Dissolved Organic Nitrogen Release 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.

Sediment release of dissolved organic matter to the oxygen minimum zone off Peru

Abstract. The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterised by a pronounced oxygen minimum zone (OMZ). Particulate organic matter (POM) that sinks out of the euphotic zone is supplied to the anoxic sediments and utilised by microbial communities, and the degradation of POM is associated with the production and reworking of dissolved organic matter (DOM). The release of DOM to the overlying waters may, therefore, represent an important organic matter escape mechanism from remineralisation within sediments but has received little attention in OMZ regions so far. Here, we combine measurements of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) with DOM optical properties in the form of chromophoric (CDOM) and fluorescent (FDOM) DOM from pore waters and near-bottom waters of the ETSP off Peru. We evaluate diffusion-driven fluxes and net in situ fluxes of DOC and DON to investigate processes affecting DOM cycling at the sediment–water interface along a transect at 12∘ S. To our knowledge, these are the first data for sediment release of DON and pore water CDOM and FDOM for the ETSP off Peru. Pore water DOC accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralisation within sediments. High DON accumulation resulted in very low pore water DOC ∕ DON ratios (≤1) which could be caused by an “uncoupling” in DOC and DON remineralisation. Diffusion-driven fluxes of DOC and DON exhibited high spatial variability and ranged from 0.2±0.1 to 2.5±1.3 mmolm-2d-1 and from -0.04±0.02 to 3.3±1.7 mmolm-2d-1, respectively. Generally low net in situ DOC and DON fluxes, as well as a steepening of spectral inclination (S) of CDOM and an increase in humic-like DOM at the sediment–water interface over time, indicated active microbial DOM utilisation. The latter may potentially be stimulated by the presence of nitrate (NO3-) and nitrite (NO2-) in the water column. The microbial DOC utilisation rates, estimated in our study, are potentially sufficient to support denitrification rates of 0.2–1.4 mmolm-2d-1, suggesting that the sediment release of DOM may on occasion contribute to nitrogen loss processes in the ETSP off Peru.

Integrated effects of Ulva prolifera bloom and decay on nutrients inventory and cycling in marginal sea of China

Ulva prolifera blooms occur annually in the Yellow Sea. Most studies focus on how U. prolifera blooming is influenced by nitrogen chemical forms and concentrations, while little concern goes to how U. prolifera bloom-decay cycle would impact local seawater nutrients structure. Therefore, we use 15N-labeled NO3 tracers and transcriptome analysis to determine N uptake, metabolism, and interconversion during U. prolifera growth and decay, so that we can quantify the conversions rate and fluxes of different nitrogen chemical forms. U. prolifera absorbes 17.37 μmol g−1·d−1 NO3–N during growth. NO3–N predominates (73.75–92.15%) in the dissolved inorganic nitrogen (DIN) in U. prolifera. During decay, NH4–N accountes for 60.87–92.13% of the in-cell DIN. The decomposing U. prolifera releases considerable amounts of NH4–N and dissolved organic nitrogen (DON) (63.8–98.2% < 1 kDa fraction and 1.8–36.2% is > 1 kDa fraction) into the ambient environment. The high DON release rate (59.57 μmol g−1 d−1) indicates active DON biosynthesis in U. prolifera. The isotope 15NO3–N tracer showes that 73.6% of the 15NO3–N is transformed to DON. The <1 kDa and the >1 kDa fractions account for 67.46–90.86% and 9.14–32.54% of the DON, respectively. The high efficiency of U. prolifera in utilizing NO3–N is explained by the responsive nitrate/nitrite transporter in cell membrane, and the DON biosynthesized capability is attributed to the up-regulated glutamine synthetase. Our study highlights the unique role of U. prolifera as a “Nitrogen-Pump” in converting nitrogen chemical forms during its bloom-decay cycle and quantifies its impacts on local N-nutrients inventory.

