Uptake Rates Of NO3 Research Articles

Effects of wastewater treatment plant discharge on ecosystem structure and function of lowland streams

Secondary and tertiary wastewater treatment is common in developed countries, but little is known about the responses of lotic ecosystems to contemporary wastewater treatment plant (WWTP) discharge. We examined the effects of WWTP discharge on various ecosystem components and functions of 2 morphologically and chemically impacted lowland streams near Berlin, Germany. We sampled one reach upstream and one reach downstream of a WWTP in each stream during each of 5 sampling campaigns. Discharge of treated wastewater resulted in increased concentrations of total organic C, total N, and total P in the sediments and in elevated macrophyte and benthic invertebrate biomasses. However, adverse effects of the WWTPs on the benthic invertebrate communities were small compared to effects reported in previous studies. This difference was a result of the higher purification efficiency of modern WWTPs, but also of significant structural degradation and eutrophication of the streams that already had impoverished the invertebrate community upstream of the WWTPs. Whole-stream community respiration (CR24) and gross primary production (GPP) were both enhanced by WWTP discharge. WWTP discharge generally caused diminished NH4- and PO4-uptake efficiencies, but did not necessarily lead to diminished NO3- uptake efficiencies of streams. Increases in areal NO3-uptake rates caused by the discharge of a large municipal WWTP were high enough to result in increased load-specific NO3-uptake efficiencies. Our study shows that the effects of present-day WWTPs on stream ecosystem functioning clearly differ from the former impacts of poorly treated wastewater. Present-day WWTP discharges mainly cause eutrophication and subsequent side effects and low nutrient-retention efficiencies relative to the high nutrient concentrations and loads of impacted streams. Our results highlight the need for efficient tertiary treatment of wastewater and for the refinement of agricultural practices to reduce diffuse nutrient loadings. We found evidence that even efficiently treated wastewater can have extensive effects on stream ecosystem structure and function. Therefore, adequate dilution rates always should be considered when routing treated wastewater through lotic networks. Our findings on the response of key ecosystem variables to present-day WWTP loading underline the importance of scientifically based stream management.

Leaf fluxes of NO and NO 2 in four herbaceous plant species: The role of ascorbic acid

Nitrogen oxides (NO x =NO+NO 2) are a common group of nitrogen-containing trace gases that interact significantly with plant canopies and play a critical role in atmospheric chemistry including the formation of ozone, smog and nitric acid. The controls over leaf–atmosphere exchange of NO x (NO x =NO+NO 2) have been characterized for a few plant species and important drivers have been identified including stomatal conductance, the partial pressure gradient from the interior to the exterior of the leaf, and, most recently the level of ultraviolet radiation. However, our ability to predict leaf fluxes has been hampered by a dearth of information describing the reactions occurring between NO x and internal constituents of the leaf (often referred to as the mesophyllic resistance). In this study, we investigated NO x uptake in four important agricultural plant species and specifically tested the hypothesis that ascorbic acid within the leaf was a significant control over mesophyllic resistance. Measurements were made using a dynamic leaf chamber at ambient mixing ratios typical of urban atmospheres. Measured NO and NO 2 leaf fluxes were lower than the potential flux calculated assuming stomatal conductance as the sole limitation (i.e., no mesophyllic resistance). The ratios of leaf intercellular to ambient concentrations ( C i / C a ) for NO across partial pressures and plant species was usually >0.99 and the leaf uptake rates of NO were negligible within the ranges examined. In contrast, leaf uptake for NO 2 was much greater and C i / C a were often as low as 0.75. For both gases, C i / C a ratios>1 were observed at low ambient mixing ratios indicating emission from leaves. Across plant species, plants with higher leaf nitrogen contents showed higher NO 2 emission rates and plants with higher leaf ascorbate concentrations had higher leaf NO 2 uptake rates. These results suggest that in addition to the known effects of stomatal conductance and incident UV radiation, leaf nitrogen content and the capacity for the scavenging of NO x by ascorbic acid may explain the variation in the ability of plants to absorb atmospheric nitrogen oxides.

