Dissolved Organic Nitrogen Production Research Articles

Photochemical Release of Dissolved Organic Nitrogen from Algal Detritus and Sediment Particles in Lake Taihu, China

Solar irradiation in aquatic systems can induce the conversion of substances from the solid to the dissolved phase (photodissolution). Yet, the photochemical release of dissolved organic nitrogen (DON) from internal particles in lakes remains largely unknown. In this study, suspensions of algal detritus and sediment particles from a shallow eutrophic lake were exposed to simulated solar irradiation, and the release and compositional changes of dissolved organic matter were explored by measuring their UV–Visible absorption spectroscopy and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The photochemical release of inorganic nitrogen during the incubations was also investigated. Results showed that light irradiation induced stronger dissolved organic carbon and DON production in the algal detritus suspensions, with release rates of 1.17 mg C L−1 h−1 and 0.14 mg N L−1 h−1, respectively, at an algal detritus concentration of 0.1 dry g L−1. Light irradiation also induced compositional changes of DON in both algal and sediment suspensions. A larger number of DON molecules with lower molecular weight were continuously released in the algal suspensions, e.g., the total number of DON formulas increased from 1349 to 4135 during an 8 h irradiation. In contrast, upon irradiation of sediment suspensions, DON showed decreased molecular diversity and increased aromaticity. The photochemical release of ammonium (photoammonification) was also higher in the algal suspensions with a rate of 0.015 mg N L−1 h−1, which may contribute to the eutrophication of the lake. This study provides new molecular insights into the photochemical release of DON from typical internal particles in eutrophic lakes.

Multiple Lenses of N-Isotopes Reveal Active Dissolved Organic Nitrogen Cycling in a Subtropical Estuary and Marginal Sea

Dissolved organic nitrogen (DON) is an important component of the marine nitrogen (N) inventory and plays an essential role in N cycling in global estuaries and marginal seas. Understanding DON cycling is important but challenging. Global estuaries and marginal seas are experiencing significant anthropogenic impacts and have intensive physical/biochemical gradients. Therefore, high-quality DON concentration and N-isotope (δ15N–DON) data are very difficult to obtain. To enrich this knowledge, we take the Changjiang Estuary and the adjacent East China Sea shelf seas as a representative example and analyzed multiple isotopes and the concentrations of nitrate (NO3−), particulate nitrogen (PN), and DON. N isotopes combined with optimum multiparameter analysis proved to be very informative. This integrated analysis discriminates active DON production and consumption from a seemingly conservative distribution pattern of DON. The study area was divided into DON production zones 1 and 2 (P-zone 1 and 2) and DON consumption zones 1 and 2 (C-zone 1 and 2). For P-zone 1, the PN-originated DON elevated the δ15N–DON, while in P-zone 2, the DON excreted by phytoplankton was characterized by low δ15N and lowered δ15N–DON. DON consumption occurred in the NO3− depleted surface waters (C-zone 1) as well as the shelf middle and bottom waters (C-zone 2). This study discovers and consolidates the active and dynamical zoning of DON cycling from the estuary to the offshore marginal sea and establishes a useful means that is of valuable reference to DON cycling studies in global estuaries and marginal seas.

Emerging role of dissolved organic nitrogen in supporting algal bloom persistence in Lake Taihu, China: Emphasis on internal transformations