Microbial uptake and regeneration of inorganic nitrogen off the coastal Namibian upwelling system

We used 15N-labeled substrates to measure microbial nitrate (NO3−) and ammonium (NH4+) uptake, regeneration and associated dissolved organic nitrogen (DON) release in a coastal upwelling system off Namibia (Benguela Current) in the austral winter of 2011 with the aim of quantifying rates of new production (Pnew) and regenerated production (Preg). These measurements were made during four consecutive coastal-offshore transects. The water parcels sampled at the different stations over the transect were classified into three groups according to the time passed from the first contact of the water with the surface during coastal upwelling (‘pseudo-age’). The average Pnew was high in freshly upwelled waters with a pseudo-age <13d (17.8mmolNm−2h−1), and decreased abruptly towards older waters (3.9 and 2.3mmolNm−2h−1 in waters with a pseudo-age of 13 to 55d, and >55d, respectively). Preg rates were similar in <13d and 13–55d waters (10.9 and 11.1mmolNm−2h−1, respectively), and decreased to 6.24mmolNm−2h−1 in waters with a pseudo-age >55d. Measuring nitrogen regeneration and DON release fluxes allowed us to correct Pnew and Preg rates. NO3− regeneration rates were low (<0.5mmolNm−2h−1), while NH4+ regeneration rates were in the range of NH4+ uptake rates (~2 to 5mmolNm−2h−1), thus influencing significantly Preg rates. Parallel studies presented in this volume indicate a relatively high abundance of dinoflagellates and mixotrophic microflagellates, which may be partly responsible for the high Preg rates observed. Our results suggest that nitrogen regeneration plays an important role in sustaining primary production in this upwelling system.

Benthic fluxes of dissolved organic nitrogen in the lower St. Lawrence estuary and implications for selective organic matter degradation

Abstract. The distribution of dissolved organic nitrogen (DON) and carbon (DOC) in sediment porewaters was determined at nine locations along the St. Lawrence estuary and in the gulf of St. Lawrence. In a previous manuscript (Alkhatib et al., 2012a), we have shown that this study area is characterized by gradients in the sedimentary particulate organic matter (POM) reactivity, bottom water oxygen concentrations, and benthic respiration rates. Based on the porewater profiles, we estimated the benthic diffusive fluxes of DON and DOC in the same area. Our results show that DON fluxed out of the sediments at significant rates (110 to 430 μmol m−2 d−1). DON fluxes were positively correlated with sedimentary POM reactivity and varied inversely with sediment oxygen exposure time (OET), suggesting direct links between POM quality, aerobic remineralization and the release of DON to the water column. DON fluxes were on the order of 30 to 64% of the total benthic inorganic fixed N loss due to denitrification, and often exceeded the diffusive nitrate fluxes into the sediments. Hence they represented a large fraction of the total benthic N exchange, a result that is particularly important in light of the fact that DON fluxes are usually not accounted for in estuarine and coastal zone nutrient budgets. In contrast to DON, DOC fluxes out of the sediments did not show any significant spatial variation along the Laurentian Channel (LC) between the estuary and the gulf (2100 ± 100 μmol m−2 d−1). The molar C / N ratio of dissolved organic matter (DOM) in porewater and the overlying bottom water varied significantly along the transect, with lowest C / N in the lower estuary (5–6) and highest C / N (&gt; 10) in the gulf. Large differences between the C / N ratios of porewater DOM and POM are mainly attributed to a combination of selective POM hydrolysis and elemental fractionation during subsequent DOM mineralization, but selective adsorption of DOM to mineral phases could not be excluded as a potential C / N fractionating process. The extent of this C- versus N- element partitioning seems to be linked to POM reactivity and redox conditions in the sediment porewaters. Our results thus highlight the variable effects selective organic matter (OM) preservation can have on bulk sedimentary C / N ratios, decoupling the primary source C / N signatures from those in sedimentary paleoenvironmental archives. Our study further underscores that the role of estuarine sediments as efficient sinks of bioavailable nitrogen is strongly influenced by the release of DON during early diagenetic reactions, and that DON fluxes from continental margin sediments represent an important internal source of N to the ocean.

