Balance of assimilative and dissimilative nitrogen processes in a diatom-rich tidal flat sediment

Tidal flat sediments are subject to repetitive mixing and resuspension events. In a short-term (24 h) <sup>15</sup>N-labelling experiment, we investigated reactive nitrogen cycling in a tidal flat sediment following an experimentally induced resuspension event. We focused on (a) the relative importance of assimilatory versus dissimilatory processes and (b) the role of benthic microalgae therein. <sup>15</sup>N-labelled substrate was added to homogenized sediment, and <sup>15</sup>N was subsequently traced into sediment and dissolved inorganic nitrogen (DIN) pools. Integration of results in a N-cycle model allowed us to quantify the proportion of major assimilatory and dissimilatory processes in the sediment. <br><br> Upon sediment disturbance, rates of dissimilatory processes like nitrification and denitrification were very high, but declined rapidly towards a steady state. Once this was reached, the balance between assimilation and dissimilation in this tidal mudflat was mainly dependent on the nitrogen source: nitrate was utilized almost exclusively dissimilatory via denitrification, whereas ammonium was rapidly assimilated, with about a quarter of this assimilation due to benthic microalgae (BMA). Benthic microalgae significantly affected the nitrogen recycling balance in sediments, because in the absence of BMA activity the recovering sediment turned from a net ammonium sink to a net source. <br><br> The driving mechanisms for assimilation or dissimilation accordingly appear to be ruled to a large extent by external physical forcing, with the entire system being capable of rapid shifts following environmental changes. Assimilatory pathways gain importance under stable conditions, with a substantial contribution of BMA to total assimilation

Seasonal variability of nitrous oxide concentrations and emissions in a temperate estuary

Nitrous oxide (N2O) is a greenhouse gas, with a global warming potential 298 times that of carbon dioxide. Estuaries can be sources of N2O, but their emission estimates have significant uncertainties due to limited data availability and high spatiotemporal variability. We investigated the spatial and seasonal variability of dissolved N2O and its emissions along the Elbe Estuary (Germany), a well-mixed temperate estuary with high nutrient loading from agriculture. During nine research cruises performed between 2017 and 2022, we measured dissolved N2O concentrations, as well as dissolved nutrient and oxygen concentrations along the estuary, and calculated N2O saturations, flux densities, and emissions. We found that the estuary was a year-round source of N2O, with the highest emissions in winter when dissolved inorganic nitrogen (DIN) loads and wind speeds are high. However, in spring and summer, N2O saturations and emissions did not decrease alongside lower riverine nitrogen loads, suggesting that estuarine in situ N2O production is an important source of N2O. We identified two hotspot areas of N2O production: the Port of Hamburg, a major port region, and the mesohaline estuary near the maximum turbidity zone (MTZ). N2O production was fueled by the decomposition of riverine organic matter in the Hamburg Port and by marine organic matter in the MTZ. A comparison with previous measurements in the Elbe Estuary revealed that N2O saturation did not decrease alongside the decrease in DIN concentrations after a significant improvement of water quality in the 1990s that allowed for phytoplankton growth to re-establish in the river and estuary. The overarching control of phytoplankton growth on organic matter and, subsequently, on N2O production highlights the fact that eutrophication and elevated agricultural nutrient input can increase N2O emissions in estuaries.</p

Nitrogen isotope dynamics and fractionation during sedimentary denitrification in Boknis Eck, Baltic Sea

The global marine nitrogen cycle is constrained by nitrogen fixation as a source of reactive nitrogen, and denitrification or anammox on the sink side. These processes with their respective isotope effects set the marine nitrate ¹⁵N-isotope value (<i>&delta;</i>¹⁵N) to a relatively constant average of 5‰. This value can be used to better assess the magnitude of these sources and sink terms, but the underlying assumption is that sedimentary denitrification and anammox, processes responsible for approximately one-third of global nitrogen removal, have little to no isotope effect on nitrate in the water column. <br><br> We investigated the isotope fractionation in sediment incubations, measuring net denitrification and nitrogen and oxygen stable isotope fractionation in surface sediments from the coastal Baltic Sea (Boknis Eck, northern Germany), a site with seasonal hypoxia and dynamic nitrogen turnover. <br><br> Sediment denitrification was fast, and regardless of current paradigms assuming little fractionation during sediment denitrification, we measured fractionation factors of 18.9‰ for nitrogen and 15.8‰ for oxygen in nitrate. While the input of nitrate to the water column remains speculative, these results challenge the current view of fractionation during sedimentary denitrification and imply that nitrogen budget calculations may need to consider this variability, as both preferential uptake of light nitrate and release of the remaining heavy fraction can significantly alter water column nitrate isotope values at the sediment–water interface

