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Abstract:
Biological N2 fixation 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 N2 fixation rates directly without a
proxy or isotopic labeling. We determined a mean N2 fixation rate of 78 ± 5 μmol N2 (g
dry weight nodule)−1 h−1 of a Medicago sativa−Rhizobium consortium by continuously
analyzing the amount of atmospheric N2 in static environmental chambers with Raman
gas spectroscopy. By simultaneously analyzing the CO2 uptake and photosynthetic plant
activity, we think that a minimum CO2 mixing ratio might be needed for natural N2
fixation and only used the time interval above this minimum CO2 mixing ratio for N2
fixation 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 N2 fluxes by denitrification from ecosystems to the atmosphere.