Heterogeneous dissolved organic nitrogen supply over a coral reef: first evidence from nitrogen stable isotope ratios

Dissolved organic nitrogen (DON) potentially plays a major role in sustaining the high productivity and biological diversity of coral reefs. However, data are scarce regarding sources and sinks of DON. This study, for the first time, determined the 15N isotopic composition of total dissolved nitrogen (δ15NTDN), reflecting the isotopic signature of DON, in the water column over a coral reef. The uniformity in δ15NTDN during high tide (3.2 ± 0.3 ‰) indicated that the DON was mainly derived from offshore waters. In contrast, higher spatial heterogeneity of δ15NTDN (3.1 ± 0.9 ‰) and DON concentrations during low tide indicated the existence of local DON sources patchily distributed over the reef. Low δ15NTDN values located mid-reef were indicative of DON release from organisms that obtained their N via N2 fixation, whereas high δ15NTDN appeared to reflect localized release of DON by organisms exposed to dissolved inorganic nitrogen with elevated 15N, such as from terrestrial and offshore inputs. Collectively, the results highlight the importance of spatial patterns in DON release from reef communities in the N cycling of coral reefs.

Longitudinal variability of size-fractionated N2fixation and DON release rates along 24.5°N in the subtropical North Atlantic

[1] Dinitrogen (N2) fixation and dissolved organic nitrogen (DON) release rates were measured on fractionated samples (>10 µm and 10 µm and 10 µm and <10 µm fractions, respectively). The sum of both fractions correlated with aerosol optical depth at 550 nm (AOD 550 nm) (rs = 0.382, p = 0.017) and phosphate (PO43−) concentrations (rs = 0.453, p = 0.018), suggesting an enhancement of diazotrophy as a response to aerosol inputs and phosphorus availability. In contrast, DON release was constant among size fractions and did not correlate with any of these variables. We also compared N2 fixation rates obtained using the 15N2 dissolved and bubble methods. The first gave average rates 50% (49% ± 39) higher than the latter, which supports the finding that previously published N2 fixation rates are likely underestimated. We suggest that by combining N2 fixation and DON release measurements using dissolved 15N2, global N2 fixation rates could increase enough to balance oceanic fixed nitrogen budget disequilibria.

Dissolved organic nitrogen and carbon release by a marine unicellular diazotrophic cyanobacterium

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 69:69-80 (2013) - DOI: https://doi.org/10.3354/ame01621 Dissolved organic nitrogen and carbon release by a marine unicellular diazotrophic cyanobacterium Mar Benavides1,*, Nona S. R. Agawin2, Javier Arístegui1, Jan Peene3, Lucas J. Stal3,4 1Instituto de Oceanografía y Cambio Global, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Spain 2Departament de Biologia, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain 3Netherlands Institute of Sea Research (NIOZ), Department of Marine Microbiology, 4400 AC Yerseke, The Netherlands 4Department of Aquatic Microbiology, IBED, University of Amsterdam, 1090 GE Amsterdam, The Netherlands *Email: mar.benavides101@doctorandos.ulpgc.es ABSTRACT: Dinitrogen (N2) fixation rates may be underestimated when recently fixed N2 is released as dissolved organic nitrogen (DON). DON release (DONr) is substantial in the filamentous cyanobacterium Trichodesmium but has never been reported in unicellular diazotrophic cyanobacteria. We used axenic cultures of the marine unicellular diazotroph Cyanothece sp. Miami BG 043511 to measure dissolved organic matter release under N2-fixing conditions. DONr was measured as the transfer of 15N2 from the culture medium to the extracellular DON pool. On average, the DON released represented ~1% of the total N2 fixed. The average release of dissolved organic carbon, as determined by 14C, represented ~2% of the total carbon fixed. These results suggest that cultured populations of unicellular diazotrophs do not release much dissolved organic matter, but it cannot be excluded that DONr is important in the field when grazers and bacteria are present, or when the organism is exposed to environmental stresses such as turbulence, excess light, temperature changes, or nutrient limitation. KEY WORDS: N2 fixation · DON release · Unicellular cyanobacteria · Cyanothece Full text in pdf format PreviousNextCite this article as: Benavides M, Agawin NSR, Arístegui J, Peene J, Stal LJ (2013) Dissolved organic nitrogen and carbon release by a marine unicellular diazotrophic cyanobacterium. Aquat Microb Ecol 69:69-80. https://doi.org/10.3354/ame01621 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 69, No. 1. Online publication date: April 18, 2013 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2013 Inter-Research.

