Water chemistry and periphyton in an alpine wetland

Remote high elevation sites are thought to be good sites to monitor global change and anthropogenic effects on ecosystems. This study was conducted during 1987–1990 in a high elevation wetland (3593 m) located in the Green Lakes Valley, Front Range, Colorado (USA). Salix spp. was the dominant riparian species in this 2 ha. wetland. Small shallow pools (<0.5 m depth) constituted a water area of 236 m³. The major source of water during the study period was snowmelt. The wetland had a well defined outlet and inlet, although an undetermined amount of water entered as groundwater from the snow patch above. Outlet discharge was 424–460 m³ during the month of July and declined thereafter as water input from the snowpatch declined. Inlet discharge was 67% of outlet discharge. Water temperatures in the outlet were always less than 6.8°C, pH 6.0–6.3, and mean conductivity 30.8 µS cm^–1. Both NO _inf3 ^sup– and SO _inf4 ^sup–2 were higher in the inlet thanin the outlet. Dominant cations in the inlet and outlet waters were Ca⁺² Mg⁺² > K⁺ + Na⁺; dominant anions were SO _inf4 ^sup–2 HCO _inf3 ^sup– > NO _inf3 ^sup– Cl^–. Nutrient limitation by P was demonstrated once using nutrient diffusing substrata. No limitation could be shown for NO _inf3 ^sup– , HCO _inf3 ^sup– , or Fe+EDTA. Slow colonization rates of periphyton on tiles were attributed to low temperatures and/or ultraviolet radiation. However, interannual differences in biomass on tiles were as much as 300% after 35 days. A minimum of 16–54 samples would be needed to detect a significant interannual change in biomass on tiles after 35 days assuming that the extreme case for periphyton patchiness. Global climate change is likely to affect discharge and water temperature in this wetland which hill have direct and indirect affects on population dynamics and ecosystem function.     

Diurnal variation in physical-chemical properties and primary production in the interconnected marine,mangrove and freshwater biotopes of Kakinada coast,Andhra Pradesh,India

Diurnal variation in hydrological variables and dissolved inorganic nutrients such as PO _inf4 ^sup3– -P, N O _inf2 ^sup– -N, NO _inf3 ^sup– -N and NH _inf4 ^sup+ -N were studied in three interconnected biotopes including freshwater, marine and mangrove brackish water of the Kakinada coastal zone, Andhra Pradesh. Samples were collected at intervals of 3 hours, for a period of 24 hours. In the marine environment salinity varied from 26 to 32 whereas in the mangrove waters it fluctuated from 12 to 20 and in both biotopes salinity showed bimodal type of oscillation. Dissolved oxygen content was high in the mangrove waters during day time but decreased rapidly during the night hours. In the marine environment PO_f4 ^p3–-P concentration varied from 0.345 to 1.195 g at l^–1, NO _inf3 ^sup– -N from 1.03 to 6.62 g at l^–1 and NO _inf2 ^sup– -N from 0.086 to 0.506 g at l^–1. The highest and the lowest concentrations of PO _inf4 ^sup3– -P, NO _inf3 ^sup– -N, NO _inf2 ^sup– -N recorded in the mangrove waters were 0.790 and 0.325 g at l^–1, 7.10 and 1.60 g at l^–1 and 0.278 and 0.060 g at l^–1, respectively. The concentration of PO _inf4 ^sup3– -P, NO _inf3 ^sup– -N and NO _inf2 ^sup– -N were high in the freshwater canal, the maximum and minimum values being 1.110 and 0.730 g at l^–1, 26.40 and 9.98 g at l^–1 and 0.520 and 0.252 g at l^–1 respectively. The concentration of ammonia was relatively high in the mangrove water. Gross and net primary production in the mangrove water was 4 times higher than in the marine biotope. There was no export of dissolved nutrients from the mangrove environment to the adjacent marine waters.     

Reduction of nitrate byPseudomonas putrefaciens entrapped in composite agar layer/microporous membrane structures

Summary Composite structures consisting of aPseudomonas putrefaciens immobilized-cell agar layer bounded by a microporous membrane filter were used for water denitrification. With methanol as the C-source, one litre of high nitrate water (3 mM) was completely freed from NO ₃ ^– and NO ₂ ^– ions in 11 days at a rate of 90 mol N–NO ₃ ^– /day/g of agar gel, while no production of ammonium ions could be detected. When acetic acid was substituted for methanol, the denitrifying activity was lower. No noticeable contamination of the treated water due to cell leakage from the biocatalytic structures occurred during the incubation periods.     

