Chlorine transport in a small catchment in southeast Sweden during two years

Previous studies have revealed that chlorine participates in a complex biogeochemical cycle in soil, which suggests that the transport of chloride through catchments may also be influenced. The present study is based on field observations of organic carbon, chloride (Cl_in), and chlorinated organic carbon (Cl_org) in precipitation, soil, and runoff over a 2-year period from a small, forested catchment in southeast Sweden. The study reveals that (1) the soil pool is dominated by Cl_org, (2) the input via wet deposition and output of Cl_in via runoff is 30 times smaller than the total storage of chlorine (Cl_in + Cl_org) in soil, and (3) the transport is dominated by Cl_in. The organic matter that entered the outlet of the catchment was more chlorinated in the autumn than during the rest of the year, and rain events taking place in low-flow periods had a greater influence on TOC, Cl_org, and Cl_in than did rain events taking place in high-flow periods. The seasonal pattern in combination with the low-flow versus high-flow pattern and previous findings of increasing chlorine-to-carbon ratios with soil depth suggests that the chlorine-to-carbon ratio variation in the leached organic matter is due that water preferentially comes from deeper layers in low-flow conditions. This study provides well-founded estimates of Cl_org and Cl_in storage and fluxes for the studied catchment; however, the processes underlying the observed seasonal Cl_org variations and transportation processes need further study.     

Effect of deep and shallow root systems on the dynamics of soil inorganic N during 3-year crop rotations

Unused inorganic nitrogen (N_inorg) left in agricultural soils will typically leach to deeper soil layers. If it moves below the root zone it will be lost from the system, but the depth of the root zone depends on the crop species grown. In this experiment we studied the effect of 3-year crop sequences, with different combinations of deep-rooted and shallow-rooted crops, on soil N_inorg dynamics to 2.5 m soil depth and the possibility of crop utilization of N leached to deep soil layers. We grew ten different crop sequences for 3 years. The crops and catch crops grown were selected to allow different sequences of deep-rooted and shallow-rooted crops. Very different rooting depths were obtained, from only 0.5 m (leek), to ∼1.0 m (ryegrass and barley), 1.5 m (red beet), 2.0 m (fodder radish and white cabbage) and more than 2.5 m by the chicory catch crop. The results showed a significant retention of N_inorg within the 2.5 m soil profile from one year to the next, but the retained N had leached to deeper parts of the profile during the winter season. Only little N_inorg was retained over two winter seasons. The retention in the deeper soil layers allowed N_inorg to be taken up by succeeding deep-rooted main crops or catch crops. The effects of crop rooting depth on N_inorg in the subsoil layers from 1.0 to 2.5 m were striking. White cabbage reduced N_inorg below 1.0 m with up to 113 kg N ha^-1 during its growth. Grown after catch crops, leek and red beet left on average 60 kg N ha⁻¹ less below 1.0 m than leek and red beet grown without a preceding catch crop. We conclude that it is possible to design crop rotations with improved nitrogen use efficiency by using the differences in crop rooting patterns; deep-rooted crops or catch crops can be used to recover N_inorg leached after previous crops, and catch crops can be grown before shallow-rooted crops to lift the deep N_inorg up to layers where these crops have their roots.     

Chloroform in runoff water—a two-year study in a small catchment in Southeast Sweden

Chloroform concentrations were observed and input and output fluxes estimated over a 2-yr period in a small coniferous catchment (0.22 km²) in southeast Sweden. Water discharge was measured daily, and runoff water was sampled bi-weekly for chloroform analysis. An approximate chloroform budget was calculated, which indicated that the annual output of 6 μg m⁻² yr⁻¹ was approximately six times higher than the input, inferring an internal source of chloroform in the catchment. To the best of our knowledge, neither flux estimates nor mass balances have previously been made for chloroform on a catchment scale, nor have data regarding natural runoff variation with time been gathered. Concentrations of chloroform in runoff were found to be generally high during wet periods, such as spring, but also peaked during summer rain events. The observed pattern suggests that chloroform is formed in surface soil layers and transported to the outlet under high-flow conditions and during dry-period rain events; it is lost through degradation or evaporation during drier periods due to longer soil water residence times. The data suggest that the variation among replicates increases with concentration; this emphasizes the need to know what the degree of on-site variation is, so one can collect a sufficient number of replicates to permit detection of spatial or temporal changes.     

