of the eastern tropical South Pacific Ocean." Biogeosciences 13, no. 10 (May 18, 2016): 2889–99. http://dx.doi.org/10.5194/bg-13-2889-2016.
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Abstract. Mesoscale eddies play a major role in controlling ocean biogeochemistry. By impacting nutrient availability and water column ventilation, they are of critical importance for oceanic primary production. In the eastern tropical South Pacific Ocean off Peru, where a large and persistent oxygen-deficient zone is present, mesoscale processes have been reported to occur frequently. However, investigations into their biological activity are mostly based on model simulations, and direct measurements of carbon and dinitrogen (N2) fixation are scarce.We examined an open-ocean cyclonic eddy and two anticyclonic mode water eddies: a coastal one and an open-ocean one in the waters off Peru along a section at 16° S in austral summer 2012. Molecular data and bioassay incubations point towards a difference between the active diazotrophic communities present in the cyclonic eddy and the anticyclonic mode water eddies.In the cyclonic eddy, highest rates of N2 fixation were measured in surface waters but no N2 fixation signal was detected at intermediate water depths. In contrast, both anticyclonic mode water eddies showed pronounced maxima in N2 fixation below the euphotic zone as evidenced by rate measurements and geochemical data. N2 fixation and carbon (C) fixation were higher in the young coastal mode water eddy compared to the older offshore mode water eddy. A co-occurrence between N2 fixation and biogenic N2, an indicator for N loss, indicated a link between N loss and N2 fixation in the mode water eddies, which was not observed for the cyclonic eddy. The comparison of two consecutive surveys of the coastal mode water eddy in November 2012 and December 2012 also revealed a reduction in N2 and C fixation at intermediate depths along with a reduction in chlorophyll by half, mirroring an aging effect in this eddy. Our data indicate an important role for anticyclonic mode water eddies in stimulating N2 fixation and thus supplying N offshore.
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Zehr, Jonathan P., and Douglas G. Capone. "Changing perspectives in marine nitrogen fixation." Science 368, no. 6492 (May 14, 2020): eaay9514. http://dx.doi.org/10.1126/science.aay9514.
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Nitrogen fixation, the reduction of atmospheric dinitrogen gas (N2) to ammonia, is critical for biological productivity but is difficult to study in the vast expanse of the global ocean. Decades of field studies and the infusion of molecular biological, genomic, isotopic, and geochemical modeling approaches have led to new paradigms and questions. The discovery of previously unknown N2-fixing (diazotrophic) microorganisms and unusual physiological adaptations, combined with diagnostic distributions of nutrients and their isotopes as well as measured and modeled biogeographic patterns, have revolutionized our understanding of marine N2 fixation and its role in the global nitrogen cycle. Anthropogenic upper-ocean warming, increased dissolved carbon dioxide, and acidification will affect the distribution and relative importance of specific subgroups of N2 fixers in the sea; these changes have implications for foodwebs and biogeochemical cycles.
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Gier, J., S. Sommer, C. R. Löscher, A. W. Dale, R. A. Schmitz, and T. Treude. "Nitrogen fixation in sediments along a depth transect through the Peruvian oxygen minimum zone." Biogeosciences Discussions 12, no. 17 (September 2, 2015): 14401–40. http://dx.doi.org/10.5194/bgd-12-14401-2015.
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Abstract. Benthic nitrogen (N2) fixation and sulfate reduction (SR) were investigated in the Peruvian oxygen minimum zone (OMZ). Sediment samples, retrieved by a multiple corer were taken at six stations (70–1025 m) along a depth transect at 12° S, covering anoxic and hypoxic bottom water conditions. Benthic N2 fixation was detected at all sites, with high rates measured in OMZ mid-waters between the 70 and 253 m and lowest N2 fixation rates below 253 m down to 1025 m water depth. SR rates were decreasing with increasing water depth, with highest rates at the shallow site. Benthic N2 fixation depth profiles largely overlapped with SR depth profiles, suggesting that both processes are coupled. The potential of N2 fixation by SR bacteria was verified by the molecular analysis of nifH genes. Detected nifH sequences clustered with SR bacteria that have been demonstrated to fix N2 in other benthic environments. Depth-integrated rates of N2 fixation and SR showed no direct correlation along the 12° S transect, suggesting that the benthic diazotrophs in the Peruvian OMZ are being controlled by additional various environmental factors. The organic matter availability and the presence of sulfide appear to be major drivers for benthic diazotrophy. It was further found that N2 fixation was not inhibited by high ammonium concentrations. N2 fixation rates in OMZ sediments were similar to rates measured in other organic-rich sediments. Overall, this work improves our knowledge on N sources in marine sediments and contributes to a better understanding of N cycling in OMZ sediments.
