Fluxes of methane and carbon dioxide from a small productive lake to the atmosphere

The fluxes of CH₄ and CO₂ to the atmosphere, and the relative contributions of ebullition and molecular diffusion, were determined for a small hypertrophic freshwater lake (Priest Pot, UK) over the period May to October 1997. The average total flux of CH₄ and CO₂ (estimated from 7 sites on the lake) was approximately 52 mmol m^–2 d^–1 and was apportioned 12 and 40 mmol m^–2 d^–1 toCH₄ and CO₂ respectively. Diffusion across the air-water interface accounted for the loss of 0.4and 40 mmol m^–2 d^–1 of CH₄ and CO₂ respectively whilst the corresponding figures for ebullition losses were 12.0 (CH₄) and 0.23 (CO₂) mmol m^–2 d^–1. Most CH₄ (96%) was lost by ebullition, and most CO₂ (99%) by diffusive processes. The ebullition of gas, measured at weekly intervals along a transect of the lake, showed high spatial and temporal variation. The CH₄ content of the trapped gas varied between 44 and 88% (by volume) and was highest at the deepest points. Pulses of gas ebullition were detected during periods of rapidly falling barometric pressure. Therelevance of the measurements to global estimates ofcarbon emission from freshwaters are discussed.     

Throughfall chemistry in a loblolly pine plantationunder elevated atmospheric CO2 concentrations

Accelerated tree growth under elevatedatmospheric CO₂ concentrations may influencenutrient cycling in forests by (i) increasingthe total leaf area, (ii) increasing the supplyof soluble carbohydrate in leaf tissue, and (iii) increasing nutrient-use efficiency. Here wereport the results of intensive sampling andlaboratory analyses of NH ₄ ⁺ , NO ₃ ^– , PO ₄ ^3– , H⁺, K⁺, Na⁺,Ca²⁺, Mg²⁺, Cl^–, SO ₄ ^2– , and dissolved organic carbon (DOC) in throughfallprecipitation during the first 2.5+ years of the DukeUniversity Free-Air CO₂ Enrichment (FACE)experiment. After two growing seasons, a largeincrease (i.e., 48%) in throughfall deposition of DOCand significant trends in throughfall volume and inthe deposition of NH ₄ ⁺ , NO ₃ ^– , H⁺, and K⁺ can be attributed to the elevatedCO₂ treatment. The substantial increase indeposition of DOC is most likely associated withincreased availability of soluble C in plant foliage,whereas accelerated canopy growth may account forsignificant trends toward decreasing throughfallvolume, decreasing deposition of NH ₄ ⁺ ,NO ₃ ^– , and H⁺, and increasing deposition of K⁺ under elevated CO₂. Despiteconsiderable year-to-year variability, there wereseasonal trends in net deposition of NO ₃ ^– ,H⁺, cations, and DOC associated with plant growthand leaf senescence. The altered chemical fluxes inthroughfall suggest that soil solution chemistry mayalso be substantially altered with continued increasesin atmospheric CO₂ concentrations in the future.     

Greenhouse gas dynamics in boreal, littoral sediments under raised CO2 and nitrogen supply

SUMMARY 1. The effects of increasing CO₂ and nitrogen loading and of a change in water table and temperature on littoral CH₄, N₂O and CO₂ fluxes were studied in a glasshouse experiment with intact sediment cores including vegetation (mainly sedges), taken from a boreal eutrophic lake in Finland. Sediments with the water table held at a level of 0 or at ?15 cm were incubated in an atmosphere of 360 or 720 p.p.m. CO₂ for 18 weeks. The experiment included fertilisation with NO₃^– and NH₄⁺ (to a total 3 g N m^?2). 2. Changes in the water table and temperature strongly regulated sediment CH₄ and cCO₂ fluxes (community CO₂ release), but did not affect N₂O emissions. Increase in the water table increased CH₄ emissions but reduced cCO₂ release, while increase in temperature increased emissions of both CO₂ and CH₄. 3. The raised CO₂ increased carbon turnover in the sediments, such that cCO₂ release was increased by 16–26%. However, CH₄ fluxes were not significantly affected by raised CO₂, although CH₄ production potential (at 22 °C) of the sediments incubated at high CO₂ was increased. In the boreal region, littoral CH₄ production is more likely to be limited by temperature than by the availability of carbon. Raised CO₂ did not affect N₂O production by denitrification, indicating that this process was not carbon limited. 4. A low availability of NO₃^– did severely limit N₂O production. The NO₃^– addition caused up to a 100‐fold increase in the fluxes of N₂O. The NH₄⁺ addition did not increase N₂O fluxes, indicating low nitrification capacity in the sediments.     

