Evidence for the direct oxidation of organic nitrogen to N2 gas in the Arabian Sea

We performed a suite of ¹⁵N incubations (¹⁵NO₂⁻, ¹⁵NO₃⁻ and ¹⁵NH₄⁺) with and without the organic-nitrogen (N) compound allylthiourea (ATU), in the suboxic waters of the Arabian Sea. Production of ²⁹N₂ in control (-ATU) incubations with either ¹⁵NH₄⁺+¹⁴NO₂⁻, or their analogues, ¹⁵NO₂⁻+¹⁴NH₄⁺, though small, confirmed the presence of anammox. In contrast, when we added ATU, along with ¹⁵NO₂⁻ and ¹⁴NH₄⁺, there was a much greater production of ²⁹N₂, with 92% of the ¹⁵N-label being recovered as ²⁹N₂ on average. Such stimulated production of ²⁹N₂ could not be due to anammox, as the addition of ATU, along with ¹⁵NH₄⁺+¹⁴NO₂⁻, only produced ²⁹N₂ equivalent to that in the controls. The ratios of ²⁹N₂ to ³⁰N₂ produced also precluded stimulation of denitrification. We present this as evidence for a hitherto uncharacterised metabolism potentially capable of oxidising organic-N (e.g. NH₂ groups) directly to N₂ gas at the expense of NO₂⁻.     

Ammonium Removal by the Oxygen-Limited Autotrophic Nitrification-Denitrification System

The present lab-scale research reveals the potential of implementation of an oxygen-limited autotrophic nitrification-denitrification (OLAND) system with normal nitrifying sludge as the biocatalyst for the removal of nitrogen from nitrogen-rich wastewater in one step. In a sequential batch reactor, synthetic wastewater containing 1 g of NH₄⁺-N liter⁻¹ and minerals was treated. Oxygen supply to the reactor was double-controlled with a pH controller and a timer. At a volumetric loading rate (B_v) of 0.13 g of NH₄⁺-N liter⁻¹ day⁻¹, about 22% of the fed NH₄⁺-N was converted to NO₂⁻-N or NO₃⁻-N, 38% remained as NH₄⁺-N, and the other 40% was removed mainly as N₂. The specific removal rate of nitrogen was on the order of 50 mg of N liter⁻¹ day⁻¹, corresponding to 16 mg of N g of volatile suspended solids⁻¹ day⁻¹. The microorganisms which catalyzed the OLAND process are assumed to be normal nitrifiers dominated by ammonium oxidizers. The loss of nitrogen in the OLAND system is presumed to occur via the oxidation of NH₄⁺ to N₂ with NO₂⁻ as the electron acceptor. Hydroxylamine stimulated the removal of NH₄⁺ and NO₂⁻. Hydroxylamine oxidoreductase (HAO) or an HAO-related enzyme might be responsible for the loss of nitrogen.     

Effect of N Source on the Steady State Growth and N Assimilation of P-limited Anabaena flos-aquae

Phosphate-limited chemostat cultures were used to study cell growth and N assimilation in Anabaena flos-aquae under various N sources to determine the relative energetic costs associated with the assimilation of NH₃, NO₃⁻, or N₂. Expressed as a function of relative growth rate, steady state cellular P contents and PO₄ assimilation rates did not vary with N-source. However, N-source did alter the maximal PO₄-limited growth rate achieved by the cultures: the NO₃⁻ and N₂ cultures attained only 97 and 80%, respectively, of the maximal growth rate of the NH₃ grown cells. Cellular biomass and C contents did not vary with growth rate, but changed with N source. The NO₃⁻-grown cells were the smallest (627 ± 34 micromoles C · 10⁻⁹ cells), while NH₃-grown cells were largest (900 ± 44 micromoles C · 10⁻⁹ cells) and N₂-fixing cells were intermediate (726 ± 48 micromoles C · 10⁻⁹ cells) in size. In the NO₃⁻-and N₂-grown cultures, N content per cell was only 57 and 63%, respectively, of that in the NH₃-grown cells. Heterocysts were absent in NH₃-grown cultures but were present in both the N₂ and NO₃⁻ cultures. In the NO₃⁻-grown cultures C₂H₂ reduction was detected only at high growth rates, where it was estimated to account for a maximum of 6% of the N assimilated. In the N₂-fixing cultures the acetylene:N₂ ratio varied from 3.4:1 at lower growth rates to 3.0:1 at growth rates approaching maximal.