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Messer, Lauren F., Mark V. Brown, Paul D. Van Ruth, Mark Doubell, and Justin R. Seymour. "Temperate southern Australian coastal waters are characterised by surprisingly high rates of nitrogen fixation and diversity of diazotrophs." PeerJ 9 (March 1, 2021): e10809. http://dx.doi.org/10.7717/peerj.10809.
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Biological dinitrogen (N2) fixation is one mechanism by which specific microorganisms (diazotrophs) can ameliorate nitrogen (N) limitation. Historically, rates of N2 fixation were believed to be limited outside of the low nutrient tropical and subtropical open ocean; however, emerging evidence suggests that N2 fixation is also a significant process within temperate coastal waters. Using a combination of amplicon sequencing, targeting the nitrogenase reductase gene (nifH), quantitative nifH PCR, and 15N2 stable isotope tracer experiments, we investigated spatial patterns of diazotroph assemblage structure and N2 fixation rates within the temperate coastal waters of southern Australia during Austral autumn and summer. Relative to previous studies in open ocean environments, including tropical northern Australia, and tropical and temperate estuaries, our results indicate that high rates of N2 fixation (10–64 nmol L−1 d−1) can occur within the large inverse estuary Spencer Gulf, while comparatively low rates of N2 fixation (2 nmol L−1 d−1) were observed in the adjacent continental shelf waters. Across the dataset, low concentrations of NO3/NO2 were significantly correlated with the highest N2 fixation rates, suggesting that N2 fixation could be an important source of new N in the region as dissolved inorganic N concentrations are typically limiting. Overall, the underlying diazotrophic community was dominated by nifH sequences from Cluster 1 unicellular cyanobacteria of the UCYN-A clade, as well as non-cyanobacterial diazotrophs related to Pseudomonas stutzeri, and Cluster 3 sulfate-reducing deltaproteobacteria. Diazotroph community composition was significantly influenced by salinity and SiO4 concentrations, reflecting the transition from UCYN-A-dominated assemblages in the continental shelf waters, to Cluster 3-dominated assemblages in the hypersaline waters of the inverse estuary. Diverse, transitional diazotrophic communities, comprised of a mixture of UCYN-A and putative heterotrophic bacteria, were observed at the mouth and southern edge of Spencer Gulf, where the highest N2 fixation rates were observed. In contrast to observations in other environments, no seasonal patterns in N2 fixation rates and diazotroph community structure were apparent. Collectively, our findings are consistent with the emerging view that N2 fixation within temperate coastal waters is a previously overlooked dynamic and potentially important component of the marine N cycle.
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Shirtliffe, Steven J., J. Kevin Vessey, B. R. Buttery, and S. J. Park. "Comparison of growth and N accumulation of common bean (Phaseolus vulgaris L.) cv. OAC Rico and its two nodulation mutants, R69 and R99." Canadian Journal of Plant Science 76, no. 1 (January 1, 1996): 73–83. http://dx.doi.org/10.4141/cjps96-012.
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Nodulation mutants are present in many species of legumes and have potential as ideal reference crops in field measurements of N2 fixation. The objective of this experiment was to characterize the growth, development, mineral-N accumulation, and N2 fixation of two nodulation mutants of common bean (Phaseolus vulgaris L.), R69 (nod+/fix−) and R99 (nod−) in comparison with the wild type parent OAC Rico. The mutants were incapable of N2 fixation. With dual dependence on N2 fixation and [Formula: see text] as N sources, OAC Rico accumulated more N than R69 and R99. However, when supplied with mineral [Formula: see text] sufficient to prevent N2 fixation in OAC Rico, its growth and N accumulation did not differ from those of the mutants. There were differences in internode length that resulted in different heights among the three lines. The addition of silver to the nutrient solution failed to restore nodulation to R99. As a result, it was determined that the nod− phenotype is not a result of an ethylene-related mutation. The nodulation mutants R69 and R99 appear to be good choices as reference crops for common bean in N2 fixation studies. Key words: Common bean, N accumulation, N2 fixation, nodulation mutants, Phaseolus vulgaris, Rhizobium leguminosarum bv. phaseoli
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Knapp, Angela N., Kelly M. McCabe, Olivier Grosso, Nathalie Leblond, Thierry Moutin, and Sophie Bonnet. "Distribution and rates of nitrogen fixation in the western tropical South Pacific Ocean constrained by nitrogen isotope budgets." Biogeosciences 15, no. 9 (May 4, 2018): 2619–28. http://dx.doi.org/10.5194/bg-15-2619-2018.