Effects of temperature and oxygenavailability on greenhouse gas and nutrient dynamics in sediment of a eutrophic mid-boreal lake

The effects of oxygen conditions and temperature on dynamics of greenhousegases (CH₄, CO₂, N₂O) and nutrients(NH₄ ⁺, NO₂ ^–+NO₃ ^–, tot-P) were studied in sediment of hyper-eutrophic LakeKevätön, Finland. Undisturbed sediment cores were incubated at 6, 11,16, and 23 °C in a laboratory microcosm using a continuouswater flowtechnique with an oxic or anoxic water flow. The production of CO₂increased with increasing temperature in both oxic (Q₁₀ 3.2 ±0.6) and anoxic (Q₁₀ 2.3 ± 0.4) flows. The release ofCH₄ increased with temperature in anoxic conditions (Q₁₀2.3 ± 0.2), but was negligible with the oxic flow at all temperatures.The release of NH₄ ⁺ increased with temperature with the oxic and anoxic flows(Q₁₀ 2.4 ± 0.1). There was a net production of NO₂ ^–, NO₃ ^– and N₂O with the oxic flow at temperatures below16 °C. The release of phosphorus was greater from the anoxicsediments and increased with temperature with both the anoxic (Q₁₀2.9 ± 0.5) and oxic (Q₁₀ 1.9 ± 0.1) flows. It isprobable that the temperature of boreal lakes and the associated oxygendeficiency will increase as the climate becomes warmer. Our experiments showedthat this change would increase the global warming potential of greenhousegasesreleased from sediments of eutrophic lakes predominately attributable to theincrease in the CH₄ production. Furthermore, warming would alsoaccelerate the eutrophication of lakes by increasing release of phosphorus andmineral nitrogen from sediments, which further enhance CH₄productionin sediments.     

Distribution of reducing power between photosynthetic carbon and nitrogen assimilation in Scenedesmus

Fixation of CO₂ and N assimilation were studied in synchronous cultures of Scenedesmus obtusiusculus Chod. under saturating and limiting light. Within the photon-flux range studied, the cells maintained C to N assimilation ratios of 7–10 with either NO ₃ ^- , NO ₂ ⁺ or NH ₄ ⁺ as the N source. Competitive interactions between C and N assimilation were pronounced under light limitation and were proportional to the oxidation status of the N source. Fixation of CO₂ at saturating light was also slightly reduced by NO ₂ ^- and NH ₄ ⁺ . In the absence of CO₂, NO ₃ ^- uptake and reduction was light-saturated at a comparatively low photon flux, whereas NO ₂ ^- uptake and reduction was considerably faster in the absence of CO₂ than in its presence. The pools of reduced pyridine nucleotides (NADPH and NADH) were largely unaffected by the presence or absence of the different N sources. The regulatory influences of CO₂ fixation on N assimilation are discussed in terms of coupling between the rates of CO₂ fixation and NH ₄ ⁺ assimilation, as well as the existance of control mechanisms for NO ₃ ^- uptake and reduction.Abbreviations Chl chlorophyll - PF photon flux     