A mass-balance study on chloride fluxes in a large central European catchment during 1900–2010

Using data on long-term monitoring of water quality, mass budgets, and empirical models, we quantified chloride (Cl) leaching from major diffuse and point sources in a large central European catchment (upper Vltava river, Czech Republic) over a 110-year period (1900–2010), with the major aim to evaluate the influence of historical changes in land use and management practices on Cl leaching from agricultural land. The Cl input to farmland in synthetic fertilizers, livestock feed, and atmospheric deposition tripled in the 1950s–1980s (from 23 to 64 kg ha^?1 year^?1 on average), and then abruptly decreased to ~14 kg ha^?1 year^?1 during 1990–2010. The proportion of drained agricultural land rapidly increased from 4 % in the 1950s to its maximum of 43 % in the 1990s. Until the 1950s, the Cl leaching from agricultural land followed a simple dose–response function. Then, agricultural soils retained on average 16 ± 4 kg ha^?1 year^?1 of Cl during 1959–1985, when the most important changes in land use and management practices occurred, and subsequently became a net Cl source of 11 ± 3 kg ha^?1 year^?1 on average during 1986–2010, when Cl input to soils declined and drainage of new land ceased. Our data suggest that the temporal changes in the Cl storage in agricultural land are associated with changes in Cl concentrations in both permanent soil water and soils. Physico-chemical conditions in freshly drained soils, namely elevated aeration and high concentrations of soil organic matter (SOM), and high Cl inputs probably resulted in a Cl immobilization in soils by formation of organic chlorine (Cl_org) and adsorption that was higher than the Cl production from Cl_org mineralization and desorption. In contrast, Cl_org mineralization and Cl desorption exceeded the Cl retention during the consecutive period of low Cl inputs and decreasing SOM concentrations in agricultural soils. Our study implies that changes in land use and agricultural management can significantly affect dose–response functions even for Cl, which is traditionally considered and modelled as a conservative ion.     

Distributed GIS for estimation of soil carbon stock of West Siberia boreal zone

Distributed GIS developed to assess the soil carbon stocks (SOC) includes digital maps “Vegetation” and “Soil” of boreal zone of West Siberia, middle-scale soil maps of the key sites of landscape provinces that have been created on the basis of interpretation of satellite images, and the software for updating and management of the database “Carbon in soils of Siberia”. According to preliminary estimates the resources of C_org in the soil of the boreal zone is 83.6 × 10⁸ t, which is 28.2% of the resources of C_org in soils of Russia or 15.2% of global carbon pool.     

Dynamics of mercury and methylmercury in forest floor and runoff of a forested watershed in Central Europe

Forested watersheds are an important part of the terrestrialmercury and methylmercury cycle, and a link between theatmospheric and aquatic environment. This study was conducted todetermine the contribution of the forest floor to the pools andfluxes of total Hg (Hg_total) and methylmercury (MeHg) in aforested catchment, and to identify factors influencing themobility of both compounds. Throughfall deposition, litterfall,runoff and fluxes with forest floor percolate of Hg_total and MeHgwere sampled during one year in a coniferous catchment inGermany. Total deposition of Hg_total was 552 mg ha_–1 a_–1 withlitterfall contributing one third. Nearly 60% of the total inputof Hg_total reached the mineral soil with the forest floorpercolate, but less than half of this fraction was found in therunoff of the catchment. Total deposition of MeHg was 2.6 mg ha_–1a_–1, with litterfall as the dominating pathway. Only 19% of theMeHg deposition was discharged from the forest floor, but theflux of MeHg with runoff was nearly twice as high. Only fewcorrelations with other solution parameters were found. Fluxes ofboth compounds with forest floor percolates depended mainly onwater fluxes, which was not true for the runoff. The forest floorof the upland soil is an effective sink for MeHg, but not forHg_total. Differences in the mobility of both compounds in theforest floor disappeared at the catchment scale, probably becauseother processes (i.e. Hg_total immobilization and MeHg formation)dominated.     

