15N-labelled Nitrate Research Articles

Convergent nitrogen uptake patterns and divergent nitrogen acquisition strategies of coexisting plant species in response to long-term nitrogen enrichment in a temperate grassland

Nitrogen (N) deposition can significantly alter plant diversity and soil N status and cycling. However, there is rare information about the responses of plant N uptake of coexisting species to N deposition, which could facilitate the understanding of these N-induced ecosystem changes, particularly when taking N addition frequency into consideration. We assessed the uptake rates of N of various chemical forms (both inorganic and organic N) among coexisting plant species in response to N enrichment, using in situ stable isotope labeling techniques (15N-labeled ammonium, 15N-labeled nitrate and 13C-15N-labeled glycine), through a long-term N deposition experiment with monthly and biannual N addition frequencies in a temperate grassland. The plants of all three targeted species took up much greater inorganic N than amino acid N (93.4–99.4 % vs. 0.6–6.6 % in relative contributions to the total N uptake). The most abundant soil N form that originally was NH4+, was shifted to be NO3− under low and moderate N additions and back to be NH4+ at high addition level. Plants of not only dominant but also minor species predominantly took up N from the most abundant soil N form along N addition gradient. The dominant species Leymus chinensis, which continued to maintain its dominance under N enrichment, exhibited the ability to capture N of all different forms to tolerate and survive in a wide range of soil N environments. In contrast, another dominant species Stipa grandis, which was losing its dominance with N addition, showed the acquisition capacity of single N form in response to increased N inputs. It should be noted that there were interaction effects of N addition frequency and level on plant N uptake rates. Plant N uptake patterns and acquisition strategies could have important consequences for changes in ecosystem biodiversity and function under current and future N deposition scenarios.

Alfalfa Biomass Yield and Nitrogen Fixation in Response to Applied Mineral Nitrogen Under Saline Soil Conditions

Soil salinity is deleterious for the growth and biological N2 fixation of alfalfa (Medicago sativa L.), but nitrogen fertilization has been beneficial in some cases. This study was conducted to investigate the potential benefit of N fertilization on alfalfa growth and N2 fixation under saline conditions. Solutions of sodium chloride (NaCl) and hydrous calcium chloride (CaCl2.2H2O) were added to induce ECe (electrical conductivity of the saturated paste extract) targeted at 0.5, 5, 10, and 15 dS m−1 in a sandy loam soil. 15N-labeled ammonium nitrate (NH4NO3) was added to the soil at 30, 60, and 120 mg N kg−1 soil. Alfalfa shoot and root dry weights, root nodule counts, shoot total nitrogen (N), potassium (K) and sodium (Na+), and the amount of N derived from air (%Ndfa) were determined. Shoot Na concentration increased, and plant biomass, shoot N and K concentration, root nodules, and nitrogen Ndfa decreased as soil salinity increased. At all salinity levels, the highest plant biomass, shoot total N content, and K/Na ratio were recorded at 120 mg N kg−1 soil. Root nodulation and %Ndfa were highest at 60 mg N kg−1 soil under nonsaline conditions while under saline conditions, the highest nodulation and %Ndfa were recorded at 30 mg N kg−1 soil. Plant biomass, nodule number, and %Ndfa were greater in the third cut than in the first cut. Addition of nitrogen fertilizers seemed to have a positive effect in alleviating salt stress in alfalfa and maintaining shoot biomass production, but N fertilization may limit the plant’s ability to fix atmospheric N.

Isolation of cold-adapted nitrate-reducing fungi that have potential to increase nitrate removal in woodchip bioreactors.

