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.

Genomic insight into strategy, interaction and evolution of nitrifiers in metabolizing key labile-dissolved organic nitrogen in different environmental niches

Ammonia-oxidizing archaea (AOA) and bacteria (AOB), nitrite-oxidizing bacteria (NOB), and complete ammonia oxidizers (comammox) are responsible for nitrification in nature; however, some groups have been reported to utilize labile-dissolved organic nitrogen (LDON) for satisfying nitrogen demands. To understand the universality of their capacity of LDON metabolism, we collected 70 complete genomes of AOA, AOB, NOB, and comammox from typical environments for exploring their potentials in the metabolism of representative LDON (urea, polyamines, cyanate, taurine, glycine betaine, and methylamine). Genomic analyses showed that urea was the most popular LDON used by nitrifiers. Each group harbored unique urea transporter genes (AOA: dur3 and utp, AOB: utp, and NOB and comammox: urtABCDE and utp) accompanied by urease genes ureABC. The differentiation in the substrate affinity of these transporters implied the divergence of urea utilization efficiency in nitrifiers, potentially driving them into different niches. The cyanate transporter (cynABD and focA/nirC) and degradation (cynS) genes were detected mostly in NOB, indicating their preference for a wide range of nitrogen substrates to satisfy high nitrogen demands. The lack of genes involved in the metabolism of polyamines, taurine, glycine betaine, and methylamines in most of nitrifiers suggested that they were not able to serve as a source of ammonium, only if they were degraded or oxidized extracellularly as previously reported. The phylogenetic analyses assisted with comparisons of GC% and the Codon Adaptation Index between target genes and whole genomes of nitrifiers implied that urea metabolic genes dur3 and ureC in AOA evolved independently from bacteria during the transition from Thaumarchaeota to AOA, while utp in terrestrial AOA was acquired from bacteria via lateral gene transfer (LGT). Cyanate transporter genes cynS and focA/nirC detected only in a terrestrial AOA Candidadus Nitrsosphaera gargensis Ga9.2 could be gained synchronously with Nitrospira of NOB by an ancient LGT. Our results indicated that LDON utilization was a common feature in nitrifiers, but metabolic potentials were different among nitrifiers, possibly being intensely interacted with their niches, survival strategies, and evolutions.

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.

Availability of Nitrogen in Soil for Irrigated Cotton Following Application of Urea and 3,4-Dimethylpyrazole Phosphate-Coated Urea in Concentrated Bands

Low nitrogen (N) fertilizer use efficiency for irrigated cotton has been attributed to the limited ability of tap roots to access N from concentrated subsurface bands, or the preferential root uptake of microbially-mineralized dissolved organic N. This work investigated how applying high-rate banded urea affects the availability of N in soil and the capacity of cotton roots to take up N. Soil was analyzed for water-extractable total dissolved N and inorganic N species after urea or urea coated with 3,4-dimethylpyrazole phosphate (DMPP) was applied at concentrations of 261, 455, 461, and 597 mg N kg^-1 of (air-dry) soil (mean bulk density: 1.01 g cm^-3). A mass balance was used to compare N applied as fertilizer and in unfertilized soil (supplied N) with the N recovered from soil within the cylinders (recovered N) at five plant growth phases. Root uptake was estimated by comparing ammonium-N (NH₄-N) and nitrate-N (NO₃-N) in soil sampled from within cylinders with soil sampled from immediately outside. Recovered N was up to 100% above supplied N within 30 days of applying urea above 261 mg N kg^-1 of soil. Significantly lower NO₃-N in soil sampled from immediately outside the cylinders suggests urea application stimulates cotton root uptake. The use of DMPP-coated urea prolonged high NH₄-N in soil and inhibited the mineralization of released organic N. These results imply the release of previously sequestered soil organic N within 30 days of applying concentrated urea enhances the availability of NO₃-N in the rhizosphere, reducing N fertilizer use efficiency.

