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1. Nitrogen fixation rates in the Guinea Dome and the equatorial upwelling regions in the Atlantic Ocean

   https://doi.org/10.1007/s10533-023-01089-w  

   Fernández-Carrera, Ana ; Kiko, Rainer ; Hauss, Helena ; Hamilton, Douglas S. ; Achterberg, Eric P. ; Montoya, Joseph P. ; Dengler, Marcus ; Brandt, Peter ; Subramaniam, Ajit ( December 2023 , Biogeochemistry)

   Biological nitrogen fixation is a key process balancing the loss of combined nitrogen in the marine nitrogen cycle. Its relevance in upwelling or high nutrient regions is still unclear, with the few available studies in these regions of the ocean reporting rates that vary widely from below detection limit to > 100 nmol N L⁻¹ d⁻¹. In the eastern tropical Atlantic Ocean, two open ocean upwelling systems are active in boreal summer. One is the seasonal equatorial upwelling, where the residual phosphorus associated with aged upwelled waters is suggested to enhance nitrogen fixation in this season. The other is the Guinea Dome, a thermal upwelling dome. We conducted two surveys along 23° W across the Guinea Dome and the Equator from 15° N to 5° S in September 2015 and August–September 2016 with high latitudinal resolution (20–60 nm between stations). The abundance ofTrichodesmiumcolonies was characterized by an Underwater Vision Profiler 5 and the total biological nitrogen fixation in the euphotic layer was measured using the¹⁵N₂technique. The highest abundances ofTrichodesmiumcolonies were found in the area of the Guinea Dome (9°–15° N) with a maximum of 3 colonies L⁻¹near the surface. By contrast, colonies were almost absent in the Equatorial band between 2° N and 5° S. The highest nitrogen fixation rate was measured at the northern edge of the Guinea Dome in 2016 (ca. 31 nmol N L⁻¹ d⁻¹). In this region, where diazotrophs thrived on a sufficient supply of both phosphorus and iron, a patchy distribution was unveiled by our increased spatial resolution scheme. In the Equatorial band, rates were considerably lower, ranging from below detection limit to ca. 4 nmol N L⁻¹ d⁻¹, with a clear difference in magnitude between 2015 (rates close to zero) and 2016 (average rates around 2 nmol N L⁻¹ d⁻¹). This difference seemed triggered by a contrasting supply of phosphorus between years. Our study stresses the importance of surveys with sampling at fine-scale spatial resolution, and shows unexpected high variability in the rates of nitrogen fixation in the Guinea Dome, a region where diazotrophy is a significant process supplying new nitrogen into the euphotic layer.

    

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2. Quantifying N2 fixation and its contribution to export production near the Tonga-Kermadec Arc using nitrogen isotope budgets

   https://doi.org/10.3389/fmars.2023.1249115  

   Forrer, Heather J. ; Bonnet, Sophie ; Thomas, Rachel K. ; Grosso, Olivier ; Guieu, Cecile ; Knapp, Angela N. ( October 2023 , Frontiers in Marine Science)

   The spatial distribution of marine di-nitrogen (N₂) fixation informs our understanding of the sensitivities of this process as well as the potential for this new nitrogen (N) source to drive export production, influencing the global carbon (C) cycle and climate. Using geochemically-derived δ¹⁵N budgets, we quantified rates of N₂fixation and its importance for supporting export production at stations sampled near the southwest Pacific Tonga-Kermadec Arc. Recent observations indicate that shallow (<300 m) hydrothermal vents located along the arc provide significant dissolved iron to the euphotic zone, stimulating N₂fixation. Here we compare measurements of water column δ¹⁵N_NO3+NO2with sinking particulate δ¹⁵N collected by short-term sediment traps deployed at 170 m and 270 m at stations in close proximity to subsurface hydrothermal activity, and the δ¹⁵N of N₂fixation. Results from the δ¹⁵N budgets yield high geochemically-based N₂fixation rates (282 to 638 µmol N m^-2d^-1) at stations impacted by hydrothermal activity, supporting 64 to 92% of export production in late spring. These results are consistent with contemporaneous¹⁵N₂uptake rate estimates and molecular work describing highTrichodesmiumspp. and other diazotroph abundances associated with elevated N₂fixation rates. Further, the δ¹⁵N of sinking particulate N collected at 1000 m over an annual cycle revealed sinking fluxes peaked in the summer and coincided with the lowest δ¹⁵N, while lower winter sinking fluxes had the highest δ¹⁵N, indicating isotopically distinct N sources supporting export seasonally, and aligning with observations from most other δ¹⁵N budgets in oligotrophic regions. Consequently, the significant regional N₂fixation input to the late spring/summer Western Tropical South Pacific results in the accumulation of low-δ¹⁵N_NO3+NO2in the upper thermocline that works to lower the elevated δ¹⁵N_NO3+NO2generated in the oxygen deficient zones in the Eastern Tropical South Pacific.