Salt impacts on organic carbon and nitrogen leaching from senesced vegetation

Senesced vegetation is exposed to a wide range of salt concentrations in surface waters resulting from human activities which include deicing salts and irrigation water chemistry. Both dissolved organic carbon (DOC) and salt concentrations are rising in northern hemisphere watersheds, yet there has been little investigation of sodium as a potential mechanism for DOC increases. The objective of this study was to investigate the impact of solution sodicity and salinity on DOC and dissolved organic nitrogen (DON) leaching from five types of senesced and cut vegetation. Vegetation was soaked for 24 h in a series of sodium chloride (NaCl)–calcium chloride (CaCl2) solutions with sodium adsorption ratios (SAR) of 2, 10, or 30 and electrical conductivities of 0.1 dS m−1 through 3.0 dS m−1. Vegetation was also soaked in a sodium bicarbonate (NaHCO3) solution at SAR = 30 and stream water from local watersheds with a range of sodicity and salinity. The mass of both DOC and DON released increased as SAR increased in the NaCl solutions, but the total salinity had inconsistent effects on DOC and DON release. NaHCO3 leached similar amounts of DOC and DON as NaCl. The SAR of the stream water solutions was able to explain 88 % of the variability in DOC leached from vegetation (p < 0.05). The results indicated that sodicity, quantified by SAR, had a significant impact on DOC and DON leaching from senesced vegetation and could be a potential mechanism to explain the observed increases in surface water DOC.

Dissolved organic matter release by an axenic culture of Emiliania huxleyi

Measurements of the release of dissolved organic nitrogen (DON) and carbon (DOC) were carried out on an axenic batch culture of the coccolithophorid Emiliania huxleyi. This unicellular marine alga was cultured using a media with nitrate as the sole N source and the changes of DOM concentrations measured over 14 days. Results showed that there was a significant release of DON, i.e.7.6 µM N day−1 during mid-exponential growth phase (days 5–7). The highest release of DOC was also recorded in the same growth phase and accounted for 24.0 μM C day−1.

Seasonal variability of inorganic and organic nitrogen in the North Sea

This study considers the cycling of nitrogen in the waters of the North Sea, particularly focussing on organic nitrogen. Dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and particulate organic nitrogen (PON) were measured in the North Sea over a one-year period (autumn 2004–summer 2005). The surface water concentrations of nitrate, ammonium, DON and PON during the present study ranged from <0.1–7.2 μM, <0.1–2.0 μM, 1.9–11.2 μM and 0.3–5.6 μM, respectively, with DON the dominant fraction of total nitrogen at all times. These nutrients concentrations were significantly lower compared to previous studies in the southern North Sea. The seasonal variations showed high mean surface concentrations of nitrate (4.7 ± 0.6 μM) and DON (8.9 ± 0.9 μM), low ammonium (<0.1 μM) and PON (0.8 ± 0.1 μM) in winter, shifting to low nitrate (0.3 ± 0.3 μM) and DON (4.2 ± 1.2 μM) in summer, with high ammonium (0.8 ± 0.4 μM) in autumn and PON (2.5 ± 1.2 μM) in spring. Highest mean surface DON concentration was measured in winter and may be due to resuspension of the organic matter from the bottom sediments. For autumn and spring, phytoplankton DON release was likely to be the most significant source of DON as shown by high concentrations of low molecular weight (LMW) DON and its positive correlation to chlorophyll a. Low total and LMW DON concentrations during summer were likely to be due to the uptake of the LMW DON fraction by phytoplankton and bacteria and the stratification of the water column. DON is therefore shown to be a potentially important source of nitrogen in shelf seas especially after the spring bloom has depleted nitrate to limiting concentrations.