Nocturnal dissolved organic matter release by turf algae and its role in the microbialization of reefs

The increased release of dissolved organic matter (DOM) by algae has been associated with the fast but inefficient growth of opportunistic microbial pathogens and the ongoing degradation of coral reefs. Turf algae (consortia of microalgae and macroalgae commonly including cyanobacteria) dominate benthic communities on many reefs worldwide. Opposite to other reef algae that predominantly release DOM during the day, turf algae containing cyanobacteria may additionally release large amounts of DOM at night. However, this night-DOM release and its potential contribution to the microbialization of reefs remains to be investigated.We first tested the occurrence of hypoxic conditions at the turf algae-water interface, as a lack of oxygen will facilitate the production and release of fermentation intermediates as night-time DOM. Second, the dissolved organic carbon (DOC) release by turf algae was quantified during day time and nighttime, and the quality of day and night exudates as food for bacterioplankton was tested. Finally, DOC release rates of turf algae were combined with estimates of DOC release based on benthic community composition in 1973 and 2013 to explore how changes in benthic community composition affected the contribution of night-DOC to the reef-wide DOC production.A rapid shift from supersaturated to hypoxic conditions at the turf algae-water interface occurred immediately after the onset of darkness, resulting in night-DOC release rates similar to those during daytime. Bioassays revealed major differences in the quality between day and night exudates: Night-DOC was utilized by bacterioplankton two times faster than day-DOC, but yielded a four times lower growth efficiency. Changes in benthic community composition were estimated to have resulted in a doubling of DOC release since 1973, due to an increasing abundance of benthic cyanobacterial mats (BCMs), with night-DOC release by BCMs and turf algae accounting for >50% of the total release over a diurnal cycle.Night-DOC released by BCMs and turf algae is likely an important driver in the microbialization of reefs by stimulating microbial respiration at the expense of energy and nutrient transfer to higher trophic levels via the microbial loop, thereby threatening the productivity and biodiversity of these unique ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

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.

A protocol for nitrogen isotopic measurement of dissolved organic nitrogen with a combination of oxidation-denitrification and gas phase diffusion methods

Isotopic tracing technique is one of the most effective methods to identify nitrogen source and fate in aquatic environments. Although dissolved organic nitrogen (DON) is a key component in nitrogen cycles, information on nitrogen stable isotope ratios in DON (δ¹⁵N-DON) is limitedly available for its low recovery through a direct measurement. Indirect measurement is based on mass balance calculations and easy to use with high recovery of DON. However, in theory, the result from mass balance calculation is sensitive to the level of DON content, and its applicability remains to be examined for waters containing a variety of DON content in total dissolved nitrogen (TDN). In this study, we established a protocol for indirect measurement of δ¹⁵N-DON values based on the combination of multiple analytical methods. Precision and accuracy in the measurement were assessed by varying the composition of DON and dissolved inorganic nitrogen, and quantitation thresholds were presented at different acceptable levels. The results illustrated an advantage of the developed protocol possibly applicable to water samples particularly with low DON content that is commonly detected in freshwater. This method is expected to expand the use of isotope tracing techniques for understanding the nitrogen cycle in water environments.

Molecular characterization of dissolved organic nitrogen during anoxic/oxic and anammox processes using ESI FT-ICR MS

Dissolved organic nitrogen (DON) is a component of wastewater with a negative influence on the environment. The removal of DON is conducted through the anoxic/oxic (A/O) and anammox processes. However, the mechanisms and chemical preferences in the removal of DON compounds have not been understood and compared so far. This study, for the first time, comparatively investigated the molecular-level characteristics of DON during both processes by using FT-ICR MS (Fourier transform ion cyclotron resonance mass spectrometry). The results indicated that the number of DON formulas increased from 1844 to 1935 during A/O process, and from 2784 to 3242 during anammox process, highlighting the increase in complexity of DON after undergoing both processes. DON with high saturation and aliphatic structures was removed by A/O process, whereas highly unsaturated and aromatic structures were removed by anammox process. For DON without S atom, Lignin-like and tannin-like ones were resistant to both processes and protein-like and condensed aromatic structures were resistant to anammox process. The complementarity of these two processes provided a sequential combination with sufficient theoretical support to improve DON removal efficiency. PRACTITIONER POINTS: Molecular components of dissolved organic nitrogen characterized by ESI FT-ICR MS. DON removal preferences of A/O and anammox processes evaluated. A/O and anammox processes are effective to remove aliphatic and aromatic DON, respectively. Complementarity in removal preferences of A/O and anammox processes can remove recalcitrant DON of each other. Sequential A/O and anammox processes can improve DON removal.