The aim of this study was to obtain cold-adapted denitrifying fungi that could be used for bioaugmentation in woodchip bioreactors to remove nitrate from agricultural subsurface drainage water. We isolated a total of 91 nitrate-reducing fungal strains belonging to Ascomycota and Mucoromycota from agricultural soil and a woodchip bioreactor under relatively cold conditions (5 and 15°C). When these strains were incubated with 15 N-labelled nitrate, 29 N2 was frequently produced, suggesting the occurrence of co-denitrification (microbially mediated nitrosation). Two strains also produced 30 N2 , indicating their ability to reduce N2 O. Of the 91 nitrate-reducing fungal strains, fungal nitrite reductase gene (nirK) and cytochrome P450 nitric oxide reductase gene (p450nor) were detected by PCR in 34 (37%) and 11 (12%) strains, respectively. Eight strains possessed both nirK and p450nor, further verifying their denitrification capability. In addition, most strains degraded cellulose under denitrification condition. Diverse nitrate-reducing fungi were isolated from soil and a woodchip bioreactor. These fungi reduced nitrate to gaseous N forms at relatively low temperatures. These cold-adapted, cellulose-degrading and nitrate-reducing fungi could support themselves and other denitrifiers in woodchip bioreactors. The cold-adapted, cellulose-degrading and nitrate-reducing fungi isolated in this study could be useful to enhance nitrate removal in woodchip bioreactors under low-temperature conditions.

Nitrogen flux into metabolites and microcystins changes in response to different nitrogen sources in Microcystis aeruginosa NIES-843.

The over-enrichment of nitrogen (N) in the environment has contributed to severe and recurring harmful cyanobacterial blooms, especially by the non-N2 -fixing Microcystis spp. N chemical speciation influences cyanobacterial growth, persistence and the production of the hepatotoxin microcystin, but the physiological mechanisms to explain these observations remain unresolved. Stable-labelled isotopes and metabolomics were employed to address the influence of nitrate, ammonium, and urea on cellular physiology and production of microcystins in Microcystis aeruginosa NIES-843. Global metabolic changes were driven by both N speciation and diel cycling. Tracing 15 N-labelled nitrate, ammonium, and urea through the metabolome revealed N uptake, regardless of species, was linked to C assimilation. The production of amino acids, like arginine, and other N-rich compounds corresponded with greater turnover of microcystins in cells grown on urea compared to nitrate and ammonium. However, 15 N was incorporated into microcystins from all N sources. The differences in N flux were attributed to the energetic efficiency of growth on each N source. While N in general plays an important role in sustaining biomass, these data show that N-speciation induces physiological changes that culminate in differences in global metabolism, cellular microcystin quotas and congener composition.

Nitrogen Rate Increase Not Required for No-Till Wheat in Cool and Humid Conditions

An increased nitrogen (N) supply was proposed to avoid grain yield (GY) reductions and successfully implement conservation agriculture (CA). We investigated interactions effects of tillage system and N supply on winter wheat (Triticum aestivum L.) at two sites in the Swiss midlands with no (0 kg N ha−1) and high (150–160 kg N ha−1) N supply using 15N-labelled ammonium nitrate in selected treatments. Wheat’s GY, yield components, N related traits and soil mineral N content (Nmin) under conventional tillage (CT), minimum tillage (MT), and no-tillage (NT) were studied following two preceding crops: oilseed rape (Brassica napus L.) and maize (Zea mays L.). Wheat after oilseed rape had significantly higher GY and biomass than after maize while a yield decrease under NT compared with CT and MT was observed regardless of N supply level. Differences in soil Nmin among tillage systems were seldom found and were inconsistent. No differences in 15N fertilizer recovery were found between NT and CT while residual Nmin after harvest was lower under NT than CT or MT. In conclusion, we did not found consistent reductions in soil N availability and N uptake under NT that would justify an increased N supply for wheat under CA.

Quantifying the overestimation of planktonic N2 fixation due to contamination of 15N2 gas stocks

AbstractThe 15N2-tracer assay [Montoya et al. (1996) A simple, high-precision, high-sensitivity tracer assay for N2 fixation. Appl. Environ. Microbiol., 62, 986–993.] is the most used method for measuring biological N2 fixation in terrestrial and aquatic environments. The reliability of this technique depends on the purity of the commercial 15N2 gas stocks used. However, Dabundo et al. [(2014) PLoS One, 9, e110335.] reported the contamination of some of these stocks with labile 15N-labeled compounds (ammonium, nitrate and/or nitrite). The contamination of commercial 15N2 gas stocks with 15N-labeled nitrate and 142 ammonium and consequences for nitrogen fixation measurements. Considering that the tracer assay relies on the conversion of isotopically labeled 15N2 into organic nitrogen, this contamination may have led to overestimated N2 fixation rates. We conducted laboratory and field experiments in order to (i) test the susceptibility of 15N contaminants to assimilation by non-diazotroph organisms and (ii) determine the potential overestimation of the N2 fixation rates estimated in the field. Our findings indicate that the contaminant 15N-compounds are assimilated by non-diazotrophs organisms, leading to an overestimation of N2 fixation rates in the field up to 16-fold under hydrographic conditions of winter mixing.

