Determination of dissolved carbon dioxide by coulometric titration in modified atmosphere systems

The solubility of carbon dioxide (CO₂) in microbiological media at different pH values, water activities ( a_w ), temperatures, buffering capacities and ratios of headspace to media volumes was determined by using a coulometer. Buffering capacity and ratio of headspace to media volume were shown to be the major factors influencing the solubility of CO₂ in modified atmosphere model systems. The growth inhibitory effects of different dissolved CO₂ concentrations (0–50 μmol ml^-1) were determined for Pseudomonas fragi at 8°C and 22 C. Pseudomonas fragi was shown to be strongly affected by the CO₂ concentration in the media. A carbon dioxide concentration of 40 μmol ml^-1 was needed to inhibit Ps. fragi at 8°C. The importance of measuring dissolved CO₂ concentrations in modified atmosphere packaging applications was shown and the coulometer proved to be an excellent tool for this purpose.     

Effects of storage and the presence of a beef microflora on the thermal resistance of Salmonella Typhimurium DT104 in beef and broth systems

AIMS: To investigate the effects of storage and the presence of a beef microflora on the thermal resistance of Salmonella serotype Typhimurium DT104 on beef surfaces and in a broth system during subsequent heat treatments after extended low-temperature storage (4 degrees C for 14 days) or mild temperature abuse (10 degrees C for 7 days). METHODS AND RESULTS: Surviving Salm. Typhimurium DT104 cells were estimated after heating in a water bath (55 degrees C) by plating beef and broth samples on tryptone soya agar and overlaying with xylose-lysine-deoxycholate agar. In beef and broth systems, D(55) values for Salm. Typhimurium DT104 stored at 4 degrees C or 10 degrees C in the presence or absence of a beef microflora were significantly lower (P < 0.01) than the D values for this organism heat-treated immediately after inoculation. In beef systems, the D(55) values were significantly lower (P < 0.05) in the presence of a beef microflora than the D(55) values obtained in 'pure' culture under all temperature/storage combinations. However, in broth systems, there was no significant difference between the D(55) values obtained in 'pure' culture and the D(55) values obtained from systems containing beef microflora. CONCLUSIONS: Storage of Salm. Typhimurium DT104 significantly reduced the thermal resistance of the pathogen in beef and broth systems. In the presence of high numbers of a Gram-negative beef microflora, the heat sensitivity of the pathogen was further increased on beef surfaces but not in broth. SIGNIFICANCE AND IMPACT OF THE STUDY: Studies investigating the survival of Salm. Typhimurium DT104 in different food systems will help define safe food preservation processes and will aid in the elimination this pathogen from the food production environments.     

Effect of a low oxygen and high carbon dioxide modified atmosphere on the mortality of different life stages of Carpophilus hemipterus

Carpophilus hemipterus is an important cosmopolitan pest species that causes high economic losses in fruits before and after harvest. In this study, 0–2 d old eggs, 12 d old mature larvae and 1 week old adults of C. hemipterus were exposed to a modified atmosphere with low oxygen and high carbon dioxide content (2.1% O₂ + 90% CO₂ + 7.9% N₂) for 48–120 h at 20 °C and 75 ± 5% relative humidity. Time-mortality responses were subjected to probit analysis, and the difference between lethal exposure times (LTs) for the different life stages were compared using a lethal dose ratio test. LT₉₅ values were recorded as 221.83 h, 82.79 h and 65.31 h for egg, larval and adult stages, respectively. The statistical difference between LT₉₅ values was significant and egg stage was found to be more tolerant than the other two life stages. In addition, the larvae were found to be more tolerant than adults. According to the results of this study, a long time was needed to control the eggs of C. hemipterus.     

The influence of leaf‐atmosphere NH3(g) exchange on the isotopic composition of nitrogen in plants and the atmosphere

The distribution of nitrogen isotopes in the biosphere has the potential to offer insights into the past, present and future of the nitrogen cycle, but it is challenging to unravel the processes controlling patterns of mixing and fractionation. We present a mathematical model describing a previously overlooked process: nitrogen isotope fractionation during leaf‐atmosphere NH_3(g) exchange. The model predicts that when leaf‐atmosphere exchange of NH_3(g) occurs in a closed system, the atmospheric reservoir of NH_3(g) equilibrates at a concentration equal to the ammonia compensation point and an isotopic composition 8.1‰ lighter than nitrogen in protein. In an open system, when atmospheric concentrations of NH_3(g) fall below or rise above the compensation point, protein can be isotopically enriched by net efflux of NH_3(g) or depleted by net uptake. Comparison of model output with existing measurements in the literature suggests that this process contributes to variation in the isotopic composition of nitrogen in plants as well as NH_3(g) in the atmosphere, and should be considered in future analyses of nitrogen isotope circulation. The matrix‐based modelling approach that is introduced may be useful for quantifying isotope dynamics in other complex systems that can be described by first‐order kinetics.     

