An alternative explanation for the post-disturbance NO3– flush in some forest ecosystems

The appearance of soil NO₃^– after forest disturbance is commonly ascribed to a higher availability of NH₄⁺ to autotrophic nitrifiers, or to a reduction in available-C resulting in lower microbial assimilation of NO₃^–. Alternatively, it has been proposed that increasing NH₄⁺ pools following disturbance could increase net nitrification by reducing microbial assimilation of NO₃^–. Forest floor material was collected from shelterwood harvest plots which displayed both low available-C and low NH₄⁺ pools, and where previous experiments had suggested the prevalence of heterotrophic nitrification. Subsamples were amended with incremental rates of glucose-C or NH₄⁺, and gross NO₃^– transformation rates were measured by isotope dilution. Glucose-C additions had little effect on the net difference between gross NO₃^– production and consumption rates. On the other hand, NH₄⁺ additions caused gross NO₃^– consumption processes to decrease sharply, while gross NO₃^– production processes remained constant. The results suggest that NH₄⁺ can have an immediate positive effect on net nitrification rates by suppressing NO₃^– assimilation and uptake systems.     

A comparison of NH4+ and NO3– net fluxes along roots of rice and maize

Net fluxes of NH₄⁺ and NO₃^– along adventitious roots of rice ( Oryza sativa L.) and the primary seminal root of maize ( Zea mays L.) were investigated under nonperturbing conditions using ion-selective microelectrodes. The roots of rice contained a layer of sclerenchymatous fibres on the external side of the cortex, whereas this structure was absent in maize. Net uptake of NH₄⁺ was faster than that of NO₃^– at 1 mm behind the apex of both rice and maize roots when these ions were supplied together, each at 0·1 mol m^–3. In rice, NH₄⁺ net uptake declined in the more basal regions, whereas NO₃^– net uptake increased to a maximum at 21 mm behind the apex and then it also declined. Similar patterns of net uptake were observed when NH₄⁺ or NO₃^– was the sole nitrogen source, although the rates of NO₃^– net uptake were faster in the absence of NH₄⁺. In contrast to rice, rates of NH₄⁺ and NO₃^– net uptake in the more basal regions of maize roots were similar to those near the root apex. Hence, the layer of sclerenchymatous fibres may have limited ion absorption in the older regions of rice roots.     

Root ammonium transport efficiency as a determinant in forest colonization patterns: an hypothesis

Ratios of ammonium (NH₄⁺) to nitrate (NO₃^–) in soils are known to increase during forest succession. Using evidence from several previous studies, we hypothesize that a malfunction in NH₄⁺ transport at the membrane level might limit the persistence of early successional tree species in later seral stages. In those studies, ¹³N radiotracing was used to determine unidirectional fluxes and pool sizes of NH₄⁺ and NO₃^– in seedlings of the late-successional species white spruce ( Picea glauca ) and in the early successional species Douglas-fir ( Pseudotsuga menziesii var. glauca ) and trembling aspen ( Populus tremuloides ). At high external NH₄⁺, the two early successional species accumulated excessive NH₄⁺ in the root cytosol, and exhibited high-velocity, low-efficiency (15% to 22%), membrane fluxes of NH₄⁺. In sharp contrast, white spruce had low cytosolic NH₄⁺ accumulation, and lower-velocity but much higher-efficiency (65%), NH₄⁺ fluxes. Because these divergent responses parallel known differences in tolerance and toxicity to NH₄⁺ amongst these species, we propose that they constitute a significant driving force in forest succession, complementing the discrimination against NO₃^– documented in white spruce (Kronzucker et al. 1997).     

Translocation of NH4+ in oilseed rape plants in relation to glutamine synthetase isogene expression and activity

Translocation of NH₄⁺ was studied in relation to the expression of three glutamine synthetase (GS, EC 6.3.1.2) isogenes and total GS activity in roots and leaves of hydroponically grown oilseed rape ( Brassica napus ). The concentration of NH₄⁺ in the stem xylem sap of NO₃⁻-fed plants was 0.55–0.70 m M , which was ≈60% higher than that in plants deprived of external nitrogen for 2 days. In NH₄⁺-fed plants, xylem NH₄⁺ concentrations increased linearly both with time of exposure to NH₄⁺ and with increasing external NH₄⁺ concentration. The maximum xylem NH₄⁺ concentration was 8 m M , corresponding to 11% of the nitrogen translocated in the xylem. In the leaf apoplastic solution, the NH₄⁺ concentration increased from 0.03 m M in N-deprived plants to 0.20 m M in N-replete plants. The corresponding values for leaf tissue water were 0.33 and 1.24 m M , respectively. The addition of either NO₃⁻ or NH₄⁺ to N-starved plants induced both cytosolic gs isogene expression and GS activity in the roots. In N-replete plants, gs isogene expression and GS activity were repressed, probably due to carbon limitations, thereby protecting the roots against the excessive drainage of photosynthates. Repressed gs isogene expression and GS activity under N-replete conditions caused enhanced NH₄⁺ translocation to the shoots.     