Underestimation of denitrification rates from field application of the <sup>15</sup>N gas flux method and its correction by gas diffusion modelling

Abstract. Common methods for measuring soil denitrification in situ include monitoring the accumulation of 15N-labelled N2 and N2O evolved from 15N-labelled soil nitrate pool in closed chambers that are placed on the soil surface. Gas diffusion is considered to be the main transport process in the soil. Because accumulation of gases within the chamber decreases concentration gradients between soil and the chamber over time, the surface efflux of gases decreases as well, and gas production rates are underestimated if calculated from chamber concentrations without consideration of this mechanism. Moreover, concentration gradients to the non-labelled subsoil exist, inevitably causing downward diffusion of 15N-labelled denitrification products. A numerical 3-D model for simulating gas diffusion in soil was used in order to determine the significance of this source of error. Results show that subsoil diffusion of 15N-labelled N2 and N2O – and thus potential underestimation of denitrification derived from chamber fluxes – increases with chamber deployment time as well as with increasing soil gas diffusivity. Simulations based on the range of typical soil gas diffusivities of unsaturated soils showed that the fraction of N2 and N2O evolved from 15N-labelled NO3- that is not emitted at the soil surface during 1 h chamber closing is always significant, with values up to >50 % of total production. This is due to accumulation in the pore space of the 15N-labelled soil and diffusive flux to the unlabelled subsoil. Empirical coefficients to calculate denitrification from surface fluxes were derived by modelling multiple scenarios with varying soil water content. Modelling several theoretical experimental set-ups showed that the fraction of produced gases that are retained in soil can be lowered by lowering the depth of 15N labelling and/or increasing the length of the confining cylinder. Field experiments with arable silt loam soil for measuring denitrification with the 15N gas flux method were conducted to obtain direct evidence for the incomplete surface emission of gaseous denitrification products. We compared surface fluxes of 15N2 and 15N2O from 15N-labelled micro-plots confined by cylinders using the closed-chamber method with cylinders open or closed at the bottom, finding 37 % higher surface fluxes with the bottom closed. Modelling fluxes of this experiment confirmed this effect, however with a higher increase in surface flux of 89 %. From our model and experimental results we conclude that field surface fluxes of 15N-labelled N2 and N2O severely underestimate denitrification rates if calculated from chamber accumulation only. The extent of this underestimation increases with closure time. Underestimation also occurs during laboratory incubations in closed systems due to pore space accumulation of 15N-labelled N2 and N2O. Due to this bias in past denitrification measurements, denitrification in soils might be more relevant than assumed to date. Corrected denitrification rates can be obtained by estimating subsurface flux and storage with our model. The observed deviation between experimental and modelled subsurface flux revealed the need for refined model evaluation, which must include assessment of the spatial variability in diffusivity and production and the spatial dimension of the chamber.

Metabolomic study of saxitoxin analogues and biosynthetic intermediates in dinoflagellates using 15N-labelled sodium nitrate as a nitrogen source

A stable-isotope-labelling method using 15N-labelled sodium nitrate as a nitrogen source was developed for the toxic dinoflagellate Alexandrium catenella. The labelled saxitoxin analogues (STXs), their precursor, and the biosynthetic intermediates were analyzed by column-switching high-resolution hydrophilic interaction liquid chromatography with mass spectrometry. The low contents on Day 0, high 15N incorporation % of Int-C’2 and Int-E’ suggested that their turn-over rates are high and that the sizes of the pool of these compounds are smaller than those of the other intermediates. The experimentally determined isotopomer distributions showed that arginine, Int-C’2, 11-hydroxy-Int-C’2, Int-E’, GTX5, GTX4, C1, and C2, each existed as a combination of three populations that consisted of the non-labelled molecules and the labelled isotopomers representing molecules newly synthesized by incorporation of 15N assimilated from the medium with two different incorporation rates. The order of 15N incorporation % values of the labelled populations predicted by this model largely agreed with the proposed biosynthetic route. The stable-isotope-labelling method will be useful for understanding the complex mechanism of nitrogen flux in STX-producing dinoflagellates.

