The dynamic transfer of <Superscript>3</Superscript>H and <Superscript>14</Superscript>C in mammals: a proposed generic model

Associated with the present debate regarding the potential revival of nuclear energy there is an increased interest in assessing the radiological risk to the public and also the environment. Tritium and ¹⁴C are key radionuclides of interest in many circumstances (e.g. heavy water reactors, waste storage and fusion reactors). Because the stable analogues of these two radionuclides are integral to most biological compounds, their modelling should follow general principles from life sciences. In this paper, a model of the dynamics of ¹⁴C and ³H in mammals is proposed on the basis of metabolic understanding and of, as far as possible, readily available data (e.g. for organ composition and metabolism). The model is described together with validation tests (without calibration) for a range of farm animals. Despite simplifications, the model tests are encouraging for a range of animal types and products (tissues and milk), and further improvements are suggested.     

The risk of leukaemia in young children from exposure to tritium and carbon-14 in the discharges of German nuclear power stations and in the fallout from atmospheric nuclear weapons testing

Towards the end of 2007, the results were published from a case–control study (the “KiKK Study”) of cancer in young children, diagnosed <5 years of age during 1980–2003 while resident near nuclear power stations in western Germany. The study found a tendency for cases of leukaemia to live closer to the nearest nuclear power station than their matched controls, producing an odds ratio that was raised to a statistically significant extent for residence within 5 km of a nuclear power station. The findings of the study received much publicity, but a detailed radiological risk assessment demonstrated that the radiation doses received by young children from discharges of radioactive material from the nuclear reactors were much lower than those received from natural background radiation and far too small to be responsible for the statistical association reported in the KiKK Study. This has led to speculation that conventional radiological risk assessments have grossly underestimated the risk of leukaemia in young children posed by exposure to man-made radionuclides, and particular attention has been drawn to the possible role of tritium and carbon-14 discharges in this supposedly severe underestimation of risk. Both ³H and ¹⁴C are generated naturally in the upper atmosphere, and substantial increases in these radionuclides in the environment occurred as a result of their production by atmospheric testing of nuclear weapons during the late 1950s and early 1960s. If the leukaemogenic effect of these radionuclides has been seriously underestimated to the degree necessary to explain the KiKK Study findings, then a pronounced increase in the worldwide incidence of leukaemia among young children should have followed the notably elevated exposure to ³H and ¹⁴C from nuclear weapons testing fallout. To investigate this hypothesis, the time series of incidence rates of leukaemia among young children <5 years of age at diagnosis has been examined from ten cancer registries from three continents and both hemispheres, which include registration data from the early 1960s or before. No evidence of a markedly increased risk of leukaemia in young children following the peak of above-ground nuclear weapons testing, or that incidence rates are related to level of exposure to fallout, is apparent from these registration rates, providing strong grounds for discounting the idea that the risk of leukaemia in young children from ³H or ¹⁴C (or any other radionuclide present in both nuclear weapons testing fallout and discharges from nuclear installations) has been grossly underestimated and that such exposure can account for the findings of the KiKK Study.     

Considerations on the behavior of long-lived radionuclides in the soil

The migration of radionuclides from waste repositories to the biosphere potentially leads to a contamination of soil. Due to the importance of food production, the mobilisation and accumulation behaviour of long-lived radionuclides in the soil plays a key role in performance assessment studies. In this paper, the main features and processes that control radionuclide behaviour in soil, such as pH, redox potential and sorption to organic and inorganic soil components, are discussed for the radionuclides ³⁶Cl, ⁷⁹Se, ¹²⁹I, ⁹⁹Tc, ²³⁷Np and ²³⁸U, that are usually most relevant in long-term safety assessments of nuclear waste. The interaction of radionuclide behaviour in soil with environmental factors, such as temperature and humidity as well as farming practices are discussed. The possible impact of future soil development on long-term behaviour in soil are taken into consideration. Due to the physiological constraints of plant growth, appropriate soil conditions for growth will probably not be substantially different from current requirements, bearing in mind that sustainable agriculture strives for optimal plant growth. Against this background, present-day parameters may in general be considered appropriate for roughly estimating the behaviour of radionuclides in the soil-plant system. Received: 18 October 2000 / Accepted: 20 March 2001     