Comparison of size‐dependent carbon, nitrate, and silicic acid uptake rates in high‐ and low‐iron waters

We compare the relative contribution of large phytoplankton (>5 and >10 µm) to uptake rates of carbon (C), nitrate (NO3−), and dissolved silicon (Si) and uptake ratios of Si :NO and Si :C in Monterey Bay, California (a high‐iron region) with three low‐iron regions in the eastern tropical Pacific: the Costa Rica upwelling dome, Humboldt current, and Peru upwelling. We also demonstrate the effect of iron enrichment on the above parameters in the latter three regions. In Monterey Bay waters, the >5‐µm size fraction accounted on average for the majority of particulate organic C and was responsible for 92% of total C uptake, 81% of total NO3− uptake, and 98% of total 23 Si uptake. In contrast, in low‐iron eastern tropical Pacific waters, the >5‐µm size fraction accounted for less than half of the particulate organic C and was responsible for a substantially smaller proportion of total C and NO3− uptake: 27—43% for C and 34—59% for NO3−. Iron enrichment experiments in the eastern tropical Pacific resulted in much higher NO3−, C, and Si uptake rates, although increases were restricted to cells in the >5‐µm size fraction. Si:NO and Si:C uptake ratios decreased after iron addition at most locations, and decreases were a direct result of lower Si:NO3− and Si:C uptake ratios in the >5‐µm size fraction. Our results suggest that iron availability is a major factor regulating primary production, new production, Si uptake, and Si:NO3− and Si :C uptake ratios in the larger phytoplankton size classes in high‐nitrate, low‐chlorophyll (HNLC) regions in the eastern tropical Pacific.

Nitrogen and phosphorus requirements of an Alexandrium minutum bloom in the Penzé Estuary, France

During an intense (up to 33 X 106 cells L−1) Alexandrium minutum bloom in the Penzé estuary (France), total NO3, NH4, and PO4 requirements of the bloom were, respectively, 184, 25, and 20 µmol L−1, with peak uptake rates of 43, 6, and 4.8 µmol L−1 d−1. The measured ambient concentrations of NH4 and PO4 were far short of this peak demand, whereas those of NO3 were far in excess, indicating that PO4 supply is important for sustaining the bloom. Comparison of the measured NO3 uptake rates with advective fluxes indicates that a reduction of NO3 concentrations in river waters to <200 µmol L−1 would be necessary to contain the bloom in the Penzé estuary. The role of NO3 was restricted to sustenance of the bloom, whereas warm conditions resulting in a water column stability seem to have triggered the bloom, and a self‐shading, probably coupled with a phosphorus limitation, caused its decline.

Nitrate and potassium uptake by greenhouse roses ( Rosa hybrida) along successive flower-cut cycles: a model and its calibration

Rose ( Rosa hybrida L.) plants grown for cut-flower production in greenhouses produce flowers in flushes year-round. Crop models for this system must handle the cyclical nature of productivity, which is determined by the horticultural production methods. Nutrition was not accounted for in previous rose growth models, since little is known about uptake of the essential nutrients by rose roots. The aim of the current study was to measure uptake rates of nitrogen and potassium by roses, to be included in a production model. Rose plants var. ‘Kardinal’ were grown in the greenhouse in aero-hydroponics nutrient solution with 3 mM nitrate (NO 3)-N and 1 mM potassium (K). After several flower growth/harvest cycles, the plants were transferred to a growth chamber in groups of three, every 10 days. The growth chamber provided 25 °C and 16 h day length. The nutrient solutions were sampled periodically while maintaining the volume constant at 5 l, and analyzed for NO 3 and K concentrations reduction. The roots were harvested at the end of each depletion series, and their lengths measured. Influx of NO 3 and K into roots was obtained by fitting a Michaelis–Menten function to the concentration depletion data. There was a cyclic rhythm of both the nutrients’ influx rates over time, with a decline in uptake after shoot harvest, and an increase during flower development, with maximal values towards flower opening. The results were incorporated in a simulation model for nutrient uptake by roses along successive flower-cutting cycles. This simulation assumes a constant number of identical flowering branches, which would be cut sequentially at flower maturity, and result in new shoot growth, assumed to follow a logistic function of time. Uptake rates of NO 3 and K were assumed to follow the changes in leaf area and shoot nutrient percentage, to compensate for N and K demand by the shoot; the root system dimensions and its effective aging are assumed constant. Simulated N and K uptake agreed with published data of their accumulation and percentage in growing rose branches along a flower-cut cycle.

Plant growth-promoting bacteria and nitrate availability: impacts on root development and nitrate uptake.

