Dominant heterocyclic composition of dissolved organic nitrogen in the ocean: A new paradigm for cycling and persistence.

Marine dissolved organic nitrogen (DON) is one of the planet's largest reservoirs of fixed N, which persists even in the N-limited oligotrophic surface ocean. The vast majority of the ocean's total DON reservoir is refractory (RDON), primarily composed of low molecular weight (LMW) compounds in the subsurface and deep sea. However, the composition of this major N pool, as well as the reasons for its accumulation and persistence, are not understood. Past characterization of the analytically more tractable, but quantitatively minor, high molecular weight (HMW) DON fraction revealed a functionally simple amide-dominated composition. While extensive work in the past two decades has revealed enormous complexity and structural diversity in LMW dissolved organic carbon, no efforts have specifically targeted LMW nitrogenous molecules. Here, we report the first coupled isotopic and solid-state NMR structural analysis of LMW DON isolated throughout the water column in two ocean basins. Together these results provide a first view into the composition, potential sources, and cycling of this dominant portion of marine DON. Our data indicate that RDON is dominated by 15N-depleted heterocyclic-N structures, entirely distinct from previously characterized HMW material. This fundamentally new view of marine DON composition suggests an important structural control for RDON accumulation and persistence in the ocean. The mechanisms of production, cycling, and removal of these heterocyclic-N-containing compounds now represents a central challenge in our understanding of the ocean's DON reservoir.

Carbon Source in Tertiary Denitrification Regulates Dissolved Organic Nitrogen in Wastewater Effluent.

With global eutrophication and increasingly stringent nitrogen discharge restrictions, dissolved organic nitrogen (DON) holds considerable potential to upgrade advanced wastewater denitrification because of its large contribution to low-nitrogen effluents and stronger stimulation effect for algae. Here, we show that DON from the postdenitrification systems dominates effluent eutrophication potential under different carbon sources. Methanol resulted in significantly lower DON concentrations (0.84 ± 0.03 mg/L) compared with the total nitrogen removal-preferred acetate (1.11 ± 0.02 mg/L) (p < 0.05, ANOVA). With our well-developed mathematical model (R2 = 0.867-0.958), produced DON instead of shared (persist in both influent and effluent) and/or removed DON was identified as the key component for effluent DON variation (Pearson r = 0.992, p < 0.01). The partial least-squares path modeling analysis showed that it is the microbial community (r = 0.947, p < 0.01) rather than the predicted metabolic functions (r = 0.040, p > 0.1) that affected produced DON. Carbon sources rebuild the microorganism-DON interaction by affecting the structure of microbial communities with different abilities to generate and recapture produced DON to finally regulate effluent DON. This study revalues the importance of carbon source selection and overturns the current rationality of pursuing only the total nitrogen removal efficiency by emphasizing DON.

2-Hydroxy-1,4-Naphthoquinone: A Promising Redox Mediator for Minimizing Dissolved Organic Nitrogen and Eutrophication Effects of Wastewater Effluent.

Researchers and engineers are committed to finding effective approaches to reduce dissolved organic nitrogen (DON) to meet more stringent effluent total nitrogen limits and minimize effluent eutrophication potential. Here, we provided a promising approach by adding specific doses of 2-hydroxy-1,4-naphthoquinone (HNQ) to postdenitrification bioreactors. This approach of adding a small dosage of 0.03-0.1 mM HNQ effectively reduced the concentrations of DON in the effluent (ANOVA, p < 0.05) by up to 63% reduction of effluent DON with a dosing of 0.1 mM HNQ when compared to the control bioreactors. Notably, an algal bioassay indicated that DON played a dominant role in stimulating phytoplankton growth, thus effluent eutrophication potential in bioreactors using 0.1 mM HNQ dramatically decreased compared to that in control bioreactors. The microbe-DON correlation analysis showed that HNQ dosing modified the microbial community composition to both weaken the production and promote the uptake of labile DON, thus minimizing the effluent DON concentration. The toxic assessment demonstrated the ecological safety of the effluent from the bioreactors using the strategy of HNQ addition. Overall, HNQ is a promising redox mediator to reduce the effluent DON concentration with the purpose of meeting low effluent total nitrogen levels and remarkably minimizing effluent eutrophication effects.

