title: Direct Raman Spectroscopic Measurements of Biological Nitrogen Fixation under Natural Conditions: An Analytical Approach for Studying Nitrogenase Activity.
abstract: Biological N2fixation is a major input of bioavailable nitrogen, which represents the most frequent factor limiting the agricultural production throughout the world. Especially, the symbiotic association between legumes and Rhizobium bacteria can provide substantial amounts of nitrogen (N) and reduce the need for industrial fertilizers. Despite its importance in the global N cycle, rates of biological nitrogen fixation have proven difficult to quantify. In this work, we propose and demonstrate a simple analytical approach to measure biological N2fixation rates directly without a proxy or isotopic labeling. We determined a mean N2fixation rate of 78 ± 5 μmol N2(g dry weight nodule)-1h-1of a Medicago sativa-Rhizobium consortium by continuously analyzing the amount of atmospheric N2in static environmental chambers with Raman gas spectroscopy. By simultaneously analyzing the CO2uptake and photosynthetic plant activity, we think that a minimum CO2mixing ratio might be needed for natural N2fixation and only used the time interval above this minimum CO2mixing ratio for N2fixation rate calculations. The proposed approach relies only on noninvasive measurements of the gas phase and, given its simplicity, indicates the potential to estimate biological nitrogen fixation of legume symbioses not only in laboratory experiments. The same methods can presumably also be used to detect N2fluxes by denitrification from ecosystems to the atmosphere.
authors.affiliation: Leibniz Institute of Photonic Technology , 07745 Jena, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Jochum
authors.affiliation: Max Planck Institute for Biogeochemistry , 07745 Jena, Germany.
authors.forename: Agnes
authors.initials: A
authors.lastname: Fastnacht
authors.affiliation: Max Planck Institute for Biogeochemistry , 07745 Jena, Germany.
authors.forename: Susan E
authors.initials: SE
authors.lastname: Trumbore
authors.affiliation: Leibniz Institute of Photonic Technology , 07745 Jena, Germany.|Institute of Physical Chemistry and Abbe Center of Photonics , 07745 Jena, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Popp
authors.affiliation: Leibniz Institute of Photonic Technology , 07745 Jena, Germany.|Institute of Physical Chemistry and Abbe Center of Photonics , 07745 Jena, Germany.
authors.forename: Torsten
authors.identifier: 0000-0003-3358-8878
authors.initials: T
authors.lastname: Frosch
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism; D000455:Medicago sativa / Q000201:enzymology* / Q000378:metabolism / Q000382:microbiology; D009584:Nitrogen / Q000378:metabolism*; D009586:Nitrogen Fixation; D009591:Nitrogenase / Q000378:metabolism*; D010788:Photosynthesis; D012231:Rhizobium / Q000201:enzymology* / Q000378:metabolism; D013059:Spectrum Analysis, Raman / Q000379:methods*
chemical_list: D002245:Carbon Dioxide; D009591:Nitrogenase; D009584:Nitrogen