Plant growth-promoting bacteria (PGPB) and NO-3 availability both affect NO-3 uptake and root architecture. The presence of external NO-3 induces the expression of NO-3 transporter genes and elicits lateral root elongation in the part of the root system exposed to the NO-3 supply. By contrast, an increase in NO-3 supply leads to a higher plant N status (low N demand), which represses both the NO-3 transporters and lateral root development. The effects of PGPB on NO-3 uptake and root development are similar to those of low NO-3 availability (concomitant stimulation of NO-3 uptake rate and lateral root development). The mechanisms responsible for the localized and long-distance regulation of NO-3 uptake and root development by NO-3 availability are beginning to be elucidated. By contrast, the signalling and transduction pathways elicited by the rhizobacteria remain totally unknown. This review will compare the effects of NO-3 availability and PGPB on root morphogenesis and NO-3 uptake, in order to determine whether interactions exist between the NO-3-dependent and the PGPB-dependent regulatory pathways.

Nitrous oxide, nitric oxide, and nitrogen dioxide fluxes from soils after manure and urea application.

Nitrous oxide is a greenhouse gas, and NO and NO2 play a key role in atmospheric chemistry. Nitrous oxide, NO, and NO2 fluxes from fertilized soils were measured six times per day by an automated flux monitoring system for one year, beginning on 21 May 1998. Pac choi (Brassica spp.) was cultivated for two months, and the plots were left fallow the remainder of the year. Two types of manure, poultry manure (PM) and swine manure (SM), and a chemical fertilizer, urea, were applied to the soil. The total amount of nitrogen applied in each case was 15 g N m(-2). The total fluxes from PM, SM, and urea for the year were 184, 61.3, and 44.8 mg N m(-2) for N2O, respectively; 9.95, 16.6, and 148 mg N m(-2) for NO, respectively; and -6.21, -7.23, and -7.84 mg N m(-2) for NO2, respectively. A negative correlation was found between the NO flux and the NO concentration of the chamber air just after the chamber was closed, when a flux from the atmosphere to soil was observed for 10 months. The mean gross NO production, the NO uptake rate constant, and the apparent compensation point for this period were 0.79 to 0.95 microg N m(-2) h(-1), 120 to 128 L m(-2) h(-1), and 5.65 to 7.35 ppbv, respectively.

Dissolved Organic Carbon Enrichment Alters Nitrogen Dynamics in a Forest Stream

Dissolved organic carbon, DOC, is an important source of carbon for stream heterotrophs. In many systems, bacteria and fungi are carbon limited. Thus, carbon availability may control both the availability and the dynamics of other nutrients (N or P) in heterotrophic systems. To test the interdependence of C and N cycles in forested streams, we continuously added DOC, as potassium acetate, to the stream draining Watershed six at the Hubbard Brook Experimental Forest, New Hampshire, for six weeks during summer 1999. For two months prior to and throughout the addition, we monitored streamwater NO3− and NH4+ concentrations throughout the 70-m upstream reference and the 70-m downstream treatment reaches. We also estimated uptake lengths and uptake rates of NO3− and NH4+ using back-to-back, short-term enrichments in both the treatment and reference reaches. The change in NO3− concentration during the NH4+ addition was used to estimate whole-stream nitrification. Once before the DOC addition and once during the DOC addition we measured denitrification rates and whole-stream respiration in both reaches. In the treatment reach, the increase in labile DOC caused nitrate concentrations to decrease from pretreatment concentrations during the DOC addition. Metabolism was higher in the treatment than in the reference reach during that addition, while metabolism had been similar in the two reaches before the DOC addition. Prior to that addition, uptake lengths for both NO3− and NH4+ were longer in the treatment reach than in the reference reach. During the DOC addition, the uptake lengths for both forms of nitrogen were much shorter in the treatment than in the reference reach. Contrary to expectations, the DOC enrichment did not lead to increased rates of denitrification. The principal effect of DOC enrichment was to stimulate bacterial growth, leading to high respiration and a corresponding increase in the assimilative demand for nitrogen. Results from this experiment demonstrate tight linkages between DOC and nitrogen availability in headwater streams.

Trace metals and nitrogenous nutrition of Antarctic phytoplankton: experimental observations in the Ross Sea