Dissolved and particulate reactive nitrogen in the Elbe River/NW Europe: a 2-yrN-isotope study

Rivers collect and transport reactive nitrogen to coastal seas as nitrate, ammonium, dissolved organic nitrogen (DON), or particulate nitrogen. DON is an important component of reactive nitrogen in rivers and is suspected to contribute to coastal eutrophication, but little is known about seasonality of DON loads and turnover within rivers. We measured the concentrations and the isotope ratios N-15/N-14 of combined DON+NH4+ (delta(DON)-D-15+NH4+), nitrate (delta N-15-NO3-) and particulate nitrogen (delta(PN)-P-15) in the non-tidal Elbe River (SE North Sea, NW Europe) over a period of 2 yr (June 2005 to December 2007) at monthly resolution. Combined DON+NH4+ concentrations ranged from 22 to 75 mu M and comprised nearly 23% of total dissolved nitrogen in the Elbe River in annual mean; PN and nitrate concentrations ranged from 11 to 127 mu M, and 33 to 422 mu M, respectively. Combined PN and DON+NH4+ concentrations were, to a first approximation, inversely correlated to nitrate concentrations. delta(DON)-D-15+NH4+, which varied between from 0.8 parts per thousand to 11.5 parts per thousand, changed in parallel to delta(PN)-P-15 (range 6 to 10 parts per thousand), and both were anti-correlated to delta N-15-NO3- (range 6 to 23 parts per thousand). Seasonal patterns of DON+ NH4+ concentrations and delta(DON)-D-15+NH4+ diverge from those expected from biological DON+NH4+ production in the river alone and suggest that the elution of organic fertilisers significantly affects the DON+NH4+ pool in the Elbe River

Turnover of combined dissolved organic nitrogen and ammonium in the Elbe estuary/NW Europe: Results of nitrogen isotope investigations

Dissolved organic nitrogen (DON) is often the dominant form of reactive nitrogen transported from land to sea by rivers, but is considered to be largely recalcitrant and behaves conservatively in many estuaries. We measured the concentration and the isotope ratio delta(15)N of combined DON and ammonium (delta(15)DON + NH(4)(+)) in the Elbe River estuary (SE North Sea, NW Europe) by a combination of a modified persulfate digestion and the denitrifier method. Measurements were made on samples taken along the salinity gradient from 1 to 32 during different seasons, in order to gauge the effects of internal biological processes and external signatures (such as pollution). Combined DON and ammonium concentrations ranged from 20 to 60 mu M, and delta(15)DON + NH(4)(+) from 0 to 11 parts per thousand. The results show that DON + NH(4)(+) contributes <20% to total reactive nitrogen in the river end member and rises to 50% in the outer estuary. By comparison with older data, the DON load in the Elbe River did not change since the 1980s, when nitrate and phosphate pollution was maximal. We find evidence that DON and/or ammonium or reactive components in DON are both consumed and produced in the estuary, indicated by changing isotope ratios and non-conservative mixing gradients. The estuarine turbidity maximum zone (TMZ) at salinities <5, which today is a significant source of nitrate from nitrification, coincides with significantly decreased DON + NH(4)(+) concentrations and delta(15)DON + NH(4)(+) in all seasons sampled. Whether this is due to selective absorption/desorption of (15)N enriched moieties onto particle surfaces, or to selective heterotrophic assimilation and nitrification is yet unclear, and the loss of DON + NH(4)(+) does not balance the added nitrate. Because DON + NH(4)(+) concentrations and delta(15)DON + NH(4)(+) rise sharply seaward of the TMZ, we consider adsorption/desorption processes most likely. In the salinity gradient 5 to 30, DON + NH(4)(+) behaves conservatively in both concentration and isotopic composition. (C) 2010 Elsevier B.V. All rights reserved