Response of nitrogen metabolism to boron toxicity in tomato plants

Boron (B) toxicity has become important in areas close to the Mediterranean Sea where intensive agriculture has been developed. The objective of this research was to study the effects of B toxicity (0.5 m m and 2.0 m m B) on nitrogen (N) assimilation of two tomato cultivars that are often used in these areas. Leaf biomass, relative leaf growth rate (RGR_L), concentration of B, nitrate (NO₃⁻), ammonium (NH₄⁺), organic N, amino acids and soluble proteins, as well as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthase (GS), glutamate synthetase (GOGAT) and glutamate dehydrogenase (GDH) activities were analysed in leaves. Boron toxicity significantly decreased leaf biomass, RGR_L, organic N, soluble proteins, and NR and NiR activities. The lowest NO₃⁻ and NH₄⁺ concentration in leaves was recorded when plants were supplied with 2.0 m m B in the root medium. Total B, amino acids, activities of GS, GOGAT and GDH increased under B toxicity. Data from the present study prove that B toxicity causes inhibition of NO₃⁻ reduction and increases NH₄⁺ assimilation in tomato plants.     

Impact of gaseous nitrogen deposition on plant functioning

Dry deposition of NH₃ and NO_x (NO and NO₂) can affect plant metabolism at the cellular and whole-plant level. Gaseous pollutants enter the plant mainly through the stomata, and once in the apoplast NH₃ dissolves to form NH₄⁺, whereas NO₂ dissolves to form NO₃⁻ and NO₂⁻. The latter compound can also be formed after exposure to NO. There is evidence that NH₃-N and NO_x-N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re-emission. Once formed, NO₃⁻ and NO₂⁻ can be reduced, and NH₄⁺ can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. Fumigation with low concentrations of atmospheric NH₃ increases in vitro glutamine synthetase activity, but whether this involves both or only one of the GS isoforms is still an open question. There seems to be no correlation between fumigation with low concentrations of NH₃ and in vitro GDH activity. The contribution of atmospheric NH₃ and NO₂ deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates ( RGRs ). It is concluded that at current ambient atmospheric N concentrations the direct impact of gaseous N uptake by foliage on plant growth is generally small.     

Time-course of inorganic nitrogen uptake and incorporation by natural populations of freshwater phytoplankton

SUMMARY. 1. Time-course measurements of NH₄⁺ and NO₃⁻uptake were made on the natural phytoplankton populations in a eutrophic lake at a time when these nutrients were at their lowest annual concentration.
2. Both NH₄⁺ and NO₃⁻ uptake was increased at least five-fold during the first 5 min of incubation following near saturating pulses of these nutrients.
3. Elevated uptake was also observed following low level (∼2μg N 1⁻¹) pulses of NH₄⁺ and NO₃⁻, but substrate depletion during the first hour of incubation may have been partially responsible for this apparent enhancement.
4. Incorporation of ^I5N into TCA-insoluble material (protein) following the saturating NH₄⁺ pulse was increased less than total cellular ¹⁵N uptake, whereas no elevation of ¹⁵N incorporation into protein was observed following a saturating NO₃⁻pulse.
5. The percentage of ^I5N incorporated into protein, with respect to total cellular uptake, was ∼32% and ∼12% for NH₄⁺ and NO₃⁻, respectively, following 5 h of incubation.     

Different effects of supplied ammonium on glutamine synthetase activity in mustard (Sinapis alba) and pine (Pinus sylvestris) seedlings

The activity of glutamine synthetase (GS) in mustard ( Sinapis alba L.) and Scots pine ( Pinus sylvestris L.) seedlings was used as an index to evaluate the capacity to cope with excessive ammonium supply. In these 2 species GS activity was differently affected by the application of nitrogen compounds (NH₄⁺ or NO₃⁻). Mustard seedlings older than 5 days showed a considerable increase in GS activity after NH₄⁺ or NO₃⁻ application. This response was independent of the energy flux, but GS activity in general was positively affected by light. Endogenous NH₄⁺ did not accumulate greatly after nitrogen supply. In contrast, seedlings of Scots pine accumulated NH₄⁺ in cotyledons and roots and showed no stimulation of GS activity after the application of ammonium. In addition, root growth was drastically reduced. Thus, the pine seedlings seem to have insufficient capacity to assimilate exogenously supplied ammonium. NO₃⁻, however, did not lead to any harmful effects.     