Dissolved organic nitrogen (DON) accounts for a significant fraction of the dissolved nitrogen (N) pool in eutrophic lakes, and substantial components of DON are available for phytoplankton. However, long-term changes in DON concentrations in eutrophic lakes have been poorly documented, and the particulate-related internal cycling of DON at the molecular level remains unclear. In this study, changes in the concentration of DON during the past 14 years of Lake Taihu, China were analyzed. Algae and sediment suspensions were incubated under summer sunlight to examine transformations of DON (e.g., photodissolution) at the bulk concentration and molecular level. Concentrations of DON in Lake Taihu ranged from 0 to 4.59 mg L−1, with a mean of 0.82 ± 0.60 mg L−1 (n = 3360). Although annual averages of the concentration of DON (n = 240) showed no significant linear variation from 2005 to 2018, the percentage of DON in total dissolved nitrogen (TDN) increased linearly (p < .05) and was maintained at >50% after 2015. Seasonally, DON concentrations were highest in spring (1.10 ± 0.56 mg L−1), followed by winter (0.87 ± 0.58 mg L−1) and summer (0.54 ± 0.37 mg L−1) concentrations, and lowest in autumn (0.44 ± 0.38 mg L−1). Outdoor incubation experiments suggest that both algae and sediment suspensions contribute to significant increases of DON concentrations but contribute to decreases of inorganic N concentrations at the end of incubations. Moreover, the production of DON by algae suspensions differed from that of sediment suspensions, with more labile components released from algae (e.g., proteins and amino acids), while more refractory components were released from sediment suspensions (e.g., lignin with low H/C and O/C). Taken together, these results emphasize the increasing percentage of DON in TDN during 2009–2018 and the different patterns of algae- and sediment-dominated internal DON transformations in Lake Taihu.

Dissolved Organic Nitrogen Production and Export by Meridional Overturning in the Eastern Subpolar North Atlantic

AbstractDissolved organic matter (DOM) is produced in the surface and exported towards the deep ocean, adding ∼2 PgC/year to the global carbon export. Due to its central role in the Meridional Overturning Circulation, the eastern subpolar North Atlantic (eSPNA) contributes largely to this export. Here we quantify the transport and budget of dissolved organic nitrogen (DON) in the eSPNA, in a box delimited by the OVIDE 2002 section and the Greenland‐Iceland‐Scotland sills. The Meridional Overturning Circulation exports &gt;15.9 TgN/year of DON downward and, contrary to the extended view that these are materials of subtropical origin, up to 33% of the vertical flux derives from a net local DON production of 7.1 ± 2.6 TgN/year. The low C:N molar ratio of DOM production (7.4 ± 4.1) and the relatively short transit times in the eSPNA (3 ± 1 year) suggest that local biogeochemical transformations result in the injection of fresh bioavailable DOM to the deep ocean.

Amino acid enantiomers in old and young dissolved organic matter: Implications for a microbial nitrogen pump

Abstract Dissolved organic nitrogen (DON) represents the largest reservoir of fixed N in the surface ocean and a significant portion accumulates in the deep sea, where it can persist for millennial time scales. However, like the dissolved organic carbon (DOC) pool, the origin and composition of long-lived, refractory DON remains largely unknown. In recent years, the “microbial carbon pump” hypothesis has emerged from abundant evidence showing that microbial processes are primarily responsible for refractory DOC accumulation. However, a similar mechanism for DON has rarely been investigated. In the study of DON, spectroscopic evidence has indicated a primarily amide composition, implying a dominant contribution from peptides. Therefore, if an analogous “microbial nitrogen pump” controls refractory DON accumulation, the amino acid component should bear increasing signatures of microbial origin with increasing age. Here we investigate the microbial sequestration of N via the production of refractory DON, for the first time considering together DOM Δ14C with amino acid (AA) molar abundance (Mol%) and D/L ratio (as a tracer for prokaryotic input). Measurements were made on a unique set of high and low molecular weight (HMW, LMW) DOM isolates with 14C ages and chemical compositions generally consistent with semi-labile and refractory DOM respectively. The samples were collected in the North Pacific Subtropical Gyre where deep waters contain some of the oldest DOC in the world ocean. We observe higher D/L ratios in older, LMW DOM isolates for almost all analyzed AAs. Using mass spectral data, we also quantify three D-AAs in all samples (D-valine, D-phenylalanine, and D-leucine), which have not previously been confirmed in ocean DOM. These newly identified D-AAs are concentrated in the LMW refractory DOM fraction and have oceanographically consistent depth profiles. Our results suggest that several novel D-AA subgroupings may be unique tracers for different prokaryotic source processes. D-alanine appears to have largely independent cycling from the other D-AAs with a connection to the production of HMW DON, which we hypothesize is linked to water column peptidoglycan. In contrast, D-leucine, D-valine, and D-phenylalanine appear to be most strongly related to the production of LMW DON. Trends in both the HMW and LMW fractions suggest a linkage to sinking particles and local microbial transformations, implying that LMW DON has a direct biological source rather than originating from successive microbial reprocessing of HMW DON. Taken together, our observations are consistent with the dominant production of refractory LMW DON by prokaryotic organisms and suggests that different AA sub-groupings that can be used to track different processes within the DON pool.