Compared with NH₃, the assimilation of NO₃⁻ and N₂ resulted either in a decrease in cellular C (NO₃⁻ and N₂ cultures) or in a lower maximal growth rate (N₂ culture only). The observed changes in cell C content were used to calculate the net cost (in electron pair equivalents) associated with growth on NO₃⁻ or N₂ compared with NH₃.

     

New Insights into Methyl Bromide Cooxidation by Nitrosomonas europaea Obtained by Experimenting with Moderately Low Density Cell Suspensions

We examined the rates and sustainability of methyl bromide (MeBr) oxidation in moderately low density cell suspensions (~6 × 10⁷ cells ml⁻¹) of the NH₃-oxidizing bacterium Nitrosomonas europaea. In the presence of 10 mM NH₄⁺ and 0.44, 0.22, and 0.11 mM MeBr, the initial rates of MeBr oxidation were sustained for 12, 12, and 24 h, respectively, despite the fact that only 10% of the NH₄⁺, 18% of the NH₄⁺, and 35% of the NH₄⁺, respectively, were consumed. Although the duration of active MeBr oxidation generally decreased as the MeBr concentration increased, similar amounts of MeBr were oxidized with a large number of the NH₄⁺-MeBr combinations examined (10 to 20 μmol mg [dry weight] of cells⁻¹). Approximately 90% of the NH₃-dependent O₂ uptake activity and the NO₂⁻-producing activity were lost after N. europaea was exposed to 0.44 mM MeBr for 24 h. After MeBr was removed and the cells were resuspended in fresh growth medium, NO₂⁻ production increased exponentially, and 48 to 60 h was required to reach the level of activity observed initially in control cells that were not exposed to MeBr. It is not clear what percentage of the cells were capable of cell division after MeBr oxidation because NO₂⁻ accumulated more slowly in the exposed cells than in the unexposed cells despite the fact that the latter were diluted 10-fold to create inocula which exhibited equal initial activities. The decreases in NO₂⁻-producing and MeBr-oxidizing activities could not be attributed directly to NH₄⁺ or NH₃ limitation, to a decrease in the pH, to the composition of the incubation medium, or to toxic effects caused by accumulation of the end products of oxidation (NO₂⁻ and formaldehyde) in the medium. Additional cooxidation-related studies of N. europaea are needed to identify the mechanism(s) responsible for the MeBr-induced loss of cell activity and/or viability, to determine what percentages of cells damaged by cooxidative activities are culturable, and to determine if cooxidative activity interferes with the regulation of NH₃-oxidizing activity.     

Mild ammonium stress increases chlorophyll content in Arabidopsis thaliana

Nitrate (NO₃⁻) and ammonium (NH₄⁺) are the main forms of nitrogen available in the soil for plants. Excessive NH₄⁺ accumulation in tissues is toxic for plants and exclusive NH₄⁺-based nutrition enhances this effect. Ammonium toxicity syndrome commonly includes growth impairment, ion imbalance and chlorosis among others. In this work, we observed high intraspecific variability in chlorophyll content in 47 Arabidopsis thaliana natural accessions grown under 1 mM NH₄⁺ or 1 mM NO₃⁻ as N-source. Interestingly, chlorophyll content increased in every accession upon ammonium nutrition. Moreover, this increase was independent of ammonium tolerance capacity. Thus, chlorosis seems to be an exclusive effect of severe ammonium toxicity while mild ammonium stress induces chlorophyll accumulation.     