[Distribution of exogenous nitrogen fractions and their fate in moss-dominated biological soil crusts]

Nitrogen (N) labeled with ¹⁵N was evenly added into plots of moss-dominated biological soil crusts (BSCs) and bare soil on the Chinese Loess Plateau. After that, the surface BSCs and bare soil samples were continuously collected within 1-30 days. The ¹⁵N content of each N fraction in soil, microorganisms, and mosses was measured for each sample. The effects of BSCs on soil N fate and cycling was determined through analyzing the differences in the distribution of ¹⁵N fractions between the BSCs and bare soil. Our results showed that: 1) The ¹⁵N content of total N (TN), microbial biomass N (MBN), and dissolved organic N (DON) in the BSCs was 2.9, 17.5, and 9.0 times higher than that in the bare soil, respectively. The ¹⁵N content of moss plants in the BSCs was 4.73 mg kg^-1. 2) The residual rate of ¹⁵N in the BSCs and bare soil was 13.0% and 3.3%, respectively, indicating that the N fixing and holding ability of BSCs was four times higher than that of bare soil. The percentage of each ¹⁵N fraction in T¹⁵N in the BSCs was in the order of MBN (54.3%)&gt;moss plant N (22.5%)&gt;DON (6.2%), while that in the bare soil was in the order of MBN (11.5%)&gt;DON (2.6%). Over all, microorganisms and mosses in the BSCs had 65.3% higher capacity of N fixation as compared with the bare soil. 3) The transferred amount and storage capacity of MB¹⁵N in the BSCs were 17.2 and 20.5 times higher than that in the bare soil, respectively. Accordingly, the turnover rate of MB¹⁵N in the BSCs and bare soil was 5.8 and 7.2 times per month, respectively, with the turnover time of MB¹⁵N in the BSCs being 1.2 times longer than that in bare soil. In conclusion, BSCs fix and hold more N than bare soil and change the distribution of each N fraction, implying that BSCs play a critical role in N cycling in dryland ecosystems.

Long-term Changes of Disinfection Byproducts in Treatment of Simulated Ballast Water

This study investigated the changes in concentrations of haloacetic acids (HAAs) and haloacetonitriles (HANs) as disinfection byproducts (DBPs) for different storage times (as long as 20 days) and temperatures (5 to 20°C). A ship's voyage after treatment of its ballast water with active substances was considered. The HAA showed a clear trend of increasing concentration only with storage time, especially for dibromoacetic acid (DBAA). Dissolved organic nitrogen concentration was increased by the decomposition of dead organisms at 10 days, and then reacted with the remaining total residual oxidants, resulting in increased concentration of DBPs. An environmental risk assessment indicated that DBAN and monochloroacetic acid (MCAA) could have a negative impact on the marine environment. This study suggests that, because all international vessels must have a ballast water management system installed by September, 2024, the conc e ntra tio ns of DBPs, especially DBAN, MCAA, and DBAA, should be monitored in the waters at major international ports.

[Effects of nitrogen deposition on carbon and nitrogen contents in soil aggregates in temperate forests of Changbai Mountain, Northeast China.]