Sels minéraux et cycles de marées dans un estuaire de Bretagne nord (Dourduff,France)

Short-term variations in nutrient concentrations of water-masses have been studied in a Brittany estuary over several tidal cycles, during winter and summer. NH _inf4 ^sup+ , NO _inf2 ^su– , NO _inf3 ^sup– , SiO₂ and PO _inf4 ^su3– have been measured at a fixed station. The Dourduff estuary is characterized by a very low river discharge (80 l · s^–1 during the minimum water runoff and 1 000 l · s^–1 during the maximum) and an important tidal range (9 m at spring tides). SiO₂ and NO _inf3 ^sup+ concentrations are directly related to freshwater flow whereas PO _inf4 ^su3– is partially adsorbed by seston in the turbid ebb waters. NH _inf4 ^sup+ concentration seems to be, in part, dependent upon sediment resuspension: late ebb and onset of flood periods liberate NH _inf4 ^sup+ into the overlying water column. Nutrient concentrations are also related to seasons. Nutrient fluxes are insignificant or negative during summer periods, so the estuary imports nutrients for its own regulation whereas during winter periods it exports NO _inf3 ^sup– and SiO₂ (ca 50 kg NO _inf3 ^sup– and ca 200 g SiO₂ during a single spring tide). The NO₃: PO₄ ratio is always above 15:1 and can reach 300:1; moreover this ratio fluctuates during the tidal cycle. This imbalance originates in terrestrial discharges of nitrogen compounds.

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Sels minéraux et cycles de marées dans un estuaire de Bretagne nord (Dourduff, France)

     

The impact of NO inf3 sup− loading on the freshwater macrophyte Littorella uniflora: N utilization strategy in a slow-growing species from oligotrophic habitats

The decline and disappearance of Littorella uniflora from oligotrophic waters which have become eutrophic has been associated with shading or reduced CO₂ supply. However NO _inf3 ^sup– concentrations can reach very high levels (100–2000 mmol m^–3 compared with <1–3 in oligotrophic habitats). To investigate the impact of NO _inf3 ^sup– loading alone, plants were grown under three NO _inf3 ^sup– regimes (very low, near-natural and high). The interactive effects of NO _inf3 ^sup– and photon flux density (low and high regimes) on N assimilation and accumulation, CO₂ concentrating mechanisms, C₃ photosynthesis and growth were also examined. The results were unexpected. Increased NO _inf3 ^sup– supply had very little effect on photosynthetic capacity, crassulacean acid metabolism (CAM) or lacunal CO₂ concentrations ([CO₂]_i), although there was considerable plasticity with respect to light regime. In contrast, increased NO _inf3 ^sup– supply resulted in a marked accumulation of NO _inf3 ^sup– , free amino acids and soluble protein in shoots and roots (up to 25 mol m^–3, 30 mol m^–3 and 9 mg g^–1 fresh weight respectively in roots), while fresh weight and relative growth rate were reduced. Total N content even under the very low NO _inf3 ^sup– regime (1.6–2.3%) was mid-range for aquatic and terrestrial species (and 3.1–4.3% under the high NO _inf3 ^sup– regime). These findings, together with field data, suggest that L. uniflora is not growth limited by low NO _inf3 ^sup– supply in natural oligotophic habitats, due not to an efficient photosynthetic nitrogen use but to a slow growth rate, a low N requirement and to the use of storage to avoid N stress. However the increased NO _inf3 ^sup– concentrations in eutrophic environments seem likely have detrimental effects on the long-term survival of L. uniflora, possibly as a consequence of N accumulation.     

Aerobic denitrification in various heterotrophic nitrifiers

Various heterotrophic nitrifiers have been tested and found to also be aerobic denitrifiers. The simultaneous use of two electron acceptors (oxygen and nitrate) permits these organisms to grow more rapidly than on either single electron acceptor, but generally results in a lower yield than is obtained on oxygen, alone. One strain, formerly known as Pseudomonas denitrificans, was grown in the chemostat and shown to achieve nitrification rates of up to 44 nmol NH₃ min^–1 mg protein^–1 and denitrification rates up to 69 nmol NO _inf3 ^sup–1 min^–1 mg protein^–1.Unlike Thiosphaera pantotropha, this strain needed to induce its nitrate reductase. However, the remainder of the denitrifying pathway was constitutive and, like T. pantotropha, Ps. denitrificans probably possesses the copper nitrite reductase.     