Effect of darkness and CO2 starvation on NH4+ and NO3− assimilation in the unicellular alga Cyanidium caldarium

Cyanidium caldarium (Tilden) Geitler, a non-vacuolate unicellular alga, resuspended in medium flushed with air enriched with 5% CO₂, assimilated NH₄⁺ at high rates both in the light and in the dark. The assimilation of NO₃⁻, by contrast, was inhibited by 63% in the dark. In cell suspensions flushed with CO₂-free air, NH₄⁺ assimilation decreased with time both in the light and in the dark and ceased almost completely after 90 min. The addition of CO₂ completely restored the capacity of the alga to assimilate NH₄⁺. NO₃⁻ assimilation, by contrast, was 33% higher in the absence of CO₂ and was linear with time. It is suggested that NO₃⁻ and NH₄⁺ metabolism in C. caldarium are differently controlled in response to the light and carbon conditions of the cell.     

Nitrogen concentration, ammonium/nitrate ratio and NaCl interaction in vegetative and reproductive growth of peanuts

Peanuts ( Arachis hypogaea L. cv. Shulamit) grown with NO₃⁻ and saline water in hydroponics responded positively to addition of nitrogen (N) in their vegetative growth, but not in desert dune sand. In order to clarify these conflicting results, peanut plants were grown in a greenhouse pot experiment with fine calcareous sand. The nutrient solution contained 0 or 50 m M NaCl and 2 or 6 m M N in the form of Ca(NO₃)₂, NH₄NO₃ or (NH₄)₂SO₄. Three replicates were harvested after 48 days (beginning of reproductive stage) and three after 109 days (pod filling). In addition, gynophores were treated with 0, 50, 100, 150 or 200 m M NaCl outside the growth pot to check their sensitivity to salt. Shoot dry weight became greater with increasing NH₄⁺/NO₃⁻ ratio. Increasing the N concentration from 2 to 6 m M did not change shoot dry weight of the NH₄NO₃ or NH₄⁺-fed plants, but caused a reduction in shoot dry weight of NO₃⁻-fed plants. Shoot dry weight was not affected by increasing the NaCl concentration to 50 m M . Salt caused an increase in the number of gynophores per plant and a reduction of the mean pod weight. A NaCl concentration of 100 m M and above reduced gynophore vitality. It is concluded that the salt sensitivity of peanut plants resides mainly in the sensitivity of the reproductive organs.     

Adaptation of plasma membrane H+-ATPase of rice roots to low pH as related to ammonium nutrition

The preference of paddy rice for NH₄⁺ rather than NO₃^- is associated with its tolerance to low pH since a rhizosphere acidification occurs during NH₄⁺ absorption. However, the adaptation of rice root to low pH has not been fully elucidated. This study investigated the acclimation of plasma membrane H⁺-ATPase of rice root to low pH. Rice seedlings were grown either with NH₄⁺ or NO₃^-. For both nitrogen forms, the pH value of nutrient solutions was gradually adjusted to pH 6.5 or 3.0. After 4 d cultivation, hydrolytic H⁺-ATPase activity, V _max, K _m, H⁺-pumping activity, H⁺ permeability and pH gradient across the plasma membrane were significantly higher in rice roots grown at pH 3.0 than at 6.5, irrespective of the nitrogen forms supplied. The higher activity of plasma membrane H⁺-ATPase of adapted rice roots was attributed to the increase in expression of OSA1, OSA3, OSA7, OSA8 and OSA9 genes, which resulted in an increase of H⁺-ATPase protein concentration. In conclusion, a high regulation of various plasma membrane H⁺-ATPase genes is responsible for the adaptation of rice roots to low pH. This mechanism may be partly responsible for the preference of rice plants to NH₄⁺ nutrition.     