Uptake and efflux of inorganic carbon in Dunaliella salina

The apparent photosynthetic K_m (CO₂) of air-grown Dunaliella salina is 2 M as measured both by the filtering centrifugation technique and by O₂ electrode. These cells are capable of accumulating inorganic carbon (C_inorg) up to 20 times its concentration in the medium. It is suggested that air-grown Dunaliella cells are able to concentrate CO₂ within the cell. Analysis of the efflux of C_inorg from cells previously loaded with H¹⁴CO ₃ ^- demonstrated the existence of an internal pool which has an half-time of depletion of 2.5–7 min depending on the conditions of the experiment. This finding indicates that the internal C_inorg pool is not readily exchangeable with the external medium. Furthermore, the influence of the presence or absence of unlabelled C_inorg in the medium during the efflux experiment on the half-time observed indicate that efflux of C_inorg is not a simple diffusion process but is rather carrier-mediated.Abbreviation C_inorg inorganic carbon     

Chloride in Soils and its Uptake and Movement within the Plant: A Review

Natural inputs of chlorine (Cl) to soils come mainly from rainwater,sea spray, dust and air pollution. In addition, human practices,such as irrigation and fertilization, contribute significantlyto Cl deposition. In the soil solution, Cl occurs predominantlyas the chloride anion (Cl^-). The Cl^-anion does not form complexesreadily, and shows little affinity (or specificity) in its adsorptionto soil components. Thus, Cl^-movement within the soil is largelydetermined by water flows. Chlorine is an essential micronutrientfor higher plants. It is present mainly as Cl^-. Chloride isa major osmotically active solute in the vacuole and is involvedin both turgor- and osmoregulation. In the cytoplasm it mayregulate the activities of key enzymes. In addition, Cl^-alsoacts as a counter anion, and Cl^-fluxes are implicated in thestabilization of membrane potential, regulation of intracellularpH gradients and electrical excitability. Chloride enters plantsthrough the roots, and there is some concern over the uptakeof the long-lived radionuclide³⁶Cl, which enters into the foodchain through plants. Chloride is thought to traverse the rootby a symplastic pathway, and Cl^-fluxes across the plasma membraneand tonoplast of root cells have been estimated. These fluxesare regulated by the Cl^-content of the root. Chloride is mobilewithin the plant. The Cl^-concentrations of xylem and phloemsaps have been determined and Cl^-fluxes through the xylem andphloem have been modelled. Measurements of transmembrane voltagesand Cl^-activities in cellular compartments suggest (1) thatactive Cl^-transport across the plasma membrane dominates Cl^-influxto root cells at low Cl^-concentrations in the soil solutionand that passive Cl^-influx to root cells occurs under more salineconditions, and (2) that both active and passive Cl^-transportoccurs at the tonoplast. Electrophysiological studies have demonstratedthe presence of an electrogenic Cl^-/2H⁺symporter in the plasmamembrane of root-hair cells and Cl^-channels mediating eitherCl^-influx or Cl^-efflux across the plasma membrane. Similarly,there is both biochemical and electrophysiological evidencethat Cl^-channels mediate Cl^-fluxes in either direction acrossthe tonoplast and that a Cl^-/nH⁺antiport mediates Cl^-influxto the vacuole. This article reviews the availability of Cl^-inthe soil, the roles and distribution of Cl^-within the plant,the magnitude of Cl^-fluxes across membranes and between tissues,the mechanisms of Cl^-transport across membranes and the electricalcharacteristics and molecular biology of Cl^-channels. Copyright2001 Annals of Botany Company Review, Arabidopsis thaliana, channel, chloride (Cl^-), influx, phloem, plasma membrane, radiochlorine (³⁶Cl), soil, tonoplast, transport, uptake, xylem     