    

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3. A coastal N2 fixation hotspot at the Cape Hatteras front: Elucidating spatial heterogeneity in diazotroph activity via supervised machine learning

   https://doi.org/10.1002/lno.11727  

   Selden, Corday R. ; Chappell, P. Dreux ; Clayton, Sophie ; Macías‐Tapia, Alfonso ; Bernhardt, Peter W. ; Mulholland, Margaret R. ( April 2021 , Limnology and Oceanography)

   In the North Atlantic Ocean, dinitrogen (N₂) fixation on the western continental shelf represents a significant fraction of basin‐wide nitrogen (N) inputs. However, the factors regulating coastal N₂fixation remain poorly understood, in part due to sharp physico‐chemical gradients and dynamic water mass interactions that are difficult to constrain via traditional oceanographic approaches. This study sought to characterize the spatial heterogeneity of N₂fixation on the western North Atlantic shelf, at the confluence of Mid‐ and South Atlantic Bight shelf waters and the Gulf Stream, in August 2016. Rates were quantified using the¹⁵N₂bubble release method and used to build empirical models of regional N₂fixation via a random forest machine learning approach. N₂fixation rates were then predicted from high‐resolution CTD and satellite data to infer the variability of its depth and surface distributions, respectively. Our findings suggest that the frontal mixing zone created conditions conducive to exceptionally high N₂fixation rates (> 100 nmol N L⁻¹d⁻¹), which were likely driven by the haptophyte‐symbiont UCYN‐A. Above and below this hotspot, N₂fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N₂uptake rates, a biomass‐independent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N₂fixation is favored offshore but occurs continuously across the shelf. Nevertheless, our model results indicate that there is a niche for diazotrophs along the coastline as phytoplankton populations begin to decline, likely due to exhaustion of coastal nutrients.

    

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4. Fire Affects Asymbiotic Nitrogen Fixation in Southern Amazon Forests

   https://doi.org/10.1029/2019JG005383  

   Bomfim, Barbara ; Silva, Lucas C. R. ; Marimon‐Júnior, Ben H. ; Marimon, Beatriz S. ; Doane, Timothy A. ; Horwath, William R. ( February 2020 , Journal of Geophysical Research: Biogeosciences)

   In this study, we investigate the biogeochemical consequences of fire in seasonally flooded Amazon forests, where recent declines in forest cover have been linked to increases in fire frequency and severity. Previous studies have hypothesized that a quasi‐permanent state‐shift transition from typical Amazon forests to open savannas can occur when fire results in further depletion of already impoverished soil nutrient pools. Asymbiotic N₂fixation (ANF) is an essential pathway for fire‐affected forests to acquire nitrogen (N) after disturbance, but ANF response to fire has yet to be quantified in Amazonia. Here, we quantify ANF through field sampling and laboratory incubations using¹⁵N‐labeled dinitrogen (¹⁵N₂) and measurement of 14 biogeochemical parameters in surface (0–10 cm) and subsurface (10–30 cm) soils. Our data represent burned and unburned replicated sampling sites, across five stands, spanning a gradient from infrequent (once in 13 years) to frequent (five times in 13 years) fire occurrences. ANF did not vary with fire frequency but was, on average, 24% lower in burned than in unburned surface soils across all stands. Burned and unburned subsurface soils had similar ANF rates. About 58% of ANF variance was explained by the joint effect of carbon (C):N ratio and available phosphorus (P) in burned and unburned soils. ANF increased linearly with C:N and P availability in unburned soils, but a highly non‐linear relationship was observed in burned soils. Our findings show that fire alters soil C‐to‐nutrient stoichiometry, which resulted in lower N inputs via ANF into burned relative to unburned tropical forest soils.

    

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5. Toward resolving disparate accounts of the extent and magnitude of nitrogen fixation in the Eastern Tropical South Pacific oxygen deficient zone

   https://doi.org/10.1002/lno.11735  

   Selden, Corday R. ; Mulholland, Margaret R. ; Widner, Brittany ; Bernhardt, Peter ; Jayakumar, Amal ( April 2021 , Limnology and Oceanography)

   Examination of dinitrogen (N₂) fixation in the Eastern Tropical South Pacific oxygen deficient zone has raised questions about the range of diazotrophs in the deep sea and their quantitative importance as a source of new nitrogen globally. However, technical considerations in the deployment of stable isotopes in quantifying N₂fixation rates have complicated interpretation of this research. Here, we report the findings of a comprehensive survey of N₂fixation within, above and below the Eastern Tropical South Pacific oxygen deficient zone. N₂fixation rates were measured using a robust¹⁵N tracer method (bubble removal) that accounts for the slow dissolution of N₂gas and calculated using a conservative approach. N₂fixation was only detected in a subset of samples (8 of 125 replicated measurements) collected within suboxic waters (< 20 μmol O₂kg⁻¹) or at the oxycline. Most of these detectable rates were measured at nearshore stations, or where surface productivity was high. These findings support the hypothesis that low oxygen/high organic carbon conditions favor non‐cyanobacterial diazotrophs. Nevertheless, this study indicates that N₂fixation is neither widespread nor quantitatively important throughout this region.

    

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