In-reservoir transformation of dissolved organic matter as a function of hydrological flow.

Reservoirs are vital to meet the ever-increasing demands for freshwater in a warming climate. Dissolved organic matter (DOM) represents an important pool of carbon and can be a major concern in drinking water sources. However, insights into DOM dynamics in temperate, semi-arid reservoirs remain limited. Therefore, we investigated the variations in DOM properties in Lake Diefenbaker, a large reservoir on the Canadian Prairies, by analyzing eight years of DOM concentrations and composition through linear mixed effect modeling. Contrary to expectations, reservoir dissolved organic carbon (DOC) concentration showed no correlation with inflow from the South Saskatchewan River (p = 0.12), while dissolved organic nitrogen (DON) increased with decreasing inflow (p = 0.002). DOM optical indices (SUVA254 and E4:E6 ratio) and DOC:DON ratio revealed a pronounced influence of inflow on reservoir DOM composition (p < 0.001), i.e., allochthonous characteristics increased with increasing flow, and autochthonous characteristics increased with declining flow. Travel time corrected comparison of approximately the same water parcel along the reservoir length revealed that increasing water residence time in downstream regions led to a significant transformation in DOM composition, favoring autochthonous characteristics (mean SUVA254 reduced by 0.52 L mg-C-1 m-1, and the E4:E6 and spectral slope ratio increased by 1.6 and 0.06, respectively). Autochthonous DOC inputs likely offset the allochthonous DOC losses, which resulted in a relatively stable DOC concentration throughout the reservoir (mean 3.7 mg L-1). Additionally, the effect of a large aquaculture operation on reservoir DOM properties was investigated, but no effect was detected. The results have significant implications for managing large river-reservoirs. Autochthonous DOM poses challenges to water processing, necessitating monitoring of DOM composition for reservoir drinking water quality. Insights on climate-induced changes in DOM properties will also assist with understanding changes to habitat conditions and contaminant transport.

Evaluation of phytoplankton-stimulating potency of effluent nitrogen depending on its chemical forms: A comparison between inorganic and organic nitrogen.

Wastewater-originated nitrogen (N) is considered a primary N source in urban waters. In order to mitigate eutrophication in such waters, decreasing N discharges from wastewater treatment plants (WWTPs) is necessary. Upgrading WWTPs from conventional activated sludge (CAS) to biological nutrient removal (BNR) is the most common measure to lower levels of effluent N. However, in spite of successful N reduction through such upgrades, eutrophication persists in numerous urban waters. In this study, we investigated why decreased N discharge resulting from upgrading CAS to BNR, particularly predenitrification BNR, cannot necessarily alleviate eutrophication. Our laboratory reactor study demonstrated that compared to CAS effluent N, predenitrification BNR effluent N contains less dissolved inorganic N (DIN) but more dissolved organic N (DON), especially low molecular weight DON (LMW-DON). Bioassay-based experimental and numerical analyses found that effluent N has dissimilar phytoplankton-stimulating potency depending on its chemical forms. In particular, effluent LMW-DON showed significantly greater potency than effluent DIN. This difference in potency makes predenitrification BNR effluent N more productive for causing primary production than CAS effluent N. These results indicate that the impact of effluent N on eutrophication should be evaluated based on not only the total quantity but also the qualitative aspect of N.

Simultaneous determination of dissolved organic nitrogen, nitrite, nitrate and ammonia using size exclusion chromatography coupled with nitrogen detector.