Low-pressure glow discharge in a Xenon/Chlorine mixture

The spatial, electrical, and optical characteristics of a transverse glow discharge and a volume discharge with a spherical anode and plane cathode in low-pressure Xe/Cl₂ mixtures are studied. It is shown that the transverse glow discharge in mixtures with a low chlorine content occupies most of the interelectrode gap and exists in the form of strata. As the total pressure (P≥300 Pa) and the partial chlorine pressure (P(Cl₂)≥80 Pa) increase, a solitary plasma domain with a volume of 1–2 cm³ forms in the discharge gap. It acts as a selective source of UV radiation in the XeCl(D-X) 236-nm, Cl₂ (D′-A′) 257-nm, and XeCl(B-X) 308-nm bands. In certain Xe/Cl₂ mixtures, plasma self-oscillations in the frequency range 1–100 kHz are observed. The current of a low-pressure volume discharge with a spherical anode and plane cathode and the emission from it have both a dc and an ac component. The pressure and composition of the working mixture, as well as the average current of the volume discharge are optimized to attain the maximum emission intensity of the XeCl(D,B-X) bands. Low-pressure volume discharges in xenon/chlorine mixtures can be used as active media in low-pressure large-aperture planar or cylindrical excimer-halogen lamps emitting modulated or repetitive pulsed UV radiation.     

Chlorine decay and bacterial inactivation kinetics in drinking water in the tropics

The decay of free chlorine (Cl₂) and combined chlorine (mostly monochloramine: NH₂Cl) and the inactivation of bacteria was examined in Dar es Salaam, Tanzania. Batch experiments, pilot-scale pipe experiments and full-scale pipe experiments were carried out to establish the kinetics for both decay and inactivation, and to compare the two disinfectants for use under tropical conditions. The decay of both disinfectants closely followed first order kinetics, with respect to the concentration of both disinfectant and disinfectant-consuming substances. Bacterial densities exhibited a kinetic pattern consisting of first order inactivation with respect to the density of the bacteria and the concentration of the disinfectant, and first order growth with respect to the bacterial density. The disinfection kinetic model takes the decaying concentration of the disinfectant into account. The decay rate constant for free chlorine was 114 lg^-1h^-1, while the decay rate constant for combined chlorine was 1.84 lg^-1h^-1 (1.6% of the decay rate for free chlorine). The average concentration of disinfectant consuming substances in the water phase was 2.6 mg Cl₂/l for free chlorine and 5.6 mg NH₂Cl/l for combined chlorine. The decay rate constant and the concentration of disinfectant consuming substances when water was pumped through pipes, depended on whether or not chlorination was continuous. Combined chlorine especially could clean the pipes of disinfectant consuming substances. The inactivation rate constant , was estimated at 3.06×10⁴ lg^-1h^-1. Based on the inactivation rate constant, and a growth rate constant determined in a previous study, the critical concentration of free chlorine was found to be 0.08 mg Cl₂/l. The critical concentration is a value below which growth rates dominate over inactivation.The authors are with the Technical University of Denmark, IMT, CDC, Build. 208, DK-2800 Lyngby, Denmark     

Nitrogen Cycling and Mass Balance for a Forested Catchment in the Italian Alps. Assessment of Nitrogen Status