Distribution and activity of microorganisms in the deep repository for liquid radioactive waste at the Siberian Chemical Combine

The physicochemical conditions, composition of microbial communities, and the rates of anaerobic processes in the deep sand horizons used as a repository for liquid radioactive wastes (LRW) at the Siberian Chemical Combine (Seversk, Tomsk oblast), were studied. Formation waters from the observation wells drilled into the horizons used for the radioactive waste disposal were found to be inhabited by microorganisms of different physiological groups, including aerobic organotrophs, anaerobic fermentative, denitrifying, sulfate-reducing, and methanogenic bacteria. The density of microbial population, as determined by cultural methods, was low and usually did not exceed 10⁴ cells/ml. Enrichment cultures of microorganisms producing gases (hydrogen, methane, carbon dioxide, and hydrogen sulfide) and capable of participation in the precipitation of metal sulfides were obtained from the waters of the disposal site. The contemporary processes of sulfate reduction and methanogenesis were assayed; the rates of these terminal processes of organic matter destruction were found to be low. The denitrifying bacteria from the deep repository were capable of reducing the nitrates contained in the wastes, provided sources of energy and biogenic elements were available. Biosorption of radionuclides by the biomass of aerobic bacteria isolated from groundwater was demonstrated. The results obtained give us insight into the functional structure of the microbial community inhabiting the waters of repository horizons. This study indicates that the numbers and activity of microbial cells are low both inside and outside the zone of radioactive waste dispersion, in spite of the long period of waste discharge.     

The effect of soil pH on rhizosphere carbon flow of Lolium perenne

Perennial rye-grass plants were grown at 15°C in microcosms containing soil sampled from field plots that had been maintained at constant pH for the last 30 years. Six soil pH values were tested in the experiment, with pH ranging from 4.3–6.5. After 3 weeks growth in the microcosms, plant shoots were exposed to a pulse of ¹⁴C-CO₂. The fate of this label was determined by monitoring ¹⁴C-CO₂ respired by the plant roots/soil and by the shoots. The ¹⁴C remaining in plant roots and shoots was determined when the plants were harvested 7 days after receiving the pulse label. The amount of ¹⁴C (expressed as a percentage of the total ¹⁴C fixed by the plant) lost from the plant roots increased from 12.3 to 30.6% with increasing soil pH from 4.3 to 6. Although a greater percentage of the fixed ¹⁴C was respired by the root/soil as soil pH increased, plant biomass was greater with increasing soil pH. Possible reasons for observed changes in the pattern of ¹⁴C distribution are discussed and, it is suggested that changes in the soil microbial biomass and in plant nitrogen nutrition may, in particular be key factors which led to increased loss of carbon from plant roots with increasing soil pH.     

Radioactive contamination of bottom sediments in the upper reaches of the Techa river: analysis of the data obtained in 1950 and 1951

A stationary sorption model has been developed for re-evaluating and analysing archive data from 1950-1951 on the radioactive contamination of Techa river bottom sediments close to the site of liquid radioactive waste discharge. In general, good agreement was obtained between calculations and measurements, which substantiates further the assumptions and conclusions in two preceding articles, on the radionuclide composition of discharged liquid radioactive waste. Estimates on the effective liquid radioactive waste discharges given here are significantly different from those deduced in the 1950s, i.e. in summer 1950 and October 1951. The results are discussed in relation to the Techa River Dosimetry System 2000 (TRDS-2000) that has recently been presented to serve as a means for estimating doses to the Techa river residents. Parameter values describing the exponential decrease of bottom sediment contamination along the river due to short-lived radionuclides, such as (106)Ru, and (144)Ce, agree reasonably with those used in TRDS-2000. However, for other radionuclides, such as (95)Zr, (95)Nb, (91)Y, (90)Sr and (137)Cs, substantial differences are found. It is demonstrated that water flow rate, width of the river, and surface area of bottom sediments are important parameters which were not adequately taken into account in TRDS-2000. Also, the stirring-up of contaminated bottom sediments and their subsequent transport by the water flow are seen to be an important mechanism that governs the radionuclide transport downstream. This mechanism was not included in the TRDS-2000 model. It is concluded that the sorption model used in TRDS-2000 for the reconstruction of radioactive contamination of water and bottom sediments of the Techa river in 1949-1951, is subject to considerable errors. While the present paper is focussed on details of the dosimetric modelling, the implications for the Techa river dosimetry are major. They will be further elucidated in a forthcoming paper.     

