Understanding denitrification with stable isotope measurements of N2: proof of concept study

Abstract

The biologically mediated removal of nitrogen from estuarine and shallow coastal waters was investigated using a new method for the determination of N₂ isotope ratios. This method employs the use of a chromatographic separation technique to purify N₂ for isotopic analysis. The method was used to correlate [DELTA]¹⁵N-N₂ isotope ratios to measured denitrification rates in benthic chambers and to examine the mechanisms supporting denitrification in the Mississippi River Plume (MRP). Dinitrogen isotope ratios were measured during time series experiments of intact estuarine sediments in Galveston Bay. Where chambers were used, the [DELTA]¹⁵N-N₂ and the N₂ concentration were used to determine the [DELTA]¹⁵N of N₂ produced during denitrification. Isotope ratios of N₂ produced during denitrification were from 14 to 26⁰/₀₀ more negative than nitrate substrate. Increased denitrification rates measured during the summer yielded lower [DELTA]¹⁵N-N₂ values than during winter when denitrification rates were lower. The average [DELTA]¹⁵N-N₂ value measured throughout Galveston Bay, without the use of chambers, was 0.80⁰/₀₀. Using chambers to prevent the effects of mixing, differences between coupled and uncoupled denitrification were distinguished in the [DELTA]¹⁵N-N₂ values over hourly to daily time scales. In situ [DELTA]¹⁵N-N₂ measurements made in the Gulf of Mexico/MRP averaged 0.59⁰/₀₀ for the surface waters of both the winter 1997 and summer 1998 cruises suggesting that the MRP remains enriched with ¹⁴N throughout the year. Isotope ratios measured at depth during the winter 1997 cruise were indicative of direct denitrification when compared to substrate availability. Dinitrogen isotope values, enriched with ¹⁴N, were also measured at deeper stations where the effects of wind driven mixing were less important. During the summer 1998 cruise decreased NH₄⁺ concentrations followed by increased NO₃⁻ concentrations were coupled to ¹⁴N enriched [DELTA]¹⁵N-N₂ values. These results show that the use of [DELTA]¹⁵N-N₂ isotope ratios can be used in conjunction with N₂ concentrations and environmental conditions to elucidate the mechanisms of denitrification in both estuarine and coastal waters. Additionally, MRP results imply that the effects of wind driven mixing and air-sea exchange play an important role in the removal of denitrified N₂ from the water column and inevitably from the entire system.