Nitrate, but not nitrite, derived from nitrogen dioxide accumulates in Arabidopsis leaves following exposure to 15N-labeled nitrogen dioxide

ABSTRACTIt is known that when plant leaves are exposed to exogenously applied nitrogen dioxide (NO2), nitrogen derived from NO2 is reduced to amino acid nitrogen. However, whether this is the sole metabolic fate of exogenously applied NO2 is unclear. In this study, Arabidopsis leaves were exposed to 4 ppm 15N-labeled NO2 for 4 h in light, followed by capillary ion analysis and elemental analysis–mass spectrometry with an elemental analyzer connected directly to a mass spectrometer. We found that leaf cells exposed to 15N-labeled NO2 accumulated a large amount of 15N-labeled nitrate. Neither 15N-labeled nitrite nor endogenous nitrite was present in exposed leaves. It is likely that exogenously applied NO2 is first converted to nitrite, and that nitrite is oxidized to nitrate in Arabidopsis leaf cells. The complete disappearance of nitrite derived from exogenously applied NO2 and endogenous nitrite supports this mechanism.

Labeling Maize (Zea mays L.) Leaves with 15 NH4+ and Monitoring Nitrogen Incorporation into Amino Acids by GC/MS Analysis.

The human body contains approximately 3.2% nitrogen (N), mainly present as protein and amino acids. Although N exists at a high concentration (78%) in the air, it is not readily available to animals and most plants. Plants are however able to take up both nitrate (NO3- ) and ammonium (NH4+ ) ions from the soil and convert them to amino acids and proteins, which are excellent sources for all animals. Most N is available as the stable isotope 14 N, but a second form, 15 N, is present in very low concentrations. 15 N can be detected in extracts of plants by gas chromatography followed by mass spectrometry (GC/MS). In this protocol, the methods are described for tracing the pathway by which plants are able to take up 15 N-labeled nitrate and ammonium and convert them into amino acids and proteins. A protocol for extracting and quantifying amino acids and 15 N enrichment in maize (Zea mays L.) leaves labeled with 15 NH4+ is described. Following amino acid extraction, purification, and separation by GC/MS, a calculation of the 15 N enrichment of each amino acid is carried out on a relative basis to identify any differences in the dynamics of amino acid accumulation. This will allow a study of the impact of genetic modifications or mutations on key reactions involved in primary nitrogen and carbon metabolism. © 2018 by John Wiley & Sons, Inc.

“Preferential” ammonium uptake by sugarcane does not increase the 15N recovery of fertilizer sources

As ammonium (NH4+) has been reported as the “preferred” mineral N source for sugarcane (Saccharum spp.), the predominant supply of NH4+ has been hypothesized as a means of increasing the 15N recovery and crop yield. We aimed to evaluate the 15N recovery by sugarcane as affected by 15N-NH4+ and 15N-NO3− addition. Under field conditions, 15N-labeled ammonium nitrate [labeling as 15NH4NO3 (15N-NH4+) and NH415NO3 (15N-NO3−)] and urea were surface-applied at a rate of 100 kg N ha−1. Under controlled conditions, sugarcane was grown in hydroponic medium (containing unlabeled N) until the maximum N demand period. Subsequently, plants received 15N-NH4+ or 15N-NO3− (as ammonium nitrate). Plants were then sampled at 1, 3, 7, and 15 d after fertilization (DAF) to estimate 15N recovery. Under field conditions, the 15N recovery for urea was lower than that for ammonium nitrate, partly explained by ammonia volatilization, but there was no difference in N use efficiency between the two fertilizer N sources, as well as between 15N-NH4+ and 15N-NO3− supply. Under controlled conditions, however, 15N-NH4+ resulted in higher 15N recovery than 15N-NO3− until 7 DAF. Nevertheless, as 15N-NH4+ became depleted in the nutrient solution, 15N-NO3− was taken up more intensely by the crop. At 15 DAF, highest 15N recovery was detected in the roots and whole plant under 15N-NO3− supply. Although uptake of NH4+ is faster than NO3−, the so-called “preference” for NH4+ by the crop did not necessarily translate into higher 15N recovery. Therefore, we suggest that the term “preference” should be used with caution to avoid misinterpretation.