Temperature and Microtopography Interact to Control Carbon Cycling in a High Arctic Fen

High arctic wetlands hold large stores of soil carbon (C). The fate of these C stores in a changing climate is uncertain, as rising air temperatures may differentially affect photosynthesis and ecosystem respiration (ER). In this study, open-top warming chambers were used to increase air and soil temperatures in contrasting microtopographic positions of a high arctic fen in NW Greenland. CO₂ exchange between the ecosystem and the atmosphere was measured on 28 dates over a 3-year period. Measurements of the normalized difference vegetation index, leaf and stem growth, leaf-level gas exchange, leaf nitrogen, leaf δ¹³C, and fine root production were made to investigate the mechanisms and consequences of observed changes in CO₂ exchange. Gross ecosystem photosynthesis (GEP) increased with chamber warming in hollows, which are characterized by standing water, and in hummocks, which extend above the water table. ER, however, increased only in hummocks, such that net ecosystem exchange (NEE) increased in hollows, but did not change in hummocks with chamber warming. Complementary measurements of plant growth revealed that increases in GEP corresponded with increases in C allocation to aboveground biomass in hummocks and belowground biomass in hollows. Our results and those of several recent studies clearly demonstrate that effects of climate change on the C balance of northern wetlands will depend upon microtopography which, in turn, may be sensitive to climate change.     

Climate Change Affects Carbon Allocation to the Soil in Shrublands

Climate change may affect ecosystem functioning through increased temperatures or changes in precipitation patterns. Temperature and water availability are important drivers for ecosystem processes such as photosynthesis, carbon translocation, and organic matter decomposition. These climate changes may affect the supply of carbon and energy to the soil microbial population and subsequently alter decomposition and mineralization, important ecosystem processes in carbon and nutrient cycling. In this study, carried out within the cross-European research project CLIMOOR, the effect of climate change, resulting from imposed manipulations, on carbon dynamics in shrubland ecosystems was examined. We performed a ¹⁴C-labeling experiment to probe changes in net carbon uptake and allocation to the roots and soil compartments as affected by a higher temperature during the year and a drought period in the growing season. Differences in climate, soil, and plant characteristics resulted in a gradient in the severity of the drought effects on net carbon uptake by plants with the impact being most severe in Spain, followed by Denmark, with the UK showing few negative effects at significance levels of p 0.10. Drought clearly reduced carbon flow from the roots to the soil compartments. The fraction of the ¹⁴C fixed by the plants and allocated into the soluble carbon fraction in the soil and to soil microbial biomass in Denmark and the UK decreased by more than 60%. The effects of warming were not significant, but, as with the drought treatment, a negative effect on carbon allocation to soil microbial biomass was found. The changes in carbon allocation to soil microbial biomass at the northern sites in this study indicate that soil microbial biomass is a sensitive, early indicator of drought- or temperature-initiated changes in these shrubland ecosystems. The reduced supply of substrate to the soil and the response of the soil microbial biomass may help to explain the observed acclimation of CO₂ exchange in other ecosystems.     

Soil microbial community structure and functionality changes in response to long-term metal and radionuclide pollution

Microbial communities are essential for a healthy soil ecosystem. Metals and radionuclides can exert a persistent pressure on the soil microbial community. However, little is known on the effect of long-term co-contamination of metals and radionuclides on the microbial community structure and functionality. We investigated the impact of historical discharges of the phosphate and nuclear industry on the microbial community in the Grote Nete river basin in Belgium. Eight locations were sampled along a transect to the river edge and one location further in the field. Chemical analysis demonstrated a metal and radionuclide contamination gradient and revealed a distinct clustering of the locations based on all metadata. Moreover, a relation between the chemical parameters and the bacterial community structure was demonstrated. Although no difference in biomass was observed between locations, cultivation-dependent experiments showed that communities from contaminated locations survived better on singular metals than communities from control locations. Furthermore, nitrification, a key soil ecosystem process seemed affected in contaminated locations when combining metadata with microbial profiling. These results indicate that long-term metal and radionuclide pollution impacts the microbial community structure and functionality and provides important fundamental insights into microbial community dynamics in co-metal-radionuclide contaminated sites.     