The Sources and Transformations of Dissolved Organic Matter in the Pearl River Estuary, China, as Revealed by Stable Isotopes

AbstractTo investigate the sources and transformation processes of bulk marine dissolved organic matter (DOM), seasonally collected water samples from the Pearl River Estuary (PRE), south China, were analyzed for concentration and stable isotopic ratios (δ13C and δ15N) of DOM. The δ13C values clearly indicated a shift of dissolved organic carbon (DOC) sources from terrigenous material in the river to predominantly plankton in the lower estuary. The distribution of δ13C during the estuarine mixing, in combination with the notable deviation of DOC concentration, provided strong evidence for a substantial DOC removal in the salinity range of 5 to 22. In contrast to δ13C, the δ15N of DOM exhibits a strong seasonality, implying a clear decoupling of estuarine DOC and dissolved organic nitrogen (DON) dynamics. During dry periods with low river discharge, sewage‐derived DON contributed significantly to the riverine loading. While freshwater‐saltwater mixing is important in regulating the distribution of DON, the distribution of δ15N along the salinity gradient provided strong evidence for active sediment‐water interaction and adsorption/desorption processes at the middle to high salinities. During wet periods, severe soil loss and erosion, and/or in situ biological production, adsorption onto suspended particles dominate the riverine DON fluxes, whereas the DON dynamics might be governed by different biogeochemical processes between spring and summer, as reflected by distinct δ15DON patterns in the estuarine zone. In spring, highly degraded DON originating from soil experienced little further biological alternation during estuarine mixing, whereas there was a tight coupling of DON production and consumption in summer.

Nitrogen excretion by copepods and its effect on ammonia‐oxidizing communities from a coastal upwelling zone

AbstractThe role of zooplankton in microbial nitrogen turnover in marine environments is poorly understood. Here, we present results from two experiments designed to determine the excretion rate of ammonium and dissolved organic nitrogen (DON) by dominant copepods, Acartia tonsa and Paracalanus cf indicus, fed with two natural sized‐fractioned diets (20–150 μm and &lt; 20 μm), and its possible effects on the transcriptional activity of ammonia monooxygenase subunit A (amoA), a functional marker for ammonia‐oxidizing archaea (AOA) and bacteria (AOB), as a response to the input of ammonium and DON by copepod excretion, during autumn and winter in central/southern Chile. Our results reveal that DON was the main excretion product, with rates up to 3.7 μmol L−1 h−1. DON production increased in copepods fed with the small‐sized food. Ammonium was also excreted, with rates up to 0.08 μmol L−1 h−1 in autumn and 0.4 μmol L−1 h−1 in winter, and rapidly consumed by the microbial community, decreasing down to 0.07 μmol L−1 between the initial time to 4 h. Ammonium consumption coincided with increased AOB and AOA amoA transcript copies in copepods fed with the larger‐sized food, while a different microbial community, probably heterotrophic, reacted to nitrogen input via excretion by copepods fed with the smaller‐sized food in autumn. AOA‐A was transcriptionally active in winter with nearly zero ammonium concentration, suggesting that AOA outcompete AOB when ammonium becomes limited. We conclude that nitrogen excreted by copepods can be used directly by microbial communities, including nitrifying ones. Zooplankton excretion may thus provide significant remineralized nitrogen for new and regenerated production in the upper ocean.