Atmospheric Nitrogen Deposition at Two Sites in an Arid Environment of Central Asia

Arid areas play a significant role in the global nitrogen cycle. Dry and wet deposition of inorganic nitrogen (N) species were monitored at one urban (SDS) and one suburban (TFS) site at Urumqi in a semi-arid region of central Asia. Atmospheric concentrations of NH₃, NO₂, HNO₃, particulate ammonium and nitrate (pNH₄ ⁺ and pNO₃ ⁻) concentrations and NH₄-N and NO₃-N concentrations in precipitation showed large monthly variations and averaged 7.1, 26.6, 2.4, 6.6, 2.7 µg N m⁻³ and 1.3, 1.0 mg N L⁻¹ at both SDS and TFS. Nitrogen dry deposition fluxes were 40.7 and 36.0 kg N ha⁻¹ yr⁻¹ while wet deposition of N fluxes were 6.0 and 8.8 kg N ha⁻¹ yr⁻¹ at SDS and TFS, respectively. Total N deposition averaged 45.8 kg N ha⁻¹ yr⁻¹at both sites. Our results indicate that N dry deposition has been a major part of total N deposition (83.8% on average) in an arid region of central Asia. Such high N deposition implies heavy environmental pollution and an important nutrient resource in arid regions.     

Effects of Carbon Substrates on Nitrite Accumulation in Freshwater Sediments

The contribution of the biochemical pathways nitrification, denitrification, and dissimilatory NO₃⁻ reduction to NH₄⁺ (DNRA) to the accumulation of NO₂⁻ in freshwaters is governed by the species compositions of the bacterial populations resident in the sediments, available carbon (C) and nitrogen (N) substrates, and environmental conditions. Recent studies of major rivers in Northern Ireland have shown that high NO₂⁻ concentrations found in summer, under warm, slow-flowing conditions, arise from anaerobic NO₃⁻ reduction. Locally, agricultural pollutants entering rivers are important C and N sources, providing ideal substrates for the aquatic bacteria involved in cycling of N. In this study a range of organic C compounds commonly found in agricultural pollutants were provided as energy sources in 48-h incubation experiments to investigate if the chemical compositions of the pollutants affected which NO₃⁻ reduction pathway was followed and influenced subsequent NO₂⁻ accumulation. Carbon stored within the sediments was sufficient to support DNRA and denitrifier populations, and the resulting NO₂⁻ peak (80 μg of N liter⁻¹ [approximate]) observed at 24 h was indicative of the simultaneous activities of both bacterial groups. The value of glycine as an energy source for denitrification or DNRA appeared to be limited, but glycine was an important source of additional N. Glucose was an efficient substrate for both the denitrification and DNRA pathways, with a NO₂⁻ peak of 160 μg of N liter⁻¹ noted at 24 h. Addition of formate and acetate stimulated continuous NO₂⁻ production throughout the 48-h period, caused by partial inhibition of the denitrification pathway. The formate treatment resulted in a high NO₂⁻ accumulation (1,300 μg of N liter⁻¹ [approximate]), and acetate treatment resulted in a low NO₂⁻ concentration (<100 μg of N liter⁻¹).     

The High Salt Requirement of the Moderate Halophile Chromohalobacter salexigens DSM3043 Can Be Met Not Only by NaCl but by Other Ions

The growth rate of Chromohalobacter salexigens DSM 3043 can be stimulated in media containing 0.3 M NaCl by a 0.7 M concentration of other salts of Na⁺, K⁺, Rb⁺, or NH₄⁺, Cl⁻, Br⁻, NO₃⁻, or SO₄²⁻ ions. To our knowledge, growth rate stimulation by a general high ion concentration has not been reported for any organism previously.     

