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International audience; Calibration, the estimation of model parameters based on fitting the model to experimental data, is among the first steps in many applications of process-based models and has an important impact on simulated values. We propose a novel method of developing guidelines for calibration of process-based models, based on development of recommendations for calibration of the phenology component of crop models. The approach was based on a multi-model study, where all teams were provided with the same data and asked to return simulations for the same conditions. All teams were asked to document in detail their calibration approach, including choices with respect to criteria for best parameters, choice of parameters to estimate and software. Based on an analysis of the advantages and disadvantages of the various choices, we propose calibration recommendations that cover a comprehensive list of decisions and that are based on actual practices.

Vegetable fields in and around urban areas in the Kurdistan region of Iraq may have higher than background concentrations of potentially toxic elements (PTEs) from contamination sources including municipal waste disposal and wastewater used for irrigation. The purpose of this study was to assess PTE concentrations in soils and the edible parts of field-grown vegetables to quantify potential health risks to the local population. In this survey, 174 soils and 26 different vegetable and fruit types were sampled from 15 areas around Sulaymaniyah and Halabja cities. Sampling was undertaken from fields in urban, peri-urban and rural locations including sites close to areas of waste disposal. The soils are calcareous (pH 7.67–8.21) and classified as silty loam, sandy or silty clay with organic matter content between 6.62 and 11.4%. Concentrations of PTEs were typically higher in waste disposal areas compared with urban, peri-urban and rural areas. Pollution load indices suggested that agricultural soils near waste disposal sites were contaminated with some trace elements. Potentially toxic element concentrations in vegetables were highly variable. Higher total concentrations of PTEs were measured in vegetables from the waste areas with decreasing concentrations in urban, peri-urban and rural areas. Risks to human health were assessed using hazard quotients (HQ). Vegetable consumption poses no risk for adults, whereas children might be exposed to Ni, As and Cd. Although HQs suggest elevated risk for children from consumption of some vegetables, these risks are likely to be lower when realistic dietary consumption levels are considered.

Iodine is an essential trace element for humans and grazing animals and is often deficient. Our aim was to investigate the role of soil properties in retaining and ‘fixing' iodine in soils and thereby controlling its phyto-availability to grass. Soils were spiked with labelled 129IO3− and rye grass (Lolium perenne L.) was grown to measure iodine uptake by grass as a function of yield, soil properties and continuous 127I inputs from irrigation water. Iodine-129 added at the start of the uptake trial was rapidly fixed (t1/2 c. 40 h) into non-labile humus-bound forms in soil. The 129I/127I isotopic ratio in grass, compared to the ratio in soil, declined over time confirming progressive 129I fixation into the soil solid phase. The rate of fixation was controlled by soil properties. A model describing iodine dynamics and uptake accounted for c. 75% of the variation in iodine concentration in grass. For most of the soils studied, the main source of iodine in herbage probably arises from the transient availability of periodic rainfall inputs rather than from soil sources. This is expected to improve biofortification strategies.

Following nuclear releases to the environment, 137Cs (half-life 30 years) is a long-term contaminant of many ecosystems, including forests. We recently sampled soils under pine forests in temperate and tropical climates to test the hypothesis that migration of 137Cs, 50 years after nuclear weapons fallout, is coupled with organic matter (OM) accumulation in these soils. Depth profiles of 137Cs, naturally-occurring 210Pb and weapons-derived 241Am were measured. After 50 years, migration of 137Cs into the temperate and tropical soils is limited to half-depths of 7–8 cm and 2–3 cm, respectively. At both locations, most 137Cs is associated with OM that accumulated from the early to mid-1960s. Illite, which immobilises radiocaesium, was undetectable by X-ray diffraction in the layer of peak 137Cs accumulation in the temperate forest soil, but apparent in the zone of peak concentration in the tropical soil. Data indicate that long-term (50 year) fate of 137Cs in organic-rich, temperate forest soil is coupled with OM accumulation; fixation of 137Cs by illite is more important in the tropical forest soil where OM is rapidly decomposed. Models of long-term radiocaesium migration in forest soils should explicitly account for the role of OM, especially when considering forests under contrasting climatic regimes.