Accurate quantification of dissolved organic nitrogen (DON) has been a challenge due to the cumulative analytical errors in the conventional method via subtracting dissolved inorganic nitrogen species (DIN) from total dissolved nitrogen (TDN). Size exclusion chromatography coupled with an organic nitrogen detector (SEC-OND) has been developed as a direct method for quantification and characterization of DON. However, the applications of SEC-OND method still subject to poor separations between DON and DIN species and unsatisfied N recoveries of macromolecules. In this study, we packed a series of SEC columns with different lengths and resin materials for separation of different N species and designed an independent vacuum ultraviolet (VUV) oxidation device for complete oxidation converting N species to nitrate. To guarantee sufficient N recoveries, the operation conditions were optimized as oxidation time ≥ 30 min, injection mass (sample concentration × injection volume) < 1000 µL × mg-N/L for macromolecular proteins, and neutral pH mobile eluent. The dissolved O2 concentration in SEC mobile phase determined the upper limit of VUV oxidation at a specific oxidation time. Compared to conventional HW50S column (20 × 250 mm), HW40S column (20 × 350 mm) with mobile phase comprising of 1.5 g/L Na2HPO4·2H2O + 2.5 g/L KH2PO4 (pH = 6.85) could achieve a better separation of DON, nitrite, nitrate, and ammonia. When applied to river water, lake water, wastewater effluent, groundwater, and landfill leachate, the SEC-OND method could quantify DON as well as DIN species accurately and conveniently even the DIN/TDN ratio reached 0.98.

Shifting stoichiometry: Long-term trends in stream-dissolved organic matter reveal altered C:N ratios due to history of atmospheric acid deposition.

Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past 50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long-term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy-nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by-products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control.

Novel insight into dissolved organic nitrogen (DON) transformation along wastewater treatment processes with special emphasis on endogenous-source DON.

Knowledge of endogenous-source dissolved organic nitrogen (esDON) produced in wastewater treatment processes is critical for evaluating its potential impacts on receiving waters because esDON is a recognized concern, as it causes eutrophication. However, differentiating esDON from influent residual DON in real wastewater is always a challenge. Here, we deciphered esDON information in DON transformation processes along a full-scale wastewater treatment train by combining multiple chemometric tools with ion-mobility separation quadrupole time-of-flight mass spectrometry (IMS-QTOF MS) analyses. In total, DON became more refractory and compact with shorter carbon chains and fewer nitrogen atoms, and esDON composed a nonnegligible fraction that dominated DON transformation and characteristics. New esDON produced in treatment processes constituted a crucial part (>35.5%) of wastewater DON, and its contributions to wastewater DON are augmented along the train. Evidence of molecular conformations further confirmed dominant roles of esDON in DON characteristics. Moreover, esDON participated in 46.7% of core biochemical reaction networks, explaining the importance of esDON in DON transformation. Our study offers a tool to gain esDON characteristics and transformation mechanisms, and highlights the importance to control esDON for alleviating adverse influences from DON in receiving waters.

Synthesis and characterization of a novel magnetic resin (m-MAR resin) and its removal performance for alkaline amino acids.

Alkaline amino acids as dissolved organic nitrogen (DON) have raised much concern in drinking water treatment due to poor removal in conventional treatment process and high potential for nitrogenous disinfection by-products (N-DBPs). This work was intended to devise a new magnetic adsorption resin (noted as m-MAR resin) for the efficient reduction of alkaline amino acids and explore the application potential of combined MIEX and m-MAR resins. The distribution and composition of DON and amino acids was clarified for different water sources in Lake Taihu basin, in which alkaline amino acids accounted for a higher proportion. The removal of different nitrogenous organics by MIEX resin was also examined, where the resin was effective in removing phycocyanin (65.6%) and glutamic acid (74.2%), reducing the generation of disinfection by-products (DBPs). The m-MAR resin was manufactured and characterized to cope with alkaline amino acids, and batch experiments were undertaken to investigate its adsorption behaviors on histidine and arginine under different operating conditions. The maximal adsorption capacities of arginine and histidine onto m-MAR resin were 2.84 mg/g and 1.62 mg/g, respectively, which was better than MIEX resin. The removal mechanism of the two basic amino acids by m-MAR resin was mainly due to the hydrogen bonding and the acid-base reaction. Moreover, the reusability of the m-MAR resin was elucidated after six successive adsorption-desorption cycles. Finally, the effectiveness of combined MIEX and m-MAR resin in treating DON derived from Microcystis aeruginosa reached 35.2% and the DON concentration in Lake Taihu could be reduced from 0.56 to 0.16 mg/L, which simultaneously decreased the generation potential of N-DBPs. The enhancement of coagulation by the combined process of m-MIER and m-MAR as pretreatment was estimated.