Nitrogen deposition is one of the most important factors affecting carbon (C) and nitrogen (N) cycling in terrestrial ecosystem. A six-year N addition experiment was carried out to explore how N deposition affected C and N fractions in soil aggregates in the secondary aspen forest (YHL) and primary Korean pine broad-leaved forest (HSL). We investigagted the effects of N addition on dissolved organic carbon and nitrogen (DOC and DON), microbial biomass carbon and nitrogen (MBC and MBN), particulate organic carbon and nitrogen (POC and PON) in soil aggregates with different particle sizes. The results showed that the contents of carbon and nitrogen fractions generally increased with the decrease of particle sizes of soil aggregates except for POC and PON. In soil aggregates of HSL, POC and PON significantly decreased by 20.7% and 22.6% in N treatment, respectively, but DOC increased by 11.6%. In YHL, N addition treatment had no signi-ficant effect on C and N fractions in soil aggregates. Total carbon or nitrogen correlated well with the active C and N fractions in soil aggregates, with a great significant negative correlation between POC and DOC in HSL (r=-0.503) and a significant positive correlation between DOC and MBC (r=0.462). In HSL, the negative effect of N treatment on POC and PON and the positive effect on DOC was mainly attributed to the accelerated decomposition of POM by stimulating microbial activity. Soil C and N pools in HSL were more vulnerable to N deposition than that in YHL.

Activation of permanganate with hydrogen sulfite for enhanced oxidation of a typical amino acid

Dissolved organic nitrogen (DON) has drawn more attention from research into drinking water treatment because of its potential to form nitrogenous disinfection byproducts. Amino acids are important DON constituents that are difficult to remove by conventional water treatment. The permanganate/hydrogen sulfite (PM/BS) system, a novel oxidation process, is used to remove a typical amino acid, glutamate. The factors that affect removal mechanism and the degradation pathways were studied. The results show that the rates of DON and glutamate removal in the PM/BS system within 10 min were 55% and 90%, respectively. The optimum pH for glutamate degradation was 5, and the optimal dosages of KMnO₄ and NaHSO₃ were 1 mM and 5 mM, respectively. Activation of MnO4- by HSO3- resulted in the oxidation of glutamate at very high rates. As shown by electronic spin resonance (ESR) experiments, Mn (III), SO3-⋅ , and •OH were involved in the degradation process. The oxidation products suggest that most of the glutamate undergoes oxidation to ammonia nitrogen within 10 min, eventually forming N₂ in the process. No evidence shows that NO3- and NO2- formed during the entire process. High-performance liquid chromatography-mass spectrometry results reveal that glutamate degradation consists of two separate pathways involving Mn(III), SO3-⋅ , and •OH, both of which convert -NH₂ into NH4+ or directly to N₂. It can be indicated that nitrogenous disinfection by-products (N-DBPs) formation decreased in the degradation process, as shown by gas chromatograph detection.

Characteristics of Dissolved Organic Nitrogen in the Sediments of Six Water Sources in Taihu Lake, China

KCl-extractable sediment dissolved organic nitrogen (KS-DON) extracted from sediments near drinking water intakes of six drinking water sources in Taihu Lake in China was partitioned into hydrophobic and hydrophilic fractions and high/low molecular weight fractions. The results showed that the total dissolved nitrogen (TDN) contents of the extracts ranged from 67.78 to 128.27 mg/kg. KS-DON was the main TDN species, accounting for more than 50%, with NH₄⁺-N and NO₃⁻-N averaging 30% and 20%, respectively. The molecular weight fractions of <1 kDa accounted for almost half of KS-DON. Hydrophilic compounds accounted for more than 75% of KS-DON. Three fluorescence peaks were identified: soluble microbial byproducts (A); protein-like substances (B); and humic acid-like substances (C). It is concluded that the KS-DON in Taihu Lake sources has higher bioavailability and higher risk of endogenous release. Ecological dredging and establishment of constructed wetlands are possible measures to reduce the release of endogenous nitrogen.

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.