Influence of filtration velocity on DON variation in BAF for micropolluted surface water treatment.

Biological aerated filters (BAFs) are widely used for the treatment of micropolluted surface water. However, the biological process produces dissolved organic nitrogen (DON), which, as precursors of nitrogenous disinfection by-products, pose potential threats to drinking water safety. Therefore, to control DON in BAF effluent, it is necessary to study the influence of BAF operation parameters on DON production. In this study, the influence of filtration velocity in a BAF on DON production was investigated. Under different filtration velocity (0.5, 2, and 4m/h) conditions, profiles of DON concentrations along the media layer were measured. The profile at a filtration velocity of 0.5m/h showed a decreasing trend, and the ones under filtration velocities of 2 and 4m/h fluctuated in a small range (from 0.1 to 0.4mg/L). Moreover, the relatively high filtration velocities of 2 and 4m/h resulted in a lower level of DON concentration. Additionally, 3D excitation-emission matrix fluorescence spectroscopy was used to characterize DON. It is found that the patterns of DON at a relatively high filtration velocity condition (4m/h) were obviously different from the ones under low filtration velocity conditions (0.5 and 2m/h).

Quantifying the production of dissolved organic nitrogen in headwater streams using 15N tracer additions

Most nitrogen (N) assimilation in lake and marine ecosystems is often subsequently released via autochthonous dissolved organic nitrogen (DON) production, but autochthonous DON production has yet to be quantified in flowing waters. We measured in‐stream DON production following 24 h 15N‐nitrate () tracer additions in 36 headwater streams, a subset of sites from the second Lotic Intersite Nitrogen eXperiment. Streams were located in five North American ecoregions and drained basins dominated by native vegetation, agriculture, or urban land use. Using a two‐compartment model, we could quantify DON production in 15 streams as a function of DO15N derived from 15N tracer in biomass compartments. The streams with detectable DON production had higher % modified land use (agriculture + urban) in their basins than did streams with undetectable DON production. Median DON production represented 8% of total uptake when we used N biomass estimates based on N assimilated over 1 d (measured directly from the 15N additions). Median DON production was 17% of total uptake when we used N assimilated over 42 d (extrapolated from previous 15N tracer studies). Variation in DON production was positively correlated with ecosystem respiration, indicating that stream heterotrophy may influence DON production. In‐stream DON production was similar in magnitude to stream denitrification and nitrification, indicating that the production of autochthonous DON can represent a substantial transformation of stream N. Our results confirm that headwater streams can quickly convert inorganic N into organic forms, although the ultimate fate of DON remains unclear.

The distinct nitrogen isotopic compositions of low and high molecular weight marine DON