Low-molecular-weight dissolved organic nitrogen transformation behavior in concentrated leachate by O3 and •OH: Partition, molecular insight, and potential precursor-product relationship.

Low-molecular-weight dissolved organic nitrogen (LMW-DON) is an emerging issue in concentrated leachate (CL). Ozonation is crucial to remove LMW-DON, but selectivity mechanisms of different reactive oxygen species were unknown. Here, reactions of O3 and •OH with LMW-DON at different dosages were determined from composition, unsaturation/redox potential, and precursor-product relationship. The molecular weight of LMW-DON in CL presented a normal distribution and 76.5% was below 450 Da. LMW-DON with 400-1000 Da increased to 55.6%-66.7% and O/Cwa increased by over 40.0% due to electrophilic substitution of O3. LMW-DON with <400 Da and 550-1000 Da were preferentially degraded by •OH at the low and high O3 dosage, respectively. O3 preferred to remove lipid-like (69.1%), protein-like (58.2%), and amino sugars-like (72.8%) LMW-DON, whereas •OH preferred to the refractory LMW-DON, such as carbohydrates-like (71.1%), lignin-like (49.6%), and tannins-like (72.5%) LMW-DON. Forty-three transformation reactions were quantified using mass difference analysis, and O3 preferred to oxygen addition (e.g., +2O) and conversed amino to nitro groups, and saturated LMW-DON increased via unsaturated bonds rupture. •OH attacked the carbon groups (e.g., -CH2) and nitrogen groups (e.g., -NH3+O, -NO2+H). These findings provide molecular evidence for the selectivity of oxidants with LMW-DON and improve the ozonation application in wastewater treatment.

Transformation mechanism and fate of dissolved organic nitrogen (DON) in a full-scale drinking water treatment.

Dissolved organic nitrogen (DON) has attracted much attention in drinking water treatment due to its potential to produce nitrogenous disinfection by-products (N-DBPs). This work was designed to explore the transformation and fate of DON and dissolved inorganic nitrogen (DIN) in drinking water treatment. The changes of DON and formation of N-DBPs were evaluated along the water treatment route (i.e., pre-ozonation and biological-contact oxidation, delivery pipes' transportation, coagulation-sedimentation, sand filtration, post-ozonation, biological activated carbon, ultrafiltration and disinfection) of drinking water treatment plant (DWTP). The transformation mechanism of DON was comprehensively investigated by molecular weight fractionation, three-dimensional fluorescence, LC-OCD (Liquid Chromatography-Organic Carbon Detection), total free amino acids. A detailed comparison was made between concentrations and variations of DON and DIN affected by seasons in the drinking water treatment. Regardless of seasonal variation in raw water concentration, the DON removal trends between different treatment processes remain constant in the present study. Compared to other treatment processes, pre-ozonation and coagulation-sedimentation exhibited the dominant DON removal in different seasons, i.e., 11.13%-14.45% and 14.98%-22.49%, respectively. Contrary, biological-contact oxidation and biological activated carbon negatively impacted the DON removal, in which DON increased by 1.76%-6.49% in biological activated carbon. This may be due to the release of soluble microbial products (SMPs) from bacterial metabolism, which was further validated by the rise of biopolymers in LC-OCD.

The interaction of dissolved organic nitrogen removal and microbial abundance in iron-filings based green environmental media for stormwater treatment.