Iodine is an essential micronutrient for human health; phytofortification is a means of improving humans’ nutritional iodine status. However, knowledge of iodine uptake and translocation in plants remains limited. In this paper, plant uptake mechanisms were assessed in short-term experiments (24 h) using labelled radioisotopes; the speciation of iodine present in apoplastic and symplastic root solutions was determined by (HPLC)-ICP-QQQ-MS. Iodine storage was investigated in spinach (Spinacia oleracea L.) treated with I− and IO3−. Finally, translocation through the phloem to younger leaves was also investigated using a radioiodine (129I−) label. During uptake, spinach roots demonstrated the ability to reduce IO3− to I−. Once absorbed, iodine was present as org-I or I− with significantly greater concentrations in the apoplast than the symplast. Plants were shown to absorb similar concentrations of iodine applied as I− or IO3−, via the roots, grown in an inert growth substrate. We found that whilst leaves were capable of absorbing radioactively labelled iodine applied to a single leaf, less than 2% was transferred through the phloem to younger leaves. In this paper, we show that iodine uptake is predominantly passive (approximately two-thirds of total uptake); however, I- can be absorbed actively through the symplast. Spinach leaves can absorb iodine via foliar fertilisation, but translocation is severely limited. As such, foliar application is unlikely to significantly increase the iodine content, via phloem translocation, of fruits, grains or tubers.

Plant phenology, which describes the timing of plant development, is a major aspect of plant response to environment and for crops, a major determinant of yield. Since climate change is projected to alter crop phenology worldwide, there is a large effort to predict phenology as a function of environment. Many studies have focused on comparing model equations for describing how phenology responds to weather but the effect of crop model calibration, also expected to be important, has received much less attention. The objective here was to obtain a rigorous evaluation of prediction capability of wheat crop phenology models, and to analyze the role of calibration. The 27 participants in this multi-model study were provided experimental data for calibration and asked to submit predictions for sites and years not represented in those data. Participants were instructed to use and document their 99usual99 calibration approach. Overall, the models provided quite good predictions of phenology (median of mean absolute error of 6.1 days) and did much better than simply using the average of observed values as predictor. Calibration was found to compensate to some extent for differences between models, specifically for differences in simulated time to emergence and differences in the choice of input variables. Conversely, different calibration approaches led to major differences in prediction error between models with the same structure. Given the large diversity of calibration approaches and the importance of calibration, there is a clear need for guidelines and tools to aid with calibration. Arguably the most important and difficult choice for calibration is the choice of parameters to estimate. Several recommendations for calibration practices are proposed. Model applications, including model studies of climate change impact, should focus more on the data used for calibration and on the calibration methods employed.

Predicting wheat phenology is important for cultivar selection, for effective crop management and provides a baseline for evaluating the effects of global change. Evaluating how well crop phenology can be predicted is therefore of major interest. Twenty-eight wheat modeling groups participated in this evaluation. Model predictions depend not only on model structure but also on the parameter values. This study is thus an evaluation of modeling groups, which choose the structure and fix or estimate the parameters, rather than an evaluation just of model structures. Our target population was wheat fields in the major wheat growing regions of Australia under current climatic conditions and with current local management practices. The environments used for calibration and for evaluation were both sampled from this same target population. The calibration and evaluation environments had neither sites nor years in common, so this is a rigorous evaluation of the ability of modeling groups to predict phenology for new sites and weather conditions. Mean absolute error (MAE) for the evaluation environments, averaged over predictions of three phenological stages and over modeling groups, was 9 days, with a range from 6 to 20 days. Predictions using the multi-modeling group mean and median had prediction errors nearly as small as the best modeling group. For a given modeling group, MAE for the evaluation environments was significantly correlated with MAE for the calibration environments, which suggests that it would be of interest to test ensemble predictors that weight individual modeling groups based on performance for the calibration data. About two thirds of the modeling groups performed better than a simple but relevant benchmark, which predicts phenology by assuming a constant temperature sum for each development stage. The added complexity of crop models beyond just the effect of temperature was thus justified in most cases. Finally, there was substantial variability between modeling groups using the same model structure, which implies that model improvement could be achieved not only by improving model structure, but also by improving parameter values, and in particular by improving calibration techniques.