Methane in maritime Antarctic freshwater lakes

Summary Methane was found to occur in all freshwater lakes, irrespective of trophic status, sampled during this preliminary investigation at Signy Island, South Orkney Islands, Antarctica. Methane accumulated in the water column of these lakes during the winter period when ice cover prevented wind-induced mixing. Maritime Antarctic lakes are usually subject to wind-induced complete mixing during the summer open-water period but two major exceptions to the rule were found during this study. Methanogenesis occurred in both littoral and profundal regions of oligotrophic Sombre Lake. The presence of a substantial algal mat stabilized the Eh status of underlying sediments at the littoral site. Methane production was confined to the sediments in both littoral and profundal sediments during the study period (December–March) but in winter probably migrated to the sediment surface at the profundal site. All Signy Island lakes sampled were sulphate-poor and addition of sulphate markedly inhibited methanogenesis. Radio-isotope studies indicated that the H₂/CO₂ pathway was probably the predominant route for methanogenesis in these sediments through the acetate pathway appeared equally important at the sediment surface. In the absence of sulphate, sulphate reducers probably acted as net hydrogen donors to the methanogens. The process rate was permanently limited by the consistent low temperature (annual range 1–3°C). Rates increased with increasing temperature over the range 4–32°C, but no evidence was found to suggest cold sensitivity or psychrophily. The optimum temperature for methanogenesis was in excess of 30°C, temperatures never experienced at Signy Island. Rates of methanogenesis during the study period (Dec–Mar) ranged from 0.29 to 0.45 mg of carbon m^-2 and on an annual basis methanogenesis was calculated equivalent to 13% of the organic carbon deposition rate.     

Spatial variation in flux, δ<Superscript>13</Superscript>C and δ<Superscript>2</Superscript>H of methane in a small Arctic lake with fringing wetland in western Greenland

Small lakes in northern latitudes represent a significant source of CH₄ to the atmosphere that is predicted to increase with warming in the Arctic. Yet, whole-lake CH₄ budgets are lacking as are measurements of δ¹³C-CH₄ and δ²H-CH₄. In this study, we quantify spatial variability of diffusive and ebullitive fluxes of CH₄ and corresponding δ¹³C-CH₄ and δ²H-CH₄ in a small, Arctic lake system with fringing wetland in southwestern Greenland during summer. Net CH₄ flux was highly variable, ranging from an average flux of 7 mg CH₄ m^?2 d^?1 in the deep-water zone to 154 mg CH₄ m^?2 d^?1 along the lake margin. Diffusive flux accounted for ~8.5 % of mean net CH₄ flux, with plant-mediated and ebullitive flux accounting for the balance of the total net flux. Methane content of emitted ebullition was low (mean ± SD 10 ± 17 %) compared to previous studies from boreal lakes and wetlands. Isotopic composition of net CH₄ emissions varied widely throughout the system, with δ¹³C-CH₄ ranging from ?66.2 to ?55.5 ‰, and δ²H-CH₄ ranging from ?345 to ?258 ‰. Carbon isotope composition of CH₄ in ebullitive flux showed wider variation compared to net flux, ranging from ?69.2 to ?49.2 ‰. Dissolved CH₄ concentrations were highest in the sediment and decreased up the water column. Higher concentrations of CH₄ in the hypoxic deep water coincided with decreasing dissolved O₂ concentrations, while methanotrophic oxidation dominated in the epilimnion based upon decreasing concentrations and increasing values of δ¹³C-CH₄ and δ²H-CH₄. The most depleted ¹³C- and ²H-isotopic values were observed in profundal bottom waters and in subsurface profundal sediments. Based upon paired δ¹³C and δ²H observations of CH₄, acetate fermentation was likely the dominant production pathway throughout the system. However, isotopic ratios of CH₄ in deeper sediments were consistent with mixing/transition between CH₄ production pathways, indicating a higher contribution of the CO₂ reduction pathway. The large spatial variability in fluxes of CH₄ and in isotopic composition of CH₄ throughout a single lake system indicates that the underlying mechanisms controlling CH₄ cycling (production, consumption and transport) are spatially heterogeneous. Net flux along the lake margin dominated whole-lake flux, suggesting the nearshore littoral area dominates CH₄ emissions in these systems. Future studies of whole-lake CH₄ budgets should consider this significant spatial heterogeneity.     