A Method to Evaluate Isotopic and Energy Turnover Rates in Larval Culex quinquefasciatus (Diptera: Culicidae) Using Stable Isotope Labeled Compounds.

The goal of this study was to evaluate the use of stable isotope labeled compounds to better understand factors influencing energy turnover in larval Culex quinquefasciatus (Say; Diptera: Culicidae). Three isotope labeled compounds were evaluated in this study, including 15N-labeled potassium nitrate, 13C-labeled glucose, and 13C-labeled leucine. Conditions were first optimized in the laboratory to determine the most appropriate concentration of isotope, as well as the half-life of enrichment. Once optimum conditions were established we used standard equations to predict and determine temperature and density-dependent energy turnover rates. Our results showed that higher concentrations of isotope had an impact on mosquito survivability, overall enrichment, and adult wing length. We predicted the half-life of to be around 0.614 to 0.971 d, and our observed half-lives were determined to be 0.72 to 1.44 d depending on temperature, larval density, and isotope compound. Both density and temperature had a strong influence on isotopic turnover rates in all isotopes evaluated. Our results suggest that stable isotopes can provide a useful tool in understanding how different stress factors influence energy turnover in larval Cx. quinquefasciatus. These data can also help lay a foundation on ways to improve larvicide efficacy under different biotic and abiotic conditions.

Anthropogenic changes of nitrogen loads in a small river: external nutrient sources vs. internal turnover processes

ABSTRACTAnthropogenic nutrient inputs increase the N-load in many aquatic systems, leading to eutrophication and potential changes of biological N-retention capacity. In this study, nitrate inputs in a small river were investigated along a gradient of anthropogenic influence. We aimed to determine changes in nitrate load and isotope signatures in the water column and to identify the anthropogenic influence on biological nitrogen assimilation and nitrification or denitrification in sediments. In seasonal sampling campaigns, we analysed dissolved inorganic nitrogen concentrations, and stable isotopes of nitrate. To differentiate rates of nitrate production and consumption in the pristine vs. agricultural river section, intact sediment cores were incubated with 15N-labelled nitrate. δ15N values of nitrate in the pristine river section were low, reflecting natural sources, but, as expected, increased with nitrate concentration in all seasons along the gradient. In general, nitrate retention and consumption were higher in the anthropogenically impacted than in the pristine river section, and nitrate consumption exceeded production. In addition to our measurements, modelled results also show that even in a small river, the anthropogenically enhanced consumption capacity is overwhelmed by surplus N-inputs, and nitrate consumption cannot increase in turn with external loads.

The bivalve Lembulus bicuspidatus may enhance denitrification in shelf sediment at the Angola–Benguela Frontal Zone

We collected living individuals of the bivalve Lembulus bicuspidatus, which shows an unusual preference for the oxygen-deficient habitat found at the Angola–Benguela Frontal Zone of the southeastern Atlantic. With a series of incubation experiments with 15N-labelled nitrate as a tracer in combination with membrane-inlet mass spectrometry, we studied the potential contribution of L. bicuspidatus to nitrate reduction in the upper sediment layer. Our preliminary results suggest that L. bicuspidatus enhances nitrate reduction if the oxygen concentration is sufficiently low. The Lembulus-mediated nitrate reduction rate is then similar to the rate of microbial nitrate reduction in the surrounding sediment.