Nonpoint sources pollution from agricultural crop fields and urbanized regions oftentimes have elevated concentrations of dissolved organic nitrogen (DON) in stormwater runoff, which are difficult for microbial communities to decompose. The impact of elevated DON can be circumvented through the use of green sorption media, such as Biosorption Activated Media (BAM) and Iron-Filing Green Environmental Media (IFGEM), which, as integral parts of microbial ecology, can contribute to the decomposition of DON. To compare the fate, transport, and transformation of DON in green sorption media relative to natural soil (control), a series of fixed-bed columns, which contain natural soil, BAM, and two types of IFGEM, respectively, were constructed to compare nutrient removal efficiency under three distinct stormwater influent conditions containing nitrogen and phosphorus. The interactions among six microbial species, including ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, complete ammonia oxidation (comammox) bacteria, anaerobic ammonium oxidation (anammox) bacteria, dissimilatory nitrate reduction to ammonium bacteria, and iron-reducing bacteria, were further analyzed from microbial ecology perspectives to determine the DON impact on nutrient removal in BAM and IFGEM. Natural soil was only able to achieve adequate DON transformation at the influent condition of lower nutrient concentration. However, the two types of IFGEM showed satisfactory nutrient removals and achieved greater transformation of DON relative to BAM when treating stormwater in all three influent conditions.

Fate and transport processes of nitrogen in biosorption activated media for stormwater treatment at varying field conditions of a roadside linear ditch.

Roadside drainage networks can result in changes to watershed hydrology and water quality. By acting as hydrological links between urban development, agricultural fields, and natural streams, roadside ditches may be modified by filling in some green sorption media to control nitrogen pollution. Biosorption activated media (BAM), one of the green sorption media, are composed of sand, tire crumb, and clay, which can remove nitrogen from stormwater and groundwater through integrated hydrological, chemophysical, and microbial processes. The fate and transport processes of interest are complicated by internal microbial processes including ammonification, nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA), each of which is controlled by different microbial species in addition to some varying field conditions. In this study, BAM was tested in a suite of columns to address site-specific physical, chemical and biological concerns driven by in situ traffic compaction, carbon availability, and animal impact (such as gopher turtles, moles, and ants) all of which impose different impacts on nitrogen fate and transport processes that may be signified by changing dissolved organic nitrogen species (DONs). The traffic compaction condition resulted in the most suitable hydraulic retention time in the hydrological process, which is beneficial for the assimilation of DONs in a long-term carbon rich environment due to biofilm expansion. Denitrifiers were the most predominant microbial population and the microbial species of DNRA were the second most predominant one in all three field conditions. However, the relationship of denitrifiers and DNRA in BAM can be shifted from commensalism to competition or even inhibition after carbon addition in microbial ecology.

Vacuum ultraviolet irradiation for mitigating dissolved organic nitrogen and formation of haloacetonitriles.

The main objective of this work was to investigate the feasibility of using vacuum ultraviolet (VUV, 185 + 254 nm) and ultraviolet (UV, 254 nm) for the reduction of dissolved organic nitrogen (DON) and haloacetonitrile formation potential (HANFP) of surface water and treated effluent wastewater samples. The results showed that the reduction of dissolved organic carbon (DOC), DON, hydrophobicity (HPO), absorbance at 254 nm (UV254), and fluorescence excitation-emission matrix (FEEM) of both water samples by VUV was higher compared to using UV. The addition of H2O2 remarkably improved the performances of VUV and UV. VUV/H2O2 exhibited the highest removal efficiency for DOC and DON. Even though HANFP increased at the early stage, its concentration decreased (19-72%) at the end of treatment (60 min). Decreases in DON (30-41%) and DOC (51-57%) led to HANFP reduction (53-72%). Moreover, FEEM revealed that substantial reduction in soluble microbial product-like compounds (nitrogen-rich organic) had a strong correlation with HANFP reduction, implying that this group of compounds act as a main precursor of HANs. The VUV/H2O2 system significantly reduced HANFP more than UV/H2O2 and therefore is suitable for controlling HAN precursors and HAN formation in drinking water and reclaimed wastewater.

Nutrient export and elemental stoichiometry in an urban tropical river.