During 1999–2001 the chemical composition and fluxes were measured in rainfall, throughfall, soil solution and stream water in a remote forested site in the Italian Alps. The analysis of temporal patterns revealed the differential behaviour of nitrogen and sulphur and suggested that different mechanisms controlled their flux. No important changes in sulphate concentration and fluxes emerged as the solution passed through the various components of the forest ecosystem, and temporal variations of SO₄ in the soil solution and stream were likely driven by the physical process of dilution. The availability of nitrate and ammonia, by contrast, was drastically reduced as throughfall water entered the soil and passed through the mineral layers, irrespective of season. The calculated hydrochemical budget based on throughfall and soil solution N fluxes revealed that ~80% N retention in the forest soil, corresponding to 12 kg ha⁻¹ yr⁻¹, despite a relatively high N deposition loading (15 kg ha⁻¹ yr⁻¹). Most of the leached nitrogen (90%) was in the organic form. Indicators of the N status of this ecosystem, such as C/N ratio in solid and solution phase of the soil and N foliage content as well as land use history were examined. Despite the strong N retention in the forested part of the catchment, the stream water N–NO₃ levels were consistently above 10 μg l⁻¹ suggesting that the Val Masino catchment as a whole was less efficient in processing atmospheric N inputs. This contrasting N behaviour illustrates the role of landscape features, such as the soil cover and vegetation type, that is characteristic of an alpine catchment.     

DGT-measured fluxes explain the chloride-enhanced cadmium uptake by plants at low but not at high Cd supply

The technique of diffusive gradients in thin films (DGT) has been shown to be a promising tool to assess metal uptake by plants in a wide range of soils. With the DGT technique, diffusion fluxes of trace metals through a diffusion layer towards a resin layer are measured. The DGT technique therefore mimics the metal uptake by plants if uptake is limited by diffusion of the free ion to the plant roots, which may not be the case at high metal supply. This study addresses the capability of DGT to predict cadmium (Cd) uptake by plants at varying Cd supply. To test the performance of DGT in such conditions, we used the chloride (Cl^?) enhancement effect, i.e. the increase in Cd solution concentrations—due to chloride complexation of Cd—and Cd uptake with increasing Cl^? concentrations, as previously characterized in pot, field and solution culture experiments. The uptake of Cd by spinach was assessed in soil amended with Cd (0.4–10.5 mg Cd kg^?1) and NaCl (up to 120 mM) in a factorial design. Treatments with NaNO₃ were included as a reference to correct for ionic strengths effects. The effect of Cl^? on the shoot Cd concentrations was significant at background Cd but diminished with increasing soil Cd. Increasing Cl^? concentrations increased the root area based Cd uptake fluxes by more than a factor of 5 at low soil Cd, but had no significant effect at high soil Cd. Short-term uptake of Cd in spinach from nutrient solutions confirmed these trends. In contrast, increasing Cl^? concentrations increased the DGT measured fluxes by a factor of 5 at all Cd levels. As a result, DGT fluxes were able to explain soil Cl^? effects on plant Cd concentrations at low but not at high Cd supply. This example illustrates under which conditions DGT mimics trace metal bioavailability. If biouptake is controlled by diffusive limitations, DGT should be a successful tool for predicting ion uptake across different conditions.     

Forest vegetation affecting the deposition of atmospheric elements to soils

Atmospheric inputs of elements/ions into the soil through bulk precipitation and throughfall (precipitation below tree canopies) were monitored monthly at two forested catchments (Lesni Potok and Liz) in central and southwestern Bohemia, respectively. The annual deposition fluxes (expressed in μg/mg m^?2 yr^?1) of Al, As, Ba, Be, Ca, Cd, Cl^?, F^?, Fe, K, Mg, Mn, N_tot, Na, Ni, Pb, Rb, SO ₄ ^2? , Sr and Zn between 1997 and 2005 were calculated from their concentrations in monthly collected samples of both precipitation types. The flux of H⁺ was calculated from the monthly pH values as well. The more pristine character of the Liz catchment was manifested in lower inputs of anions of strong inorganic acids (mostly of anthropogenic origin) and of H⁺ in spite of higher precipitation amounts at the site. The comparison of fluxes in bulk precipitation (BP) and throughfall (TH) has shown significantly higher values for Rb, K, Mg, Mn, F^?, Ca, SO ₄ ^2? , Sr, Ba and Cl^? in the latter flux. It is declared that high fluxes of these elements/ions in TH significantly affect the forest soil water chemistry and that the forest vegetation significantly contributes to the mobilization of several elements in soil and to their redistribution throughout the soil profile.     