Cytosolic and chloroplastic glutamine synthetase of sugarbeet (Beta vulgaris) respond differently to organ ontogeny and nitrogen source

Changes in the activity and subunit composition of cytosolic glutamine synthetase (GS 1; EC 6.3.1.2) and chloroplastic GS (GS 2) were studied in response to an internal (organ ontogeny) and external signal (N source: NO₃⁻ or NH₄⁺). Maximum GS 1 activity of all organs examined was measured in the fibre roots, irrespective of the N source. The response of GS 1 to the N source was, however, organ specific. In the fibre roots, NH₄⁺ nutrition resulted in a 2- to 7-fold (based on protein or freshweight, respectively) increase of GS 1 activity compared to NO₃⁻-grown plants. In contrast to the roots, GS 1 activity in the leaf blades was 2-fold lower with NH₄⁺ nutrition, whereas only minor changes occurred in the petioles. GS 2 activity was highest in the mature and senescing leaf blade; activity was 2-fold higher with NH₄⁺ than with NO₃⁻ nutrition. Not only activity, but also subunit composition of GS 1 changed during organ ontogeny as well as in response to the N source. In contrast to GS 1, only minor changes were evident in GS 2 subunit composition, despite significant changes in GS 2 activity. Up to 5 different GS 1 subunits of ≈41–43 kDa were separated; they were identical in all organs examined. GS 2 was composed of 4 different subunits of ≈48 kDa.     

A reaction diffusion model of C-N-S-O species in some arctic sediments

Abstract A reaction diffusion model was used to simulate the mineralization processes in an Arctic sediment. The simulation and the actual sediment were compared in relation to profiles of O₂, NO₃ and NH₄⁺. The site of particulate organic matter (POM) degradation was the single most important factor in fitting the simulation profiles to those of the sediment. It was deduced that most POM degradation occurred close to the sediment surface. When a reasonably good simulation had been obtained, the sensitivity of the model to changes in other parameters was investigated. Increases in POM degradation in the upper sediment resulted in increases in concentration of NH₄⁺ and NO₃⁻, but further increases in POM degradation created anoxic conditions below 3 mm, resulting in decreases in NO₃⁻ concentrations. The model was relatively intensive to changes in POM degradation in the lower sediment layers; increases led to more anoxic conditions and to less NO₃⁻. Increases in the C/N ratio of the POM in the lower sediment layers had little effect; increases in C/N in the upper layers led to a decrease in NH₄⁺ and NO₃⁻. The model was sensitive to changes in the first order rate constant for nitrification, but not for denitrification. Decreases in the K _m for O₂ of the nitrifying bacteria had no effect on the profiles.     

Methane-dependent nitrate production by a microbial consortium enriched from a cultivated humisol

Abstract A consortium was enriched from a humisol incubated with 3.6 kPa CH₄ and NH₄⁺. This consortium oxidized NH₄⁺ to NO₂⁻ and NO₃⁻ (NO₃⁻/NO₂⁻ ratio about 20) with smaller amounts of N₂O. This oxidation stopped in the stationary phase after depletion of CH₄. CH₃OH or CO₂ did not support oxidation. Growth and resting cell experiments suggested that nitrification was associated with methanotrophic activity and that chemoautotrophic nitrifiers were absent.     

Photosynthate costs associated with the utilization of different nitrogen–forms: influence on the carbon balance of plants and shoot–root biomass partitioning

The photosynthate costs of processes (amino acid and protein synthesis and turnover, and pH regulation) associated with the utilization of nitrate (NO₃⁻), ammonium (NH₄⁺) or glutamine (Gln) for plant growth were estimated. Based on these estimates, the effects of these forms of nitrogen (N) on the carbon balance of plants and on shoot–root biomass allocation were evaluated. The results indicated that NO₃⁻ as an N source for plant growth is not substantially more expensive to utilize than either NH₄⁺ or Gln, particularly in the long term when costs due to protein turnover dominate the total costs of N utilization. It is also suggested that the photosynthate use in processes associated with N assimilation has little impact on the carbon balance of plants, and hence on shoot–root biomass allocation.     

Net fluxes and pools of nitrogenous compounds during suspension culture of photoautotrophic Chenopodium rubrum cells

Abstract. Suspension cultured cells of Chenopodium rubrum were grown photoautotrophically under a diurnal light-dark cycle of 16-8h. The following phases of the batch culture were differentiated: a short lag, a cell division phase terminated by a pronounced transition to stationary maintenance which finally gradually passed into senescence. Nitrogen fluxes typical of these stages were followed by measuring uptake of NO₃ and NH₄⁺ from the medium and their incorporation into the cellular fractions of nitrogenous compounds. Activities of seven N-metabolizing enzymes were determined. Compartmentation of enzymes and nitrogenous compounds was analysed after isolation of intact chloroplasts and vacuoles from protoplasts. Eighty-two per cent of the N originally present in the medium was taken up and incorporated to an extent of 80% into protein until the end of the division phase. Net protein synthesis ceased upon transition to the stationary phase. During the division phase a vacuolar pool of NO₃ was established and then maintained throughout the resting phase. Free cellular NH₄⁺ was not localized within the vacuole and responded to the ammonium content of the medium. Amino acids accumulated in the cells especially during the stationary phase, during which they were present in the vacuole. Typical nitrogen relations are portrayed as flux diagrams for one day of each of the essential developmental phases. The enzyme activities were easily sufficient to account for the observed flow rates of the corresponding nitrogenous compounds. Hence, uptake of NO₃ and NH₄⁺ must be considered as steps limiting N metabolism in Chenopodium rubrum cell suspensions.     