To constrain the sources and cycling of bulk and size-fractionated marine dissolved organic nitrogen (DON), samples were collected for concentration and isotopic (δ15N) analysis from the western tropical North Atlantic Ocean as well as over the North Australian shelf. Bulk DON concentrations are typically between 4 and 6μM, with low molecular weight (LMW, i.e., <1000Da) DON accounting for 60 to 70% of total DON. The δ15N of both bulk and high molecular weight (HMW, i.e., >1000Da to <0.2μm) DON from the western tropical North Atlantic is similar to previous measurements, as well as to bulk and HMW DON collected off the North Australian Shelf, ~4‰. Here we report the first measurement of marine LMW DON δ15N, and a coherent pattern emerges where LMW DON δ15N<HMW DON δ15N. An analytical concern is that the bulk DON δ15N as calculated from our coupled HMW and LMW measurements is often lower than that measured directly for the bulk DON. Despite this discrepancy, the self-consistency of the data in other regards argues for the robustness of the basic observation of lower δ15N in LMW relative to HMW DON. One explanation for this isotopic difference is considered most likely, based on the model that DON δ15N elevation relative to PON is fundamentally due to DON production from PON without isotopic fractionation, coupled with DON destruction occurring with fractionation. In this model, HMW DON loss must occur predominantly through the breakage of N-containing bonds, for which isotope fractionation should be substantial, whereas LMW DON loss is partly due to direct assimilation by phytoplankton and other microbes, which appears to occur with only minor isotopic fractionation. However, an alternative hypothesis exists: LMW and HMW DON may have distinct sources, for example, prokaryotic and eukaryotic plankton, respectively. In either case, it appears that most LMW DON has not passed through the HMW DON pool, as this would require an unrealistically large isotope fractionation at the HMW to LMW conversion. Finally, neither the concentration nor the δ15N of the bulk or either size fraction of DON varies with in situ N2 fixation rates measured at the time of sample collection. However, the δ15N of both bulk and LMW DON from the western tropical North Atlantic shows significant differences between samples collected six months apart, which may be due to seasonally variable stimulation of primary production by the Amazon River.

An exploration of how litter controls drainage water DIN, DON and DOC dynamics in freely draining acid grassland soils

Surface and subsurface litter fulfil many functions in the biogeochemical cycling of C and N in terrestrial ecosystems. These were explored using a microcosm study by monitoring dissolved inorganic nitrogen (DIN) (NH4 +–N + NO3 −–N), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations and fluxes in drainage water under ambient outdoor temperatures. Subsurface litter remarkably reduced the DIN concentrations in winter, probably by microbial N uptake associated with higher C:N ratio of added litter compared with soil at 10–25 cm depth. Fluxes of DIN were generally dominated by NO3 −–N; but NH4 +–N strongly dominated DIN fluxes during freeze–thaw events. Appreciable concentrations of NH4 +–N were observed in the drainage from the acid grassland soils throughout the experiment, indicating NH4 +–N mobility and export in drainage water especially during freeze–thaw. Litter contributed substantially to DOC and DON production and they were correlated positively (p < 0.01) for all treatments. DOC and DON concentrations correlated with temperature for the control (p < 0.01) and surface litter (p < 0.001) treatments and they were higher in late summer. The subsurface litter treatment, however, moderated the effect of temperature on DOC and DON dynamics. Cumulative N species fluxes confirmed the dominance of litter as the source of DON and DOC in the drainage water. DON constituted 42, 46 and 62% of cumulative TDN flux for control, surface litter and subsurface litter treatments respectively.

Dissolved Organic Nitrogen Characterization and Bioavailability in Wastewater Effluents

This project addresses the dissolved organic nitrogen (DON) content in the effluents of different wastewater treatment plants (WWTPs). In this report, information is presented on the occurrence and levels of DON in WWTP effluents and its bioavailability. The information presented was generated by a survey of existing data from full scale plants achieving TN removal, direct sampling and monitoring of effluents from selected plants, and laboratory investigations on DON production in biological processes of WWTPs and its bioavailability to algae and bacteria. The results showed that there is a wide variation in effluent DON at different plants, ranging from non-detectable levels to as high as 2.5 mg N/L. The DON fraction of the effluent TN increases with decrease in effluent TN values, and could be as high as 85% of effluent TN. Size fractionation of effluent organic nitrogen (ON) fraction passing through a 1.2 μm filter showed that a significant colloidal fraction of ON exists between 0.1 and 1.2 μm particle size range in some plants. A fraction of the effluent DON is readily bioavailable to algae and bacteria in laboratory bioassays, and the presence of both algae and bacteria species in the bioassay increases the bioavailable DON fraction. This title belongs to WERF Research Report Series . ISBN: 9781843393542 (eBook)

Changes in DOC and DON fluxes in response to harvest intensity of black-spruce-dominated forest ecosystems in northwestern Ontario