Apoplastic pH and Fe3+ Reduction in Intact Sunflower Leaves

It has been hypothesized that under NO₃⁻ nutrition a high apoplastic pH in leaves depresses Fe³⁺ reductase activity and thus the subsequent Fe²⁺ transport across the plasmalemma, inducing Fe chlorosis. The apoplastic pH in young green leaves of sunflower (Helianthus annuus L.) was measured by fluorescence ratio after xylem sap infiltration. It was shown that NO₃⁻ nutrition significantly increased apoplastic pH at distinct interveinal sites (pH ≥ 6.3) and was confined to about 10% of the whole interveinal leaf apoplast. These apoplastic pH increases presumably derive from NO₃⁻/proton cotransport and are supposed to be related to growing cells of a young leaf; they were not found in the case of sole NH₄⁺ or NH₄NO₃ nutrition. Complementary to pH measurements, the formation of Fe²⁺-ferrozine from Fe³⁺-citrate was monitored in the xylem apoplast of intact leaves in the presence of buffers at different xylem apoplastic pH by means of image analysis. This analysis revealed that Fe³⁺ reduction increased with decreasing apoplastic pH, with the highest rates at around pH 5.0. In analogy to the monitoring of Fe³⁺ reduction in the leaf xylem, we suggest that under alkaline nutritional conditions at interveinal microsites of increased apoplastic pH, Fe³⁺ reduction is depressed, inducing leaf chlorosis. The apoplastic pH in the xylem vessels remained low in the still-green veins of leaves with intercostal chlorosis.     

Short Term Studies of Nitrate Uptake into Barley Plants Using Ion-Specific Electrodes and ClO(3): II. Regulation of NO(3) Efflux by NH(4)

The influence of NH₄⁺, in the external medium, on fluxes of NO₃⁻ and K⁺ were investigated using barley (Hordeum vulgare cv Betzes) plants. NH₄⁺ was without effect on NO₃⁻ (³⁶ClO₃⁻) influx whereas inhibition of net uptake appeared to be a function of previous NO₃⁻ provision. Plants grown at 10 micromolar NO₃⁻ were sensitive to external NH₄⁺ when uptake was measured in 100 micromolar NO₃⁻. By contrast, NO₃⁻ uptake (from 100 micromolar NO₃⁻) by plants previously grown at this concentration was not reduced by NH₄⁺ treatment. Plants pretreated for 2 days with 5 millimolar NO₃⁻ showed net efflux of NO₃⁻ when roots were transferred to 100 micromolar NO₃⁻. This efflux was stimulated in the presence of NH₄⁺. NH₄⁺ also stimulated NO₃⁻ efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO₃⁻ occur via effects upon efflux. By contrast to the situation for NO₃⁻, net K⁺ uptake and influx of ³⁶Rb⁺-labeled K⁺ was inhibited by NH₄⁺ regardless of the nutrient history of the plants. Inhibition of net K⁺ uptake reached its maximum value within 2 minutes of NH₄⁺ addition. It is concluded that the latter ion exerts a direct effect upon K⁺ influx.     

Anaerobic Ammonia Oxidation in the Presence of Nitrogen Oxides (NOx) by Two Different Lithotrophs

The anaerobic ammonia-oxidizing activity of the planctomycete Candidatus “Brocadia anammoxidans” was not inhibited by NO concentrations up to 600 ppm and NO₂ concentrations up to 100 ppm. B. anammoxidans was able to convert (detoxify) NO, which might explain the high NO tolerance of this organism. In the presence of NO₂, the specific ammonia oxidation activity of B. anammoxidans increased, and Nitrosomonas-like microorganisms recovered an NO₂-dependent anaerobic ammonia oxidation activity. Addition of NO₂ to a mixed population of B. anammoxidans and Nitrosomonas induced simultaneous specific anaerobic ammonia oxidation activities of up to 5.5 mmol of NH₄⁺ g of protein⁻¹ h⁻¹ by B. anammoxidans and up to 1.5 mmol of NH₄⁺ g of protein⁻¹ h⁻¹ by Nitrosomonas. The stoichiometry of the converted N compounds (NO₂⁻/NH₃ ratio) and the microbial community structure were strongly influenced by NO₂. The combined activity of B. anammoxidans and Nitrosomonas-like ammonia oxidizers might be of relevance in natural environments and for technical applications.     