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Abstract. Constraining the rates and spatial distribution of dinitrogen (N2) fixation fluxes to the ocean informs our understanding of the environmental sensitivities of N2 fixation as well as the timescale over which the fluxes of nitrogen (N) to and from the ocean may respond to each other. Here we quantify rates of N2 fixation as well as its contribution to export production along a zonal transect in the western tropical South Pacific (WTSP) Ocean using N isotope (“δ15N”) budgets. Comparing measurements of water column nitrate + nitrite δ15N with the δ15N of sinking particulate N at a western, central, and eastern station, these δ15N budgets indicate high, modest, and low rates of N2 fixation at the respective stations. The results also imply that N2 fixation supports exceptionally high, i.e. ≥ 50 %, of export production at the western and central stations, which are also proximal to the largest iron sources. These geochemically based rates of N2 fixation are equal to or greater than those previously reported in the tropical North Atlantic, indicating that the WTSP Ocean has the capacity to support globally significant rates of N2 fixation, which may compensate for N removal in the oxygen-deficient zones of the eastern tropical Pacific.
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Löscher, C. R., A. Bourbonnais, J. Dekaezemacker, C. N. Charoenpong, M. A. Altabet, H. W. Bange, R. Czeschel, C. Hoffmann, and R. A. Schmitz. "N<sub>2</sub> fixation in eddies of the eastern tropical South Pacific Ocean." Biogeosciences Discussions 12, no. 22 (November 27, 2015): 18945–72. http://dx.doi.org/10.5194/bgd-12-18945-2015.
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Abstract. Mesoscale eddies play a major role in controlling ocean biogeochemistry. By impacting nutrient availability and water column ventilation, they are of critical importance for oceanic primary production. In the eastern tropical South Pacific Ocean off Peru, where a large and persistent oxygen deficient zone is present, mesoscale processes have been reported to occur frequently. However, investigations on their biological activity are mostly based on model simulations, and direct measurements of carbon and dinitrogen (N2) fixation are scarce. We examined an open ocean cyclonic eddy and two anticyclonic mode water eddies: a coastal one and an open ocean one in the waters off Peru along a section at 16° S in austral summer 2012. Molecular data and bioassay incubations point towards a difference between the active diazotrophic communities present in the cyclonic eddy and the anticyclonic mode water eddies. In the cyclonic eddy, highest rates of N2 fixation were measured in surface waters but no N2 fixation signal was detected at intermediate water depths. In contrast, both anticyclonic mode water eddies showed pronounced maxima in N2 fixation below the euphotic zone as evidenced by rate measurements and geochemical data. N2 fixation and carbon (C) fixation were higher in the young coastal mode water eddy compared to the older offshore mode water eddy. A co-occurrence between N2 fixation and biogenic N2, an indicator for N loss, indicated a link between N loss and N2 fixation in the mode water eddies, which was not observed for the cyclonic eddy. The comparison of two consecutive surveys of the coastal mode water eddy in November and December 2012 revealed also a reduction of N2 and C fixation at intermediate depths along with a reduction in chlorophyll by half, mirroring an aging effect in this eddy. Our data indicate an important role for anticyclonic mode water eddies in stimulating N2 fixation and thus supplying N offshore.
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Garcias-Bonet, Neus, Raquel Vaquer-Sunyer, Carlos M. Duarte, and Núria Marbà. "Warming effect on nitrogen fixation in Mediterranean macrophyte sediments." Biogeosciences 16, no. 1 (January 17, 2019): 167–75. http://dx.doi.org/10.5194/bg-16-167-2019.
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Abstract. The Mediterranean Sea is warming faster than the global ocean, with important consequences for organisms and biogeochemical cycles. Warming is a major stressor for key marine benthic macrophytes. However, the effect of warming on marine N2 fixation remains unknown, despite the fact that the high productivity of macrophytes in oligotrophic waters is partially sustained by the input of new nitrogen (N) into the system by N2 fixation. Here, we assess the impact of warming on the N2 fixation rates of three key marine macrophytes: Posidonia oceanica, Cymodocea nodosa, and Caulerpa prolifera. We experimentally measured N2 fixation rates in vegetated and bare sediments at temperatures encompassing current summer mean (25 and 27 ∘C), projected summer mean (29 and 31 ∘C), and projected summer maximum (33 ∘C) seawater surface temperatures (SSTs) by the end of the century under a scenario of moderate greenhouse gas emissions. We found that N2 fixation rates in vegetated sediments were 2.8-fold higher than in bare sediments at current summer mean SST, with no differences among macrophytes. Currently, the contribution of N2 fixation to macrophyte productivity could account for up to 7 %, 13.8 %, and 1.8 % of N requirements for P. oceanica, C. nodosa, and C. prolifera, respectively. We show the temperature dependence of sediment N2 fixation rates. However, the thermal response differed for vegetated sediments, in which rates showed an optimum at 31 ∘C followed by a sharp decrease at 33 ∘C, and bare sediments, in which rates increased along the range of the experimental temperatures. The activation energy and Q10 were lower in vegetated than bare sediments, indicating the lower thermal sensitivity of vegetated sediments. The projected warming is expected to increase the contribution of N2 fixation to Mediterranean macrophyte productivity. Therefore, the thermal dependence of N2 fixation might have important consequences for primary production in coastal ecosystems in the context of warming.