Ecosystem carbon stocks and their changes in China’s grasslands

The knowledge of carbon (C) stock and its dynamics is crucial for understanding the role of grassland ecosystems in China’s terrestrial C cycle. To date, a comprehensive assessment on C balance in China’s grasslands is still lacking. By reviewing published literature, this study aims to evaluate ecosystem C stocks (both vegetation biomass and soil organic C) and their changes in China’s grasslands. Our results are summarized as follows: (1) biomass C density (C stock per area) of China’s grasslands differed greatly among previous studies, ranging from 215.8 to 348.1 g C m⁻² with an average of 300.2 g C m⁻². Likewise, soil C density also varied greatly between 8.5 and 15.1 kg C m⁻². In total, ecosystem C stock in China’s grasslands was estimated at 29.1 Pg C. (2) Both the magnitude and direction of ecosystem C changes in China’s grasslands differed greatly among previous studies. According to recent reports, neither biomass nor soil C stock in China’s grasslands showed a significant change during the past 20 years, indicating that grassland ecosystems are C neutral. (3) Spatial patterns and temporal dynamics of grassland biomass were closely correlated with precipitation, while changes in soil C stocks exhibited close associations with soil moisture and soil texture. Human activities, such as livestock grazing and fencing could also affect ecosystem C dynamics in China’s grasslands.     

A metabolic approach to simulating the dynamics of C-14, H-3 and S-35 in sheep tissues

The results of a study in which groups of sheep were given single oral administrations of ¹⁴C, ³H and ³⁵S and then slaughtered over a period of 1 year are reported. The experimental data were used to investigate the potential of metabolically based models for describing the transfer of the three radionuclides to sheep tissues. The structure of these models is based upon a simplified understanding of the transfer of the macro-elements C, H and S by processes such as respiration and protein synthesis/degradation. A consequence of this approach is that the three models have many common parameters. The models reproduced the general trends of the observations, accounting for 74%, 66%, and 58% of the observed variation in the ¹⁴C, ³H and ³⁵S data, respectively, suggesting that they may provide a useful alternative approach to modelling the transfer of these radionuclides. The models presented are limited to the particular experimental situation for which they were developed, and further experimental work would be required to extend them. However, such metabolically based models have great potential: for example, they should be able to account for the influence of dietary intake, physiological status or the form of the radionuclide in the animals diet (e.g. tritiated water or organically bound tritium). Received: 19 June 1997 / Accepted in revised form: 20 August 1997     

树种水平的大兴安岭地上生物量变化特征及其与气候和干扰的关系

树种水平地上生物量(每个树种的地上生物量)是衡量森林生态系统结构功能的重要指标。为揭示树种水平森林地上生物量变化机制及其与气候变化和干扰的关系,运用KNN (k-nearest neighbor distance)方法将森林调查数据和MODIS数据相结合,估算了黑龙江大兴安岭2000、2010和2015年树种水平的森林地上生物量,在此基础上运用典型对应分析和随机森林方法,分析了研究区树种水平地上生物量变化特征及其与气候和干扰因素的关系。研究结果表明:2000—2015年期间,研究区总的森林地上生物量增加了8.9%(0.41×10~8 t),其中2010—2015年期间地上生物量的增加速度要明显高于2000—2010年;地上生物量增加最多的树种为白桦(Betula platyphylla Suk.),与2000年相比生物量增加了0.40×10~8 t,其次为樟子松(Pinus sylvestris var.mongolica Litv.)、山杨(Populus davidiana Dode)和蒙古栎(Quercus mongolica Fisch. ex Ledeb.),落叶松(Larix gmelinii(Rupr.) Kuzen)地上生物量下降了0.08×10~8 t,柳树(Chosenia arbutifolia(Pall.) A. Skv.)和云杉(Picea koraiensis Nakai)基本上没有变化;林火、采伐和造林等森林干扰均对树种水平地上生物量影响显著,林火对树种水平地上生物量的影响要高于造林和采伐;气候要素显示出了比干扰要素更为重要的作用,多年平均温度和降水解释了最多的树种水平地上生物量变异。年均温度与阔叶树种的生物量以及林火干扰有显著的正相关性,与总的森林地上生物量呈现出显著的负相关,与落叶松和白桦表现出微弱的负相关,预示着气候变暖将影响该区域的树种组成并降低该区域的森林生产力。     