As part of the US JGOFS Antarctic Environment Southern Ocean Process Study (AESOPS), shipboard incubation experiments were conducted to investigate the effects of iron and zinc addition on ammonium (NH 4 + ) and nitrate (NO 3 − ) uptake by natural assemblages of Antarctic phytoplankton. Samples were collected from the Ross Sea during austral summer (January–February, 1997) within the mixed layer at two stations: a continental shelf station—Orca, and a high NO 3 − , low chlorophyll (HNLC) station located further offshore—Blue. Twenty-liter bottle incubations were sampled repeatedly during an 8-day period at Orca, and a 13-day period at Blue for short-term 15 N-tracer uptake experiments. Ambient concentrations of NO 3 − were elevated at both locations. Biomass, measured as chlorophyll a (chl a ), was relatively high at Orca (chl a =2.31 μg l −1 ), whereas true HNLC conditions were observed at Blue (chl a =0.52 μg l −1 ). All Fe enrichments produced an acceleration in NO 3 − decline, and accumulation in biomass. Chl a increased 2–3-fold at Orca and 4–5-fold at Blue; these increases were directly correlated with increasing Fe enrichment. Unambiguous responses to zinc addition were not evident at Blue, whereas increased total biomass accumulation and nitrate drawdown were observed at Orca. Rates of biomass (particulate nitrogen) specific NO 3 − -uptake in the Fe-enriched samples were up to 2-fold greater than un-enriched controls at both sites. There were no significant changes in specific uptake rates of NH 4 + at the HNLC site, and only a 30–40% increase at Orca with the highest Fe enrichments. These results clearly indicate that Fe additions resulted in faster rates of NO 3 − consumption per unit phytoplankton biomass at both sites. The N-uptake response to zinc enrichment was not as evident as with Fe, presumably due in part to the relatively high dissolved concentrations of zinc (ambient concentrations=ca. 2 nM). However, during the later sampling periods, zinc addition resulted in a 40% increase in the specific uptake rates of NO 3 − but not NH 4 + , whereas the specific uptake rates of NO 3 − and NH 4 + increased by 4–16% and 18–49%, respectively, at Orca. Absolute uptake rates of NO 3 − and NH 4 + (corrected for isotopic dilution) were 4–5 and 2–3 times greater, respectively, than the controls at Blue, whereas at Orca both NO 3 − - and NH 4 + -uptake rates doubled as a result of Fe enrichment. Post-incubation, size-fractionation uptake experiments demonstrated that the higher rates of nitrogen uptake were primarily due to larger phytoplankton (>5 μm). The f -ratio [ f =NO 3 − -uptake/(NO 3 − +NH 4 + -uptake)] increased from ca. 0.7 to 0.8 as a result of Fe enrichment at the HNLC site, but declined from an average of 0.84 to 0.52–0.69 in both the controls and the Fe-amended samples collected further onshore at Orca . This decrease may be due to the inhibition of NO 3 − -uptake by elevated NH 4 + concentrations resulting from increased heterotrophic remineralization within the carboys over time. Based on the apparent half-saturation constant ( K s ) of 0.09 nM Fe estimated for community planktonic NO 3 − -uptake, the availability of dissolved Fe (ambient concentration=0.03–0.04 nM) limits the uptake of NO 3 − by phytoplankton at the HNLC and continental shelf regions of the western Ross Sea during austral summer.

Nitrogen relations of a low nitrate uptake inbred line of white clover (Trifolium repens L.)

Macduff, J. H., Abberton, M. T., Raistrick, N., Michaelson-Yeates, T. P. T. (2002). Low nitrate uptake white clover genotype - Nitrogen relations of a low nitrate uptake inbred line of white clover (Trifolium repens L.). Plant and Soil, 238, (2), 191-204 Sponsorship: MAFF; BBSRC

Root growth and function of three Mojave Desert grasses in response to elevated atmospheric CO2 concentration

Root growth and physiological responses to elevated CO2 were investigated for three important Mojave Desert grasses: the C3 perennial Achnatherum hymenoides, the C4 perennial Pleuraphis rigida and the C3 annual Bromus madritensis ssp. rubens. Seeds of each species were grown at ambient (360 μl l−1) or elevated (1000 μl l−1) CO2 in a glasshouse and harvested at three phenological stages: vegetative, anthesis and seed fill. Because P. rigida did not flower during the course of this study, harvests for this species represent three vegetative stages. Primary productivity was increased in both C3 grasses in response to elevated CO2 (40 and 19% for A. hymenoides and B. rubens, respectively), but root biomass increased only in the C3 perennial grass. Neither above‐ground nor below‐ground biomass of the C4 perennial grass was significantly affected by the CO2 treatment. Elevated CO2 did not significantly affect root surface area for any species. Total plant nitrogen was also not statistically different between CO2 treatments for any species, indicating no enhanced uptake of N under elevated CO2. Physiological uptake capacities for NO3 and NH4 were not affected by the CO2 treatment during the second harvest; measurements were not made for the first harvest. However, at the third harvest uptake capacity was significantly decreased in response to elevated CO2 for at least one N form in each species. NO3 uptake rates were lower in A. hymenoides and P. rigida, and NH4 uptake rates were lower in B. rubens at elevated CO2. Nitrogen uptake on a whole root‐system basis (NO3+NH4 uptake capacity × root biomass) was influenced positively by elevated CO2 only for A. hymenoides after anthesis. These results suggest that elevated CO2 may result in a competitive advantage for A. hymenoides relative to species that do not increase root‐system N uptake capacity. Root respiration measurements normalized to 20 °C were not significantly affected by the CO2 treatment. However, specific root respiration was significantly correlated with either root C∶N ratio or root water content when all data per species were included within a simple regression model. The results of this study provide little evidence for up‐regulation of root physiology in response to elevated CO2 and indicate that root biomass responses to CO2 are species‐specific.