High temporal resolution of ion fluxes in semi-natural ecosystems – gain of information or waste of resources?

Monitoring programs of ion concentrations and fluxes in semi-natural ecosystems are confronted with the task to gain as much information as possible with simultaneously minimizing costs and efforts. The aim of this study was (i) to assess how much of the heterogeneity of solution concentrations is lost because of temporal integration of measurements and (ii) to estimate the error in ion fluxes due to temporal integration. High resolution measurements (daily interval) of ion concentrations (sulfate, nitrate, chloride, pH and EC) in throughfall, soil solutions and runoff at the catchment Lehstenbach (Fichtelgebirge, Northeast Bavaria, Germany) were compared over a two year period with the reference monitoring program (biweekly measurement interval). Evaluation of the maximum temporal heterogeneity of ion concentrations in throughfall, soil solution and runoff (expressed as minimum, maximum, median and 25–75% percentile) did not result in an overall higher heterogeneity of the high resolution measurements compared to the reference program. The calculation of runoff fluxes from the reference data (biweekly concentration) resulted in significant errors of up to 25% for time periods < 1 year (high resolution data was considered the "true" value and set as 100%). However, errors became minor (< 10%) if longer time periods were considered. The suitability of different interpolation methods to up-scale biweekly concentration data for the calculation of runoff fluxes was evaluated in this study. We concluded for the monitoring programs at the Lehstenbach catchment that a biweekly measurement interval seemed to be suitable to capture the heterogeneity of ion concentrations and fluxes (and thus temporal trends). In comparison, high resolution measurements with a daily measurement interval were higher in cost, work and time resources and had a relatively low information gain. While the introduced methods are applicable in all monitoring programs, conclusions on temporal resolution of measurements are most likely not valid for systems where ion concentrations have a low autocorrelation length (e.g., agricultural or urban systems with nitrate or pesticide treatment; tropical systems with extreme temperature or hydrological events).     

Biogeochemistry of trimethyllead and lead in a forested ecosystem in Germany

Lead compounds, especially ionic organolead compounds (OLC), are highly toxic and mobile pollutants strongly affecting many ecosystems. Soil pools and fluxes with precipitation, litterfall and runoff of trimethyllead (TML), one of the dominant ionic OLC in the environment, and Pb_total were investigated in a forested ecosystem in NE-Bavaria, Germany. In addition, ad/desorption of TML to soils was studied in batch experiments and its degradation in soils was investigated using long term incubations. Total soil storage in the catchment was 11.56?mg Pb?ha^?1 for TML and 222?kg Pb?ha^?1 for Pb_total. More than 90% of the soil storage of TML was found in the wetland soils of the catchment representing only 30% of the area. Most Pb_total (>90%) was found in the upland soils. In upland soils, TML was only detectable in the forest floor. The annual total deposition from the atmosphere, estimated as throughfall?+?litterfall fluxes, amounted to 3.7?mg Pb?ha^?1 year^?1 for TML and 52?g Pb?ha^?1 year^?1 for Pb_total. The contribution of litterfall was 1.5 and 32%, respectively. The concentrations of TML and Pb_total in wet precipitation were: fog?>?throughfall?>?bulk precipitation. The annual fluxes with runoff from the catchment was 0.5?mg Pb?ha^?1 year^?1 for TML and 2.8?g Pb?ha^?1 year^?1 for Pb_total. TML degraded rapidly in the forest floor (Oa horizon) with a half-life (t) of 33.5 days. The degradation of TML in Fen (t?=?421 days) and in the mineral soil (Bw-C horizon, t?=?612 days) was much slower. Emission of tetramethyllead from wetland soils was not observed during the 1 year incubation. The adsorption affinity of TML to different soils was Fen?>?Oa?>?A?≥?Bw-C. The ratio of total soil storages to the present annual input were 3.6 years for TML. TML and Pb_total are still deposited in remote areas even after the use of tetraalkyllead as additives has been terminated for years. The rates of deposition are, however, much lower than in the past. Forest soils act as a sink for deposited TML and Pb_total. TML is accumulated mostly in wetland soils and seems to be stable under anoxic conditions for a long time. In upland soils, TML decomposes rapidly. Only small amounts of TML are transferred from soils into runoff.     

