The contamination of commercial 15N2 gas with 15N-labeled nitrate/nitrite and ammonium and consequences for nitrogen fixation measurements

<p>The data contained herein are associated with the manuscript, "The contamination of commercial 15N2 gas stocks with 15N-labeled nitrate and ammonium and consequences for nitrogen fixation measurements", which is available at PLOS ONE.</p> <p>We report on the presence of 15N-labeled nitrate/nitrite, ammonium, and nitrous oxide in commercial 15N2 gas and the enrichment of particulate nitrogen as a result of the biological assimilation of 15N labeled contaminants.</p

Mineralization of RDX-Derived Nitrogen to N₂ via Denitrification in Coastal Marine Sediments

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a common constituent of military explosives. Despite RDX contamination at numerous U.S. military facilities and its mobility to aquatic systems, the fate of RDX in marine systems remains largely unknown. Here, we provide RDX mineralization pathways and rates in seawater and sediments, highlighting for the first time the importance of the denitrification pathway in determining the fate of RDX-derived N. ¹⁵N nitro group labeled RDX (¹⁵N-[RDX], 50 atom %) was spiked into a mesocosm simulating shallow marine conditions of coastal Long Island Sound, and the ¹⁵N enrichment of N₂ (δ¹⁵N₂) was monitored via gas bench isotope ratio mass spectrometry (GB-IRMS) for 21 days. The ¹⁵N tracer data were used to model RDX mineralization within the context of the broader coastal marine N cycle using a multicompartment time-stepping model. Estimates of RDX mineralization rates based on the production and gas transfer of ¹⁵N₂O and ¹⁵N₂ ranged from 0.8 to 10.3 μmol d^–1. After 22 days, 11% of the added RDX had undergone mineralization, and 29% of the total removed RDX-N was identified as N₂. These results demonstrate the important consideration of sediment microbial communities in management strategies addressing cleanup of contaminated coastal sites by military explosives

Tracing the Cycling and Fate of the Explosive 2,4,6-Trinitrotoluene in Coastal Marine Systems with a Stable Isotopic Tracer, ¹⁵N‑[TNT]

2,4,6-Trinitrotoluene (TNT) has been used as a military explosive for over a hundred years. Contamination concerns have arisen as a result of manufacturing and use on a large scale; however, despite decades of work addressing TNT contamination in the environment, its fate in marine ecosystems is not fully resolved. Here we examine the cycling and fate of TNT in the coastal marine systems by spiking a marine mesocosm containing seawater, sediments, and macrobiota with isotopically labeled TNT (¹⁵N-[TNT]), simultaneously monitoring removal, transformation, mineralization, sorption, and biological uptake over a period of 16 days. TNT degradation was rapid, and we observed accumulation of reduced transformation products dissolved in the water column and in pore waters, sorbed to sediments and suspended particulate matter (SPM), and in the tissues of macrobiota. Bulk δ¹⁵N analysis of sediments, SPM, and tissues revealed large quantities of ¹⁵N beyond that accounted for in identifiable derivatives. TNT-derived N was also found in the dissolved inorganic N (DIN) pool. Using multivariate statistical analysis and a ¹⁵N mass balance approach, we identify the major transformation pathways of TNT, including the deamination of reduced TNT derivatives, potentially promoted by sorption to SPM and oxic surface sediments

<b>Dataset: High methane concentrations in tidal salt marsh soils: where does the methane go?</b>

Tidal salt marshes produce and emit CH4. Therefore, it is critical to understand the biogeochemical controls that regulate CH4 spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH4 production, and higher salinity concentrations inhibit CH4 production in salt marshes. Recent evidence shows that CH4 is produced within salt marshes via methylotrophic methanogenesis, a process not inhibited by sulfate reduction. To further explore this conundrum, we performed measurements of soil-atmosphere CH4 and CO2 fluxes coupled with depth profiles of soil CH4 and CO2 pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH4 production and fate within a temperate tidal salt marsh. We found unexpectedly high CH4 concentrations up to 145,000 μmol mol-1 positively correlated with S2- (salinity range: 6.6 to 14.5 ppt). Despite large CH4 production within the soil, soil-atmosphere CH4 fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 ± 684.4 nmol m-2 s-1). CH4 and CO2 within the soil pore water were produced from young carbon, with most Δ14C-CH4 and Δ14C-CO2 values at or above modern. We found evidence that CH4 within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH4 is produced, including diffusion into the atmosphere, CH4 oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH4 production and fluxes are biogeochemically heterogeneous, with multiple processes and pathways that can co-occur and vary in importance over the year. This study highlights the potential for high CH4 production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH4 budgets and blue carbon in salt marshes.</p