SFS Annual Meeting

6/04/2024  |   13:30 - 13:45   |  Salon 10

THE AQUATIC NITROGEN FIXATION RESEARCH COORDINATION NETWORK: GOALS, PLANS AND PROGRESS The Aquatic N2 Fixation Research Coordination Network (ANF RCN) was established in 2021 to cultivate a new paradigm of the fundamental, yet understudied, role of N2 fixation in ecosystems across the freshwater to marine continuum. The ANF RCN is addressing three research coordination opportunities (RCOs) via the formation of working groups that work over 24 months to set and conduct research tasks, participate in a workshop, and produce paper and data products. RCO1 synthesized the current knowledge of the rates and biodiversity of diazotrophs (organisms that fix N2) for ecosystems along the freshwater-marine continuum. The RCO1 workshop was held in June 2022, and produced a data synthesis of rates from the published literature, and a synthesis of diazotroph biodiversity from archived nifH amplicon and metagenome data. The goal of RCO2 was to characterize the variable stoichiometries of diazotrophs with different metabolic strategies. The RCO2 workshop was held in October 2023 and produced mathematical models describing the stoichiometry of photosynthetic and organo-heterotrophic diazotrophs, and a synthetic overview of the state of knowledge of chemotrophic diazotrophs. RCO3 will develop a set of common mathematical and statistical tools enabling the upscaling of N2 fixation for comparison within and among diverse aquatic ecosystems and in regional and global N budgets, with a workshop planned for March 2025. The ANF RCN invites applicants for RCO3 participation, and also provides opportunities for the entire community of researchers studying nitrogen fixation across the aquascape to share research via networking, focused publications and special conference sessions.

Amy Marcarelli (Primary Presenter/Author), Michigan Technological University, ammarcar@mtu.edu;

Robinson Fulweiler (Co-Presenter/Co-Author), Boston University, rwf@bu.edu;

Thad Scott (Co-Presenter/Co-Author), Baylor University, Thad_Scott@baylor.edu ;

Megan Berberich (Co-Presenter/Co-Author), Michigan Technological University, meberer@mtu.edu;

Julian Damashek (Co-Presenter/Co-Author), Hamilton College, jdamashe@hamilton.edu;

Jason Taylor (Co-Presenter/Co-Author), USDA ARS National Sedimentation Laboratory, jason.taylor@usda.gov;

Peter Groffman (Co-Presenter/Co-Author), City University of New York, Peter.Groffman@asrc.cuny.edu ;

Halvor Halvorson (Co-Presenter/Co-Author), University of Central Arkansas, hhalvorson@uca.edu;

Angela Knapp (Co-Presenter/Co-Author), Florida State University, anknapp@fsu.edu;

Robert Sterner (Co-Presenter/Co-Author), University of Minnesota Duluth, stern007@d.umn.edu;

John Harrison (Co-Presenter/Co-Author), Washington State University, john_harrison@wsu.edu;

Emily Stanley (Co-Presenter/Co-Author), University of Wisconsin - Madison, ehstanley@wisc.edu;

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6/04/2024  |   13:45 - 14:00   |  Salon 10

CURRENT BARRIERS TO SIMULTANEOUS QUANTIFICATION OF N2 FIXATION AND DENITRIFICATION FROM OPEN-CHANNEL DIEL N2 FLUX (AND SOME IDEAS FOR BREAKING THEM) The contribution of dinitrogen (N2) fixation to stream nutrient budgets could be better quantified with whole-reach scale rate estimates. While the “opposite” of N2 fixation (denitrification) can be estimated from open-channel diel N2 flux models, this approach needs refinement to account for all the ways that N2 enters, exits, or is transformed in ecosystems. Therefore, we ask: how can we more accurately estimate N2 fixation and denitrification rates from diel N2 concentrations, either by better parameterization of model unknowns or by changes to the model structure? Using syntheses of denitrification and N2 fixation rates in streams, we investigate the degree of divergence between process rates estimated with published N2 flux model structures and those derived from other methods, such as 15N tracers or acetylene chambers. We review the N2 flux and transformation pathways that are not captured in current model structures, and review ways to (a) integrate them into the N2 flux model equation, and (b) estimate rates of these processes in rivers to better constrain Bayesian model priors. Using example data, we build on published N2 flux model equations, demonstrating the impact of changes to the model structure and Bayesian priors on the fit between measured and predicted N2 concentrations. Improved N2 flux models could provide a structure to measure N2 fixation and denitrification on the same temporal and spatial scale as other reach and ecosystem-scale process measurements (eg. metabolism, nutrient uptake), building a framework to understand controls on these processes across ecosystems.