Exchange of gaseous nitrogen compounds between agricultural systems and the atmosphere

Crop and livestock agricultural production systems are important contributors to local, regional and global budgets of NH₃, NO_x (NO + NO₂) and N₂O. Emissions of NH₃ and NO_x (which are biologically and chemically active) into the atmosphere serve to redistribute fixed N to local and regional aquatic and terrestrial ecosystems that may otherwise be disconnected from the sources of the N gases. The emissions of NO_x also contribute to local elevated ozone concentrations while N₂O emissions contribute to global greenhouse gas accumulation and to stratospheric ozone depletion.Ammonia is the major gaseous base in the atmosphere and serves to neutralize about 30% of the hydrogen ions in the atmosphere. Fifty to 75% of the 55 Tg N yr^–1 NH₃ from terrestrial systems is emitted from animal and crop-based agriculture from animal excreta and synthetic fertilizer application. About half of the 50 Tg N yr^–1 of NO_x emitted from the earth surface annually arises from fossil fuel combustion and the remainder from biomass burning and emissions from soil. The NO_x emitted, principally as nitric oxide (NO), reacts rapidly in the atmosphere and in a complex cycle with light, ozone and hydrocarbons, and produces nitric acid and particulate nitrate. These materials can interact with plants and the soil locally or be transported form the site and interact with atmospheric particulate to form aerosols. These salts and aerosols return to fertilize terrestrial and aquatic systems in wet and dry deposition. A small fraction of this N may be biologically converted to N₂O. About 5% of the total atmospheric greenhouse effect is attributed to N₂O from which 70% of the annual global anthropogenic emissions come from animal and crop production.The coupling of increased population with a move of a large sector of the world population to diets that require more energy and N input, will lead to continued increases in anthropogenic input into the global N cycle. This scenario suggests that emissions of NH₃, NO_x and N₂O from agricultural systems will continue to increase and impact global terrestrial and aquatic systems, even those far removed from agricultural production, to an ever growing extent, unless N resources are used more efficiently or food consumption trends change.     

The interdependence of the reactive species of oxygen, nitrogen, and carbon

This mini-review tries to summarize the main interdependences between the free radicals of oxygen, nitrogen, and carbon. Also, the main metabolic pathways for these radical species are described, as well as how these affect their interaction and functional implications. Emphasis is made on the metabolic disturbances induced by stressing aggressions that produce radical species. In this way, cellular oxidative imbalances created by the superiority of reactive oxygen species over the antioxidant systems produce both activation of nitroxide synthases and the oxidation of terminal nitrogen from l-arginine, as well as the metabolization of heme until carbon monoxide by nitric oxide-activated hemoxygenase. Also, multiple cellular protein and nucleoprotein alterations determined by these three kinds of radical species are completed by the involvement of hydrogen sulfide, which results from the degradation of l-cysteine by cistationine-γ-lyase. In this way, sufficient experimental data tend to demonstrate the involvement of hydrogen sulfide and other thiol derivatives in the interrelations between oxygen, nitrogen, and carbon, which results in a true radical cascade. Thus, oxidative stress, together with nitrosative and carbonilic stress, may constitute a central point where other factors of vulnerability meet, and their interactions could have an important impact in many modern diseases. Considering that the actions of reactive species can be most of the time corrected, future studies need to establish the therapeutical importance of various agents which modulate oxidative, nitrosative, or carbonilic stress.     