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Löscher, Carolin R., Wiebke Mohr, Hermann W. Bange, and Donald E. Canfield. "No nitrogen fixation in the Bay of Bengal?" Biogeosciences 17, no. 4 (February 18, 2020): 851–64. http://dx.doi.org/10.5194/bg-17-851-2020.
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Abstract. The Bay of Bengal (BoB) has long stood as a biogeochemical enigma, with subsurface waters containing extremely low, but persistent, concentrations of oxygen in the nanomolar range which – for some, yet unconstrained, reason – are prevented from becoming anoxic. One reason for this may be the low productivity of the BoB waters due to nutrient limitation and the resulting lack of respiration of organic material at intermediate waters. Thus, the parameters determining primary production are key in understanding what prevents the BoB from developing anoxia. Primary productivity in the sunlit surface layers of tropical oceans is mostly limited by the supply of reactive nitrogen through upwelling, riverine flux, atmospheric deposition, and biological dinitrogen (N2) fixation. In the BoB, a stable stratification limits nutrient supply via upwelling in the open waters, and riverine or atmospheric fluxes have been shown to support only less than one-quarter of the nitrogen for primary production. This leaves a large uncertainty for most of the BoB's nitrogen input, suggesting a potential role of N2 fixation in those waters. Here, we present a survey of N2 fixation and carbon fixation in the BoB during the winter monsoon season. We detected a community of N2 fixers comparable to other oxygen minimum zone (OMZ) regions, with only a few cyanobacterial clades and a broad diversity of non-phototrophic N2 fixers present throughout the water column (samples collected between 10 and 560 m water depth). While similar communities of N2 fixers were shown to actively fix N2 in other OMZs, N2 fixation rates were below the detection limit in our samples covering the water column between the deep chlorophyll maximum and the OMZ. Consistent with this, no N2 fixation signal was visible in δ15N signatures. We suggest that the absence of N2 fixation may be a consequence of a micronutrient limitation or of an O2 sensitivity of the OMZ diazotrophs in the BoB. Exploring how the onset of N2 fixation by cyanobacteria compared to non-phototrophic N2 fixers would impact on OMZ O2 concentrations, a simple model exercise was carried out. We observed that both photic-zone-based and OMZ-based N2 fixation are very sensitive to even minimal changes in water column stratification, with stronger mixing increasing organic matter production and export, which can exhaust remaining O2 traces in the BoB.
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Fernández, A., B. Mouriño-Carballido, A. Bode, M. Varela, and E. Marañón. "Latitudinal distribution of <i>Trichodesmium</i> spp. and N<sub>2</sub> fixation in the Atlantic Ocean." Biogeosciences 7, no. 10 (October 15, 2010): 3167–76. http://dx.doi.org/10.5194/bg-7-3167-2010.
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Abstract. We have determined the latitudinal distribution of Trichodesmium spp. abundance and community N2 fixation in the Atlantic Ocean along a meridional transect from ca. 30° N to 30° S in November–December 2007 and April–May 2008. The observations from both cruises were highly consistent in terms of absolute magnitude and latitudinal distribution, showing a strong association between Trichodesmium abundance and community N2 fixation. The highest Trichodesmium abundances (mean = 220 trichomes L−1,) and community N2 fixation rates (mean = 60 μmol m−2 d−1) occurred in the Equatorial region between 5° S–15° N. In the South Atlantic gyre, Trichodesmium abundance was very low (ca. 1 trichome L−1) but N2 fixation was always measurable, averaging 3 and 10 μmol m2 d−1 in 2007 and 2008, respectively. We suggest that N2 fixation in the South Atlantic was sustained by other, presumably unicellular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale latitudinal variability of Trichodesmium spp. abundance and N2 fixation in the Atlantic Ocean. We observed a marked South to North decrease in surface phosphate concentration, which argues against a role for phosphorus availability in controlling the large scale distribution of N2 fixation. Scaling up from all our measurements (42 stations) results in conservative estimates for total N2 fixation of ∼6 TgN yr−1 in the North Atlantic (0–40° N) and ~1.2 TgN yr−1 in the South Atlantic (0–40° S).