The Behavior of Organic Phosphorus under Non-Point Source Wastewater in the Presence of Phototrophic Periphyton

To understand the role of ubiquitous phototrophic periphyton in aquatic ecosystem on the biogeochemical cycling of organic phosphorus, the conversion and removal kinetic characteristics of organic phosphorus (Porg) such as adenosine triphosphate (ATP) were investigated in the presence of the periphyton cultured in artificial non-point source wastewater. The preliminary results showed that the periphyton was very powerful in converting P_org evidenced by the fact that inorganic phosphorus (P_inorg) content in solution increased from about 0.7 to 14.3 mg P L⁻¹ in 48 hours in the presence of 0.6 g L⁻¹ periphyton. This was because the periphyton could produce abundant phosphatases that benefited the conversion of P_org to P_inrog. Moreover, this conversion process was described more suitable by the pseudo-first-order kinetic model. The periphyton was also effective in removing P_org, which showed that the P_org can be completely removed even when the initial P_org concentration was as high as 13 mg P L⁻¹ in 48 hours in the presence of 1.6 g L⁻¹ periphyton. Furthermore, it was found that biosorption dominated the P_org removal process and exhibited the characteristics of physical adsorption. However, this biosorption process by the periphyton was significantly influenced by biomass (absorbent dosage) and temperature. This work provides insights into P_org biogeochemical circulation of aquatic ecosystem that contained the periphyton or similar microbial aggregates.     

Chloride Transport in Porous Lipid Bilayer Membranes

This paper describes dissipative Cl_- transport in "porous" lipid bilayer membranes, i.e., cholesterol-containing membranes exposed to 1–3 x 10^-7 M amphotericin B. P_DCl (cm·s^-1), the diffusional permeability coefficient for Cl^-, estimated from unidirectional ³⁶Cl^- fluxes at zero volume flow, varied linearly with the membrane conductance (Gm, Ω^-1·cm^-2) when the contributions of unstirred layers to the resistance to tracer diffusion were relatively small with respect to the membranes; in 0.05 M NaCl, P_DCl was 1.36 x 10^-4 cm·s^-1 when Gm was 0.02 Ω^-1·cm^-2. Net chloride fluxes were measured either in the presence of imposed concentration gradients or electrical potential differences. Under both sets of conditions: the values of P_DCl computed from zero volume flow experiments described net chloride fluxes; the net chloride fluxes accounted for ~90–95% of the membrane current density; and, the chloride flux ratio conformed to the Ussing independence relationship. Thus, it is likely that Cl^- traversed aqueous pores in these anion-permselective membranes via a simple diffusion process. The zero current membrane potentials measured when the aqueous phases contained asymmetrical NaCl solutions could be expressed in terms of the Goldman-Hodgkin-Katz constant field equation, assuming that the P_DNa/P_DCl ratio was 0.05. In symmetrical salt solutions, the current-voltage properties of these membranes were linear; in asymmetrical NaCl solutions, the membranes exhibited electrical rectification consistent with constant-field theory. It seems likely that the space charge density in these porous membranes is sufficiently low that the potential gradient within the membranes is approximately linear; and, that the pores are not electrically neutral, presumably because the Debye length within the membrane phase approximates the membrane thickness.     