[Enhanced Coagulation as a Pretreatment for Low Temperature Wastewater]

The effects of low temperature on enhanced coagulation were studied. A new composite coagulant called SynthA was synthesized. The effects of enhanced coagulation on the removals of dissolved organic matter, dissolved organic nitrogen, and so on under room temperature or low temperature (2-5℃) were determined, and their influences on biological treatments were investigated by using membrane fractionation distribution, three-dimensional fluorescence spectrum (3DEEM), and differential ultraviolent absorbance. The results showed that, under room temperature, the removals of particulate COD, particulate nitrogen, colloidal COD, and colloidal nitrogen were highly correlated with turbidity reduction by coagulation using aluminum chloride (AlCl₃), poly aluminum chloride (PACl), and SynthA as coagulants separately, while the relationship was not clear between the dissolved parameters and turbidity reduction. The reduction of fluorescence value of dissolved organic matter after coagulation was much higher than that of dissolved COD. Dissolved organic nitrogen (DON) is removed to the greatest extent by preset coagulation along with particulate nitrogen (PN) and colloidal nitrogen (CN). Low temperature affected enhanced coagulation in many aspects. It inhibited turbidity reduction and COD removal by the three coagulants with the order being AlCl₃ > PACl > SynthA. It exhibited differential influences on the removals of particulate, colloidal and dissolved COD, and nitrogen, and it showed greater adverse effects on particulate and colloidal COD and nitrogen. The fluorescence value of dissolved organic matter in low temperature water showed a significant increase, and its reduction by coagulation was high, compared with that in room temperature water. Low temperature coagulation exerted greater impacts on ultraviolet differential absorbance than did room temperature. Under low temperatures, slight increases of total nitrogen (TN) removal, DN, and DON removals were achieved by using SynthA as coagulant, and removals of PN and CN were maintained, compared with room temperature. As an example, when SynthA dosage was above 30 mg ·L^-1, DON removal reached 28.5%-41.7% at low temperature, while the removal was only 17%-31.4% at room temperature. A large portion of the COD and some TN were removed by coagulation as a pretreatment, indicating that a large amount of the time in an aeration pond could be reduced, and the removal efficiency of TN would be stabilized. Therefore, in winter, the decrease of biological treatment efficiency could be alleviated to some extent by using enhanced coagulation with an adaptable coagulant, such as SynthA as a pretreatment, which would relieve the stress of denitrogen and stabilize treatment efficiency.

[Effects of nitrogen addition and elevated CO2 concentration on soil dissolved organic carbon and nitrogen in rhizosphere and non-rhizosphere of Bothriochloa ischaemum]

A pot experiment was conducted to study soil dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in the rhizosphere and non-rhizosphere of Bothriochloa ischaemum in loess hilly-gully region under the different treatments of CO₂ concentrations (400 and 800 μmol·mol^-1) and nitrogen addition (0, 2.5, 5.0 g N·m^-2·a^-1). The results showed that eleva-ted CO₂ treatments had no significant effect on the contents of DOC, dissolved total nitrogen (DTN), DON, dissolved ammonium nitrogen (NH₄⁺-N) and dissolved nitrate nitrogen (NO₃^--N) in the soil of rhizosphere and non-rhizosphere of B. ischaemum. The contents of DTN, DON, and NO₃^--N in the rhizosphere soil were significantly increased with the nitrogen application and the similar results of DTN and NO₃^--N also were observed in the non-rhizosphere of B. ischaemum. Nitrogen application significantly decreased DOC/DON in the rhizosphere of B. ischaemum. The contents of DTN, NO₃^--N and DON in the soil of rhizosphere were significantly lower than that in the non-rhizosphere soil, and DOC/DON was significantly higher in the rhizosphere soil than that in the non-rhizosphere soil. It indicated that short-term elevated CO₂ concentration had no significant influence on the contents of soil dissolved organic carbon and nitrogen. Simulated nitrogen deposition, to some extent, increased the content of soil dissolved nitrogen, but it was still insufficient to meet the demand of dissolved nitrogen for plant growing.

