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

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 microg-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 microg-atoms of N m(-2) h(-1)) were lower than nitrogen fixation rates (average, 60 microg-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.

FIG. 1

Relationships between different AMU signals in liquid samples measured with the quadruple mass spectrometer. Solid symbols, standard water (▪, 30 ppt, 21°C; ▴, 30 ppt, 30°C); open symbols, samples (□, outflow water; ○, inflow water). The line represents the regression among the standard water samples. The signal represents partial pressure of gases (in pascals [10²]) inside the mass spectrometer. Data are from the work of An and Gardner (submitted).

FIG. 2

Production rates of N₂ isotope species in a potential-denitrification experiment with sediments enriched with different proportions of ¹⁴NO₃⁻ and ¹⁵NO₃⁻. The line represents the expected production rate of the three N₂ isotope species based on the proportion of each NO₃⁻ species. Symbols represent measured production rates of three N₂ gases (solid symbols, day 1; open symbols, day 2). Total amounts of NO₃⁻ were constant in each treatment (32 μmol bottle⁻¹). About 90% of added NO₃⁻ was recovered as N₂ gas in this experiment.

FIG. 3

Denitrification based on ¹⁴NO₃⁻ (D₁₄′) and ¹⁵NO₃⁻ (D₁₅′) during potential denitrification experiment. Error bar represents 1 SE for two incubation bottles.

FIG. 4

Total denitrification (DNF) and nitrogen fixation (NF) rates under different light and carbon enrichment conditions. Data are averages ± SE for two sediment cores. Duplicate measurements were not available for day 4 and 7 of treatment GD and day 3 of treatment GL.

FIG. 5

Nitrogen gas flux changes during sediment incubation experiments. The arrow shows the time at which ¹⁵NO₃⁻ was added. Total denitrification is calculated as ²⁸N₂ + ²⁹N₂ + ³⁰N₂ flux. Data are averages ± 1 SE for four replicate cores. Note the scale change in panel C.