Asparagine and sugars are both required to sustain secondary axis elongation after bud outgrowth in Rosa hybrida

Nitrogen is required for optimal plant growth, especially in young organs such as secondary axes (axes II) after axillary bud outgrowth. Several studies have shown an increase of nitrogen concentration in xylem sap concomitantly with bud outgrowth, but the relation between nitrogen, sugars and plant hormones in axis II still remains unclear. We investigated in Rosa hybrida the involvement of nitrogen nutrition in axis II elongation in relation with sugars and cytokinins using 15N-labeled nitrate and sugars, amino acids and cytokinin quantifications. Besides, we measured the effect of the exogenous supply of these compounds on axis II elongation using in vitro excised bud culture. We demonstrated that nitrogen in the axis II comes mainly from new root uptake after decapitation. Asparagine, which concentration increases in sap exudates and tissues during axis II elongation, was the sole amino acid able to sustain an efficient elongation in vitro when supplied in combination with sucrose.

Fate of 15N-labelled ammonium nitrate with or without the new nitrification inhibitor DMPSA in an irrigated maize crop

Nitrification inhibitors, originally proposed for nitrate leaching mitigation, are recommended as effective nitrous oxide (N2O) mitigation strategies. Several compounds have been trialled and used in the past including dicyandiamide (DCD) or 3,4-dimethylpyrazole phosphate (DMPP). Yet, little is known about the new nitrification inhibitor 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA). A field experiment using 15N single-labelled ammonium nitrate (15NH4NO3 or NH415NO3) was conducted to understand the effectiveness of DMPSA on a biochemical basis in an irrigated maize (Zea mays L.) crop. Gaseous fluxes, i.e. N2O, 15N2O, 15N2, methane (CH4), and carbon dioxide (CO2) were measured, as well as soil mineral N, yield components and 15N recovery in plant and soil. During the maize cropping period, the use of DMPSA significantly reduced cumulative N2O emissions (118 g N ha−1) compared to ammonium nitrate without inhibitor (231 g N ha−1). The 15N analyses revealed that most N2O losses (particularly during the emission peak) came from 15NH4NO3 (i.e. nitrification, nitrifier denitrification and/or coupled nitrification denitrification) rather than NH415NO3 in this calcareous low C-content soil. As expected, DMPSA decreased N2O losses from 15NH4+ oxidation, but an effect on non-target microorganisms was noticed, as shown by the significant reduction of respiration rates and N2O losses coming from 15NO3−. No significant effect of DMPSA on CH4 oxidation or 15N2 fluxes was observed. The DMPSA did not lead to a significant improvement of the dry weights of grain or biomass, although an increment of root biomass by 64% was found. This compound also tended to increase plant N recovery (average 67.8%) and to decrease soil N recovery (average 18.3%) but differences were not statistically significant. Conversely, the nitrification inhibitor significantly reduced the residual fertilizer-N in the 15–30 cm and 30–45 cm soil layers. The use of DMPSA was confirmed as a highly effective tool to reduce N2O emissions from irrigated crops in semi-arid areas.

Elevated CO2 Increases Nitrogen Fixation at the Reproductive Phase Contributing to Various Yield Responses of Soybean Cultivars.

Nitrogen deficiency limits crop performance under elevated CO2 (eCO2), depending on the ability of plant N uptake. However, the dynamics and redistribution of N2 fixation, and fertilizer and soil N use in legumes under eCO2 have been little studied. Such an investigation is essential to improve the adaptability of legumes to climate change. We took advantage of genotype-specific responses of soybean to increased CO2 to test which N-uptake phenotypes are most strongly related to enhanced yield. Eight soybean cultivars were grown in open-top chambers with either 390 ppm (aCO2) or 550 ppm CO2 (eCO2). The plants were supplied with 100 mg N kg−1 soil as 15N-labeled calcium nitrate, and harvested at the initial seed-filling (R5) and full-mature (R8) stages. Increased yield in response to eCO2 correlated highly (r = 0.95) with an increase in symbiotically fixed N during the R5 to R8 stage. In contrast, eCO2 only led to small increases in the uptake of fertilizer-derived and soil-derived N during R5 to R8, and these increases did not correlate with enhanced yield. Elevated CO2 also decreased the proportion of seed N redistributed from shoot to seeds, and this decrease strongly correlated with increased yield. Moreover, the total N uptake was associated with increases in fixed-N per nodule in response to eCO2, but not with changes in nodule biomass, nodule density, or root length.