Assessing the reactions of iodine (I) in soil is critical to evaluate radioiodine exposure and understand soil-to-crop transfer rates. Our mechanistic understanding has been constrained by method limitations in assessing the dynamic interactions of iodine between soil solution and soil solid phase over short periods (hours). We use microdialysis to passively extract soil solution spiked with radioiodine (129I- and 129IO3-) to monitor short-term (≤40 h) in situ fixation and speciation changes. We observed greater instantaneous adsorption of 129IO3- compared to 129I- in all soils and the complete reduction of 129IO3- to 129I- within 5 h of addition. Loss of 129I from solution was extremely rapid; the average half-lives of 129I- and 129IO3- in soil solution were 4.06 and 10.03 h, respectively. We detected the presence of soluble organically bound iodine (org-129I) with a low molecular weight (MW) range (0.5-5 kDa) in all soils and a slower (20-40 h) time-dependent formation of larger MW org-I compounds (12-18 kDa) in some samples. This study highlights the very short window of immediate availability in which I from rainfall or irrigation remains in soil solution and available to crops, thus presenting significant challenges to phytofortification strategies in soil-based production systems.

Conversion of tropical peat swamp forest to drainage-based agriculture alters greenhouse gas (GHG) production, but the magnitude of these changes remains highly uncertain. Current emissions factors for oil palm grown on drained peat do not account for temporal variation over the plantation cycle and only consider CO2 emissions. Here, we present direct measurements of GHGs emitted during the conversion from peat swamp forest to oil palm plantation, accounting for CH4 and N2O as well as CO2. Our results demonstrate that emissions factors for converted peat swamp forest is in the range 70–117 t CO2 eq ha−1 yr−1 (95% confidence interval, CI), with CO2 and N2O responsible for ca. 60 and ca. 40% of this value, respectively. These GHG emissions suggest that conversion of Southeast Asian peat swamp forest is contributing between 16.6 and 27.9% (95% CI) of combined total national GHG emissions from Malaysia and Indonesia or 0.44 and 0.74% (95% CI) of annual global emissions., The magnitude of greenhouse gas emissions from land use change on tropical peatlands is unclear. Here, the authors measure greenhouse gas fluxes throughout the conversion from peat swamp forest to oil palm plantation, and estimate the contribution to regional and global emissions.

Uranium may pose a hazard to ecosystems and human health due to its chemotoxic and radiotoxic properties. The long half-life of many U isotopes and their ability to migrate raise concerns over disposal of radioactive wastes. This work examines the long-term U bioavailability in aerobic soils following direct deposition or transport to the surface and addresses two questions: (i) to what extent do soil properties control the kinetics of U speciation changes in soils and (ii) over what experimental timescales must U reaction kinetics be measured to reliably predict long-term of impact in the terrestrial environment? Soil microcosms spiked with soluble uranyl were incubated for 1.7 years. Changes in UVI fractionation were periodically monitored by soil extractions and isotopic dilution techniques, shedding light on the binding strength of uranyl onto the solid phase. Uranyl sorption was rapid and strongly buffered by soil Fe oxides, but UVI remained reversibly held and geochemically reactive. The pool of uranyl species able to replenish the soil solution through several equilibrium reactions is substantially larger than might be anticipated from typical chemical extractions and remarkably similar across different soils despite contrasting soil properties. Modelled kinetic parameters indicate that labile UVI declines very slowly, suggesting that the processes and transformations transferring uranyl to an intractable sink progress at a slow rate regardless of soil characteristics. This is of relevance in the context of radioecological assessments, given that soil solution is the key reservoir for plant uptake., This work was carried out within the TREE (Transfer-Exposure-Effects) consortium within the Radioactivity and the Environment (RATE) programme, funded jointly by the Natural Environment Research Council, Radioactive Waste Management Ltd. and the Environment Agency (grant no. NE/L000504/1). We thank the numerous individuals and agencies who gave permission to sample soils.