Simulating soil freeze/thaw cycles typical of winter alpine conditions: Implications for N and P availability

Seasonally snow-covered alpine soils may be subjected to freeze/thaw cycles, particularly during years having little snow and during the late winter and early spring periods. Freeze/thaw cycles can stimulate soil mineralization and could therefore be one factor regulating nitrogen (N) and phosphorus (P) availability and cycling. In this study laboratory incubation experiments using four soils having contrasting properties have been used to characterize the change in N and P forms (microbial and soluble inorganic/organic) that occur after simulated freeze/thaw cycles. Soil samples were collected from locations representing extreme examples of either direct human management (grazed meadow (site M) and extensive grazing beneath larch (site L)) or those disturbed by more natural events (recent avalanche and colonisation by alder (site A)) and from beneath the expected forest climax vegetation beneath fir (site F). Topsoil from these sites, maintained at two different water contents (20 and 30%, w/w), were exposed to either a single (SF) or four sequential (4SF) freeze/thaw cycles. Each cycle consisted of 12 h at −9 °C and 12 h at +4 °C mimicking a diurnal pattern. A SF cycle reduced microbial N for soils from sites F and A and was accompanied by a significant increase in dissolved organic nitrogen (DON) at both moisture contents. In contrast, the microbial N of soils from M and L was not affected by the freeze/thaw cycles, suggesting a particular adaptation of soil microbes to these extremes in temperature. Freeze/thaw cycles resulted in a significant increase in the net ammonification in all soils. Extractable total dissolved N (TDN) and total dissolved P (TDP) increased in all soils after a SF cycle, however, the relative importance of the different N and P forms differed. At the lower soil moisture content, NO 3 − concentrations remained constant or slightly decreased in all soils, except that from site M. In all other soils DON appeared to replace NO 3 − as the potentially mobile N source after the freeze/thaw cycles. The relative contribution of dissolved organic P to TDP after freeze/thaw remained significant, and greater than 50% in all soils. Freeze/thaw cycles, in seasonally snow covered soils, are likely to have a selective effect on the microbial biomass. Freezing and thawing resulted in a pulse of net ammonification and DON release, which represent an important influence upon N cycling in these alpine systems.

Dissolved Organic Nitrogen Release and Bacterial Activity in the Upper Layers of the Atlantic Ocean

The variability of the percentage of extracellular dissolved organic nitrogen (DON) release (PER), along with the relationship between DON release and bacterioplankton activity, was examined during five oceanographic cruises, carried out in the upwelling region of the NW Iberian Peninsula, the SW Bay of Biscay, and a latitudinal transect in the Atlantic Ocean (50 degrees N-35 degrees S). Rates of nitrogen uptake, DON release, and bacterial production were measured at 66 stations and sampled between August 1998 and October 2000. The percentage of DON release relative to the gross uptake of ammonium (PERNH4+) ranged from 3 to 46%, whereas that relative to total nitrogen (NH4+ + NO3- + urea) gross uptake (PERtotal) varied between 21 and 82%. The highest values for both PERNH4+ and PER(total) were found in oligotrophic oceanic waters (< 0.25 mg chlorophyll a m(-3)). In oceanic environments, a positive relationship was found between nitrogen uptake and DON release rates, with a log-log linear regression slope significantly lower than 1, suggesting an inverse relationship between PER and gross nitrogen uptake rates. In contrast, in areas with higher productivity levels (> 6 mg chlorophyll a m(-3)), such as the continental shelf off the NW Iberian Peninsula, PER held constant as nitrogen uptake increased. These results suggest the dominance of different processes controlling DON release in oceanic and neritic zones. DON release rates accounted for less than 15% of the variability observed in bacterial production rates, suggesting a weak response of bacterioplankton to phytoplankton on short time scales (hours). Furthermore, nitrogen budgets showed an excess of DON release in relation to bacterial requirements.