Nitrate transport processes in Fagus-Laccaria-Mycorrhizae

The contribution of influx and efflux of NO3 - on NO3 - net uptake has been studied in excised mycorrhizae of 18–20 week old beech (Fagus sylvatica L.) trees. Net uptake rates of NO3 - followed uniphasic Michaelis-Menten kinetics in the concentration range between 10 μM and 1.0 mM external NO3 -, with an apparent Km of 88±7 μM, and a Vmax of 110±7 nmol g-1 root f.wt. h-1. The relative xylem loading of N, i.e. the portion of NO3 - taken up that was loaded into the xylem vessels as NO3 - plus reduced N, was constant over the concentration range tested (4.6–7.7%). NO3 - influx proceeded linearly with increasing external NO3 - supply. When the assumed regulators of net NO3 - uptake, i.e. NH4 + or L-glutamate, were applied together with NO3 -, net uptake rates of NO3 - decreased. This inhibitory effect was caused by a reduction of NO3 - influx rather than an enhanced efflux. The comparison of the present data with a recent study with non-mycorrhizal beech roots (Kreuzwieser et al., 1997; J. Exp. Bot. 48, 1431–1438) revealed that mycorrhization leads to reduced rates of NO3 - net uptake. This effect is caused by reduced influx plus enhanced efflux of NO3 - as compared with non-mycorrhizal beech roots.

Nitrogen and base cation uptake in seedlings of Acer pseudoplatanus and Calamagrostis villosa exposed to an acidified environment

The effects of solution acidity and form of nitrogen on net nutrient uptake rates in Acer pseudoplatanus and Calamagrostis villosa seedlings were examined as part of a complex ecological study. Uptake rates were measured by the depletion method under controlled conditions (temperature 20 °C, irradiance 400 µmol m−2 s−1 PAR) from a nutrient solution containing 1.5 mM nitrogen in the form of nitrate or ammonium or an equimolar mixture of both. The solution acidity was kept constant at pH 5.5 (control treatment), 4.5 or 3.5 (low pH treatments). Strongly acid pH decreased or stopped the uptake rates of NO3−, Mg2+ and Ca2+, but the uptake of NH4 + was not changed in both species Ammonium ions reduced the uptake rate of NO3 − in Acer but increased the uptake rate in Calamagrostis Ammonium as the sole source of nitrogen had a strong negative impact on the uptake rates of calcium and magnesium and this effect was independent of the media acidification usually connected with NH4 + uptake and assimilation. However, the negative effect of ammonium ions on the base cation uptake was more pronounced at low pH values.

Seasonal variability in nitrogenous nutrition of phytoplankton assemblages in the northeastern subarctic Pacific Ocean

Nitrogen uptake rates, and physical, chemical and biological characteristics of the euphotic zone were studied in winter, spring and late summer during the period 1992–1994 along a transect (Line P) extending from the continental slope off the southwest corner of Vancouver Island (British Columbia, Canada; station P4; 49°N, 127°W) to open waters in the NE Pacific (OSP; 50°N, 145°W). Nitrate (NO 3 −) and silicic acid (Si(OH) 4) concentrations increased offshore during every season. Lowest NO 3 − and Si(OH) 4 values were observed during late summer and spring, and highest during winter throughout the euphotic zone. For spring and late summer, surface depletion of NO 3 − was observed at the inshore end of the transect, while offshore concentrations were never limiting for phytoplankton growth. Silicic acid was never depleted at any depth or station during the period covered by this study. Ammonium (NH 4 +) and urea concentrations exhibited a patchy distribution along the transect, with no seasonal variations. Chlorophyll a and particulate nitrogen did not show a consistent longitudinal pattern from year to year. In general, the highest concentrations of chlorophyll a and particulate nitrogen were measured during the late summer cruises, with lower values in spring and lowest in winter. Phytoplankton assemblages were numerically dominated by flagellates <5 μm throughout the water column on each cruise transect. Ammonium, urea and NO 3 − uptake rates represented on average 55, 24 and 21% of the depth-integrated total nitrogen uptake, both longitudinally and seasonally; hence, phytoplankton utilized nitrogen in the following order: NH 4 +>urea>NO 3 − along Line P. Ammonium may have inhibited the uptake rates of NO 3 − and urea. Urea uptake rates were lower than those of NH 4 +, but higher values were occasionally observed at a few depths along the transect, particularly during the spring of 1993. Depth-integrated NH 4 + uptake rates were generally higher inshore, while NO 3 − uptake rates showed higher values offshore during most seasons. In contrast, urea uptake rates did not exhibit a consistent longitudinal trend. The depth-integrated f-ratio ranged from 0.05 to 0.37 with an average of 0.21 for all stations and cruises, and was overestimated on average by 36% when urea was excluded from the calculation. On a yearly basis, primary productivity in the NE subarctic Pacific was based on regenerated nitrogen.