Technetium-99 is a significant and long-lived component of spent nuclear fuel relevant to long-term assessments of radioactive waste disposal. Whilst 99Tc behaviour in poorly aerated environments is well known, the long-term bioavailability in aerobic soils following direct deposition or transport to the surface is less well understood. This work addresses two questions: (i) to what extent do soil properties control 99Tc kinetics in aerobic soils and (ii) over what experimental timescales must 99Tc kinetics be measured to make reliable long-term predictions of impact in the terrestrial environment? Soil microcosms spiked with 99TcO4 − were incubated for 2.5 years and 99Tc transformations were periodically monitored by a sequential extraction, which enabled quantification of the reaction kinetics. Reduction in soluble 99Tc was slow and followed a double exponential kinetic model including a fast component enhanced by low pH, a slow component controlled by pH and organic matter, and a persistently soluble 99Tc fraction. Complexation with soil humus was key to the progressive immobilisation of 99Tc. Evidence for slow transfer to an unidentified ‘sink’ was found, with estimated decadal timeframes. Our data suggest that short-term experiments may not reliably predict long-term 99Tc solubility in soils with low to moderate organic matter contents. © 2019 The Authors, This work was carried out within the TREE (Transfer-Exposure-Effects) consortium, one of three projects within the Radioactivity and the Environment (RATE) programme. This study was funded jointly by the Natural Environment Research Council Radioactive Waste Management Ltd . and the Environment Agency in the United Kingdom (grant no. NE/L000504/1 ). We also thank the numerous individuals and agencies in the UK who gave permission to sample soils on their land. Appendix A

The Chernobyl Exclusion Zone (CEZ) represents a unique natural laboratory that received significant 129I contamination across a range of soils and land-use types in a short time period in 1986. Data are presented on 129I and 127I in soil samples collected from highly contaminated areas in the CEZ in 2015. The geometric mean (GM) total concentration of stable iodine (127I) was 6.7 × 10−7 g g−1 and the (GM) total concentration of 129I was 2.39 × 10−13 g g−1, equivalent to 1.56 mBq kg−1. GM total 127I concentration is below the European average soil concentration of 3.94 × 10−6 g g−1, while 129I is significantly higher than the pre-Chernobyl activity concentration for 129I of 0.094 mBq kg−1. Significant differences were found in the extractability of native, stable 127I and 129I almost 30 years after the introduction of 129I to the soils. Both 127I and 129I were predominantly associated with alkaline-extractable soil organic matter, established using a three-step sequential extraction procedure. Whereas 127I was significantly correlated with gross soil organic matter (measured by loss on ignition), however, 129I was not. The ratio of 129I/127I was significantly lower in extracts of soil organic matter than in more labile (soluble and adsorbed) fractions, indicating incomplete equilibration of 129I with native 127I in soil humic substances after 29 years residence time in the CEZ soils. The initial physico-chemical form of 129I in the CEZ soils is unknown, but the widespread presence of uranium oxide fuel particles is unlikely to have influenced the environmental behaviour of 129I. Our findings have implications for long-term radiation dose from 129I in contaminated soils and the use of native, stable 127I as a proxy for the long-term fate of 129I. © 2019 Elsevier B.V., This work was carried out within the TREE (Transfer-Exposure-Effects) consortium, one of three projects within the Radioactivity and the Environment (RATE) programme funded jointly by the Natural Environment Research Council, Radioactive Waste Management Ltd. and the Environment Agency (grant no. NE/L000504/1). We thank M. D. Bondarkov and J. Makluk (Chornobyl Center for Nuclear Safety, Radioactive Waste and Radioecology, Slavutych) and Thawatchai Itthipoonthanakorn (Office of Atoms for Peace, Bangkok, Thailand) for their kind assistance. Finally, we thank four anonymous reviewers for their help in improving the manuscript.

