Simultaneous measurement of denitrification and nitrogen fixation using isotope pairing with membrane inlet mass spectrometry analysis

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 N-2 gas species (N-28(2), N-29(2), and N-30(2)) were monitored during incubation experiments after the addition of (NO3-)-N-15. Formulas were developed to estimate the production (denitrification) and consumption (N-2 fixation) of N-2 gas from the fluxes of the different isotopic forms of N-2. Proportions of the three isotopic forms produced from (NO3-)-N-15 and (NO3-)-N-14 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 mug-atoms of N m(-2) h(-1). They were enhanced by light and organic matter enrichment. In this environment of high nitrogen fixation, low N-2 production rates due to denitrification could be separated from high N-2 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 mug-atoms of N m(-2) h(-1)) were lower than nitrogen fixation rates (average, 60 mug-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 N-2 isotope capability, it was possible to do isotope-pairing experiments,vith the MIMS system.