The role of the atmosphere in nitrogen cycling

In this work an analysis of the sources, atmospheric concentration, chemical reactions and sinks of the principal atmospheric nitrogen compounds is made. Atmospheric emissions of N₂O and NH₃ are almost entirely due to biological activity on the continents and in the oceans. The combustion of fossil fuels and biomass is the principal source of NO_x. The only relevant chemical transformations in the troposphere are the oxidation of NO_x to NO₃ ^? and the formation of ammonium salts. Only 10% of the NH₃ emitted is oxidized. Washout of NH₄ ⁺ and NO₃ ^? by rainfall is the principal mechanism for removing nitrogen compounds from the atmosphere. Part of the N₂O enters the stratosphere and part must be removed in the biosphere by processes not yet established. NO_x produced in the atmosphere by the burning of fossil fuels and biomass and by lightning represents between 30 and 40% of the total nitrogen fixed. A complete nitrogen balance for the troposphere is presented. Since the photochemical oxidation of NO_x is rapid and atmospheric transport is relatively slow with respect to the cycling of water in the troposphere, nitrogen compounds return to the earth's surface close to where they were emitted. Fixed-nitrogen inputs to the continents and oceans due to biological and industrial fixation are slightly greater than those due to rain water. However, since rain falls everywhere, input from this source is only important on soils not subject to intensive agriculture.     

Mineralization in the Ems-Dollard estuary effects on carbon and oxygen budgets

Summary The Ems estuary is a tidal system, showing typical estuaries characteristics, such as gradients of salinity and suspended matter in the water phase. The tidal amplitude is 2.5–3 m. Tidal flats cover 40% of the total area in the outer part and 75% in the innermost part of the estuary, the Dollard. In the sediment aerobic heterotrophic bacteria are concentrated in the upper 2 cm, the numbers rapidly decreasing with depth.Oxygen production and consumption rates in the sediment, and oxygen consumption in the water are measured, together with environmental parameters and numbers of aerobic heterotrophic bacteria. Using a conversion factor of 12/32, oxygen measurements are translated to organic carbon. Input of organic carbon from external sources (particulate carbon from the river Ems and the North Sea, and organic waste discharge) is calculated for the Dollard. An organic carbon budget for the Dollard, using these calculated figures from one year's measurements, showed that most of the organic carbon entering the systems, originated from external sources.Mineralization was quantitatively more important in the sediment than it was in the water phase. The calculated import and production of organic carbon in the system was larger than the calculated consumption. An explanation for this difference is probably the export of soluble organic carbon, which was not measured, to the adjoining Waddensea, which must be considerable. It was concluded that heterotrophic bacteria must play an important role in carbon fluxes in the Dollard and that studies of growth yield under in situ conditions are necessary for a better understanding of their role in the ecosystem.     

The effects of nitrogen fertilisation and elevated CO2 on the lipid biosynthesis and carbon isotopic discrimination in birch seedlings (Betula pendula)

The effects of nitrogen (N) fertilisation and elevated [CO₂] on lipid biosynthesis and carbon isotope discrimination in birch (Betula pendula Roth.) transplants were evaluated using seedlings grown with and without N fertiliser, and under two concentrations of atmospheric CO₂ (ambient and ambient+250 μmol mol^-1) in solar dome systems. N fertilisation decreased n-fatty acid chain length (18:0/16:0) and the ratios of α-linolenate (18:2)/linoleate (18:1), whereas elevated [CO₂] showed little effect on n-fatty acid chain length, but decreased the unsaturation (18:2+18:1)/18:0. Both N fertilisation and elevated [CO₂] increased the quantity of leaf wax n-alkanes, whilst reducing that of n-alkanols by 20–50%, but had no simple response in fatty acid concentrations. ¹³C enrichment by 1–2.5‰ under N fertilisation was observed, and can be attributed to both reduced leaf conductance and increased photosynthetic consumption of CO₂. Individual n-alkyl lipids of different chain length show consistent pattern of δ¹³C values within each homologue, but are in general 5–8‰ more depleted in ¹³C than the bulk tissues. Niether nitrogen fertilisation and elevated CO₂ influenced the relationship between carbon isotope discrimination of the bulk tissue and the individual lipids. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effect of starvation on oxygen consumption and nitrogen excretion inLepidogalaxias salamandroides (Mees)

Summary Measurement of the rates of oxygen consumption and nitrogen excretion in fed and starvedLepidogalaxias salamandroides showed that starvation has no effect on these basal metabolic parameters. A low metabolic rate (50 ml O₂ kg^–1 h^–1) is suggested to account for the lack of such an effect.A high proportion of the total waste nitrogen is excreted as urea and the magnitude of this fraction is also unaffected by starvation. The importance of urea synthesis is consistent with this species' ability to aestivate.The results are discussed with reference to current concepts in fish aestivation.     