Characterization of the denitrifying bacterial community in a full-scale rockwool biofilter for compost waste-gas treatment.

The potential denitrification activity and the composition of the denitrifying bacterial community in a full-scale rockwool biofilter used for treating livestock manure composting emissions were analyzed. Packing material sampled from the rockwool biofilter was anoxically batch-incubated with 15N-labeled nitrate in the presence of different electron donors (compost extract, ammonium, hydrogen sulfide, propionate, and acetate), and responses were compared with those of activated sludge from a livestock wastewater treatment facility. Overnight batch-incubation showed that potential denitrification activity for the rockwool samples was higher with added compost extract than with other potential electron donors. The number of 16S rRNA and nosZ genes in the rockwool samples were in the range of 1.64-3.27×109 and 0.28-2.27×108 copies/g dry, respectively. Denaturing gradient gel electrophoresis analysis targeting nirK, nirS, and nosZ genes indicated that the distribution of nir genes was spread in a vertical direction and the distribution of nosZ genes was spread horizontally within the biofilter. The corresponding denitrifying enzymes were mainly related to those from Phyllobacteriaceae, Bradyrhizobiaceae, and Alcaligenaceae bacteria and to environmental clones retrieved from agricultural soil, activated sludge, freshwater environments, and guts of earthworms or other invertebrates. A nosZ gene fragment having 99% nucleotide sequence identity with that of Oligotropha carboxidovorans was also detected. Some nirK fragments were related to NirK from micro-aerobic environments. Thus, denitrification in this full-scale rockwool biofilter might be achieved by a consortium of denitrifying bacteria adapted to the intensely aerated ecosystem and utilizing mainly organic matter supplied by the livestock manure composting waste-gas stream.

Periphyton support for littoral secondary production in a highly humic boreal lake

AbstractSteep stratification and poor light penetration in highly humic lakes typically restrict oxygenated littoral areas to narrow lake margins. However, in some instances, surrounding floating vegetation mats can sustain highly productive periphyton and more diverse invertebrate communities than pelagic areas. Little is known about how these littoral food webs function or the extent to which the pelagic and littoral food webs are coupled. We added 15N-labeled ammonium nitrate to the floating moss mat surrounding the littoral zone of Mekkojarvi, a small highly humic and fishless lake in southern Finland. Our goal was to increase the δ15N values of periphyton to investigate the diets of littoral invertebrates and possible pelagic–littoral coupling in the lake. We divided the lake in 2 with a plastic curtain and added European Perch (Perca fluviatilis) to 1 basin while the other remained fishless. δ15N of periphyton and most littoral invertebrates increased well above the natural abundance levels. δ15N of...

Effect of iron oxide on nitrification in two agricultural soils with different pH

Abstract. Iron (Fe) affects soil nitrogen (N) cycling processes both in anoxic and oxic environments. The role of Fe in soil N transformations including nitrification, mineralization, and immobilization, is influenced by redox activity, which is regulated by soil pH. The effect of Fe minerals, particularly oxides, on soil N transformation processes depends on soil pH, with Fe oxide often stimulating nitrification activity in the soil with low pH. We conducted lab incubations to investigate the effect of Fe oxide on N transformation rates in two subtropical agricultural soils with low pH (pH 5.1) and high pH (pH 7.8). 15N-labeled ammonium and nitrate were used separately to determine N transformation rates combined with Fe oxide (ferrihydrite) addition. Iron oxide stimulated net nitrification in low-pH soil (pH 5.1), while the opposite occurred in high-pH soil (pH 7.8). Compared to the control, Fe oxide decreased microbial immobilization of inorganic N by 50 % in low-pH soil but increased it by 45 % in high-pH soil. A likely explanation for the effects at low pH is that Fe oxide increased NH3-N availability by stimulating N mineralization and inhibiting N immobilization. These results indicate that Fe oxide plays an important role in soil N transformation processes and the magnitude of the effect of Fe oxide is dependent significantly on soil pH.