Michelle Catherine Kelly (Primary Presenter/Author), Michigan Technological University, mckelly1@mtu.edu;

Megan Berberich (Co-Presenter/Co-Author), Michigan Technological University, meberer@mtu.edu;

Jason Taylor (Co-Presenter/Co-Author), USDA ARS National Sedimentation Laboratory, jason.taylor@usda.gov;

Amy Marcarelli (Co-Presenter/Co-Author), Michigan Technological University, ammarcar@mtu.edu;

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6/04/2024  |   14:00 - 14:15   |  Salon 10

ECOLOGICAL AND ENVIRONMENTAL INFLUENCES ON NITROGEN FIXATION EVOLUTION

Morgan Sobol (Primary Presenter/Author), , msobol@wisc.edu;

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6/04/2024  |   14:15 - 14:30   |  Salon 10

A GLOBAL SYNTHESIS OF AQUATIC DIAZOTROPH DISTRIBUTIONS AND METABOLIC DIVERSITY FROM INLAND FRESHWATERS TO THE COASTAL OCEAN

Julian Damashek (Primary Presenter/Author), Hamilton College, jdamashe@hamilton.edu;

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6/04/2024  |   14:30 - 14:45   |  Salon 10

RIVERINE NITROGEN FIXATION: AN UPDATED SYNTHESIS Dinitrogen (N2) fixation is a fundamental process that has historically been understudied in riverine ecosystems. In 2008, Marcarelli and colleagues published a synthesis of riverine N2 fixation rates from 22 streams across 9 studies. Over the last fifteen years, the number of publications that report riverine N2 fixation rates has increased with 30+ new studies that help fill this gap in research on stream nitrogen cycling. We present an updated synthesis of N2 fixation rates in streams and rivers with data derived from published literature and public data repositories, including a dataset that was generated by the Aquatic Nitrogen Fixation Research Coordination Network. This synthesis includes over 800 N2 fixation rates across >100 riverine sites. Median fixation rates are approximately 4 µg-N m-2 h-1, which is comparable to the median reported in the 2008 paper of 10 µg-N m-2 h-1. The updated dataset spans 4 continents and a wide range of environmental conditions, including nitrate concentrations ranging from 0.9 - 3600 µg-N L-1. Preliminary results show a negative relationship between nitrate availability and N2 fixation rates, while in 2008 the limited available data did not detect this relationship. For a subset of streams, we compare N2 fixation to denitrification and dissolved inorganic nitrogen uptake rates to evaluate the contribution of N2 fixation to other nitrogen cycle fluxes. This updated synthesis shows how 15 years of additional studies results in a more complete understanding of the ecological significance and environmental conditions affecting N2 fixation in streams.

Megan E. Berberich (Primary Presenter/Author), Michigan Technological University, meberber@mtu.edu ;

Michelle Kelly (Co-Presenter/Co-Author), Michigan Technological University, mckelly1@mtu.edu;

Robinson Fulweiler (Co-Presenter/Co-Author), Boston University, rwf@bu.edu;

Thad Scott (Co-Presenter/Co-Author), Baylor University, Thad_Scott@baylor.edu ;

Amy Marcarelli (Co-Presenter/Co-Author), Michigan Technological University, ammarcar@mtu.edu;

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6/04/2024  |   14:45 - 15:00   |  Salon 10

N-fixing trees as a source of nitrate for tropical streams High abundance of nitrogen fixing trees in tropical areas has been hypothesized to drive higher stream nitrate concentrations compared to temperate streams. However, few studies have examined annual variation in concentration and isotopes of nitrate in tropical watersheds. We examined the del15N and del18O in NO3 for one year in throughfall and stream water in three sites in the Salto watershed in La Selva Biological Station, Costa Rica. del15N of throughfall was more depleted (-4.15‰) than stream water (4.65 - 4.90‰), while there was little difference in del18O across all sites (2.05-2.99‰). Stream water del15N and del18O did not change from a first order stream site to a third order stream, even though the third order stream receives solute rich interbasin groundwater inputs. There were no differences in throughfall or stream del15N and del18O in dry vs wet season. Evaluation of the proportion of N lost via denitrification for this watershed using isotopic models suggest that N-fixation in tropical watersheds can sustain high N exports.

Marcelo Ardon (Primary Presenter/Author), North Carolina State University, mlardons@ncsu.edu;

Nicholas Marzolf (Co-Presenter/Co-Author), J.W. Jones Ecological Research Center, nmarzolf@jonesctr.org;

Alonso Ramirez (Co-Presenter/Co-Author), Department of Applied Ecology, North Carolina State University, alonso.ramirez@ncsu.edu;

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