Microbial population dynamics in an extreme environment: controlling factors in talus soils at 3750 m in the Colorado Rocky Mountains

Variation of CH₄ emissions over a three-year period was studied in a reed-dominated (Phragmites australis) littoral transect of a boreal lake undergoing shoreline displacement due to postglacial rebound. The seasonal variation in plant-mediated CH₄ emissions during open-water periods was significantly correlated with sediment temperature. The highest plant-mediated emission rates (up to 2050 mg CH₄ m^–2 d^–1) were found in the outermost reed zone, where culms of the previous growing seasons had accumulated and free-floating plants grew on the decomposing culms. In reed zones closer to the shoreline as well as in mixed stands of reed and cattail, the maximum daily rates were usually > 500 mg CH₄ m^–2 d^–1. The total plant-mediated CH₄ emission during the open-water period was significantly correlated with the seasonal maximum of green shoot biomass. This relationship was strongest in the continuously flooded (water depth > 25 cm) outermost zones. In this area, emissions through ebullition were of greatest importance and could exceed plant-mediated emissions. In general, total emissions of the open-water periods varied from ca. 20 to 50 g CH₄ m^–2 a^–1, but in the outermost reed zone, the plant-mediated emissions could be as high as 123 g CH₄ m^–2 a^–1; ebullition emissions from this zone reached > 100 g CH₄ m^–2 a^–1. The proportion of CH₄ released in winter was usually < 10% of annual emissions. Emissions of CH₄ were higher in this flooded transgression shore the than those measured in boreal peatlands, but the role of ancient carbon stores as a substrate supply compared with recent anthropogenic eutrophication is unknown.     

Nutrient cycling in shallow,oligotrophic Lake Kvie,Denmark

The importance of isoetids for the exchange of dissolved inorganic nitrogen (DIN) between sediment and water was studied in shallow Lake Kvie, Denmark. Vegetated sediments from the littoral zone (55% of lake area) were compared to unvegetated sediments from the littoral and profundal zone. Clear effects of the isoetids were found on DIN in the porewater. At the vegetated station, NH₄ ⁺ showed the highest concentrations just below the surface (< 40 µM) whereas NO₃ ^- was dominating below 5 cm depth with concentrations up to 100 µM during the spring. The unvegetated littoral sediment showed a distinct change between winter where NH₄ ⁺ dominated and summer where NO₃ ^- was most abundant. NH₄ ⁺ dominated in the profundal sediment and showed increasing concentration with depth. The E_h was high (> 400 mV) in the vegetated sediment, indicating isoetid release of O₂ in the rhizosphere. A low DIN uptake was observed at the vegetated station while, based on porewater data, a diffusive release from the sediment was expected. This difference was due to plant assimilation. In general a release of NH₄ ⁺ and an uptake of NO₃ ^- was seen in all sediments. The denitrification rate calculated from the mass balance for the entire lake was 0.4 mmol m - 2 d^-1 and accounted for removal of 77% of the annual N-input to Lake Kvie.     

Denitrification and methane production in sediment of Hamilton Harbour (Canada)