Nitrogen and phosphorus uptake by the Brazilian kelp Laminaria abyssalis (Phaeophyta) in culture

The uptake of ammonium, nitrate and phosphate by laboratory-grown young sporophytes of Laminaria abyssalis was measured in a perturbed system (batch mode) at 18 °C and 35 ± 5 µE m^–2 s^–1 photon flux density. Uptake of all appeared to follow saturation-type nutrient uptake kinetics. The NO _inf3 ^sup– (K _s = 14.0 µM, V _max = 5.0 µmol h^–1 g^–1 dry wt) and NH _inf4 ^sup+ (K _s = 4.6 µM, V _max= 2.0 µmol h^–1 g^–1 dry wt) were taken up simultaneously, although NH _inf4 ^sup+ was taken up more rapidly. Values of K ₃ and V _max for phosphate were, respectively, 2.21 µM and 0.83 µmol h^–1 g^–1 dry wt. Nitrate and phosphate were both consumed in similar rates (V _max /Ks 0.37) at low concentrations. NH _inf4 ^sup+ , thus, might be a more efficient form of N fertilizer if artificial enrichment of seawater is used.     

Hydrobiology and organic production in a marl lake of Kashmir Himalayan Valley

L. Naranbagh (alt. 1587 m) is a polymictic, shallow marl lake in the flood-plain valley of Kashmir, India. Macrofloral affinities resemble Potamogeton Type of Forsberg (1965) with alkaline waters, not rich in phosphorus. CaCO₃ precipitation coupled with decline in Ca²⁺ and alkalinity values are characteristic of the lake. Fluctuations in Mg²⁺, Na⁺, K⁺, and Cl^– were relatively conservative. The levels of PO _inf4 ^sup3– -P and NO _inf3 ^sup– -N indicate moderate fertility of the lake water.Persistence of a summer-autumn planktonic algal pulse is related to favourable irradiance, high water temperatures, and increased photosynthetic efficiency values. The most striking seasonality in photosynthetic rates (m^–2 h^–1) between winter minimum (3 mg C_assim) and summer maximum (75.4 mg C_assim) is determined by mainly climatic changes. Energy flow gave annual phytoplankton production of 51.95 × 10² KJ m^–2 for the ecosystem.The nutrient levels and productivity rates suggest mesotrophic status of L. Naranbagh in classic oligoeutrophic classification of lake types.     

Mixed culture hydrogenotrophic nitrate reduction in drinking water

Isolation and identification of the bacteria from a hydrogenotrophic reactor for the denitrification of drinking water revealed that several microorganisms are involved. Acinetobacter sp., Aeromonas sp., Pseudomonas sp. and Shewanella putrefaciens were repeatedly isolated from the hydrogenotrophic sludge and postulated to be of primary importance in the process. Nitrate reduction to nitrite appears to be a property of a diverse group of organisms. Nitrite reduction was found to be stimulated by the presence of organic growth factors. Thus, in a mixed culture, hydrogenotrophic denitrification reactor, NO _inf2 ^sup– formed by NO _inf3 ^sup– -reducers can be converted by true denitrifiers thriving on organic growth factors either present in the raw water, or excreted by the microbial community. Mixotrophic growth also contributes to NO _inf2 ^sup– reduction. Finally, chemolithotrophic bacteria participate in the nitrite to nitrogen gas conversion.Offprint requests to: W. Verstraete.     

Nitrite uptake by Chlamydomonas reinhardtii cells immobilized in calcium alginate

When an initial cell loading of about 30–40 µg chlorophyll (Chl)·g^–1 gel and alginate suspension of 3% (w/v) were used for immobilization of Chlamydomonas reinhardtii, the resulting cell beads showed optimum nitrite uptake rate, at 30° C and pH 7.5, of 14 µmol NO _inf2 ^sup– ·mg^–1 Chl·h^–1, the photosynthetic and respiratory activities being about 120 µmol O₂ produced·mg^–1 Chl·h^–1, and 40 µmol O₂ consumed ·mg^–1 Chl·h^–1, respectively. The nitrite uptake activity required CO₂ in the culture and persisted after 8 days of cells immobilization, or in the presence of 0.2 mm ammonium in the medium. Our data indicate that alginate-entrapped C, reinhardtii cells may provide a stable and functional system for removing nitrogenous contaminants from waste-waters.Correspondence to: C. Vílchez     