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.

Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies

This study examines the role of tree canopies in processing atmospheric nitrogen (Ndep ) for four forests in the United Kingdom subjected to different Ndep : Scots pine and beech stands under high Ndep (HN, 13-19 kg N ha(-1)  yr(-1) ), compared to Scots pine and beech stands under low Ndep (LN, 9 kg N ha(-1)  yr(-1) ). Changes of NO3 -N and NH4 -N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines δ(18) O, Δ(17) O and δ(15) N in NO3 (-) and δ(15) N in NH4 (+) , were used to assess N transformations by the canopies. Generally, HN sites showed higher NH4 -N and NO3 -N concentrations in RF compared to the LN sites. Similar values of δ(15) N-NO3 (-) and δ(18) O in RF suggested similar source of atmospheric NO3 (-) (i.e. local traffic), while more positive values for δ(15) N-NH4 (+) at HN compared to LN likely reflected the contribution of dry NHx deposition from intensive local farming. The isotopic signatures of the N-forms changed after interacting with tree canopies. Indeed, (15) N-enriched NH4 (+) in TF compared to RF at all sites suggested that canopies played an important role in buffering dry Ndep also at the low Ndep site. Using two independent methods, based on δ(18) O and Δ(17) O, we quantified for the first time the proportion of NO3 (-) in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Δ(17) O, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of Ndep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ(17) O.

Challenging the paradigm of nitrogen cycling: no evidence of in situ resource partitioning by coexisting plant species in grasslands of contrasting fertility

In monoculture, certain plant species are able to preferentially utilize different nitrogen (N) forms, both inorganic and organic, including amino acids and peptides, thus forming fundamental niches based on the chemical form of N. Results from field studies, however, are inconsistent: Some showing that coexisting plant species predominantly utilize inorganic N, while others reveal distinct interspecies preferences for different N forms. As a result, the extent to which hypothetical niches are realized in nature remains unclear. Here, we used in situ stable isotope tracer techniques to test the idea, in temperate grassland, that niche partitioning of N based on chemical form is related to plant productivity and the relative availability of organic and inorganic N. We also tested in situ whether grassland plants vary in their ability to compete for, and utilize peptides, which have recently been shown to act as an N source for plants in strongly N-limited ecosystems. We hypothesized that plants would preferentially use NO3 (-)-N and NH4 (+)-N over dissolved organic N in high-productivity grassland where inorganic N availability is high. On the other hand, in low-productivity grasslands, where the availability of dissolved inorganic N is low, and soil availability of dissolved organic N is greater, we predicted that plants would preferentially use N from amino acids and peptides, prior to microbial mineralization. Turves from two well-characterized grasslands of contrasting productivity and soil N availability were injected, in situ, with mixtures of (15)N-labeled inorganic N (NO3 (-) and NH4 (+)) and (13)C(15)N labeled amino acid (l-alanine) and peptide (l-tri-alanine). In order to measure rapid assimilation of these N forms by soil microbes and plants, the uptake of these substrates was traced within 2.5 hours into the shoots of the most abundant plant species, as well as roots and the soil microbial biomass. We found that, contrary to our hypothesis, the majority of plant species across both grasslands took up most N in the form of NH4 (+), suggesting that inorganic N is their predominant N source. However, we did find that organic N was a source of N which could be utilized by plant species at both sites, and in the low-productivity grassland, plants were able to capture some tri-alanine-N directly. Although our findings did not support the hypothesis that differences in the availability of inorganic and organic N facilitate resource partitioning in grassland, they do support the emerging view that peptides represent a significant, but until now neglected, component of the terrestrial N cycle.