Systematic sampling of 21 sites covering Hamilton Harbour (Lake Ontario, Canada) was carried out during the summer in 1990 and 1991 in order to study how well environmental factors, such as O₂, NO ₃ ^– , and organic carbon, and the spatial structure can explain observed variation of potential denitrification, CH₄ and CO₂ production, as well as N₂ fixation in sediment slurries. Using canonical redundancy analysis and an extension of this method to partial out the variance into spatial and environmental components, we found that most of the explained fraction of potential microbial activities (70–90%) was accounted for by the significant environmental variables (NH ₄ ⁺ , particulate carbon, dissolved organic carbon, dissolved O₂, depth, and temperature) and not much by the spatial polynomial trend surface. We found significant path coefficients (0.53 and 0.57 in 1990 and 1991) between CO₂ production and potential denitrification, which suggests that denitrifiers are dependent upon a heterotrophic bacterial population for directly assimilable carbon sources. We also found significant path coefficients between particulate carbon and both CH₄ production (0.67 and 0.33) and CO₂ production (0.50 and 0.38), while significant path coefficients were also found between dissolved organic carbon and CO₂ production (0.34 and 0.47). We conclude that beside well-known abiotic factors such as O₂, NO ₃ ^– , and organic carbon, a biotic factor involved in carbon metabolism may be important in explaining the spatial variation of denitrification capacity in the sediment of Hamilton Harbour.Correspondence to: R. Roy.     

Effects of inorganic nitrogen on C2H2 reduction and CO2 exchange in the Peltigera praetextata-Nostoc and Peltigera aphthosa-Coccomyxa-Nostoc symbioses

Uptake of NH ₄ ⁺ and NO ₃ ^- by the N₂-fixing lichens Peltigera praetextata (two-component lichen) and P. aphthosa (three-component lichen) was studied. In addition, the effects of these ions, separately and in combination, on C₂H₂ reduction and CO₂ exchange were examined. Both NH ₄ ⁺ and NO ₃ ^- were utilized by the lichens. NH₄NO₃ caused an increased liberation of NO ₃ ^- from the lichens as compared to the release observed in untreated lichen thalli. NH ₄ ⁺ and NO ₃ ^- led to reduced C₂H₂ reduction by P. praetextata, which, however, was less pronounced than when the two ions were given in combination. In P. aphthosa the C₂H₂ reduction was inhibited by NH ₄ ⁺ and NH₄NO₃, but not by NO ₃ ^- alone. NH ₄ ⁺ and NO ₃ ^- had no effect on the net photosynthesis of P. praetextata, while, in combination, they led to inhibition, although only at a concentration higher than that inhibitory to the C₂H₂ reduction of P. aphthosa. The photsynthesis was inhibited by all salts, but only initially, probably a salt effect. Effects of NH ₄ ⁺ on the membrane potential of the cyanobiont are suggested as an important factor causing the depression of net photosynthesis.     

Methylamine,an inhibitor of ammonium oxidation and chemoautotrophic growth in the marine nitrifying bacteriumNitrosococcus oceanus

Methylamine (CH₃NH ₃ ⁺ ) appeared to utilize the same transport mechanism as ammonium (NH ₄ ⁺ ) to enter cells ofNitrosococcus oceanus. Methylamine uptake did not show clear evidence of saturable kinetics and was not fully saturated at 20 mM. Assimilated CH₃NH ₃ ⁺ was not incorporated into macromolecular constituents, but inhibited rates of nitrification, chemoautotrophic CO₂ fixation and growth. The degree of inhibition was dependent on the relative concentrations of NH ₄ ⁺ and CH₃NH ₃ ⁺ . Rates of CO₂ fixation and growth were inhibited four times more than the rate of nitrification.     

Microbial oxidation of methane,ammonium and carbon monoxide,and turnover of nitrous oxide and nitric oxide in soils