Nutrient inputs to surface waters are particularly varied in urban areas, due to multiple nutrient sources and complex hydrologic pathways. Because of their close proximity to coastal waters, nutrient delivery from many urban areas can have profound impacts on coastal ecology. Relatively little is known about the temporal and spatial variability in stoichiometry of inorganic nutrients such as dissolved silica, nitrogen, and phosphorus (Si, N, and P) and dissolved organic matter in tropical urban environments. We examined nutrient stoichiometry of both inorganic nutrients and organic matter in an urban watershed in Puerto Rico served by municipal sanitary sewers and compared it to two nearby forested catchments using samples collected weekly from each river for 6 yr. Urbanization caused large increases in the concentration and flux of nitrogen and phosphorus (2- to 50-fold), but surprisingly little change in N:P ratio. Concentrations of almost all major ions and dissolved silica were also significantly higher in the urban river than the wildland rivers. Yield of dissolved organic carbon (DOC) was not increased dramatically by urbanization, but the composition of dissolved organic matter shifted toward N-rich material, with a larger increase in dissolved organic nitrogen (DON) than DOC. The molar ratio of DOC:DON was about 40 in rivers draining forested catchments but was only 10 in the urban river. Inclusion of Si in the assessment of urbanization's impacts reveals a large shift in the stoichiometry (Si:N and Si:P) of nutrient inputs. Because both Si concentrations and watershed exports are high in streams and rivers from many humid tropical catchments with siliceous bedrock, even the large increases in N and P exported from urban catchments result in delivery of Si, N, and P to coastal waters in stoichiometric ratios that are well in excess of the Si requirements of marine diatoms. Our data suggest that dissolved Si, often neglected in watershed biogeochemistry, should be included in studies of urban as well as less developed watersheds due to its potential significance for marine and lacustrine productivity.

Subtropical urban turfs: Carbon and nitrogen pools and the role of enzyme activity.

Urban grasslands not only provide a recreational venue for urban residents, but also sequester organic carbon in vegetation and soils through photosynthesis, and release carbon dioxide through respiration, which largely contribute to carbon storage and fluxes at regional and global scales. We investigated organic carbon and nitrogen pools in subtropical turfs and found that dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) were regulated by several factors including microbial activity which is indicated by soil enzymatic activity. We observed a vertical variation and different temporal patterns in both soil DOC, DON and enzyme activities, which decreased significantly with increasing soil depths. We further found that concentration of soil DON was linked with turf age. There were correlations between grass biomass and soil properties, and soil enzyme activities. In particular, soil bulk density was significantly correlated with soil moisture and soil organic carbon (SOC). In addition, DOC correlated significantly with DON. Significant negative correlations were also observed between soil total dissolved nitrogen (TDN) and grass biomass of Axonopus compressus and Zoysia matrella. Specifically, grass biomass was significantly correlated with the soil activity of urease and ß-glucosidase. Soil NO3-N concentration also showed negative correlations with the activity of both ß-glucosidase and protease but there were no significant correlations between cellulase and soil properties or grass biomass. Our study demonstrated a relationship between soil C and N dynamics and soil enzymes that could be modulated to enhance SOC pools through management and maintenance practices.

Initial nitrogen enrichment conditions determines variations in nitrogen substrate utilization by heterotrophic bacterial isolates.

BACKGROUND: The nitrogen (N) cycle consists of complex microbe-mediated transformations driven by a variety of factors, including diversity and concentrations of N compounds. In this study, we examined taxonomic diversity and N substrate utilization by heterotrophic bacteria isolated from streams under complex and simple N-enrichment conditions. RESULTS: Diversity estimates differed among isolates from the enrichments, but no significant composition were detected. Substrate utilization and substrate range of bacterial assemblages differed within and among enrichments types, and not simply between simple and complex N-enrichments. CONCLUSIONS: N substrate use patterns differed between isolates from some complex and simple N-enrichments while others were unexpectedly similar. Taxonomic composition of isolates did not differ among enrichments and was unrelated to N use suggesting strong functional redundancy. Ultimately, our results imply that the available N pool influences physiology and selects for bacteria with various abilities that are unrelated to their taxonomic affiliation.