Degradation of typical N-nitrosodimethylamine (NDMA) precursors and its formation potential in anoxic-aerobic (AO) activated sludge system

N-nitrosodimethylamine (NDMA) is an emerging disinfection byproduct. Removal of its potential precursors is considered as an effective method to control NDMA. In this study, four typical NDMA precursors (dimethylamine (DMA), trimethylamine (TMA), dimethylformamide (DMFA) and dimethylaminobenzene (DMAB)) were selected, and their removal capacities by activated sludge were investigated. Batch experiments indicated that removal of NDMA precursors was better under aerobic condition than anoxic condition; and their specific degradation rates follow the order of DMA > TMA > DMFA > DMAB. In anoxic-aerobic (AO) activated sludge system, the optimal hydraulic retention time and sludge retention time were 10 h and 20 d, respectively, for the removal of both NDMA precursors (four selected NDMA precursors and NDMA formation potential (NDMA FP)) and nutrients. Our results also suggested that there was a positive correlation between NDMA FP and dissolved organic nitrogen (DON) in wastewater. The removal efficiency of NDMA FP was in the range of 46.8-72.5% in the four surveyed wastewater treatment plants except the one which adopted chemically enhanced primary process. The results revealed that the AO system had the advantage of removing NDMA FP. Our results are helpful for the knowledge of the removals of NDMA precursors during activated sludge treatment processes.

Competition between plant and bacterial cells at the microscale regulates the dynamics of nitrogen acquisition in wheat (Triticum aestivum)

The ability of plants to compete effectively for nitrogen (N) resources is critical to plant survival. However, controversy surrounds the importance of organic and inorganic sources of N in plant nutrition because of our poor ability to visualize and understand processes happening at the root-microbial-soil interface. Using high-resolution nano-scale secondary ion mass spectrometry stable isotope imaging (NanoSIMS-SII), we quantified the fate of ¹⁵N over both space and time within the rhizosphere. We pulse-labelled the soil surrounding wheat (Triticum aestivum) roots with either ¹⁵NH₄⁺ or ¹⁵N-glutamate and traced the movement of ¹⁵N over 24 h. Imaging revealed that glutamate was rapidly depleted from the rhizosphere and that most ¹⁵N was captured by rhizobacteria, leading to very high ¹⁵N microbial enrichment. After microbial capture, approximately half of the ¹⁵N-glutamate was rapidly mineralized, leading to the excretion of NH₄⁺, which became available for plant capture. Roots proved to be poor competitors for ¹⁵N-glutamate and took up N mainly as ¹⁵NH₄⁺. Spatial mapping of ¹⁵N revealed differential patterns of ¹⁵N uptake within bacteria and the rapid uptake and redistribution of ¹⁵N within roots. In conclusion, we demonstrate the rapid cycling and transformation of N at the soil-root interface and that wheat capture of organic N is low in comparison to inorganic N under the conditions tested.

Formation and speciation of nine haloacetamides, an emerging class of nitrogenous DBPs, during chlorination or chloramination

Haloacetamides (HAcAms) are an emerging class of nitrogenous disinfection by-products (N-DBPs) of health concern. However, there are very limited data on the formation and speciation of the nine bromine- and chlorine-containing haloacetamides (HAcAm9). In the study, their formation and speciation during chlor(am)ination were investigated for a group of waters with a range of specific ultraviolet absorbance at 254 nm (SUVA₂₅₄), dissolved organic nitrogen (DON), and bromide levels. The waters that were the least impacted by anthropogenic pollution had the lowest DON levels, the highest ratios of dissolved organic carbon (DOC) to DON, and exhibited the least HAcAm9 formation. DON/DOC may act as an indicator of HAcAm yields during chlorination. HAcAm9 exhibited more formation during chloramination in the low-SUVA waters with no bromide, relative to high-SUVA waters with bromide. The selected waters all formed primarily dihalogenated (di-) HAcAms, followed by trihalogenated (tri-) species and, to a much lesser extent, monohalogenated (mono-) HAcAms. Di-HAcAm formation had similar trends as that of HAcAm9; whereas chloramination formed more mono- and less tri-HAcAms than chlorination. Bromine utilization factors and bromine incorporation factor increased with decreasing and increasing bromide during either chlorination or chloramination, and bromine was easier to incorporate into tri-HAcAms during chloramination than chlorination.

