Simultaneous Measurement of Denitrification and Nitrogen Fixation Using Isotope Pairing with Membrane Inlet Mass Spectrometry Analysis

Abstract

ABSTRACT 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 ( 28 N 2 , 29 N 2 , and 30 N 2 ) were monitored during incubation experiments after the addition of 15 NO 3 − . 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 15 NO 3 − and 14 NO 3 − 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 μg-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 μg-atoms of N m −2 h −1 ) were lower than nitrogen fixation rates (average, 60 μg-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 with the MIMS system.