Growth and Nitrogen Uptake Kinetics in Cultured Prorocentrum donghaiense

We compared growth kinetics of Prorocentrum donghaiense cultures on different nitrogen (N) compounds including nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), urea, glutamic acid (glu), dialanine (diala) and cyanate. P. donghaiense exhibited standard Monod-type growth kinetics over a range of N concentraions (0.5–500 μmol N L⁻¹ for NO₃ ⁻ and NH₄ ⁺, 0.5–50 μmol N L⁻¹ for urea, 0.5–100 μmol N L⁻¹ for glu and cyanate, and 0.5–200 μmol N L⁻¹ for diala) for all of the N compounds tested. Cultures grown on glu and urea had the highest maximum growth rates (μ_m, 1.51±0.06 d⁻¹ and 1.50±0.05 d⁻¹, respectively). However, cultures grown on cyanate, NO₃ ⁻, and NH₄ ⁺ had lower half saturation constants (K_μ, 0.28–0.51 μmol N L⁻¹). N uptake kinetics were measured in NO₃ ⁻-deplete and -replete batch cultures of P. donghaiense. In NO₃ ⁻-deplete batch cultures, P. donghaiense exhibited Michaelis-Menten type uptake kinetics for NO₃ ⁻, NH₄ ⁺, urea and algal amino acids; uptake was saturated at or below 50 μmol N L⁻¹. In NO₃ ⁻-replete batch cultures, NH₄ ⁺, urea, and algal amino acid uptake kinetics were similar to those measured in NO₃ ⁻-deplete batch cultures. Together, our results demonstrate that P. donghaiense can grow well on a variety of N sources, and exhibits similar uptake kinetics under both nutrient replete and deplete conditions. This may be an important factor facilitating their growth during bloom initiation and development in N-enriched estuaries where many algae compete for bioavailable N and the nutrient environment changes as a result of algal growth.     

Seasonal Photochemical Transformations of Nitrogen Species in a Forest Stream and Lake

The photochemical release of inorganic nitrogen from dissolved organic matter is an important source of bio-available nitrogen (N) in N-limited aquatic ecosystems. We conducted photochemical experiments and used mathematical models based on pseudo-first-order reaction kinetics to quantify the photochemical transformations of individual N species and their seasonal effects on N cycling in a mountain forest stream and lake (Plešné Lake, Czech Republic). Results from laboratory experiments on photochemical changes in N speciation were compared to measured lake N budgets. Concentrations of organic nitrogen (N_org; 40–58 µmol L⁻¹) decreased from 3 to 26% during 48-hour laboratory irradiation (an equivalent of 4–5 days of natural solar insolation) due to photochemical mineralization to ammonium (NH₄ ⁺) and other N forms (N_x; possibly N oxides and N₂). In addition to N_org mineralization, N_x also originated from photochemical nitrate (NO₃ ⁻) reduction. Laboratory exposure of a first-order forest stream water samples showed a high amount of seasonality, with the maximum rates of N_org mineralization and NH₄ ⁺ production in winter and spring, and the maximum NO₃ ⁻ reduction occurring in summer. These photochemical changes could have an ecologically significant effect on NH₄ ⁺ concentrations in streams (doubling their terrestrial fluxes from soils) and on concentrations of dissolved N_org in the lake. In contrast, photochemical reactions reduced NO₃ ⁻ fluxes by a negligible (<1%) amount and had a negligible effect on the aquatic cycle of this N form.     