Past greenhouse intervals of the Mesozoic were repeatedly punctuated by Ocean Anoxic Events (OAEs), major perturbations to the global carbon cycle and abrupt climate changes that may serve as relevant analogs for Earth’s greenhouse gas-forced climate future. The key to better understanding these transient climate disruptions and possible CO2 forced tipping-points resides in high-resolution, precise, and accurate estimates of atmospheric CO2 for individual OAEs. Here we present a high-temporal resolution, multi-proxy pCO2 reconstruction for the onset of mid-Cretaceous (Albian-Cenomanian Boundary) OAE1d. Coupling of pCO2 estimates with carbon isotopic compositions (δ13C) of charcoal, vitrain, and cuticle from the Rose Creek Pit (RCP), Nebraska, reveals complex phasing, including a lag between the well-documented negative δ13C excursion defining the onset of OAE1d and the CO2 increase. This lag indicates that increased CO2 or other C-based greenhouse gases may not have been the primary cause of the negative excursion. Our study reveals a pCO2 increase within the interval of the negative δ13C excursion, reaching a maximum of up to ~840 ppm (95% confidence interval -307 ppm/+167 ppm) toward its end. The reconstructed magnitude of CO2 increase (~357 ppm) is similar to that of Late Cretaceous OAE2 but of smaller magnitude than that of other major carbon cycle perturbations of the Mesozoic assessed via stomatal methods (e.g., the Toarcian OAE [TOAE], Triassic-Jurassic boundary event, Cretaceous-Paleogene Boundary event). Furthermore, our results indicate a possible shared causal or developmental mechanism with OAE1a and the TOAE.

The disposal of biosolids poses a major environmental and economic problem. Agricultural use is generally regarded as the best means of disposal. However, its impact on soil ecosystems remains uncertain. Biosolids can improve soil properties by supplying nutrients and increasing organic matter content but there is also a potentially detrimental effect arising from the introduction of heavy metals into soils. To assess the balance between these competing effects on soil health, we investigated soil bacterial and fungal diversity and community structure at a site that has been dedicated to the disposal of sewage sludge for over 100 years. Terminal restriction fragment length polymorphism (T-RFLP) was used to characterize the soil microbial communities. The most important contaminants at the site were Ni, Cu, Zn, Cd, and Pb. Concentrations were highly correlated and Zn concentration was adopted as a good indicator of the overall (historical) biosolids loading. A biosolids loading, equivalent to 700–1000 mg kg−1 Zn appeared to be optimal for maximum bacterial and fungal diversity. This markedly exceeds the maximum soil Zn concentration of 300 mg kg−1permitted under the current UK Sludge (use in agriculture) Regulations. Redundancy analysis (RDA) suggested that the soil microbial communities had been altered in response to the accumulation of trace metals, especially Zn, Cd, and Cu. We believe this is the first time the trade-off between positive and negative effects of long term (>100 years) biosolids disposal on soil microorganisms have been observed in the field situation.

© 2020 Elsevier B.V. Biosolids application to arable land is a common, and cost-effective, practice but the impact of prolonged disposal remains uncertain. We evaluated the dynamics of potentially toxic elements (PTEs) at a long-established ‘dedicated’ sewage treatment farm. Soil metal concentrations exceeded regulations governing application of biosolids to non-dedicated arable land. However, measurement of isotopic exchangeability of Ni, Cu, Zn, Cd and Pb demonstrated support for the ‘protection hypothesis’ in which biosolids constituents help immobilise potential toxic metals (PTMs). Metal concentrations in a maize crop were strongly, and almost equally, correlated with all ‘capacity-based’ and ‘intensity-based’ estimates of soil metal bioavailability. This was attributable to high correlations between soil factors controlling bioavailability (organic matter, phosphate etc.) on a site receiving a single source of PTMs. Isotopic analysis of the maize crop suggested contributions to foliar Pb from soil dust originating from neighbouring fields. There was also clear evidence of metal-specific effects of biosolids on soil metal lability. With increasing metal concentrations there was both decreasing lability of Cd and Pb, due to interaction with increasing phosphate concentrations, and increasing lability of Ni, Cu and Zn due to weaker soil binding. Such different responses to prolonged biosolids disposal to arable soil should be considered when setting regulatory limits.