The current study was conducted to quantify and compare dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) fluxes through black-spruce-dominated forests, to compare the source/sink characteristics of sphagnum- versus feathermoss-dominated forest floors, and to identify changes in DOC and DON flow patterns occurring as a result of clearcut harvesting. After 2 yr of pre-harvest monitoring, replicated, experimental harvests of varying intensities of biomass removals were conducted followed by 4 yr of post-harvest sampling. Prior to harvest, the upland site type, dominated by feathermoss, was a significant source of DOC and DON, whereas, the wet-sphagnum-dominated sites exported minor amounts of these solutes. After harvest, DOC and DON fluxes peaked in the second year, but then dropped off significantly to at or below pre-harvest levels. On the upland site type, chipper debris appeared to be a major source of DOC and DON generating fluxes well above the pre-harvest levels. On the wetter site types, it appeared that microclimate differences between harvest treatments had a stronger influence on DOC and DON production than did the amount or type of harvest residue. Full-tree harvesting did not significantly alter the production of DOC and DON when compared with stem-only harvesting on the sites included in this study. Key words: DOC, DON, forest floor leachate, black spruce, harvesting response

Tidal pumping drives nutrient and dissolved organic matter dynamics in a Gulf of Mexico subterranean estuary

We hypothesize that nutrient cycling in a Gulf of Mexico subterranean estuary (STE) is fueled by oxygen and labile organic matter supplied by tidal pumping of seawater into the coastal aquifer. We estimate nutrient production rates using the standard estuarine model and a non-steady-state box model, separate nutrient fluxes associated with fresh and saline submarine groundwater discharge (SGD), and estimate offshore fluxes from radium isotope distributions. The results indicate a large variability in nutrient concentrations over tidal and seasonal time scales. At high tide, nutrient concentrations in shallow beach groundwater were low as a result of dilution caused by seawater recirculation. During ebb tide, the concentrations increased until they reached a maximum just before the next high tide. The dominant form of nitrogen was dissolved organic nitrogen (DON) in freshwater, nitrate in brackish waters, and ammonium in saline waters. Dissolved organic carbon (DOC) production was two-fold higher in the summer than in the winter, while nitrate and DON production were one order of magnitude higher. Oxic remineralization and denitrification most likely explain these patterns. Even though fresh SGD accounted for only ∼5% of total volumetric additions, it was an important pathway of nutrients as a result of biogeochemical inputs in the mixing zone. Fresh SGD transported ∼25% of DOC and ∼50% of total dissolved nitrogen inputs into the coastal ocean, with the remainder associated with a one-dimensional vertical seawater exchange process. While SGD volumetric inputs are similar seasonally, changes in the biogeochemical conditions of this coastal plain STE led to higher summertime SGD nutrient fluxes (40% higher for DOC and 60% higher for nitrogen in the summer compared to the winter). We suggest that coastal primary production and nutrient dynamics in the STE are linked.

Large Loss of Dissolved Organic Nitrogen from Nitrogen-Saturated Forests in Subtropical China

Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very high longterm ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/ broadleaf forest in subtropical southern China. All three forests have previously been shown to have high nitrate (NO3 ) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results showed that under ambient deposition (35‐50 kg N ha -1 y -1 as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5‐16.9 kg N ha -1 y -1 ). DON leaching increased 35‐162% following 2.5 years of experimental input of 50‐150 kg N ha -1 y -1 . The fertilizer-driven increase of DON leaching comprised 4‐17% of the added N. A concurrent increase in DOC loss was observed only in the pine forest, even though DOC:DON ratios declined in all three forests. Our data showed that DON accounted for 23‐38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had the highest DON loss under ambient conditions. DON leaching was highly correlated with NO3 leaching in all three forests. We hypothesize that abiotic incorporation of excess NO3 (through chemically reactive NO2 ) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the consistent and large DON loss in the N-saturated subtropical forests of southern China.