The influence of nitrogen availability on carbon and nitrogen storage in the biennial Cirsium vulgare (Savi) Ten. II. The cost of nitrogen storage

The cost of nitrogen storage to current growth was examined in relation to N availability in the biennial Cirsium vulgare. Plants were grown outdoors, in sand culture, with continuous diel drip irrigation of fertilization medium containing one of five different N concentrations. Plants grown at the highest N concentration stored twice as much N in their tap roots as did plants grown at the lowest N concentration. In high-N-grown plants, the storage of N reserves occurred during the period of maximum growth, at the same time as tap-root production. At the time of maximum biomass, stored N was also at a maximum. During the period following maximum biomass, no additional storage of N occurred. This pattern was observed despite frequent late-season leaf senescence which resulted in a large pool of potentially mobile N which could have been stored at no cost to growth. In low-N-grown plants, the production of tap-root storage tissue and the filling of that tissue with stored N were staggered. Tap-root production and growth occurred during the period of maximum growth, as in the high-N-grown plants. However, filling of the storage tissue with N occurred late in the growing season, when the pool of mobile N from senescent leaves was large. The utilization of this late-season N source occurred with little or no cost to growth, and this N is labelled, according to previous definitions, as ‘accumulated’. The costs of storing N in plants of the different N treatments were calculated using two models based on different growth constraints. In one model, the cost of N storage was represented as lost growth due to allocation of N to storage, rather than to the photosynthetic shoot (i.e. growth was assumed to be limited by carbon acquisition). In the second model, the storage cost was calculated as lost growth due to allocation of N to storage, rather than to the nitrogen-acquiring fine-root system (i.e. growth was assumed to be limited by nitrogen acquisition). In both models, the total cost of N storage was predicted to decrease as N availability decreased due to smaller storage pool sizes in plants of the low-N treatments. The cost of filling the tap root with stored N as a percentage of the total storage cost was also reduced as N availability decreased due to the occurrence of late-season accumulation. By relying, at least in part, on late-season accumulation, plants grown at the lowest three levels of N availability reduced total storage costs by 15 to 22%. The results demonstrate that plants are capable of adjusting their storage patterns in response to low nitrogen availability such that the costs of storage are reduced.     

Modeling the effects of fire and climate change on carbon and nitrogen storage in lodgepole pine (Pinus contorta) stands

The interaction between disturbance and climate change and resultant effects on ecosystem carbon (C) and nitrogen (N) fluxes are poorly understood. Here, we model (using CENTURY version 4.5) how climate change may affect C and N fluxes among mature and regenerating lodgepole pine ( Pinus contorta var. latifolia Engelm. ex S. Wats.) stands that vary in postfire tree density following stand-replacing fire. Both young (postfire) and mature stands had elevated forest production and net N mineralization under future climate scenarios relative to current climate. Forest production increased 25% [Hadley (HAD)] to 36% [Canadian Climate Center (CCC)], compared with 2% under current climate, among stands that varied in stand age and postfire density. Net N mineralization increased under both climate scenarios, e.g., +19% to 37% (HAD) and +11% to 23% (CCC), with greatest increases for young stands with sparse tree regeneration. By 2100, total ecosystem carbon (live+dead+soils) in mature stands was higher than prefire levels, e.g., +16% to 19% (HAD) and +24% to 28% (CCC). For stands regenerating following fire in 1988, total C storage was 0–9% higher under the CCC climate model, but 5–6% lower under the HAD model and 20–37% lower under the Control. These patterns, which reflect variation in stand age, postfire tree density, and climate model, suggest that although there were strong positive responses of lodgepole pine productivity to future changes in climate, C flux over the next century will reflect complex relationships between climate, age structure, and disturbance-recovery patterns of the landscape.     