Carbon exchange of a mature,naturally regenerated pine forest in north Florida

We used eddy covariance and biomass measurements to quantify the carbon (C) dynamics of a naturally regenerated longleaf pine/slash pine flatwoods ecosystem in north Florida for 4 years, July 2000 to June 2002 and 2004 to 2005, to quantify how forest type, silvicultural intensity and environment influence stand‐level C balance. Precipitation over the study periods ranged from extreme drought (July 2000–June 2002) to above‐average precipitation (2004 and 2005). After photosynthetic photon flux density (PPFD), vapor pressure deficit (VPD) >1.5 kPa and air temperature <10 °C were important constraints on daytime half‐hourly net CO₂ exchange (NEE_day) and reduced the magnitude of midday CO₂ exchange by >5 μmol CO₂ m^?2 s^?1. Analysis of water use efficiency indicated that stomatal closure at VPD>1.5 kPa moderated transpiration similarly in both drought and wet years. Night‐time exchange (NEE_night) was an exponential function of air temperature, with rates further modulated by soil moisture. Estimated annual net ecosystem production (NEP) was remarkably consistent among the four measurement years (range: 158–192 g C m^?2 yr^?1). In comparison, annual ecosystem C assimilation estimates from biomass measurements between 2000 and 2002 ranged from 77 to 136 g C m^?2 yr^?1. Understory fluxes accounted for approximately 25–35% of above‐canopy NEE over 24‐h periods, and 85% and 27% of whole‐ecosystem fluxes during night and midday (11:00–15:00 hours) periods, respectively. Concurrent measurements of a nearby intensively managed slash pine plantation showed that annual NEP was three to four times greater than that of the Austin Cary Memorial Forest, highlighting the importance of silviculture and management in regulating stand‐level C budgets.     

Nuclear waste repositories in salt mines: a new approach to safety assessment

The long-term safety of radioactive waste repositories in rock-salt mines in the deep underground benefits significantly from the barrier effect of overlying rocks. The concentrations of radioactive substances released from the repository and migrating in the aquifer up to the biosphere are greatly reduced during passage through these rocks. In former safety analyses of waste repositories this transport has generally been modelled as a combination of the involved phenomena, e.g. convection, dispersion, adsorption, etc. The data required for a numerical evaluation of the overall effect are obtained either as (conservative) estimates based on experience or are empirical, based mainly on laboratory experiments. The approach presented here is much simpler and entirely empirical, and therefore more transparent. It makes use of the fact that the groundwater in the overlying rocks always contains dissolved salt from the salt formation and carries it continuously into the receiving channels or the drainage system. The relation between the total amount of dissolved solids present in a certain subsurface catchment area and their steady-state concentration in the receiving channels is assumed to be equivalent to the relation between the given amount of radionuclides released from the repository and their concentration in the receiving channels, the latter leading to a certain radiation exposure of the population. Two versions of this approach are discussed: version (a) assumes a continuous stream of radionuclides released from the repository, and version (b) assumes a pulse release of radionuclides from the repository. A simple calculation using data from the Gorleben exploration leads to the inequality C _max(W)≤7.85 ⋅ 10^–15 W(Bq/l) (1) where C _max is the maximum radionuclide concentration (with respect to time) in the receiving channels and W (Bq) is the amount of radionuclides released from the repository in a very short time. C _max obtained from (1), is supposed to be an upper limit of the radionuclide concentration in the receiving channel and, therefore, a conservative estimate. In some catchment areas the salt concentration in a small region near the surface is higher by a factor of <2 to 3 than the concentration in the receiving channel. In order to be conservative, this empirical factor may be used to calculate the concentration of radioactive substances. Surprisingly, the values of radiation exposure resulting from both versions of the new approach are far below those calculated by applying elaborated models of the transport processes. The respective factors are 10–100 in case (a) and as much as 1000 in case (b). The reasons for these significant differences are discussed. Received: 21 September 1995 / Accepted in revised form: 13 June 1996     