Effects of Feedstock and Pyrolysis Temperature on Biochar Adsorption of Ammonium and Nitrate

Biochar produced by pyrolysis of biomass can be used to counter nitrogen (N) pollution. The present study investigated the effects of feedstock and temperature on characteristics of biochars and their adsorption ability for ammonium N (NH₄ ⁺-N) and nitrate N (NO₃ ⁻-N). Twelve biochars were produced from wheat-straw (W-BC), corn-straw (C-BC) and peanut-shell (P-BC) at pyrolysis temperatures of 400, 500, 600 and 700°C. Biochar physical and chemical properties were determined and the biochars were used for N sorption experiments. The results showed that biochar yield and contents of N, hydrogen and oxygen decreased as pyrolysis temperature increased from 400°C to 700°C, whereas contents of ash, pH and carbon increased with greater pyrolysis temperature. All biochars could sorb substantial amounts of NH₄ ⁺-N, and the sorption characteristics were well fitted to the Freundlich isotherm model. The ability of biochars to adsorb NH₄ ⁺-N followed: C-BC>P-BC>W-BC, and the adsorption amount decreased with higher pyrolysis temperature. The ability of C-BC to sorb NH₄ ⁺-N was the highest because it had the largest cation exchange capacity (CEC) among all biochars (e.g., C-BC400 with a CEC of 38.3 cmol kg⁻¹ adsorbed 2.3 mg NH₄ ⁺-N g⁻¹ in solutions with 50 mg NH₄ ⁺ L⁻¹). Compared with NH₄ ⁺-N, none of NO₃ ⁻-N was adsorbed to biochars at different NO₃ ⁻ concentrations. Instead, some NO₃ ⁻-N was even released from the biochar materials. We conclude that biochars can be used under conditions where NH₄ ⁺-N (or NH₃) pollution is a concern, but further research is needed in terms of applying biochars to reduce NO₃ ⁻-N pollution.     

Influence of Nitrate and Ammonium Nutrition on the Uptake, Assimilation, and Distribution of Nutrients in Ricinus communis

Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO₃⁻ or NH₄⁺, the solutions being maintained at pH 5.5. In NO₃⁻-fed plants excess nutrient anion over cation uptake was equivalent to net OH⁻ efflux, and the total charge from NO₃⁻ and SO₄²⁻ reduction equated to the sum of organic anion accumulation plus net OH⁻ efflux. In NH₄⁺-fed plants a large H⁺ efflux was recorded in close agreement with excess cation over anion uptake. This H⁺ efflux equated to the sum of net cation (NH₄⁺ minus SO₄²⁻) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO₃⁻ that is taken up and reduced in NO₃⁻-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH₄⁺-fed plants absorbed NH₄⁺ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO₃⁻-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients.     

A N and N Nuclear Magnetic Resonance Study of Nitrogen Metabolism in Shoot-Forming Cultures of White Spruce (Picea glauca) Buds

Nitrogen-14 and nitrogen-15 nuclear magnetic resonance (NMR) spectra were recorded for freshly dissected buds of Picea glauca and for buds grown for 3, 6 and 9 weeks on shoot-forming medium. Resonances for Glu (and other αNH₂ groups), Pro, Ala, and the side chain groups in Gln, Arg, Orn, and γ-aminobutyric acid could be detected in in vivo¹⁵N NMR spectra. Peaks for α-amino groups, Pro, NO₃⁻ and NH₄⁺ could also be identified in ¹⁴N NMR spectra. Perfusion experiments performed for up to 20 hours in the NMR spectrometer showed that ¹⁵N-labeled NH₄⁺ and NO₃⁻ are first incorporated into the amide group of Gln and then in the αNH₂ pool. Subsequently, it also emerges in Ala and Arg. These data suggest that the glutamine synthetase/ glutamate synthase pathway functions under these conditions. The assimilation of NH₄⁺ is much faster than that of NO₃⁻. Consequently after 10 days of growth more than 70% of the newly synthesized internal free amino acid pool derives its nitrogen from NH₄⁺ rather than NO₃⁻. If NH₄⁺ is omitted from the medium, no NO₃⁻ is taken up during 9 weeks and the buds support limited growth by utilizing their endogenous amino acid pools. It is concluded that NH₄⁺ and NO₃⁻ are both required for the induction of nitrate- and nitrite reductase.     