Evidence for efficient regenerated production and dinitrogen fixation in nitrogen-deficient waters of the South Pacific Ocean: impact on new and export production estimates

Abstract. One of the major objectives of the BIOSOPE cruise, carried out on the R/V Atalante from October-November 2004 in the South Pacific Ocean, was to establish productivity rates along a zonal section traversing the oligotrophic South Pacific Gyre (SPG). These results were then compared to measurements obtained from the nutrient – replete waters in the Chilean upwelling and around the Marquesas Islands. A dual 13C/15N isotope technique was used to estimate the carbon fixation rates, inorganic nitrogen uptake (including dinitrogen fixation), ammonium (NH4) and nitrate (NO3) regeneration and release of dissolved organic nitrogen (DON). The SPG exhibited the lowest primary production rates (0.15 g C m−2 d−1), while rates were 7 to 20 times higher around the Marquesas Islands and in the Chilean upwelling, respectively. In the very low productive area of the SPG, most of the primary production was sustained by active regeneration processes that fuelled up to 95% of the biological nitrogen demand. Nitrification was active in the surface layer and often balanced the biological demand for nitrate, especially in the SPG. The percentage of nitrogen released as DON represented a large proportion of the inorganic nitrogen uptake (13–15% in average), reaching 26–41% in the SPG, where DON production played a major role in nitrogen cycling. Dinitrogen fixation was detectable over the whole study area; even in the Chilean upwelling, where rates as high as 3 nmoles l−1 d−1 were measured. In these nutrient-replete waters new production was very high (0.69±0.49 g C m−2 d−1) and essentially sustained by nitrate levels. In the SPG, dinitrogen fixation, although occurring at much lower daily rates (≈1–2 nmoles l−1 d−1), sustained up to 100% of the new production (0.008±0.007 g C m−2 d−1) which was two orders of magnitude lower than that measured in the upwelling. The annual N2-fixation of the South Pacific is estimated to 21×1012g, of which 1.34×1012g is for the SPG only. Even if our "snapshot" estimates of N2-fixation rates were lower than that expected from a recent ocean circulation model, these data confirm that the N-deficiency South Pacific Ocean would provide an ideal ecological niche for the proliferation of N2-fixers which are not yet identified.

Isotopic heterogeneity and cycling of organic nitrogen in the oligotrophic ocean

We measured the nitrogen (N) isotopic composition (δ15N) of a large set (n = 38) of high‐molecular‐weight (HMW) dissolved organic nitrogen (DON) samples isolated from the tropical and subtropical North Atlantic and Pacific Oceans. The δ15N signature of surface HMW DON is relatively invariable in both oligotrophic basins (4.1 ± 0.6‰ in the Atlantic; 5.4 ± 0.8‰ in the Pacific) and shows little correlation with sources or concentrations of N supporting new production in the euphotic zone. While large variations in δ15N of bulk HMW DON are not apparent, δ15N of proteins isolated from sites with relatively high rates of N2 fixation (&gt;80 µmol N m−2 d−1) were consistently depleted in 15N relative to bulk HMW DON and to proteins isolated from sites where N2 fixation does not routinely occur. This small component of HMW DON appears to be cycling more rapidly than bulk HMW DON and may be indicative of fresh DON contributed by organisms in the surface ocean. Furthermore, δ15N of DNA extracted from the bacterial size fraction (0.2‐0.5 µm) revealed that free‐living bacteria may be an important sink for isotopically depleted N produced during N2 fixation. We suggest that there exists a tight coupling between the production and uptake of DON contributed by diazotrophs (N2 fixers) in regions where N2 fixation provides a major input of new nitrogen.