The effect of soil microbial processes on production and/or consumption of atmospheric trace gases was studied in four different soils which were preincubated in the presence of elevated concentrations of CH₄, NH ₄ ⁺ or CO, to simulate the growth of the resident populations of methanotrophic, nitrifying, or carboxydotrophic bacteria, respectively. Oxidation of CH₄, both at atmospheric (1.8 ppmv) and at elevated (3500 ppmv) CH₄ mixing ratios, was stimulated after preincubation with CH₄, but not with NH ₄ ⁺ or CO, indicating that CH₄ was oxidized by methanotrophic, but not by nitrifying or carboxydotrophic bacteria. However, the oxidation of CH₄ was partially inhibited by addition of NH ₄ ⁺ and CO. Analogously, oxidation of NH ₄ ⁺ was partially inhibited by addition of CH₄. Oxidation of CO at elevated mixing ratios (2300 ppmv) was stimulated after preincubation with CO, indicating oxidation by carboxydotrophs, but was also stimulated at a small extent after preincubation with CH₄, suggesting the involvement of methanotrophs. At atmospheric CO mixing ratios (0.13 ppmv), on the other hand, oxidation of CO was stimulated after preincubation with NH ₄ ⁺ , indicating that the activity was due to nitrifiers. NO uptake was stimulated in soils preincubated with CH₄, indicating the involvement of methanotrophs. However, production of N₂O was only stimulated, if CH₄ was added as a substrate. The results indicate that especially the methanotrophic and nitrifying populations in soil not only oxidize their specific substrates, but are also involved in the metabolism of other compounds.     

Colonization of acidic mining lakes: <Emphasis Type="Italic">Chydorus sphaericus</Emphasis> and other Cladocera within a dynamic horizontal pH gradient (pH 3−7) in Lake Senftenberger See (Germany)

Spatio-temporal distribution of nitrogen was examined along the gradient from open water to lakeshore in Lake Taihu, China. Two types of undulating littoral zones were selected: natural reed belt and bare lakeshore. The reed belt affected nitrogen transformation and was sink for internal-lake nitrogen, whereas the bare lakeshore showed little effect. During the growing season, NO ₃ ⁻ -N concentration increased by up to 3–5 times from open water to reed belt, while NH ₄ ⁺ -N concentration decreased. It suggested that nitrification was the main nitrogen process in reed belt. Total dissolved nitrogen (TDN) showed little spatial variation, indicating that most of nitrogen released from sediment did not move into open water again. Significant temporal variation of dissolved nitrogen occurred and was similar in both the littoral zone and the open water. Maximum TDN and NH ₄ ⁺ -N concentrations occurred in January, and NO ₃ ⁻ -N in March. Minimum NH ₄ ⁺ -N and NO ₃ ⁻ -N concentrations occurred in July and August, respectively. An increasing total soil nitrogen was found in the surface sediments from reed belt to open water. This further suggested that the reed-covered littoral zone had strong nitrogen transformation potential.     

Processes affecting molecular and stable isotope compositions of sediment gas in estuarine waters along the southern Baltic coast (Poland)

This paper investigates the molecular and stable isotope compositions of sediment gases from seven coastal lakes along the southern Baltic coast in Poland. The aim is to extend the knowledge of the genesis and distribution of microbial gases in the zone of mixing of fresh and salt waters with special attention to the effect of salinity, climate-related seasonality, and vertical sediment mixing. We found differences in the compositions of gas between the studied lakes and within each lake. These differences are mainly controlled by lake water depth and the presence of macrophytes. Due to the dissolution of rising bubbles in highly oxygenated water, the concentrations of CH₄ and CO₂ show up to 67% decline along the water column in favor of N₂ and O₂. On the other hand, in vegetated parts of the lakes, the CH₄ is depleted in favor of CO₂, and the residual CH₄ and CO₂ are enriched in ¹³C. Despite the fact that the coastal lakes display highly oxidizing conditions in the water column and that the bottom sediments are mixed by wind waves the CH₄ reveals rather low oxidation. On the basis of the CH₄/N₂ ratio we established that there are differences in the intensity of ebullition throughout the lakes. Higher intensities of ebullition were found in shallower parts of the lakes. Salinity has no effect on the stable C and H isotope composition of sediment gas. It seems, however, that salinity affects the molecular composition of hydrocarbons via preferential oxidation of CH₄ under higher salinity conditions.     