Simulated Nitrogen Deposition Reduces CH4 Uptake and Increases N2O Emission from a Subtropical Plantation Forest Soil in Southern China

To date, few studies are conducted to quantify the effects of reduced ammonium (NH₄ ⁺) and oxidized nitrate (NO₃ ⁻) on soil CH₄ uptake and N₂O emission in the subtropical forests. In this study, NH₄Cl and NaNO₃ fertilizers were applied at three rates: 0, 40 and 120 kg N ha⁻¹ yr⁻¹. Soil CH₄ and N₂O fluxes were determined twice a week using the static chamber technique and gas chromatography. Soil temperature and moisture were simultaneously measured. Soil dissolved N concentration in 0–20 cm depth was measured weekly to examine the regulation to soil CH₄ and N₂O fluxes. Our results showed that one year of N addition did not affect soil temperature, soil moisture, soil total dissolved N (TDN) and NH₄ ⁺-N concentrations, but high levels of applied NH₄Cl and NaNO₃ fertilizers significantly increased soil NO₃ ⁻-N concentration by 124% and 157%, respectively. Nitrogen addition tended to inhibit soil CH₄ uptake, but significantly promoted soil N₂O emission by 403% to 762%. Furthermore, NH₄ ⁺-N fertilizer application had a stronger inhibition to soil CH₄ uptake and a stronger promotion to soil N₂O emission than NO₃ ⁻-N application. Also, both soil CH₄ and N₂O fluxes were driven by soil temperature and moisture, but soil inorganic N availability was a key integrator of soil CH₄ uptake and N₂O emission. These results suggest that the subtropical plantation soil sensitively responses to atmospheric N deposition, and inorganic N rather than organic N is the regulator to soil CH₄ uptake and N₂O emission.     

Anaerobic Ammonium Oxidation Measured in Sediments along the Thames Estuary, United Kingdom

Until recently, denitrification was thought to be the only significant pathway for N₂ formation and, in turn, the removal of nitrogen in aquatic sediments. The discovery of anaerobic ammonium oxidation in the laboratory suggested that alternative metabolisms might be present in the environment. By using a combination of ¹⁵N-labeled NH₄⁺, NO₃⁻, and NO₂⁻ (and ¹⁴N analogues), production of ²⁹N₂ and ³⁰N₂ was measured in anaerobic sediment slurries from six sites along the Thames estuary. The production of ²⁹N₂ in the presence of ¹⁵NH₄⁺ and either ¹⁴NO₃⁻ or ¹⁴NO₂⁻ confirmed the presence of anaerobic ammonium oxidation, with the stoichiometry of the reaction indicating that the oxidation was coupled to the reduction of NO₂⁻. Anaerobic ammonium oxidation proceeded at equal rates via either the direct reduction of NO₂⁻ or indirect reduction, following the initial reduction of NO₃⁻. Whether NO₂⁻ was directly present at 800 μM or it accumulated at 3 to 20 μM (from the reduction of NO₃⁻), the rate of ²⁹N₂ formation was not affected, which suggested that anaerobic ammonium oxidation was saturated at low concentrations of NO₂⁻. We observed a shift in the significance of anaerobic ammonium oxidation to N₂ formation relative to denitrification, from 8% near the head of the estuary to less than 1% at the coast. The relative importance of anaerobic ammonium oxidation was positively correlated (P < 0.05) with sediment organic content. This report of anaerobic ammonium oxidation in organically enriched estuarine sediments, though in contrast to a recent report on continental shelf sediments, confirms the presence of this novel metabolism in another aquatic sediment system.     