The fate of Amazonian ecosystems over the coming century arising from changes in climate,atmospheric CO2, and land use

There is considerable interest in understanding the fate of the Amazon over the coming century in the face of climate change, rising atmospheric CO₂ levels, ongoing land transformation, and changing fire regimes within the region. In this analysis, we explore the fate of Amazonian ecosystems under the combined impact of these four environmental forcings using three terrestrial biosphere models (ED2, IBIS, and JULES) forced by three bias‐corrected IPCC AR4 climate projections (PCM1, CCSM3, and HadCM3) under two land‐use change scenarios. We assess the relative roles of climate change, CO₂ fertilization, land‐use change, and fire in driving the projected changes in Amazonian biomass and forest extent. Our results indicate that the impacts of climate change are primarily determined by the direction and severity of projected changes in regional precipitation: under the driest climate projection, climate change alone is predicted to reduce Amazonian forest cover by an average of 14%. However, the models predict that CO₂ fertilization will enhance vegetation productivity and alleviate climate‐induced increases in plant water stress, and, as a result, sustain high biomass forests, even under the driest climate scenario. Land‐use change and climate‐driven changes in fire frequency are predicted to cause additional aboveground biomass loss and reductions in forest extent. The relative impact of land use and fire dynamics compared to climate and CO₂ impacts varies considerably, depending on both the climate and land‐use scenario, and on the terrestrial biosphere model used, highlighting the importance of improved quantitative understanding of all four factors – climate change, CO₂ fertilization effects, fire, and land use – to the fate of the Amazon over the coming century.     

Plant- and soil related controls of the flow of carbon from roots through the soil microbial biomass

The flow of carbon from plant roots through the microbial biomass is one of the key processes in terrestrial ecosystems. Roots release considerable amounts of organic materials which are utilized by microbes as substrate for biosynthesis and energy supply. The fate of photosynthates and other organic material in the soil-root environment under different conditions was studied using¹⁴C-tracers. Soil structure and texture had a large effect on the turnover of the¹⁴C-labelled materials through the microbial biomas. Finer, clayey soils tended to be more preservative than coarser, sandy soils,i.e., larger amounts of¹⁴C were incorporated in microbial biomass and soil organic matter fractions in clayey soils than in sandy soils.The soil nutrient status also appeared to affect organic matter turnover. At limiting plant-nutrient concentrations the utilization of¹⁴C-labelled photosynthates seem to be hampered. Plant roots influenced the transformation of glucose and crop residues and the effect was attributed to plant-induced changes in mineral nutrient status. The mechanisms of this process and the consequences are discussed.A number of areas for future research are identified, including the potentials for manipulating rhizodeposition.     

Energy metabolism used as a tool to model the transfer of 14C and 3H in animals

The transfer through the environment of ³H and ¹⁴C must be modelled differently than that of other radionuclides released from nuclear reactors because hydrogen and carbon enter straight into the life cycle. A solid understanding of the behaviour of ³H and ¹⁴C in the food chain is essential because ³H may be released in large quantities from future thermonuclear reactors, and ¹⁴C accumulates in the environment because of its long half-life. For the present study, the hypothesis that both ³H and ¹⁴C metabolism in mammals can be modelled based on the understanding of energy metabolism has been tested. Recently published results demonstrate that the loss rate of organically bound tritium and ¹⁴C from tissues of laboratory and farm animals can be assessed based upon their specific metabolic rates and enthalpy of combustion; the same is true for human beings. The improved model presented here relates the dynamics of organically bound tritium and ¹⁴C within organs to the whole body and has been expanded to account for the growth of ruminants. The improved model has been expanded and applied for ¹⁴C transfer in wild mammals and has been modified to apply to birds.     