Function and regulation of the avian caecal bulb: influence of dietary NaCl and aldosterone on water and electrolyte fluxes in the hen (Gallus domesticus) perfused in vivo

Summary The function of the caecal bulb, and its adaptation to chronic high- or low-Na⁺ intake, was investigated by in vivo perfusion of anaesthetised birds. Effects of acute aldosterone injection (125 g·kg^–1 body mass) were also measured.Evidence was found for primary active net absorption of Na⁺, inducing parallel Na-linked absorption of water and Cl^– and secretion of K⁺. Around 20–35% of total Cl^– absorption and K⁺ secretion were independent of Na⁺ fluxes, and these components appear to be driven by passive processes with apparent conductances of 6.3×10^–3 (G _Cl) and 1.1×10^–3 (G _K) S·cm^–2.Acetate (40mM) stimulated Na⁺ fluxes (8.5–9.9 Eq·cm^–2·h^–1) and Na-linked water fluxes (27–44 l·cm^–2·h^–1). Increased coupling ratios (2.9–4.6 l·Eq^–1) and other data indicate that these effects may be due to increased osmotic permeabilities of barriers involved in the Na-linked water transfer pathway.Low-Na⁺ maintenance enhanced EPD (49–69 mV, serosa positive) and all net fluxes:J _Na (6.8–11.6);J _K (–3.2––4.3);J _Cl (4.3–5.6 Eq·cm serosal area^–2·h^–1);J _v (28–43 l·cm^–2·h^–1) (mucosal-serosal fluxes positive).Acute aldosterone enhancedJ _Na (10.8–14.0 Eq·cm^–2·h^–1) and EPD (54–66 mV) by 3 h after injection, but had no effect on the Na-linked components ofJ _K orJ _Cl.Abbreviations ECPD, EPD Electrochemical or electrical potential difference - G _Cl ,G _K ionic conductances (Cl^–, K⁺) - J _v ,J _ion net volume or ion flux rate, mucosa-serosa positive;P _d (Cl) diffusive permeability coefficient (of Cl^–) - SEDM standard error of difference between means     

Nitrate induction in spruce: an approach using compartmental analysis

Using ¹³NO ₃ ^– -efflux analysis, the induction of nitrate uptake by externally supplied nitrate was monitored in roots of intact Picea glauca (Moench) Voss. seedlings over a 5-d period. In agreement with our earlier studies, efflux analysis revealed three compartments, which have been identified as surface adsorption, apparent free space, and cytoplasm. While induction of nitrate uptake was pronounced, NO ₃ ^– fluxes in induced plants were decidedly lower and the induction response was slower than in other species. Influx rose from 0.1 mol·g^–1·h^–1 (measured at 100 M [NO ₃ ^– _o) in uninduced plants to a maximum of 0.5 mol·g^–1h^–1 after 3 d of exposure to 100 M [NO ₃ ^– _o and declined to 0.3–0.4 mol·g^–1h^–1 at the end of the 5-d period. Efflux remained relatively constant around 0.02-0.04 mol·g^–1h^–1, but its percentage with respect to influx declined from initially high values (around 30%) to steady-state values of 4–7%. Cytoplasmic [NO ₃ ^– ] ranged from the low micromolar in uninduced plants to a maximum of 2 mM in plants fully induced at 100 M [NO ₃ ^– ]_o. In-vivo root nitrate reductase activity (NRA) was measured over the same time period, and was found to follow a similar pattern of induction as influx. The maximum response in NRA slightly preceded that of influx. It increased from 25 nmol·g^–1·h^–1 without prior exposure to NO ₃ ^– to peak values around 150 nmol· g^–1h^–1 after 2 d of exposure to 100 M [NO ₃ ^– ]_o. Subsequently, NRA declined by about 50%. The dynamics of flux partitioning to reduction, to the vacuole, the xylem, and to efflux during the induction process are discussed.The research was supported by an Natural Sciences and Engineering Research Council, Canada, grant to Dr. A.D.M. Glass and by a University of British Columbia Graduate Fellowship to Herbert J. Kronzucker. Our thanks go to Dr. M. Adam and Mr. P. Culbert at the particle accelerator facility TRIUMF on the University of British Columbia campus for providing ¹³N, to Drs. R.D. Guy and S. Silim for providing plant material, and to Dr. M.Y. Wang, Mr. J. Bailey, Mr. J. Mehroke and Mr. J. Vidmar for essential assistance in experiments.     