Ammonium as a Driving Force of Plant Diversity and Ecosystem Functioning: Observations Based on 5 Years' Manipulation of N Dose and Form in a Mediterranean Ecosystem

Enhanced nitrogen (N) availability is one of the main drivers of biodiversity loss and degradation of ecosystem functions. However, in very nutrient-poor ecosystems, enhanced N input can, in the short-term, promote diversity. Mediterranean Basin ecosystems are nutrient-limited biodiversity hotspots, but no information is available on their medium- or long-term responses to enhanced N input. Since 2007, we have been manipulating the form and dose of available N in a Mediterranean Basin maquis in south-western Europe that has low ambient N deposition (<4 kg N ha⁻¹ yr⁻¹) and low soil N content (0.1%). N availability was modified by the addition of 40 kg N ha⁻¹ yr⁻¹ as a 1∶1 NH₄Cl to (NH₄)₂SO₄ mixture, and 40 and 80 kg N ha⁻¹ yr⁻¹ as NH₄NO₃. Over the following 5 years, the impacts on plant composition and diversity (richness and evenness) and some ecosystem characteristics (soil extractable N and organic matter, aboveground biomass and % of bare soil) were assessed. Plant species richness increased with enhanced N input and was more related to ammonium than to nitrate. Exposure to 40 kg NH₄ ⁺-N ha⁻¹ yr⁻¹ (alone and with nitrate) enhanced plant richness, but did not increase aboveground biomass; soil extractable N even increased under 80 kg NH₄NO₃-N ha⁻¹ yr⁻¹ and the % of bare soil increased under 40 kg NH₄ ⁺-N ha⁻¹ yr⁻¹. The treatment containing less ammonium, 40 kg NH₄NO₃-N ha⁻¹ yr⁻¹, did not enhance plant diversity but promoted aboveground biomass and reduced the % of bare soil. Data suggest that enhanced NH_y availability affects the structure of the maquis, which may promote soil erosion and N leakage, whereas enhanced NO_x availability leads to biomass accumulation which may increase the fire risk. These observations are relevant for land use management in biodiverse and fragmented ecosystems such as the maquis, especially in conservation areas.     

Growth and Nitrogen Uptake Characteristics Reveal Outbreak Mechanism of the Opportunistic Macroalga Gracilaria tenuistipitata

Macroalgae has bloomed in the brackish lake of Shenzhen Bay, China continuously from 2010 to 2014. Gracilaria tenuistipitata was identified as the causative macroalgal species. The aim of this study was to explore the outbreak mechanism of G. tenuistipitata, by studying the effects of salinity and nitrogen sources on growth, and the different nitrogen sources uptake characteristic. Our experimental design was based on environmental conditions observed in the bloom areas, and these main factors were simulated in the laboratory. Results showed that salinity 12 to 20 ‰ was suitable for G. tenuistipitata growth. When the nitrogen sources'' (NH₄ ⁺, NO₃ ⁻) concentrations reached 40 µM or above, the growth rate of G. tenuistipitata was significantly higher. Algal biomass was higher (approximately 1.4 times) when cultured with NH₄ ⁺ than that with NO₃ ⁻ addition. Coincidentally, macroalgal bloom formed during times of moderate salinity (∼12 ‰) and high nitrogen conditions. The NH₄ ⁺ and NO₃ ⁻ uptake characteristic was studied to understand the potential mechanism of G. tenuistipitata bloom. NH₄ ⁺ uptake was best described by a linear, rate-unsaturated response, with the slope decreasing with time intervals. In contrast, NO₃ ⁻ uptake followed a rate-saturating mechanism best described by the Michaelis-Menten model, with kinetic parameters V_max = 37.2 µM g⁻¹ DM h⁻¹ and K_s = 61.5 µM. Further, based on the isotope ¹⁵N tracer method, we found that ¹⁵N from NH₄ ⁺ accumulated faster and reached an atom% twice than that of ¹⁵N from NO₃ ⁻, suggesting when both NH₄ ⁺ and NO₃ ⁻ were available, NH₄ ⁺ was assimilated more rapidly. The results of the present study indicate that in the estuarine environment, the combination of moderate salinity with high ammonium may stimulate bloom formation.