NITROGEN UTILIZATION AND METABOLISM RELATIVE TO PATTERNS OF N2 FIXATION IN CULTURES OF TRICHODESMIUM NIBB1067

We measured uptake kinetics for four combined N sources, ambient rates of N uptake and N2 fixation, glutamine synthetase activity (transferase and biosynthetic), and concentrations of intracellular pools of glutamate (glu) and glutamine (gln) in cultures of Trichodesmium NIBB1067. N dynamics and metabolism were examined to assess the relative importance of N2 fixation and N uptake to Trichodesmium nutrition. Comparisons were made between cultures grown on medium without added N, with excess NO, or with excess urea. Of the combined N sources tested, Trichodesmium NIBB1067 had the highest affinity for NH; high uptake capacities for NH, urea, and glu; and little capacity for NO uptake. In cultures grown on medium without added N, NH accumulated in the medium during growth, resulting in high NH uptake rates relative to rates of N2 fixation. Glu uptake rates were low but consistent throughout the diel period. In cultures grown on excess NO or urea, uptake of these compounds supplied the majority of the daily N demand, although some N2 fixation occurred during the light period. NO uptake rates were reduced when N2‐fixation rates were high. In all of the cultures, the highest gln/glu ratios and the lowest glutamine synthetase transferase/biosynthetic ratios were observed during the period when rates of total N uptake were highest. In cultures growing exponentially on medium without added N, N2 fixation accounted for 14%– 16% of the total daily N uptake. Uptake of NH and glu, presumably regenerated within the culture vessels, represented 84%–86% of the daily N uptake. Because these systems were closed, net growth was constrained by the rate at which N2 could be fixed into the system. However, total daily N turnover was greater than that necessary to accommodate the observed increase in culture biomass. The rapid N turnover rates observed in these cultures may support gross productivity and balance the high rates of C fixation observed in natural populations of Trichodesmium.

Partitioning of Nitrate Assimilation between Shoots and Roots of Kentucky Bluegrass

Turfgrass roots play an important role in the overall metabolism of NO−3 partly because leaves of turfgrasses are partially lost during mowing. Nitrate transported to shoots stimulates shoot growth and decreases N use efficiency through clipping removal. This study was conducted to quantify NO−3 reduction by shoots and roots of Kentucky bluegrass (Poa pratensis L.). Two cultivars, Livingston, which performed better under high N fertility, and Merit, which performed better under low N fertility, were grown in aerated nutrient solution containing 0.1 to 5.2 mM NO−3. These grasses were analyzed for relative growth rate (RGR), shoothoot ratio (S/R),in situ NO−3 uptake rate (NUR), in vivo NO−3 reductase activity (NRA), metabolic NO−3 pool (MNP), and storage NO−3 pool (SNP) in shoots and roots. Under low NO−3 levels, Merit exhibited a higher NUR, a larger root SNP, a smaller shoot SNP and a greater RGR than Livingston. Under high NO−3 levels, Livingston exhibited a greater shoot growth rate and reduced a greater proportion of NO−3 in its shoots than did Merit. In both cultivars, root contribution to the plant total NO−3 reduction (PTNR), estimated from NRA, was <5%. When dissolved O2 was decreased and root carbohydrate content increased, root contribution to PTNR increased to 40 and 15%, respectively. Our results suggest that root carbohydrate status and root-zone O2 levels strongly influence root contribution to PTNR and N use efficiency of Kentucky bluegrass in response to N fertility. This in turn is a function of the partitioning of NO−3 assimilation between shoots and roots.