Ammonium and nitrate uptake in gap,generalist and understory species of the genus Piper

Summary We studied root net uptake of ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) in species of the genus Piper (Piperaceae) under high, intermediate and low photosynthetically active photon flux densities (PFD). Plants were grown hydroponically, and then transferred to temperature controlled (25° C) root cuvettes for nutrient uptake determinations. Uptake solutions provided NH ₄ ⁺ and NO ₃ ^– simultaneously (both) or separately (single). In the first experiment, seven species of Piper, from a broad range of rainforest light habitats ranging from gap to understory, were screened for mineral nitrogen preference (100 M NH ₄ ⁺ and/or 100 M NO ₃ ^– ) at intermediate PFD (100 mol m^–2 s^–1). Preference for NH ₄ ⁺ relative to NO ₃ ^– , defined as the ratio of NH ₄ ⁺ (both):NO ₃ ^– (both) net uptake, was higher in understory species than in gap species. Ammonium repression of NO ₃ ^– uptake, defined as the ratio of NO ₃ ^– (single): NO ₃ ^– (both) net uptake, was also higher in understory species as compared to gap species. In a second set of experiments, we examined the effect of nitrogen concentration (equimolar, 10 to 1000 M) on NH ₄ ⁺ preference and NH ₄ ⁺ repression of NO ₃ ^– net uptake at high (500 mol m^–2 s^–1) and low (50 mol m^–2 s^–1) PFD in a gap (P. auritum), generalist (P. hispidum) and understory species (P. aequale). All species exhibited negligible NH ₄ ⁺ repression of NO ₃ ^– net uptake at high PFD. At low PFD, NH ₄ ⁺ preference and repression of NO ₃ ^– net uptake occurred in all species (understory > generalist > gap), but only at intermediate nitrogen concentrations, i.e. between 10 and 200 M. Ammonium repression of net NO ₃ ^– uptake decreased or increased rapidly (in < 48 h) after transitions from low to high or from high to low PFD respectively. No significant diurnal patterns in NO ₃ ^– or NH ₄ ⁺ net uptake were observed.CIWDPB publication # 1130     

A field method of determining NH 4 + and NO 3 - uptake kinetics in intact roots: Effects of CO2 enrichment on trees and crop species

Models describing plant and ecosystem N cycles require an accurate assessment of root physiological uptake capacity for NH ₄ ⁺ and NO ₃ ^- under field conditions. Traditionally, rates of ion uptake in field-grown plants are determined by using excised root segments incubated for a short period in an assay solution containing N either as a radioactive or stable isotope tracer (e.g., ³⁶ClO₃ as a NH ₄ ⁺ analogue, ¹⁴CH₃NH₃ as an NO ₃ ^- analogue or ¹⁵NH ₄ ⁺ and ¹⁵NO ₃ ^- ). Although reliable, this method has several drawbacks. For example, in addition to radioactive safety issues, purchase and analysis of radioactive and stable isotopes is relatively expensive and can be a major limitation. More importantly, because excision effectively interrupts exchange of compounds between root and shoot (e.g., carbohydrate supply to root and N transport to shoot), the assay must be conducted quickly to avoid such complications. Here we present a novel field method for simultaneous measurements of NH ₄ ⁺ and NO ₃ ^- uptake kinetics in intact root systems. The application of this method is demonstrated using two tree species; red maple (Acer rubrum) and sugar maple (Acer saccharum) and two crop species soybean (Glycine max) and sorghum (Sorghum bicolor). Plants were grown in open-top chambers at either ambient or elevated levels of atmospheric CO₂ at two separate US national sites involved in CO₂ research. Absolute values of net uptake rates and the kinetic parameters determined by our method were found to be in agreement with the literature reports. Roots of the crop species exhibited a greater uptake capacity for both N forms relative to tree species. Elevated CO₂ did not significantly affect kinetics of N uptake in species tested except in red maple where it increased root uptake capacity, V, for NH ₄ ⁺ . The application, reliability, advantages and disadvantages of the method are discussed in detail. This revised version was published online in June 2006 with corrections to the Cover Date.     