How significant to plant N nutrition is the direct consumption of soil microbes by roots?

The high degree to which plant roots compete with soil microbes for organic forms of nitrogen (N) is becoming increasingly apparent. This has culminated in the finding that plants may consume soil microbes as a source of N, but the functional significance of this process remains unknown. We used (15) N- and (14) C-labelled cultures of soil bacteria to measure rates of acquisition of microbes by sterile wheat roots and plants growing in soil. We compared these rates with acquisition of (15) N delivered as nitrate, amino acid monomer (l-alanine) and short peptide (l-tetraalanine), and the rate of decomposition of [(14) C] microbes by indigenous soil microbiota. Acquisition of microbe (15) N by both sterile roots and roots growing in soil was one to two orders of magnitude slower than acquisition of all other forms of (15) N. Decomposition of microbes was fast enough to account for all (15) N recovered, but approximately equal recovery of microbe (14) C suggests that microbes entered roots intact. Uptake of soil microbes by wheat (Triticum aestivum) roots appears to take place in soil. If wheat is typical, the importance of this process to terrestrial N cycling is probably minor in comparison with fluxes of other forms of soil inorganic and organic N.

Nitrogen regulation of root branching

BACKGROUND

Many plant species can modify their root architecture to enable them to forage for heterogeneously distributed nutrients in the soil. The foraging response normally involves increased proliferation of lateral roots within nutrient-rich soil patches, but much remains to be understood about the signalling mechanisms that enable roots to sense variations in the external concentrations of different mineral nutrients and to modify their patterns of growth and development accordingly.

SCOPE

In this review we consider different aspects of the way in which the nitrogen supply can modify root branching, focusing on Arabidopsis thaliana. Our current understanding of the mechanism of nitrate stimulation of lateral root growth and the role of the ANR1 gene are summarized. In addition, evidence supporting the possible role of auxin in regulating the systemic inhibition of early lateral root development by high rates of nitrate supply is presented. Finally, we examine recent evidence that an amino acid, L-glutamate, can act as an external signal to elicit complex changes in root growth and development.

CONCLUSIONS

It is clear that plants have evolved sophisticated pathways for sensing and responding to changes in different components of the external nitrogen supply as well as their own internal nitrogen status. We speculate on the possibility that the effects elicited by external L-glutamate represent a novel form of foraging response that could potentially enhance a plant's ability to compete with its neighbours and micro-organisms for localized sources of organic nitrogen.

Inorganic and organic losses of nitrogen from upland regions of Britain: concentrations and fluxes

The nitrogen (N) composition of streams draining eight upland regions of Britain was compared using monthly samples collected between April 1997 and April 1998. Stream samples were analysed for total N (TN), particulate N (PN), nitrate (NO3), ammonium (NH4), and dissolved organic nitrogen (DON). Concentrations of TN were small, generally less than 1.5 mg N l(-1), were dominated by dissolved forms of N, and varied significantly between regions. NO3 accounted for the majority of variability. Concentrations of DON also varied between regions but to a smaller extent than those of NO3. There were considerable variations in TN fluxes between upland regions, which ranged between 3.8 and 16.1 kg N ha(-1) year(-1). The majority of the variation was due to NO3 fluxes, which were largest in regions receiving largest inputs of atmospheric N deposition and ranged between 1.4 and 13.5 kg N ha(-1) year(-1). Fluxes of DON ranged between 1 and 3.5 kg N ha(-1) year( -1), while fluxes of PN were generally less than 0.5 kg N ha(-1) year(-1) , and NH4 fluxes ranged between 0.1 and 0.4 kg N ha(-1) year(-1). NO3 was the dominant fraction (47-84%) of N exported from all upland regions except the Highlands, where DON accounted for 52% of the TN flux. This study has shown that the DON fraction is an important component of the total N transported by upland streams in Britain.