Temperature regulation of nitrate uptake: A novel hypothesis about nitrate uptake and reduction in cool‐water diatoms

Marine diatoms generally form large blooms during periods of cool temperature (&lt;20°C), high NO3− fluxes (&gt;25 μM‐N), and turbulent mixing, but the adaptations that allow diatoms to bloom under these conditions are not well understood. We have conducted both NO3− uptake kinetics and direct short‐term temperature manipulation studies on field diatom‐dominated populations from Chesapeake and Delaware Bays during both spring and fall blooms. Absolute rates of NO3− uptake by a Rhizoseleni‐dominated population did not appear to saturate even at concentrations as high as 180 µM‐N. We observed contrasting patterns of NO2−, NH+, and urea utilization as a function of experimental temperature (ambient ± 9°C). Over the temperature range of 7–25°C, absolute uptake rates of NO3− (ρNO3−) decreased an average of 46% with increasing temperature from 7 to 25°C (nine individual experiments), while ρNH4+ and ρUREA increased with increasing temperature by an average of 179 and 86% (eight individual experiments), respectively. Based on these observations and the nature of the physical environment, we hypothesize that these diatom‐dominated populations were taking up NO3− in excess of nutritional requirements, the reduction of which may serve as a sink for electrons during transient periods of imbalance between light energy harvesting and utilization. We suggest that the increase in non‐nutritional NO3− uptake increases proportionately with the magnitude of the imbalance between light energy harvesting and imbalance. This hypothesis reconciles previous observations of low C:N uptake ratios, high release rates of dissolved organic nitrogen or NO2− by diatom‐dominated assemblages, other observations of nonsaturating NO3− kinetics in field populations, and the apparent "preference" for NO3− by the netplankton size fraction. The two phenomena described here, nonsaturable kinetics and a negative relationship between NO3− uptake and short‐term temperature shifts, have important ecological implications. The hypothesized ability of these diatom‐dominated populations to better modulate the flow of photosynthetic electron energy, via NO3− reduction, in variable environments may provide a competitive advantage to diatoms and could potentially explain why diatoms frequently dominate in regions of cool temperature, high NO3− flux, and turbulent mixing. Also, models of new production may need to incorporate terms for temperature dependence of NO3− uptake. Finally, if a significant fraction of NO3− uptake is regulated by non‐nutritional mechanisms in the cell, and if some fraction of nitrogen reduced by this mechanism is subsequently released in the form of NO2−, NH+, or dissolved organic nitrogen (DON), then estimates of new production based solely on NO3− uptake could be seriously biased.

Inorganic and organic nitrogen cycling in Chesapeake Bay: autotrophic versus heterotrophic processes and relationships to carbon flux

To define the role of dissolved organic nitrogen (DON) in Chesapeake Bay (USA) nitrogen cycling, we performed experiments in the mesohaline Chesapeake Bay dunng 3 seasons: May, during the mid to late spring bloom, August, when patchy summer blooms often develop, and October, after the fall overturn. Every 3 h for 30 h we measured ambient nitrogen concentrations and used tracers to determine uptake and regeneration rates of NH,' and urea, uptake rates of NO,(NO; was measured in October only), and rates of DON release due to NH,' uptake. Autotrophic production and dissolved organic carbon (DOC) release were determined sunultaneously using 'v tracer techniques to compare with nitrogen flux rates We found that first, the water column nitrogen demand exceeded nitrogen regeneration in May but n~ t rogen regeneration was over 3 times greater than the nitrogen demand in October. Second, mean rates of autotrophic DON release were highest in May but heterotrophic release was more important in summer and fall. Third, dunng all 3 studies, the C N ratio of DOC to DON release rates was 3.4 to 4.5, indicating release of nitrogen-rich compounds. Release of dissolved organic matter (DOM) \nth a low C:N ratio was corroborated by an observed increase in DON concentrations from 7 to 4 2 pg-at. N I-' from May to August and a concomitant decrease in the C . N ratio of the ambient DOM pool from 36.6 to 8.8. These data suggest that Chesapeake Bay IS primarily autotrophic In the spring, but becomes progressively more heterotrophic later in the year. These data also suggest that DON is an Important intermediate in this transition and serves as a link between the allochthonous nitrogen-based autotrophic production In the spring and the autochthonous nitrogen-based heterotrophic system in the summer and fall.

Potential role of bacterial mats in the nitrogen budget of marine sediments:the case of Thioploca spp.