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Fernández, A., B. Mouriño-Carballido, A. Bode, M. Varela, and E. Marañón. "Latitudinal distribution of <i>Trichodesmium</i> spp. and N<sub>2</sub> fixation in the Atlantic Ocean." Biogeosciences Discussions 7, no. 2 (March 26, 2010): 2195–225. http://dx.doi.org/10.5194/bgd-7-2195-2010.
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Abstract. We have determined the latitudinal distribution of Trichodesmium spp. abundance and community N2 fixation in the Atlantic Ocean along a meridional transect from ca. 30° N to 30° S in November–December 2007 and April–May 2008. The observations from both cruises were highly consistent in terms of absolute magnitude and latitudinal distribution, showing a strong association between Trichodesmium abundance and community N2 fixation. The highest Trichodesmium abundances (mean = 220 trichomes L−1) and community N2 fixation rates (mean = 60 μmol m−2 d−1) occurred in the Equatorial region between 5° S–15° N. In the South Atlantic gyre, Trichodesmium abundance was very low (ca. 1 trichome L−1) but N2 fixation was always measurable, averaging 3 and 10 μmol m2 d−1 in 2007 and 2008, respectively. We suggest that N2 fixation in the South Atlantic was sustained by other, presumably unicellular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale latitudinal variability of Trichodesmium spp. abundance and N2 fixation in the Atlantic Ocean. We observed a marked South to North decrease in surface phosphate concentration, which argues against a role for phosphorus availability in controlling the large scale distribution of N2 fixation. Scaling up from all our measurements (42 stations) results in conservative estimates for total N2 fixation of ~6 TgN yr−1 in the North Atlantic (0–40° N) and 1.2 TgN yr−1 in the South Atlantic (0–40° S).
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Montoya, J. P., M. Voss, and D. G. Capone. "Spatial variation in N<sub>2</sub>-fixation rate and diazotroph activity in the Tropical Atlantic." Biogeosciences 4, no. 3 (June 22, 2007): 369–76. http://dx.doi.org/10.5194/bg-4-369-2007.
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Abstract. A variety of N2-fixers occur in oligotrophic waters and these diazotrophs make a substantial contribution to the nitrogen budget of the upper water column. A synthesis of previously published and new rate measurements for the North Atlantic provides insight into the role of two different groups of N2-fixers (Trichodesmium and small diazotrophs) in supporting N2 fixation in the tropical Atlantic. The highest rates of N2 fixation occurred in the western part of the basin, but the full data set showed no significant difference between the eastern and western parts of the basin in overall rates of N2-fixation by the two groups of diazotrophs. N2-fixation by Trichodesmium was strongly dominant in the western part of the basin while small diazotrophs played a much larger role to the east of 40° W. The reasons for this shift in dominance are unclear, as is the identity of the small organisms fixing N2 in the water column.
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Roper, M. M., and V. V. S. R. Gupta. "Enhancing Non-symbiotic N2 Fixation in Agriculture." Open Agriculture Journal 10, no. 1 (May 13, 2016): 7–27. http://dx.doi.org/10.2174/1874331501610010007.
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Much of the demand for nitrogen (N) in cereal cropping systems is met by using N fertilisers, but the cost of production is increasing and there are also environmental concerns. This has led to a growing interest in exploring other sources of N such as biological N2fixation. Non-symbiotic N2fixation (by free-living bacteria in soils or associated with the rhizosphere) has the potential to meet some of this need especially in the lower input cropping systems worldwide. There has been considerable research on non-symbiotic N2fixation, but still there is much argument about the amount of N that can potentially be fixed by this process largely due to shortcomings of indirect measurements, however isotope-based direct methods indicate agronomically significant amounts of N2fixation both in annual crop and perennial grass systems. New molecular technologies offer opportunities to increase our understanding of N2-fixing microbial communities (many of them non-culturable) and the molecular mechanisms of non-symbiotic N2fixation. This knowledge should assist the development of new plant-diazotrophic combinations for specific environments and more sustainable exploitation of N2-fixing bacteria as inoculants for agriculture. Whilst the ultimate goal might be to introduce nitrogenase genes into significant non-leguminous crop plants, it may be more realistic in the shorter-term to better synchronise plant-microbe interactions to enhance N2fixation when the N needs of the plant are greatest. The review explores possibilities to maximise potential N inputs from non-symbiotic N2fixation through improved management practices, identification of better performing microbial strains and their successful inoculation in the field, and plant based solutions.