Hydrological and biological processes modulate carbon,nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec,Canada)

Changes in atmospheric deposition, stream water chemistry, and solute fluxes were assessed across 15 small forested catchments. Dramatic changes in atmospheric deposition have occurred over the last three decades, including a 70% reduction in sulphur (S) deposition. These changes in atmospheric inputs have been associated with expected changes in levels of acidity, sulphate and base cations in streams. Soil retention of S appeared to partially explain rates of chemical recovery. In addition to these changes in acid–base chemistry we also observed unexpected changes in nitrogen (N) biogeochemistry and nutrient stoichiometry of stream water, including decreased stream N concentrations. Among all catchments the average flux of dissolved inorganic nitrogen (DIN) was best predicted by average runoff, soil chemistry (forest floor C/N) and levels of acid deposition (both S and N). The rate of change in stream DIN flux, however, was much more closely correlated with reductions in rates of S deposition rather than those of DIN. Unlike DIN fluxes, the average concentrations as well as the rates of decline in streamwater nitrate (NO₃) concentration over time were tightly linked to stream dissolved organic carbon/dissolved organic nitrogen ratios DOC/DON and DON/TP rather than catchment characteristics. Declines in phosphorus adsorption with increasing soil pH appear to contribute to the relationship between C, N, and P in our study catchments. Our observations suggest that catchment P availability and its alteration due to environmental changes (e.g. acidification) might have profound effects on N cycling and catchment N retention that have been largely unrecognized.     

The natural chlorine cycle – fitting the scattered pieces

Chlorine is one of the most abundant elements on the surface of the earth. Until recently, it was widely believed that all chlorinated organic compounds were xenobiotic, that chlorine does not participate in biological processes and that it is present in the environment only as chloride. However, over the years, research has revealed that chlorine takes part in a complex biogeochemical cycle, that it is one of the major elements of soil organic matter and that the amount of naturally formed organic chlorine present in the environment can be counted in tons per km(2). Interestingly enough, some of the pieces of the chlorine puzzle have actually been known for decades, but the information has been scattered among a number of different disciplines with little or no exchange of information. The lack of communication appears to be due to the fact that the points of departure in the various fields have not corresponded; a number of paradoxes are actually revealed when the known pieces of the chlorine puzzle are fit together. It appears as if a number of generally agreed statements or tacit understandings have guided perceptions, and that these have obstructed the understanding of the chlorine-cycle as a whole. The present review enlightens four paradoxes that spring up when some persistent tacit understandings are viewed in the light of recent work as well as earlier findings in other areas. The paradoxes illuminated in this paper are that it is generally agreed that: (1) chlorinated organic compounds are xenobiotic even though more than 1,000 naturally produced chlorinated compounds have been identified; (2) only a few, rather specialised, organisms are able to convert chloride to organic chlorine even though it appears as if the ability among organisms to transform chloride to organic chlorine is more the rule than the exception; (3) all chlorinated organic compounds are persistent and toxic even though the vast majority of naturally produced organic chlorine is neither persistent nor toxic; (4) chlorine is mainly found in its ionic form in the environment even though organic chlorine is as abundant or even more abundant than chloride in soil. Furthermore, the contours of the terrestrial chlorine cycle are outlined and put in a concrete form by constructing a rough chlorine budget over a small forested catchment. Finally, possible ecological roles of the turnover of chlorine are discussed.     

The Fractionation of Chlorine Isotopes by the Aerobic Methylotrophic Bacterium Methylobacterium dichloromethanicum Grown on Dichloromethane

Methylobacterium dichloromethanicum was found to be able to utilize dichloromethane (DCM) as the source of carbon and energy with the production of biomass, CO₂, and HCl. A comparative analysis of the abundances of the major DCM isotopomers ³⁵Cl₂ ¹²C¹H₂, ³⁵Cl³⁷Cl¹²C¹H₂, and ³⁷Cl₂ ¹²CH₂1H₂ made it possible to estimate the fractionation of chlorine isotopes during the bacterial metabolism of DCM. The kinetic chlorine isotope effects for ³⁵Cl³⁷Cl¹²C¹H₂ (m/z 86) and ³⁷Cl₂ ¹²C¹H₂ (m/z 88) relative to ³⁵Cl₂ ¹²C¹H₂ (m/z 84) were characterized by _86/84 = 1.006 ± 0.002 and _88/84 = 1.023 ± 0.003, respectively. The inference is made that the growth of M. dichloromethanicum on DCM is accompanied by the mass-independent fractionation of the DCM isotopomers.