The main source of organic matter to estuarine and shelf sediments is the deposition of detrital material from plankton in the water column (Suess 1980). The release of nutrients from the sediments is considered to be regulated (feedback mechanism) by the rate at which organic detritus reaches the bottom, the rate at which this detritus is decon~posed (mainly by heterotrophic bacteria), and the rate at which nutrients released to pore waters are transported to overlying waters by diffusion and bioturbation. Furthermore, the regeneration of nutrients from the sediments of shallow, near-shore marine environments could be supplying more than half of the required nitrogen for primary production (Rowe et al. 1975). However, recent observations (Fossing et al. 1995) suggest that prokaryotic metabolic processes associated with benthic microbial mats of the genus Thioploca could be responsible for converting the most abundant form of new nitrogen (NO3-) into biologically (intracellular) available substrate. This would be an important mechanism for introducing dissolved N into the seabed of the eastern Pacific shelf, representing about a third of the total N input into the sediments, when compared to the input via the rain of detritus and assuming that 10% of the primary production of 9.6 g C n r 2 d-' reaches the botton~. These and other observations (e.g. hydrothermal vents) demand a reassessment of the classic view of the benthic-pelagic coupling in marine systems. Mats of giant filamentous bacteria were first observed off northern Chile under the Peru-Chile Undercurrent (Gallardo 1963). They were later identified from central Chile as being made up of dense populations of Thioploca spp. (Beggiatoaceae) with biomasses of between 10 and 1000 g n1r2, potentially covering more than 40000 km2 of the sea bottom off southern Peru to central Chile (Gallardo 1977). The undercurrent is associated with the Equatorial Subsurface Water (ESSW) and is characterised by high salinity, low oxygen and nutrient-rich waters, which are source waters for wind-driven coastal upwelling that fertilises the Chilean and Peruvian coasts (Wooster & Gilrnartin 1961). The ESSW impinges on the seabed between depths of 50 and 280 m, creating a benthic environment with extended periods of suboxic and anoxic conditions that fluctuate intra-annually and interannually (e.g. with El Nilio) and have a strong influence on the mats' distribution and biomass. The metabolism of these marine bacteria has remained a mystery long after their discovery. An intensive field study of biogeochemical processes in sediments of the continental shelf in central Chile (36 S) was carried out in 1994 through a joint effort between German (Max Planck Institute for Marine Microbiology) and Chilean (University of Concepcion) researchers with the goal of understanding the metabolic pathways of Thioploca spp. Fossing et al. (1995) reported that Thioploca cells are able to concentrate nitrate up to 500 mM in a liquid vacuole that occupies >80% of the cell volume. This vacuole is surrounded by numerous sulphur globules embedded in the cytoplasm. They observed NO3uptake rates by Thioploca of ca 2.1 mm01 m-2 d-l, suggesting that they oxidise hydrogen sulphide using nitrate and behave as sulphide-oxidising denitrifiers. These observations were accompanied by flume studies that show chemotactic response towards nitrate-rich overlying waters with low oxygen conditions (Huettel et al. 1996). The fate of the large NO3pool inside the vacuoles of Thioploca spp, in this area has remained unclear. Thamdrup & Canfield (1996) found that sediment NO3consumption during incubations was not directly coupled to carbon oxidation (i.e. dissimilatory nitrate reduction), suggesting that the NO3consumed may be reduced to NH,' rather than to NZ, which would

An experimental set-up to study carbon, water, and nitrate uptake rates by hydroponically grown plants.

The experimental system described allows concomitant hourly measurements of CO2, H2O, and NO3 uptake rates by plants grown hydroponically in a greenhouse. Plants are enclosed in an airtight chamber through which air flows at a controlled speed. Carbon dioxide exchange and transpiration rates are determined from respective differences of concentrations of CO2 and water vapor of the air at the system inlet and outlet. This set-up is based on the "open-system" principle with improvements made on existing systems. For instance, propeller anemometers are used to monitor air flow rates in the chamber. From their signal it is possible to continuously adjust air speed to changing environmental conditions and plant activity. The air temperature inside the system therefore never rises above that outside. Water and NO3 uptake rates are calculated at time intervals from changes in the volume and the NO3 concentration of the nutrient solution in contact with the roots. The precise measurement of the volume of solution is achieved using a balance which has a higher precision than any liquid level sensors. Nitrate concentration is determined in the laboratory from aliquots of solution sampled at time intervals. A number of test runs are reported which validate the measurements and confirm undisturbed conditions within the system. Results of typical diurnal changes in CO2, H2O, and NO3 uptake rates by fruiting tomato plants are also presented.