Fate and Transport of Organic Nitrogen in Minimally Disturbed Montane Streams of Colorado, USA

In two montane watersheds that receive minimal deposition of atmospheric nitrogen, 15–71% of dissolved organic nitrogen (DON) was bioavailable in stream water over a 2-year period. Discharge-weighted concentrations of bulk DON were between 102 and 135 μg/l, and the C:N ratio differed substantially between humic and non-humic fractions of DON. Approximately 70% of DON export occurred during snowmelt, and 40% of that DON was biologically available to microbes in stream sediments. Concentrations of bioavailable DON in stream water were 2–16 times greater than dissolved inorganic nitrogen (DIN) during the growing season, and bioavailable DON was depleted within 2–14 days during experimental incubations. Uptake of DON was influenced by the concentration of inorganic N in stream water, the concentration of non-humic DON in stream water, and the C:N ratio of the non-humic fraction of dissolved organic matter (DOM). Uptake of DON declined logarithmically as the concentration of inorganic N in stream water increased. Experimental additions of inorganic N also caused a decline in uptake of DON and net production of DON when the C:N ratio of non-humic DOM was high. This study indicates that the relative and absolute amount of bioavailable DON can vary greatly within and across years due to interactions between the availability of inorganic nutrients and composition of DOM. DOM has the potential to be used biotically at a high rate in nitrogen-poor streams, and it may be generated by heterotrophic microbes when DIN and labile DOM with low relative nitrogen content become abundant.

Determining15N enrichment of dissolved organic nitrogen in environmental waters by gas chromatography/negative-ion chemical ionization mass spectrometry

To minimize the required sample volume and simplify the overall procedure for determining 15N enrichment of dissolved organic nitrogen (DON) in environmental waters, we employed derivatization of NO3− with pentafluorobenzyl bromide and gas chromatography/negative-ion chemical ionization mass spectrometry (GC-NICI-MS). The pretreatment processes to remove inorganic nitrogen were optimized to ensure recovery of labile DON molecules, and the derivatization procedure for NO2− and NO3− was optimized for environmental samples. The required sample volume is 10 mL or less and the detection limit is about 0.015 µmol 15N L−1, or 0.3 atomic percent excess of 15N for water samples containing 5 µmol DON L−1. This method can also be applied to 15N enrichment analysis of particulate organic nitrogen. We applied the developed method to measure DON production rate by the hermatypic coral Porites cylindrica during incubation with 15N-labeled NO3−.

Dinitrogen fixation and release of ammonium and dissolved organic nitrogen by Trichodesmium IMS101

Two methods used to measure dinitrogen (N 2 ) fixation (acetylene reduction and 15 N 2 uptake) often result in different N 2 fixation rates. Part of the discrepancy may arise from the observation that Trichodesmium can release a fraction of their recently fixed N 2 as dissolved organic nitrogen (DON) and/or ammonium (NH 4 + ). To resolve outstanding issues regarding N 2 fixation and the production of dissolved combined nitrogen (N) by Trichodesmium, we conducted a comprehensive analysis of N 2 fixation and the production of DON and NH 4 + in cultures of Trichodesmium IMS101. We performed 15 N 2 uptake experiments in parallel with acetylene (C 2 H 2 ) reduction assays, and measured production of 15 NH 4 + and DO 15 N from 15 N 2 , and 15 NH 4 + uptake and regeneration by isotope dilution. Four main results are highlighted. First, 15 N 2 uptake appears to provide a better approximation of net N-specific growth rates than N 2 fixation estimates made using C 2 H 2 reduction. Second, the C 2 H 2 reduction method provides a closer approximation of gross N 2 fixation. Third, simultaneous measurements of relevant N pools and pathways by several methods enabled us to rigorously evaluate deviations from theoretical conversion factors and to interpret the basis for those deviations. Our results suggest that a conversion ratio (mol C 2 H 2 reduced: mol N 2 reduced to PON, ammonium and DON) of 4:1 may be more appropriate for total N 2 fixation. Fourth, the difference between estimates of gross N 2 fixation, made using the C 2 H 2 reduction technique, and net 15 N 2 uptake into particulate N may be a good indicator of N release from N 2 fixation.