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Shiau, Yo-Jin, Ming-Fen Lin, Chen-Chung Tan, Guanglong Tian, and Chih-Yu Chiu. "Assessing N2 fixation in estuarine mangrove soils." Estuarine, Coastal and Shelf Science 189 (April 2017): 84–89. http://dx.doi.org/10.1016/j.ecss.2017.03.005.
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Spence, D. W., and W. D. P. Stewart. "Proline inhibits N2-fixation in Anabaena 7120." Biochemical and Biophysical Research Communications 139, no. 3 (September 1986): 940–46. http://dx.doi.org/10.1016/s0006-291x(86)80268-6.
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Walsh, Kerry B., J. Kevin Vessey, and David B. Layzell. "Carbohydrate Supply and N2 Fixation in Soybean." Plant Physiology 85, no. 1 (September 1, 1987): 137–44. http://dx.doi.org/10.1104/pp.85.1.137.
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Kyei-Boahen, S., A. E. Slinkard, and F. L. Walley. "Isotopic fractionation during N2 fixation by chickpea." Soil Biology and Biochemistry 34, no. 3 (March 2002): 417–20. http://dx.doi.org/10.1016/s0038-0717(01)00188-2.
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Miller, J. C., K. W. Zary, and G. C. J. Fernandez. "Inheritance of N2 fixation efficiency in cowpea." Euphytica 35, no. 2 (June 1986): 551–60. http://dx.doi.org/10.1007/bf00021864.
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Marron, Nicolas, Cécilia Gana, Dominique Gérant, Pascale Maillard, Pierrick Priault, and Daniel Epron. "Estimating symbiotic N2 fixation in Robinia pseudoacacia." Journal of Plant Nutrition and Soil Science 181, no. 2 (February 1, 2018): 296–304. http://dx.doi.org/10.1002/jpln.201700503.
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Gallon, J. R. "Reconciling the incompatible: N2 fixation And O2." New Phytologist 122, no. 4 (April 28, 2006): 571–609. http://dx.doi.org/10.1111/j.1469-8137.1992.tb00087.x.
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Hendzel, L. L., R. E. Hecky, and D. L. Findlay. "Recent Changes of N2-Fixation in Lake 227 in Response to Reduction of the N:P Loading Ratio." Canadian Journal of Fisheries and Aquatic Sciences 51, no. 10 (October 1, 1994): 2247–53. http://dx.doi.org/10.1139/f94-228.
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Continuous artificial additions of nitrogen (N) and phosphorus (P) to Lake 227 at a loading ratio of 11:1 (atom: atom; 5:1 by mass) from 1975 to 1989, combined with natural inputs, resulted in variable mean annual total N and total P concentrations and changes in phytoplankton species composition. Summer phytoplankton occasionally shifted from N2-fixing cyanobacteria to non-N2-fixing forms. In 1990, additions of N fertilizer were stopped in order to test the hypothesis that low N:P loading rates cause selection for heterocystous cyanobacteria and stimulate N2-fixation. Five years of phytoplankton nutrient-status measurements, together with estimates of N2-fixation before and after lowering the N:P loading ratio, verified the importance of this ratio in producing blooms of N2-fixing cyanobacteria. Nitrogen input via biological N2 fixation increased, and summer dominance by heterocystous cyanobacteria returned as a consequence of lowering the N:P ratio of external inputs. Seasonally, N return from the epilimnetic sediments increased after a period of high planktonic N2-fixation rates. This regenerated N reduced the abundance of heterocystous cyanobacteria and lowered late summer N2-fixation rates. Over several years, preferential regeneration of N relative to P maintained the N:P ratio of the internally cycling nutrient pool, despite the reduction in the external N:P loading ratio.
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Kuhn, Angela M., Katja Fennel, and Ilana Berman-Frank. "Modelling the biogeochemical effects of heterotrophic and autotrophic N<sub>2</sub> fixation in the Gulf of Aqaba (Israel), Red Sea." Biogeosciences 15, no. 24 (December 14, 2018): 7379–401. http://dx.doi.org/10.5194/bg-15-7379-2018.