Wildfire effects on carbon and nitrogen in inland coniferous forests

A ponderosa pine/Douglas-fir forest (Pinus ponderosa Dougl., Pseudotsuga menziesii (Mirb.) Franco; PP/DF) and a lodgepole pine/Engelmann spruce forest (Pinus contorta Loud., Picea engelmannii Parry ex Engelm.; LP/ES) located on the eastern slopes of the Cascade Mountains in Washington state, USA, were examined following severe wildfire to compare total soil carbon and nitrogen capitals with unburned (control) forests. One year after fire, the average C content (60 cm depth) of PP/DF and LP/ES soil was 30% (25 Mg ha^-1) and 10% (7 Mg ha^-1) lower than control soil. Average N content on the burned PP/DF and LP/ES plots was 46% (3.0 Mg ha^-1) and 13% (0.4 Mg ha^-1) lower than control soil. The reduction in C and N in the PP/DF soil was largely the result of lower nutrient capitals in the burned Bw horizons (12–60 cm depth) relative to control plots. It is unlikely that the 1994 fire substantially affected nutrient capitals in the Bw horizons; however, natural variability or past fire history could be responsible for the varied nutrient capitals observed in the subsurface soils. Surface erosion (sheet plus rill) removed between 15 and 18 Mg ha^-1 of soil from the burned plots. Nutrient losses through surface erosion were 280 kg C ha^-1 and 14 kg N ha^-1 in the PP/DF, whereas LP/ES losses were 640 and 22 kg ha^-1 for C and N, respectively. In both forests, surface erosion of C and N was _∼1% to 2% of the A-horizon capital of these elements in unburned soil. A bioassay (with lettuce as an indicator plant) was used to compare soils from low-, moderate- and high-severity burn areas relative to control soil. In both forests, low-severity fire increased lettuce yield by 70–100% of controls. With more severe fire, yield decreased in the LP/ES relative to the low-intensity burn soil; however, only in the high-severity treatment was yield reduced (14%) from the control. Moderate- and high-severity burn areas in the PP/DF were fertilized with _∼56 kg ha^-1 of N four months prior to soil sampling. In these soils, yield was 70–80% greater than the control. These results suggest that short-term site productivity can be stimulated by low-severity fire, but unaffected or reduced by more severe fire in the types of forests studied. Post-fire fertilization with N could increase soil productivity where other environmental factors do not limit growth. This revised version was published online in June 2006 with corrections to the Cover Date.     

Growth/survival of Salmonella enteritidis on fresh poultry and fish stored under vacuum or modified atmosphere

G.-J.E. NYCHAS AND C.C. TASSOU. 1996. The effect of vacuum and modified atomosphere packaging on the growth/survival of Samonella entertidis on fresh poultry and fish (Boops boos) is described. Salmonella enteritidis survived but did not grow significantly in all samples (poultry or fish) at 3^oC. At 10^oC the numbers of Salm. enteritidis increased rapidly in vacuum-packed samples and in samples flushed with 100% N₂, 20%, CO²/80% O² of both types of proteinaceous food. Growth was also evident in fish and poultry flushed with 100% CO²; however the rate of growth was greater in fish samples rather than in poultry.     

Effect of organic carbon flux and dissolved oxygen on the benthic foraminiferal oxygen index (BFOI)

Variations in oceanic primary productivity, flux of organic carbon to the sediments, and dissolved-oxygen levels in the water column are thought to be important in the control of benthic foraminiferal test size, wall thickness, morphology, and species composition of assemblages by many foraminiferal paleontologists. Aspects of these processes should be reflected by the benthic foraminiferal oxygen index (BFOI) based on these foraminiferal characteristics. However, analyses indicate that the BFOI correlates most strongly with dissolved-oxygen levels in overlying water (R²=0.81), weakly with oceanic primary productivity (R²=0.55), and weakly with organic carbon flux to the sediments (R²=0.51). Although both dissolved oxygen and organic carbon flux are main controlling factors for benthic foraminiferal assemblages, the BFOI is a useful indicator extracted from benthic foraminiferal assemblages for estimating the condition of dissolved oxygen in Cretaceous and Cenozoic oceans.     

Incubation study on effects of adding varying levels of nickel (as sulphate) on nitrogen and carbon mineralisation in soil

The effects were studied of adding 10, 100 and 1000 ppm Ni (as sulphate) to a sandy soil (pH 5·9) on N and C mineralisation during subsequent aerobic incubation for 6 weeks at 30° C. With increasing Ni level, nitrification decreased to a greater extent than did N and C mineralisation. The toxic effects of 1000 ppm Ni were not much greater than those of 100 ppm Ni. Nitrification and N and C mineralisation were decreased by 68, 36 and 35% respectively with 1000 ppm Ni. Results are discussed in relation to exchangeable and EDTA-extractable Ni levels.