Numerous radioecological models have been developed to predict radionuclides transfer from contaminated soils to the food chain, which is an essential step in preparing and responding to nuclear emergencies. However, the lessons learned from applying these models to predict radiocaesium (RCs) soil-to-plant transfer following the Fukushima accident in 2011 renewed interest in RCs transfer modelling. To help guide and prioritise further research in relation to modelling RCs transfer in terrestrial environments, we reviewed existing models focussing on transfer to food crops and animal fodders. To facilitate the review process, we categorised existing RCs soil-to-plant transfer models into empirical, semi-mechanistic and mechanistic, though several models cross the boundaries between these categories. The empirical approach predicts RCs transfer to plants based on total RCs concentration in soil and an empirical transfer factor. The semi-mechanistic approach takes into account the influence of soil characteristics such as clay and exchangeable potassium content on RCs transfer. It also uses ʻbioavailableʼ rather than total RCs in soil. The mechanistic approach considers the physical and chemical processes that control RCs distribution and uptake in soil-plant systems including transport in the root zone and root absorption kinetics. Each of these modelling approaches has its advantages and disadvantages. The empirical approach is simple and requires two inputs, but it is often associated with considerably uncertainty due to the large variability in the transfer factor. The semi-mechanistic approach factorises more soil and plant parameters than the empirical approach; therefore, it is applicable to a wider range of environmental conditions. The mechanistic approach is instrumental in understanding RCs mobility and transfer in soil-plant systems; it also helps to identify influential soil and plant parameters. However, the comlexity and the large amount of specific parameters make this approach impractical for nuclear emergency preparedness and response purposes. We propose that the semi-mechanistic approach is sufficiently robust and practical, hence more fit for the purpose of planning and responding to nuclear emergencies compared with the empirical and mechanistic approaches. We recommend further work to extend the applicability of the semi-mechanistic approach to a wide range of plants and soils.

Element cycling in the terrestrial environment is heavily reliant upon processes that occur in soil solution. Here we present the first application of microdialysis to sample iodine from soil solution. In comparison to conventional soil solution extraction methods such as Rhizon™ samplers, centrifugation, and high-pressure squeezing, microdialysis can passively sample dissolved compounds from soil solution without altering the in-situ speciation of trace elements at realistic soil moisture conditions. In order to assess the suitability of microdialysis for sampling iodine, the permeability factors and effect of perfusion flowrate on I− and IO3− recovery was examined in stirred solutions. Furthermore, microdialysis was used to sample native soluble iodine at a range of water contents and iodine-enriched soils to investigate iodine soil dynamics. Total iodine concentrations were measured using ICP-MS. Inorganic species and the molecular weight distribution of organically bound iodine were determined by anion exchange and size exclusion chromatography (SEC) coupled to an ICP-MS, respectively. The most effective recovery rates in stirred solution were observed with the slowest perfusion flowrate yielding 66.2 ± 7.1 and 70.5 ± 7.1% for I− and IO3−, respectively. Microdialysis was proven to be capable of sampling dissolved iodine from the soil solution, which accounted for I− > IO3−. The use of SEC coupled to (i) UV and (ii) ICP-MS analysis provided detail regarding the molecular weight distribution of dissolved org-I compounds. Dissolved org-I was detected with approximate molecular weights between 0.1 and 4.5 kDa. The results in this study show that microdialysis is a suitable technique for sampling dissolved iodine species from soils maintained at realistic moisture contents. In addition, inorganic iodine added to soils was predominately bound with relatively low molecular weight (