Nitrogen uptake and dissolved organic nitrogen release in planktonic communities characterised by phytoplankton size–structure in the Central Atlantic Ocean

Ammonium, nitrate and urea uptake rates, as well as the release of dissolved organic nitrogen (DON), were measured along a latitudinal transect (49°N–33°S) in the Atlantic Ocean during the 2000 summer-to-autumn transition period and subsequently related to phytoplankton-size structure. The transect included upwelling areas, oligotrophic gyres and temperate regions and, as a result, euphotic layer integrated uptake rates of ammonium (18–355 mmol m −2 h −1 ), nitrate (5–2830 mmol m −2 h −1 ) and urea (26–1212 mmol m −2 h −1 ) covered a wide productivity spectrum. The percentage of extracellular DON release relative to total nitrogen uptake (PER) was highest when nitrate was used as the substrate (mean 42±SE 3.1%) whereas the uptake of ammonium and urea produced lower PER values (22±2.1 and 26±1.6%, respectively). Small cells (<2 μm) contributed between 20% and 80% of the total primary production; however, they accounted for 50–80% of total chlorophyll- a. In upwelling areas of the ocean, dominated by large phytoplankton, PER accounted for less than 30% of total nitrogen uptake and could be predicted from a regression equation relating the contribution of <2 μm phytoplankton to either total biomass or production. In contrast, in oligotrophic areas, small cells formed the bulk of the phytoplankton biomass, but not of primary production. In these areas, PER often exceeded 50% and was very variable, displaying a low correlation with phytoplankton size, which suggests that processes not directly related to the size of the primary producers influence DON release in the oligotrophic ocean.

Solute Release from Weathering of Spent Mushroom Substrate under Controlled Conditions

Weathering of piled material in the field is a popular method to treat spent mushroom substrate (SMS) before reuse. During the weathering process, rainfall and snowmelt pass through SMS piles and a large amount of solutes is released in the leachate. To investigate solute release patterns, the field weathering process was simulated under controlled conditions in the laboratory. Fresh SMS was packed in an acrylic column (20 cm i.d.) to 150 cm height and leached intermittently with a cumulative total of 230 cm of deionized water over 180 days. Leachate was collected and analyzed for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), electrical conductivity (EC), and inorganic salts. Solute release patterns were described using first order models, and total released solutes were calculated. The SMS leachate had DOC, DON and EC values ranging from 450 to 15,500 mg L−1, 50 to 1,700 mg L−1, and 3 to 50 dS m−1, respectively. The major inorganic cations were K+, Na+, Ca2+, Mg2+ and NH4+, and anions were Cl− and SO42−. Release of DOC, DON, and bivalent cations Ca2+ and Mg2+ were described by a first order Exponential Rise to Maximum model, while releases of monovalent ions Cl−, K+, Na+ and NH4+ were described as a first order Sigmoidal Logistic process, and SO42− release was best modeled by a Sigmoidal Chapman equation. Following six months and 230 cm applied water, 3.1 kg of DOC, 0.58 kg of dissolved N, and 8.6 kg of inorganic salts were leached per cubic meter of bulk SMS (220 kg oven dry mass). Weathering of SMS involves a significant removal of nutrients from the composted material, which can contribute to pollution of soil and groundwater.

Planktonic carbon and nitrogen cycling off northwest Spain: variations in production of particulate and dissolved organic pools

The coupling of carbon and nitrogen fluxes between dissolved and particulate organic pools was examined at a coastal location off Galicia (northwest Spain). Monthly measurements of dis- solved and particulate carbon and nitrogen stocks, along with inorganic carbon and nitrogen uptake and release of dissolved organic substances were obtained between October 1998 and September 1999. Carbon fluxes were determined using incubations of plankton with inorganic 14 C and nitrogen fluxes were determined using incubations of plankton with 15 N-ammonium. Dissolved organic car- bon (DOC) was often released near the surface, while maximum release of dissolved organic nitro- gen (DON) generally occurred near the base of the euphotic zone. Both rates were linearly related to the corresponding uptake rates; however, the percentages of extracellular release relative to total uptake (PER) were highest during low-productivity periods, when both carbon and nitrogen PER exceeded 50% in deep layers. PER values were generally below 20% near the surface. Despite max- imum values of both DOC (0.63 µmol C l -1 h -1 ) and DON release (0.06 µmol N l -1 h -1 ) being measured during spring and summer, volumetric values of both rates were not significantly correlated through the year. Such a differential behaviour caused a mismatch between DOC:DON release ratios and the C:N ratios of seston at discrete depths. In contrast, euphotic-zone integrated release rates showed average DOC:DON ratios reflecting the C:N ratios of seston. These results suggest that most of the organic matter available for microbial plankton near the coast derives from phytoplankton, which in turn is sustained by a rapid recycling of organic matter when external nutrient inputs are reduced.