Microbial community structure in autotrophic nitrifying granules characterized by experimental and simulation analyses

This study evaluates the community structure in nitrifying granules (average diameter of 1600 μm) produced in an aerobic reactor fed with ammonia as the sole energy source by a multivalent approach combining molecular techniques, microelectrode measurements and mathematical modelling. Fluorescence in situ hybridization revealed that ammonia-oxidizing bacteria dominated within the first 200 μm below the granule surface, nitrite-oxidizing bacteria a deeper layer between 200 and 300 μm, while heterotrophic bacteria were present in the core of the nitrifying granule. Presence of these groups also became evident from a 16S rRNA clone library. Microprofiles of NH₄⁺, NO₂^–, NO₃^– and O₂ concentrations measured with microelectrodes showed good agreement with the spatial organization of nitrifying bacteria. One- and two-dimensional numerical biofilm models were constructed to explain the observed granule development as a result of the multiple bacteria–substrate interactions. The interaction between nitrifying and heterotrophic bacteria was evaluated by assuming three types of heterotrophic bacterial growth on soluble microbial products from nitrifying bacteria. The models described well the bacterial distribution obtained by fluorescence in situ hybridization analysis, as well as the measured oxygen, nitrite, nitrate and ammonium concentration profiles. Results of this study are important because they show that a combination of simulation and experimental techniques can better explain the interaction between nitrifying bacteria and heterotrophic bacteria in the granules than individual approaches alone.     

Causes of episodic acidification in Alpine streams

SUMMARY 1. Despite their global importance as mountain landscapes, the effect of acid deposition on running waters in the Alps is still incompletely described. The acid–base status of 30 clearwater streams in Canton Ticino (Switzerland) was therefore assessed at low and high flow across a south/north gradient of acid deposition in 2000.
2. At low flow, no stream was acidic, alkalinity being ≥29 μeq L⁻¹ and pH ≥6.4. However, NO₃^– was present in streamwater at these flows in concentrations that suggested anthropogenic enrichment, and its concentration correlated with spatial patterns of N deposition from the atmosphere.
3. Severe loss of alkalinity occurred in most streams at high flow during snowmelt (spring) or rainstorms (spring and autumn). Autumn episodes were because of dilution of streamwater alkalinity by rainwater. By contrast, spring episodes involved dilution and NO₃^– titration, the increase in nitrate being correlated with patterns of N deposition. SO₄^2– declined during most episodes.
4. High NO₃^– leaching at low flow suggests that catchment soils in Canton Ticino are approaching saturation in N. These data are among the first to illustrate that NO₃^– can drive episodic acidification in Europe, paralleling locations in north-eastern United States.     

Growth, photosynthesis, and respiration of Chlorella sorokiniana after N-starvation. Interactions between light, CO2 and NH4+ supply

N-sufficient cells of Chlorella sorokiniana Shihira and Krauss, strain 211/8k, absorbed NH₄⁺ under light plus CO₂ conditions, when growth occurred, but not in darkness or in the absence of CO₂, when growth was inhibited. N-sufficient cells subjected to conditions of N-starvation for a 24-h period showed a marked loss of photosynthetic activity. Upon supply of NH₄⁺, N-starved cells sufflated with CO₂ air exhibited a time-dependent recovery of photosynthetic activity, both when suspended in light and in darkness. By contrast, growth only occurred in cells suspended in light. N-starved cells absorbed NH₄⁺ in darkness, but at a lower rate than in light. All of these data suggest that dark NH₄⁺ uptake is driven by N assimilation to recover from N-starvation and that the light-dependent NH₄⁺ uptake is driven by growth, being then influenced by conditions that affect recovery or growth. Unlike CO₂ conditions, in a CO₂-free atmosphere, absorption of NH₄⁺ by N-starved cells occurred at a higher rate in darkness than in light. Accordingly, resumption of photosynthetic potential after NH₄⁺ supply occurred in darkened cells, but not in illuminated cells. Respiratory activity of N-starved cells was enhanced up to 3-fold by NH₄⁺ and 2-fold by methylammonium, with different patterns, suggesting that respiratory enzymes were affected by N-metabolism, especially through short-term control mechanisms triggered by the expenditure of metabolic energy involved in N-metabolism.