Nitrite consumption and associated isotope changes during a river flood event

In oceans, estuaries, and rivers, nitrification is an important nitrate source, and stable isotopes of nitrate are often used to investigate recycling processes (e.g. remineralisation, nitrification) in the water column. Nitrification is a two-step process, where ammonia is oxidised via nitrite to nitrate. Nitrite usually does not accumulate in natural environments, which makes it difficult to study the single isotope effect of ammonia oxidation or nitrite oxidation in natural systems. <br><br> However, during an exceptional flood in the Elbe River in June 2013, we found a unique co-occurrence of ammonium, nitrite, and nitrate in the water column, returning towards normal summer conditions within 1 week. Over the course of the flood, we analysed the evolution of <i>δ</i>¹⁵N–NH₄⁺ and <i>δ</i>¹⁵N–NO₂⁻ in the Elbe River. In concert with changes in suspended particulate matter (SPM) and <i>δ</i>¹⁵N SPM, as well as nitrate concentration, <i>δ</i>¹⁵N–NO₃⁻ and <i>δ</i>¹⁸O–NO₃⁻, we calculated apparent isotope effects during net nitrite and nitrate consumption. <br><br> During the flood event, &gt; 97 % of total reactive nitrogen was nitrate, which was leached from the catchment area and appeared to be subject to assimilation. Ammonium and nitrite concentrations increased to 3.4 and 4.4 µmol L⁻¹, respectively, likely due to remineralisation, nitrification, and denitrification in the water column. <i>δ</i>¹⁵N–NH₄⁺ values increased up to 12 ‰, and <i>δ</i>¹⁵N–NO₂⁻ ranged from −8.0 to −14.2 ‰. Based on this, we calculated an apparent isotope effect ¹⁵<i>ε</i> of −10.0 ± 0.1 ‰ during net nitrite consumption, as well as an isotope effect ¹⁵<i>ε</i> of −4.0 ± 0.1 ‰ and ¹⁸<i>ε</i> of −5.3 ± 0.1 ‰ during net nitrate consumption. On the basis of the observed nitrite isotope changes, we evaluated different nitrite uptake processes in a simple box model. We found that a regime of combined riparian denitrification and 22 to 36 % nitrification fits best with measured data for the nitrite concentration decrease and isotope increase

Changes in atmospheric nitrate deposition in Germany - An isotopic perspective

We investigated the isotopic composition of atmospheric NO3- deposition at a moderately polluted site in Western Europe over an annual cycle from December 2011 to November 2012. On average, we measured load-weighted delta N-15 values of +0.1 and +3.0 parts per thousand in wet and dry deposition, respectively. A comparison to source-specific N emission trends and to isotope data from the 1980s reveals distinct changes in delta N-15-NO3- values: In contrast to the increasing relative importance of isotopically depleted natural NOx sources, we find an increase of isotope values in comparison to historical data. We explore the role of land-based N sources, because backward trajectories reveal a correlation of higher delta N-15 to air mass origin from industrialized areas. Nowadays isotopically enriched NOx of coal-fired power plants using selective catalytic converters and land-based vehicle emissions, which use same technology, are apparently the main driver of rising delta N-15 values in nitrate deposition. (C) 2014 The Authors. Published by Elsevier Ltd

Stable isotope composition and turnover of nitrate in the German Bight

The German Bight is a hot-spot of eutrophication in the North Sea due to nitrate loads discharged by several large rivers into this semi-isolated embayment. River nitrate loads have a distinctly higher N-15/N-14 ratio than nitrate in open North Sea waters, and to trace the sphere of river influence we analysed stable isotope signatures of water column nitrate in the area on a grid of stations in winter and early spring 2007. Overall spatial patterns of N-15/N-14 and O-18/O-16 in nitrate reflect the predominant influence of nitrate discharged by the Rhine and Elbe rivers on the German Bight nitrate pool. On a smaller scale, however, and in offshore stations, nitrate assimilation of an incipient phytoplankton bloom is indicated by parallel enrichment of N-15 and O-18 in nitrate. Intriguingly, the enrichment ratio of O-18(NO3) to N-15(NO3) is 1.6:1, thus differing from the ratio of 1:1 associated with uptake by marine phytoplankton. This suggests that nitrate isotopic composition is not solely affected by phytoplankton assimilation, but that a substantial portion of nitrate in the outer regions of the German Bight is derived from nitrification, despite low ambient temperatures. Moreover, the data identify remineralisation and nitrification of particulate N in sediments as important sources of dissolved inorganic nitrogen to the German Bight water column, and underscore the role of sediments in recharging water column nutrient inventories