Microplanktonic regeneration of ammonium and dissolved organic nitrogen in the upwelling area of the NW of Spain: relationships with dissolved organic carbon production and phytoplankton size-structure

Ammonium regeneration and dissolved organic nitrogen (DON) release were studied experimentally in the euphotic zone of shelf and oceanic waters of NW Spain in relation to coastal upwelling dynamics and the size-structure of phytoplankton communities. Incubations of plankton labelled with [15N]ammonium were made during four cruises, two of which also included size-fractionated determinations of chlorophyll a and primary production, and experimental determinations of production rates of dissolved organic carbon (DOC) using 14C. Inorganic nitrogen concentrations were mainly related to nitrate enrichment by upwelling pulses, while ammonium concentrations were generally low in all situations. Ammonium did not accumulate in the study area, suggesting a daily time scale coupling between regeneration and uptake. In contrast, DON largely exceeded inorganic nitrogen in all situations and generally increased from spring to autumn. Ammonium regeneration was positively correlated with DON release and both rates showed the largest variation in summer, with minimum values during active upwelling and maximum values when upwelling relaxed. Comparison of DON stocks and rates in different shelf areas suggests that DON release near the coast during summer was more persistent in the water than DON release in off-shelf and oceanic areas. The carbon:nitrogen ratio of DOC and DON release rates was highly variable, revealing a large excess of DOC compared with DON. This excess can be attributed to either an underestimate of total DON release (as only release from ammonium was measured) or to an enhanced production of carbon-rich organic substances by diatoms in coastal areas. By considering a broad range of trophic situations, this study reveals a fundamental difference between short term release of DOC and DON by plankton. Physiological processes (such as carbohydrate exudation by diatoms) seem to be the cause of large DOC excess, whereas trophic processes (such as grazing) are more likely the cause of enhanced DON release.

Vegetation and climate controls on potential CO2, DOC and DON production in northern latitude soils

AbstractClimatic change may influence decomposition dynamics in arctic and boreal ecosystems, affecting both atmospheric CO2 levels, and the flux of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) to aquatic systems. In this study, we investigated landscape‐scale controls on potential production of these compounds using a one‐year laboratory incubation at two temperatures (10° and 30 °C). We measured the release of CO2, DOC and DON from tundra soils collected from a variety of vegetation types and climatic regimes: tussock tundra at four sites along a latitudinal gradient from the interior to the north slope of Alaska, and soils from additional vegetation types at two of those sites (upland spruce at Fairbanks, and wet sedge and shrub tundra at Toolik Lake in northern Alaska). Vegetation type strongly influenced carbon fluxes. The highest CO2 and DOC release at the high incubation temperature occurred in the soils of shrub tundra communities. Tussock tundra soils exhibited the next highest DOC fluxes followed by spruce and wet sedge tundra soils, respectively. Of the fluxes, CO2 showed the greatest sensitivity to incubation temperatures and vegetation type, followed by DOC. DON fluxes were less variable. Total CO2 and total DOC release were positively correlated, with DOC fluxes approximately 10% of total CO2 fluxes. The ratio of CO2 production to DOC release varied significantly across vegetation types with Tussock soils producing an average of four times as much CO2 per unit DOC released compared to Spruce soils from the Fairbanks site. Sites in this study released 80–370 mg CO2‐C g soil C−1 and 5–46 mg DOC g soil C−1 at high temperatures. The magnitude of these fluxes indicates that arctic carbon pools contain a large proportion of labile carbon that could be easily decomposed given optimal conditions. The size of this labile pool ranged between 9 and 41% of soil carbon on a g soil C basis, with most variation related to vegetation type rather than climate.