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Abstract. Recent studies demonstrate that marine N2 fixation can be carried out without light by heterotrophic N2 fixers (diazotrophs). However, direct measurements of N2 fixation in aphotic environments are relatively scarce. Heterotrophic as well as unicellular and colonial photoautotrophic diazotrophs are present in the oligotrophic Gulf of Aqaba (northern Red Sea). This study evaluates the relative importance of these different diazotrophs by combining biogeochemical models with time series measurements at a 700 m deep monitoring station in the Gulf of Aqaba. At this location, an excess of nitrate, relative to phosphate, is present throughout most of the water column and especially in deep waters during stratified conditions. A relative excess of phosphate occurs only at the water surface during nutrient-starved conditions in summer. We show that a model without N2 fixation can replicate the observed surface chlorophyll but fails to accurately simulate inorganic nutrient concentrations throughout the water column. Models with N2 fixation improve simulated deep nitrate by enriching sinking organic matter in nitrogen, suggesting that N2 fixation is necessary to explain the observations. The observed vertical structure of nutrient ratios and oxygen is reproduced best with a model that includes heterotrophic as well as colonial and unicellular autotrophic diazotrophs. These results suggest that heterotrophic N2 fixation contributes to the observed excess nitrogen in deep water at this location. If heterotrophic diazotrophs are generally present in oligotrophic ocean regions, their consideration would increase current estimates of global N2 fixation and may require explicit representation in large-scale models.
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Garcias-Bonet, Neus, Marco Fusi, Muhammad Ali, Dario R. Shaw, Pascal E. Saikaly, Daniele Daffonchio, and Carlos M. Duarte. "High denitrification and anaerobic ammonium oxidation contributes to net nitrogen loss in a seagrass ecosystem in the central Red Sea." Biogeosciences 15, no. 23 (December 11, 2018): 7333–46. http://dx.doi.org/10.5194/bg-15-7333-2018.
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Abstract. Nitrogen loads in coastal areas have increased dramatically, with detrimental consequences for coastal ecosystems. Shallow sediments and seagrass meadows are hotspots for denitrification, favoring N loss. However, atmospheric dinitrogen (N2) fixation has been reported to support seagrass growth. Therefore, the role of coastal marine systems dominated by seagrasses in the net N2 flux remains unclear. Here, we measured denitrification, anaerobic ammonium oxidation (anammox), and N2 fixation in a tropical seagrass (Enhalus acoroides) meadow and the adjacent bare sediment in a coastal lagoon in the central Red Sea. We detected high annual mean rates of denitrification (34.9±10.3 and 31.6±8.9 mg N m−2 d−1) and anammox (12.4±3.4 and 19.8±4.4 mg N m−2 d−1) in vegetated and bare sediments. The annual mean N loss was higher (between 8 and 63-fold) than the N2 fixed (annual mean = 5.9±0.2 and 0.8±0.3 mg N m−2 d−1) in the meadow and bare sediment, leading to a net flux of N2 from sediments to the atmosphere. Despite the importance of this coastal lagoon in removing N from the system, N2 fixation can contribute substantially to seagrass growth since N2 fixation rates found here could contribute up to 36 % of plant N requirements. In vegetated sediments, anammox rates decreased with increasing organic matter (OM) content, while N2 fixation increased with OM content. Denitrification and anammox increased linearly with temperature, while N2 fixation showed a maximum at intermediate temperatures. Therefore, the forecasted warming could further increase the N2 flux from sediments to the atmosphere, potentially impacting seagrass productivity and their capacity to mitigate climate change but also enhancing their potential N removal.
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Gier, Jessica, Stefan Sommer, Carolin R. Löscher, Andrew W. Dale, Ruth A. Schmitz, and Tina Treude. "Nitrogen fixation in sediments along a depth transect through the Peruvian oxygen minimum zone." Biogeosciences 13, no. 14 (July 18, 2016): 4065–80. http://dx.doi.org/10.5194/bg-13-4065-2016.
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Abstract. The potential coupling of nitrogen (N2) fixation and sulfate reduction (SR) was explored in sediments of the Peruvian oxygen minimum zone (OMZ). Sediment samples were retrieved by a multiple corer at six stations along a depth transect (70–1025 m water depth) at 12° S, covering anoxic and hypoxic bottom water conditions. Benthic N2 fixation, determined by the acetylene reduction assay, was detected at all sites, with highest rates between 70 and 253 m and lower rates at greater depth. SR rates decreased with increasing water depth. N2 fixation and SR overlapped in sediments, suggesting a potential coupling of both processes. However, a weak positive correlation of their activity distribution was detected by principle component analysis. A potential link between N2 fixation and sulfate-reducing bacteria was indicated by the molecular analysis of nifH genes. Detected nifH sequences clustered with the sulfate-reducing bacteria Desulfonema limicola at the 253 m station. However, nifH sequences of other stations clustered with uncultured organisms, Gammaproteobacteria, and Firmicutes (Clostridia) rather than with known sulfate reducers. The principle component analysis revealed that benthic N2 fixation in the Peruvian OMZ is controlled by organic matter (positive) and free sulfide (negative). No correlation was found between N2 fixation and ammonium concentrations (even at levels > 2022 µM). N2 fixation rates in the Peruvian OMZ sediments were in the same range as those measured in other organic-rich sediments.