Compartmentation and flux characteristics of nitrate in spruce

The radiotracer¹³N was used to undertake compartmental analyses for NO ₃ ^– in intact non-mycorrhizal roots ofPicea glauca (Moench) Voss. seedlings. Three compartments were defined, with half-lives of exchange of 2.5 s, 20 s, and 7 min. These were identified as representing surface adsorption, apparent free space, and cytoplasm, respectively. Influx, efflux, and net flux as well as cytoplasmic and apparent-free-space nitrate concentrations were estimated for three different concentration regimes of external nitrate. After exposure to external NO ₃ ^– for 3 d, influx was calculated to be 0.09 mol·g^–1·h^–1 (at 10 M [NO ₃ ^– ]_o), 0.5mol·g^–1·h^–1 (at 100 M [NO _inf3 ^sup– ]_o), and 1.2 mol · g^–1· h^–1 (at 1.5 mM [NO ₃ ^– ]_o). Efflux increased with increasing [NO ₃ ^– ]_o, constituting 4% of influx at 10 M, 6% at 100 M, and 21% at 1.5 mM. Cytoplasmic [NO ₃ ^– ] was estimated to be 0.3 mM at 10 uM [NO ₃ ^– ]_o, 2mM at 100 M [NO ₃ ^– ]_o, and 4mM at 1.5 mM [NO ₃ ^– ]_o, while free-space [NO ₃ ^– ] was 16 M, 173 M, and 2.2 mM, respectively. A series of experiments was carried out to confirm the identity of the compartments resolved by efflux analysis. Pretreatment at high temperature or application of 2-chloro-ethanol, sodium dodecyl sulphate or hydrogen peroxide made it possible to distinguish the metabolic (cytoplasmic) phase from the remaining two (physical) phases. Likewise, varying [Pi] of the medium altered efflux and thereby [NO ₃ ^– ]_cyt, but did not affect [NO ₃ ^– ]_{free space}.Abbreviations and Symbols [NO ₃ ^– ]_cyt cytoplasmic NO ₃ ^– concentration - [NO ₃ ^– ]_{free space} apparent-free-space NO ₃ ^– concentration - [NO ₃ ^– ]_o concentration of NO ₃ ^– in the external solution - NO ₃ ^– flux - _co efflux from the cytoplasm - _oc influx to the cytoplasm - _net net flux - _xylem flux to the xylem - _red/vac combined flux to reduction and the vacuole The research was supported by a Natural Sciences and Engineering Research Council, Canada, grant to Dr. A.D.M. Glass and by a University of British Columbia Graduate Fellowship to Herbert J. Kronzucker. Our thanks go to Dr. M. Adam and Mr. P. Culbert at the particle accelerator facility TRIUMF on the University of British Columbia Campus for providing¹³NO ₃ ^– , Drs. R.D. Guy and S. Silim for providing plant material, and Dr. M.Y. Wang, Mr. J. Mehroke and Mr. P. Poon for assistance in experiments and for helpful discussions.     

Physiology and kinetics of autotrophic denitrification by Thiobacillus denitrificans

Summary A strain of Thiobacillus denitrificans was isolated after enrichment under anaerobic conditions by the continuous culture technique using thiosulfate as energy source and nitrate as electron acceptor and nitrogen source. The isolate was an active denitrifyer, the optimal conditions being 30°C and pH 7.5–8.0. Denitrification was inhibited by sulfate (the reaction product) above 5 g SO ₄ ⁼ /l, whereas high concentrations of the substrates nitrate and thiosulfate were less harmful; nitrite affected denitrification above 0.2 g NO ₂ ^– /l. During the time course of denitrification in a batch culture growth and substrate consumption slowed down already after only half the substrate was utilized due to product inhibition. The following parameters were determined in continuous culture under nitrate limitation: _max=0.11 h^–1, K _S=0.2 mg NO ₃ ^– /l, maximum denitrification rate=0.78 g NO ₃ ^– /g cells·h, g cells/g NO ₃ ^– , g cells/g S₂O ₃ ⁼ . Nitrite did not accumulate during steady state denitrification; the denitrification gas was almost pure N₂. The concentrations of N₂O and NO were below 1 ppm.     