The relationship between inorganic nitrogen oxidation and organic carbon production in batch and chemostat cultures of marine nitrifying bacteria

Rates of nitrification and organic C production were determined in batch and chemostat cultures of marine nitrifying bacteria; two NH ₄ ⁺ -oxidizing species and one NO ₂ ^– -oxidizing spezies. With increasing age in batch cultures and with decreasing flow rates in chemostats, cellular organic C and N concentrations declined while the intracellular ratio of C:N remained constant. With decreasing flow rates in chemostats, there was a reduction in (a) carboxylating enzyme activity per unit of cellular organic C (the potential for chemoautotrophic CO₂ fixation), and (b) the yield of organic C. For both NH ₄ ⁺ and NO ₂ ^– oxidizers, rates of nitrification and C yield were lowest at very slow chemostat growth rates, when compared with optimal growth rates in batch cultures. For both NH ₄ ⁺ and NO ₂ ^– -oxidizing species, the stoichiometric relationship between nitrification and organic C production did not remain constant and appeared to be dependent on the availability of the inorganic N substrate. The organic C yield from NH ₄ ⁺ oxidation and hence the free energy efficiency declined with increasing age in batch cultures and with decreasing flow rates in chemostats. The C yield from NO ₂ ^– oxidation and the free energy efficiency at slow chemostat growth rates was also lower than that at the optimal growth rate in batch culture.     

Methane turnover in exposed sediments of an Amazon floodplain lake

In the Amazon floodplain large areas of unvegetatedlake sediments are exposed to air during low water. Imeasured methane fluxes from exposed sediments of anAmazonian floodplain lake and assessed the regulatingfactors. Methane emission decreased from values between 10 and 40 mol CH₄ m^{minus 2} h^{minus 1}to zero when the sediments were exposed to air. Meanfluxes were about 3.7 mol CH₄m^{minus 2} h^{minus 1}. Fluxes were low compared tomeasurements taken from flooded habitats makingexposed lake sediments a negligible source ofatmospheric CH₄. This was mainly due to the lackof ebullition which governs methane flux in theflooded phase. Methane oxidation at the sedimentsurface consumed up to 75% of the methane enteringthe oxidised zone. Compared with CO₂ emissionmethane turnover was of minor importance for thecarbon budget of the sediments.     

Resource limitations to nitric oxide emissions from a sagebrush-steppe ecosystem

We monitored soil emissions of NO, NO₂, N₂O, and CO₂ throughout the summer dry season at a remote North American sagebrush-steppe ecosystem following application of several resources, including water, NH ₄ ⁺ , NO ₃ ^– and sucrose. Despite low levels of soil NH ₄ ^– (5.60±0.95 mg NH ₄ ^– -N per kg soil, mean ± S.E.), and NO ₃ ^– -N (1.34±0.20 mg NO ₃ ^– -N per kg soil), NO emissions ranged from about 0.2 to 2.8 ng NO-N m^–2 s^–1, comparable to rates measured from many agricultural, tropical, and other undisturbed ecosystems. Soil wetting increased NO emissions as much as 400-fold when initial gravimetric soil moisture contents were less than about 50 mg kg _soil ^–1 and soil temperature was greater than or equal to 20 °C. Wetting treatments with 20 mg NH ₄ ⁺ -N kg _soil ^–1 raised NO emission rates to a level that was nearly an order of magnitude higher than that observed after water addition alone. Wetting treatments with 20 mg NO ₃ ^– -N kg _soil ^–1 , 240 mg sucrose-C kg _soil ^–1 , or NO ₃ ^– plus sucrose had no statistically significant effect upon NO emissions. Soil denitrifying enzyme activity was low at this site, and N₂O emissions in the field were below detection limits. Soil nitrifying enzyme activity was extremely high at this site, indicating that the NH ₄ ⁺ released by ammonification would be consumed at least once every 1.7 days. These observations indicate that NO emissions from this undisturbed ecosystem were likely a consequence of high nitrification activity, and that sagebrush-steppe ecosystems may be a more important NO source than has been previously assumed.