Accumulation of radionuclids in amphibians (<Emphasis Type="Italic">Pelophylax ridibundus</Emphasis> Pall.) in the Middle Urals

The accumulation levels of anthropogenic ⁹⁰Sr and ¹³⁴Cs and ¹³⁷Cs radionuclides in the marsh frog have been studied in the areas of the Beloyarskii water-storage reservoir (an industrial storm-water discharge channel of the nuclear power station) and the Verkhnii Tagil water-storage reservoir (the Tagil River down-stream of the dam). No significant distinction in the radionuclide accumulation (⁹⁰Sr and ¹³⁷Cs) depending on the amphibian sex and age is detected. Comparable levels of the accumulation of radionuclides in the marsh frog, when compared to the other representatives of the water ecosystem, are estimated. An assumption of the presence of some unidentified source of radioactive contamination of marsh frogs has been made; cesium-137 may be transferred from it to the Tagil River by the frogs.     

A Novel Dual-Compartment,Continuous-Flow Wetland Microcosm to Assess Cis-Dichloroethene Removal from the Rhizosphere

The anaerobic biodegradation of tetrachloroethene commonly results in the accumulation of chlorinated intermediates such as cis-1,2-dichloroethene (cDCE). Frequently, groundwater contaminated with chlorinated ethenes discharges to natural wetlands. The goal of this study was to quantitatively evaluate the effects of wetland plants and microorganisms on the fate of cDCE in the wetland rhizosphere. To accomplish this goal, a novel dual-compartment wetland microcosm was designed. A Phragmites australis individual was maintained in the microcosm, which was operated with continuous flows of air and mineral medium through the foliar and rhizosphere compartments, respectively, to incorporate mass transfer/transport processes that are important in natural wetlands and allow steady-state assessment of changes in dissolved O₂ and cDCE or [1,2–¹⁴C]cDCE levels. Substantial amounts of [¹⁴C]cDCE were phytovolatilized through a healthy P. australis individual to the foliar chamber. Rhizodegradation by native microorganisms associated with P. australis roots also converted substantial amounts of [¹⁴C]cDCE to ¹⁴C-labeled CO₂ and non-volatile compounds, presumably through cometabolic reactions that could be enhanced by the release of O₂ and exudates by P. australis. These results suggest that, in some cases, the intrinsic capacity of native wetland plants and microorganisms to remove cDCE from the rhizosphere may be substantial.     

Warming and drought alter C and N concentration,allocation and accumulation in a Mediterranean shrubland

We investigated the effects of warming and drought on C and N concentrations, nitrogen use efficiency (NUE), and C and N accumulation in different ecosystem compartments. We conducted a 6-year (1999–2005) field experiment to simulate the climate conditions projected by IPCC models for the coming decades in a Mediterranean shrubland. We studied the two dominant species, Globularia alypum and Erica multiflora, and an N-fixing species, Dorycnium pentaphyllum, also abundant in this shrubland. Warming (1 °C) decreased N leaf concentrations by 25% and increased N stem concentrations by 40% in G. alypum. Although warming changed the available ammonium in soil in some seasons, it did not increase total soil N contents. Drought (19% average reduction in soil moisture) decreased leaf N concentrations in the two dominant shrub species, E. multiflora and G. alypum by 16% and 19%, respectively, and increased stem N concentrations by 56% and 40%, respectively. Neither warming nor drought changed the leaf N concentrations in the N-fixing species D. pentaphyllum, although warming increased stem N concentration by 9%. In G. alypum, the increase of stem N concentrations contributed to the observed increase of N accumulation in stem biomass in drought treatments with respect to control plots (8 kg N ha⁻¹). Neither warming nor drought changed NUE in the period 1999–2005. Warming increased soil organic C relative to drought. The effects of warming and drought on C and N concentrations, on N accumulation and on leaf/stem N distribution were not the result of dilution or concentration effects produced by changes in biomass accumulation. Other factors such as the changes in soil N availability, photosynthetic capacity, and plant internal C and N remobilization must be involved. These changes which differed depending on the species and the plant tissue show that the climate change projected for the coming decades will have significant effects on the C and N cycle and stoichiometry, with probable implications for ecosystem structure and function, such as changes in plant–herbivore relationships, decomposition rates or community species composition.