Nitrogen cycling in lake sediments bioturbated by Chironomus plumosus larvae, under different degrees of oxygenation

Sediment cores containing different densities of Chironomus plumosus, ranging from 0 to 12 000 ind. m^–2, were incubated in the laboratory, with 100 and 39% O₂ saturation in the overlying water. Rates of O₂ uptake, and fluxes of the various inorganic N species were measured after addition of ¹⁵NO _inf3 ^su– to the overlying water. The animals enhanced O₂ and NO _inf3 ^su– uptake, due to irrigation. Denitrification of NO _inf3 ^su– coming from the overlying water (Dw) and dissimilatory NO _inf3 ^su– reduction to NH _inf4 ^sup+ (DNRA) represented 20–30 and 4–10% of the NO _inf3 ^su– uptake, respectively. Only 20–40% of the measured NH _inf4 ^sup+ effluxes corresponded to DNRA, the rest was probably due to animal excretion. Nitrite production, mostly from dissimilatory NO _inf3 ^sup– reduction, was detected at both 39 and 100% oxygen saturation. Higher rates of NO _inf2 ^su– production at the lower oxygen concentrations, were probably due to a thinner oxic layer, compared to fully oxygenated waters. The presence of Chironomus plumosus increased nitrification rates, relative to non-inhabited microcosms. However, nitrification rates were low compared to Dw, probably due to low numbers of nitrifiers in the sediment. At 39% oxygen saturation, rates of nitrification and denitrification of NO _inf3 ^su– generated within the sediment were not measurable.     

Application of N-15 dilution for simultaneous estimation of nitrification and nitrate reduction in soil-water columns

Nitrification and denitrification rates were estimated simultaneously in soil-floodwater columns of a Crowley silt loam (Typic Albaqualfs) rice soil by an¹⁵N isotopic dilution technique. Labeled NO ₃ ^– was added to the floodwater of soil-water columns, half were treated with urea fertilizer. The (NO ₃ ^– +NO ₂ ^– )–N and (NO ₃ ^– +NO ₂ ^– )–N concentrations in the floodwater were measured over time and production and reduction rates for NO ₃ ^– calculated. Nitrate reduction in the urea amended columns averaged 515 mol N m^–2h^–1 and nitrification averaged 395 mol N m^–2h^–1 over the 35–153 d incubation. The nitrification rate for 4–19 d sampling period (1,560 mol N m^–2h^–1) in the urea amended columns was almost 9 times greater than the reduction rate (175 mol N m^–2h^–1) over the same period. Without the addition of urea the NO ₃ ^– production rate averaged 32 mol N m^–2h^–1 and reduction 101 mol N m^–2h^–1.     

Ulva rigida (Ulvales,Chlorophyta) tank culture as biofilters for dissolved inorganic nitrogen from fishpond effluents

Ulva rigida was cultivated in 7501 tanks at different densities with direct and continuous inflow (at 2, 4, 8 and 12 volumes d^–1) of the effluents from a commercial marine fishpond (40 metric tonnes, Tm, of Sparus aurata, water exchange rate of 16 m³ Tm^–1) in order to assess the maximum and optimum dissolved inorganic nitrogen (DIN) uptake rate and the annual stability of the Ulva tank biofiltering system. Maximum yields (40 g DW m^–2 d^–1) were obtained at a density of 2.5 g FW 1^–1 and at a DIN inflow rate of 1.7 g DIN m^–2 d^–1. Maximum DIN uptake rates were obtained during summer (2.2 g DIN M^–2 d^–1), and minimum in winter (1.1 g DIN m^–2 d^–1) with a yearly average DIN uptake rate of 1.77 g DIN m^–2 d^–1 At yearly average DIN removal efficiency (2.0 g DIN m^–2 d^–1, if winter period is excluded), 153 m² of Ulva tank surface would be needed to recover 100% of the DIN produced by 1 Tm of fish.Abbreviations DIN= dissolved inorganic nitrogen (NH _inf4 ^sup+ + NO _inf3 ^sup– + NO _inf2 ^sup– ); - FW= fresh weight; - DW= dry weight; - PFD= photon flux density; - V= DIN uptake rate     

The ability of several high arctic plant species to utilize nitrate nitrogen under field conditions