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An, Soonmo, Wayne S. Gardner, and Todd Kana. "Simultaneous Measurement of Denitrification and Nitrogen Fixation Using Isotope Pairing with Membrane Inlet Mass Spectrometry Analysis." Applied and Environmental Microbiology 67, no. 3 (March 1, 2001): 1171–78. http://dx.doi.org/10.1128/aem.67.3.1171-1178.2001.
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ABSTRACT A method for estimating denitrification and nitrogen fixation simultaneously in coastal sediments was developed. An isotope-pairing technique was applied to dissolved gas measurements with a membrane inlet mass spectrometer (MIMS). The relative fluxes of three N2 gas species (28N2,29N2, and 30N2) were monitored during incubation experiments after the addition of15NO3 −. Formulas were developed to estimate the production (denitrification) and consumption (N2 fixation) of N2 gas from the fluxes of the different isotopic forms of N2. Proportions of the three isotopic forms produced from15NO3 − and14NO3 − agreed with expectations in a sediment slurry incubation experiment designed to optimize conditions for denitrification. Nitrogen fixation rates from an algal mat measured with intact sediment cores ranged from 32 to 390 μg-atoms of N m−2 h−1. They were enhanced by light and organic matter enrichment. In this environment of high nitrogen fixation, low N2 production rates due to denitrification could be separated from high N2 consumption rates due to nitrogen fixation. Denitrification and nitrogen fixation rates were estimated in April 2000 on sediments from a Texas sea grass bed (Laguna Madre). Denitrification rates (average, 20 μg-atoms of N m−2 h−1) were lower than nitrogen fixation rates (average, 60 μg-atoms of N m−2 h−1). The developed method benefits from simple and accurate dissolved-gas measurement by the MIMS system. By adding the N2 isotope capability, it was possible to do isotope-pairing experiments with the MIMS system.
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Mills, Matthew M., Kendra A. Turk-Kubo, Gert L. van Dijken, Britt A. Henke, Katie Harding, Samuel T. Wilson, Kevin R. Arrigo, and Jonathan P. Zehr. "Unusual marine cyanobacteria/haptophyte symbiosis relies on N2 fixation even in N-rich environments." ISME Journal 14, no. 10 (June 10, 2020): 2395–406. http://dx.doi.org/10.1038/s41396-020-0691-6.
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Abstract The microbial fixation of N2 is the largest source of biologically available nitrogen (N) to the oceans. However, it is the most energetically expensive N-acquisition process and is believed inhibited when less energetically expensive forms, like dissolved inorganic N (DIN), are available. Curiously, the cosmopolitan N2-fixing UCYN-A/haptophyte symbiosis grows in DIN-replete waters, but the sensitivity of their N2 fixation to DIN is unknown. We used stable isotope incubations, catalyzed reporter deposition fluorescence in-situ hybridization (CARD-FISH), and nanoscale secondary ion mass spectrometry (nanoSIMS), to investigate the N source used by the haptophyte host and sensitivity of UCYN-A N2 fixation in DIN-replete waters. We demonstrate that under our experimental conditions, the haptophyte hosts of two UCYN-A sublineages do not assimilate nitrate (NO3−) and meet little of their N demands via ammonium (NH4+) uptake. Instead the UCYN-A/haptophyte symbiosis relies on UCYN-A N2 fixation to supply large portions of the haptophyte’s N requirements, even under DIN-replete conditions. Furthermore, UCYN-A N2 fixation rates, and haptophyte host carbon fixation rates, were at times stimulated by NO3− additions in N-limited waters suggesting a link between the activities of the bulk phytoplankton assemblage and the UCYN-A/haptophyte symbiosis. The results suggest N2 fixation may be an evolutionarily viable strategy for diazotroph–eukaryote symbioses, even in N-rich coastal or high latitude waters.
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Gao, Xiang, Li An, Dan Qu, Wenshuai Jiang, Yanxiao Chai, Shaorui Sun, Xingyuan Liu, and Zaicheng Sun. "Enhanced photocatalytic N2 fixation by promoting N2 adsorption with a co-catalyst." Science Bulletin 64, no. 13 (July 2019): 918–25. http://dx.doi.org/10.1016/j.scib.2019.05.009.
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