Release of dissolved organic nitrogen from Scenedesmus quadricauda (Chlorophyta) and Microcystis novacekii (Cyanobacteria)

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 27:275-284 (2002) - doi:10.3354/ame027275 Release of dissolved organic nitrogen from Scenedesmus quadricauda (Chlorophyta) and Microcystis novacekii (Cyanobacteria) Fumitaka Nagao1, Tatsuo Miyazaki2,* 1Laboratory of Marine Environmental Microbiology, Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan 2Marine Biosystems Research Center, Chiba University, Amatsu-Kominato, Awa-gun, Chiba-ken 299-5502, Japan *Corresponding author. E-mail: myzkt@beige.ocn.ne.jp ABSTRACT: We measured the time-course of dissolved organic nitrogen (DON) release and inorganic nitrogen assimilation in the freshwater phytoplankton Scenedesmus quadricauda (Turp.) Brébission and Microcystis novacekii (Kom.) Comp. by the 15N tracer method. The algae were cultivated under nitrogen-limited or -replete conditions. We estimated the total DON release, release of recently assimilated nitrogen (RANR) and net nitrogen assimilation (NNA), using ammonium chloride or sodium nitrate as the nitrogen source. In the 15N tracer incubation of S. quadricauda prior to 1 h and in M. novacekii prior to 3-6 h, minimal release of total DON was observed under the nitrogen-limited conditions. This result suggests that nitrogen-limited cells efficiently use incorporated nitrogen. Appreciable release of total DON was observed in the 15N incubation of nitrogen-replete S. quadricauda and M. novacekii. RANR in both nitrogen-limited and -replete algae was smaller than, and lagged behind, release of total DON, indicating that newly assimilated nitrogen was less easily released. NNA by nitrogen-limited S. quadricauda increased from the onset of the tracer incubation, showing that ambient nitrogen was needed for nitrogen-limited cells from the beginning of the incubation. Nitrogen-replete S. quadricauda showed no NNA during the first 1 h of the tracer incubation, suggesting that the alga did not require external nitrogen immediately after being transferred from nitrogen-replete conditions. This study shows that nitrogen release and assimilation are markedly affected by the nutritional states of the algae. KEY WORDS: Ammonium · Nitrogen assimilation · Organic nitrogen release · Microcystis novacekii · Nitrate · Scenedesmus quadricauda Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 27, No. 3. Online publication date: April 16, 2002 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2002 Inter-Research.

Impact of drying and re-wetting on N, P and K dynamics in a wetland soil

As increased nutrient availability due to drainage is considered a major cause of eutrophication in wetlands rewetting of drained wetlands is recommended as a restoration measure. The effect of soil drying and rewetting on the contribution of various nutrient release or transformation processes to changed nutrient availability for plants is however weakly understood. We measured effects of soil drying and re-wetting on N mineralization, and denitrification, as well as on release of dissolved organic nitrogen (DON), phosphorus, and potassium in incubated soil cores from a wet meadow in southern Sweden. Additionally, the impact of re-wetting with sulphate-enriched water was studied. Soil drying stimulated N mineralization (3 times higher) and reduced denitrification (5 times lower) compared to continuously wet soil. In the wet cores, denitrification increased to 20 mg N m(-2) d(-1), which was much higher than denitrification measured in the field. In the field, increased inorganic-N availability for plants due to drainage seemed primarily to be caused by increased N mineralization, and less by decreased denitrification. Soil drying also stimulated the release of DON and K, but P release was not affected. Re-wetting of dried soil cores strongly stimulated denitrification (up to 160 mg N m(-2) d(-1)), but N mineralization was not significantly decreased, neither were DON or K release. In contrast, the extractable P pool increased upon soil wetting. Re-wetting with sulphate-enriched water had no effect on any of the nutrient release or transformation rates. We conclude that caution is required in re-wetting of drained wetlands, because it may unintendently cause internal eutrophication through an increased P availability for plants.