The ability to utilize NO _inf3 ^sup– in seven high arctic plant species from Truelove Lowland, Devon Island, Canada was investigated, using an in vivo assay of maximum potential nitrate reductase (NR) activity and applications of ¹⁵N. Plant species were selected on the basis of being characteristic of nutrient-poor and nutrient-rich habitats. In all species leaves were the dominant site of NR activity. Root NR activity was negligible in all species except Saxifraga cernua. NO _inf3 ^sup– availability per se did not appear to limit NR activity of the species typically found on nutrient-poor sites (Dryas integrifolia, Saxifraga oppositifolia, and Salix arctica), or in Cerastium alpinum, as leaf NR activities remained low, even after NO _inf3 ^sup– addition. ¹⁵NO _inf3 ^sup– uptake was limited in D. integrifolia and Salix arctica. However, the lack of field induction of NR activity in C. alpinum and Saxifraga oppositifolia was not due to restricted nitrate uptake, as ¹⁵NO _inf3 ^sup– labelled NO _inf3 ^sup– entered the roots and shoots of both species. Leaf NR activity rates were low in three of the species typical of nutrient-rich habitats (O. digyna, P. radicatum and Saxifraga cernua), sampled from a site containing low soil NO _inf3 ^sup– . Additions of NO _inf3 ^sup– significantly increased leaf NR activity in these latter species, suggesting that potential NR activity was limited by the availability of NO _inf3 ^sup– . ¹⁵N labelled NO _inf3 ^sup– was taken up by O. digyna. P. radicatum and Saxifraga cernua. Although two species (D. integrifolia and Salix arctica) showed little utilization of NO _inf3 ^sup– , we concluded that five of the seven selected high arctic plant species (C. alpinum, O. digyna, P. radicatum, Saxifraga cernua and Saxifraga oppositifolia) do have the potential to utilize NO _inf3 ^sup– as a nitrogen source under field conditions, with the highest potential to utilize NO _inf3 ^sup– occurring in three of the species typically found on fertile habitats.     

Seasonal denitrification in flooded and exposed sediments from the Amazon floodplain at Lago Camaleão

Denitrification processes were measured by the acetylene-blockage technique under changing flood conditions along the aquatic/terrestrial transition zone on the Amazon floodplain at Lago Camaleão, near Manaus, Brazil. In flooded sediments, denitrification was recorded after the amendment with NO ₃ ^– (100 mol liter^–1) throughout the whole study period from August 1992 to February 1993. It ranged from 192.3 to 640.7 mol N m^–2 h^–1 in the 0- to 5-cm sediment layer. Without substrate amendment, denitrification was detected only during low water in November and December 1992, when it occurred at a rate of up to 12.2 mol N m^–2 h^–1 Higher rates of denitrification at an average rate of 73.3 mol N m^–2 h^–1 were measured in sediments from the shallow lake basin that were exposed to air at low water. N₂O evolution was never detected in flooded sediments, but in exposed sediments, it was detected at an average rate of 28.3 mol N m^–2 h^–1 during the low-water period. The results indicate that under natural conditions there is denitrification and hence a loss in nitrogen from the Amazon floodplain to the atmosphere. Rates of denitrification in flooded sediments were one to two orders of magnitude smaller than in temperate regions. However, the nitrogen removal of exposed sediments exceeded that of undisturbed wetland soils of temperate regions, indicating a considerable impact of the flood pulse on the gaseous turnover of nitrogen in the Amazon floodplain.     

Response of Salvinia spp. to different nitrogen sources: the acid-base regulation approach

This paper shows the importance of acid-base analyses and ¹³C measurements in the evaluation of the responses of Salvinia species' responses to different N sources. It also highlights the importance of these methodologies as potential tools in the study of differences between habitats and nutrient acquisition, particularly N. This study used three different species of Salvinia cultured in the absence of combined N or in the presence of either NO _inf3 ^sup– or NH _inf4 ^sup+ as N sources. The interaction between NO _inf3 ^sup– or NH _inf4 ^sup+ as N source and organic acid metabolism, and the information on diazotrophy from the organic acid measurements, were also examined. Nevertheless, the results presented may not be used per se to assign diazotrophy. Carboxylate (C-A) levels in the different Salvinia species are much lower than the norm for bryophytes and tracheophytes, consistent with previously published work on Azolla. This might be related to the aquatic life form of these plants, since they seem to have no potential to increase the availability of Fe or P by the acidification of their rooting medium (water) that a larger net synthesis of organic acids, with cation-H⁺ exchange, could achieve.