Soil warming opens the nitrogen cycle at the alpine treeline.

Climate warming may alter ecosystem nitrogen (N) cycling by accelerating N transformations in the soil, and changes may be especially pronounced in cold regions characterized by N-poor ecosystems. We investigated N dynamics across the plant-soil continuum during 6 years of experimental soil warming (2007-2012; +4 °C) at a Swiss high-elevation treeline site (Stillberg, Davos; 2180 m a.s.l.) featuring Larix decidua and Pinus uncinata. In the soil, we observed considerable increases in the NH4+ pool size in the first years of warming (by >50%), but this effect declined over time. In contrast, dissolved organic nitrogen (DON) concentrations in soil solutions from the organic layer increased under warming, especially in later years (maximum of +45% in 2012), suggesting enhanced DON leaching from the main rooting zone. Throughout the experimental period, foliar N concentrations showed species-specific but small warming effects, whereas δ15 N values showed a sustained increase in warmed plots that was consistent for all species analysed. The estimated total plant N pool size at the end of the study was greater (+17%) in warmed plots with Pinus but not in those containing Larix, with responses driven by trees. Irrespective of plot tree species identity, warming led to an enhanced N pool size of Vaccinium dwarf shrubs, no change in that of Empetrum hermaphroditum (dwarf shrub) and forbs, and a reduction in that of grasses, nonvascular plants, and fine roots. In combination, higher foliar δ15 N values and the transient response in soil inorganic N indicate a persistent increase in plant-available N and greater cumulative plant N uptake in warmer soils. Overall, greater N availability and increased DON concentrations suggest an opening of the N cycle with global warming, which might contribute to growth stimulation of some plant species while simultaneously leading to greater N losses from treeline ecosystems and possibly other cold biomes.

Bioavailability of wastewater derived dissolved organic nitrogen to green microalgae Selenastrum capricornutum, Chlamydomonas reinhardtii, and Chlorella vulgaris with/without presence of bacteria.

Effluent dissolved organic nitrogen (DON) is problematic in nutrient sensitive surface waters and needs to be reduced to meet demanding total dissolved nitrogen discharge limits. Bioavailable DON (ABDON) is a portion of DON utilized by algae or algae+bacteria, while biodegradable DON (BDON) is a portion of DON decomposable by bacteria. ABDON and BDON in a two-stage trickling filter (TF) wastewater treatment plant was evaluated using three different microalgal species, Selenastrum capricornutum, Chlamydomonas reinhardtii and Chlorella vulgaris and mixed cultured bacteria. Results showed that up to 80% of DON was bioavailable to algae or algae+bacteria inoculum while up to 60% of DON was biodegradable in all the samples. Results showed that C. reinhardtii and C. vulgaris can be used as a test species the same as S. capricornutum since there were no significant differences among these three algae species based on their ability to remove nitrogen species.

An aquatic vertebrate can use amino acids from environmental water.

Conventional food-web theory assumes that nutrients from dissolved organic matter are transferred to aquatic vertebrates via long nutrient pathways involving multiple eukaryotic species as intermediary nutrient transporters. Here, using larvae of the salamander Hynobius retardatus as a model system, we provide experimental evidence of a shortcut nutrient pathway by showing that H. retardatus larvae can use dissolved amino acids for their growth without eukaryotic mediation. First, to explore which amino acids can promote larval growth, we kept individual salamander larvae in one of eight different high-concentration amino acid solutions, or in control water from which all other eukaryotic organisms had been removed. We thus identified five amino acids (lysine, threonine, serine, phenylalanine, and tyrosine) as having the potential to promote larval growth. Next, using 15N-labelled amino acid solutions, we demonstrated that nitrogen from dissolved amino acids was found in larval tissues. These results suggest that salamander larvae can take up dissolved amino acids from environmental water to use as an energy source or a growth-promoting factor. Thus, aquatic vertebrates as well as aquatic invertebrates may be able to use dissolved organic matter as a nutrient source.