In the original publication of the article, the affiliation of the first author. ‘‘Karzan A. M. Hawrami’’ was incorrectly published as ‘‘Sulaymaniyah Polytechnic University, Technical Institute of Halabja, Zamaqi, Halabja, Iraq’’. The correct affiliation is ‘‘Sulaimani Polytechnic University (SPU), Technical Institute of Halabja, Zamaqi, Halabja, Iraq’’.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Wheat farming provides 28.5% of global cereal production. After steady growth in average crop yield from 1950 to 1990, wheat yields have generally stagnated, which prompts the question of whether further improvements are possible. Statistical studies of agronomic parameters such as crop yield have so far exclusively focused on estimating parameters describing the whole of the data, rather than the highest yields specifically. These indicators include the mean or median yield of a crop, or finding the combinations of agronomic traits that are correlated with increasing average yields. In this paper, we take an alternative approach and consider high yields only. We carry out an extreme value analysis of winter wheat yield data collected in England and Wales between 2006 and 2015. This analysis suggests that, under current climate and growing conditions, there is indeed a finite upper bound for winter wheat yield, whose value we estimate to be 17.60 tonnes per hectare. We then refine the analysis for strata defined by either location or level of use of agricultural inputs. We find that there is no statistical evidence for variation of maximal yield depending on location, and neither is there statistical evidence that maximum yield levels are improved by high levels of crop protection and fertilizer use.

To understand the dynamic mechanisms governing soil-to-plant transfer of selenium (Se), technetium-99 (99Tc) and iodine (I), a pot experiment was undertaken using 30 contrasting soils after spiking with 77Se, 99Tc and 129I, and incubating for 2.5 years. Two grass species (Agrostis capillaris and Lolium perenne) were grown under controlled conditions for 4 months with 3 cuts at approximately monthly intervals. Native (soil-derived) 78Se and127I, as well as spiked 77Se, 99Tc and 129I, were assayed in soil and plants by ICP-MS. The grasses exhibited similar behaviour with respect to uptake of all three elements. The greatest uptake observed was for 99Tc, followed by 77Se, with least uptake of 129I, reflecting the transformations and interactions with soil of the three isotopes. Unlike soil-derived Se and I, the available pools of 77Se, 99Tc and 129I were substantially depleted by plant uptake across the three cuts with lower concentrations observed in plant tissues in each subsequent cut. Comparison between total plant offtake and various soil species suggested that 77SeO42−, 99TcO4− and 129IO3−, in soluble and adsorbed fractions were the most likely plant-available species. A greater ratio of 127I/129I in the soil solid phase compared to the solution phase confirmed incomplete mixing of spiked 129I with native 127I in the soil, despite the extended incubation period, leading to poor buffering of the spiked available pools. Compared to traditional expressions of soil-plant transfer factor (TFtotal), a transfer factor (TFavailable) expressed using volumetric concentrations of speciated ‘available’ fractions of each element showed little variation with soil properties.

Metal bioavailability and phytotoxicity may be exaggerated when derived from studies based on amending soils with soluble metal salts. It is therefore important to evaluate soil tests for their consistency in estimating plant uptake and phytotoxicity in both field-contaminated and freshly-spiked soils. This study aimed to compare the effects of zinc (Zn) on plant growth in soils (i) recently spiked with soluble Zn and (ii) historically amended with biosolids. The objective was to reconcile methods for determining bioavailability in both cases by testing a range of ‘quantity-based’ and ‘intensity-based’ assays. Soils with a range of Zn concentrations, from an arable farm used for biosolids disposal for over a century, were further amended with Zn added in solution, and were incubated for one month prior to planting with barley seeds in a glasshouse pot trial. The majority (67–90%) of the added Zn remained isotopically exchangeable after 60 days. Zinc in the solution phase of a soil suspension was present mainly as free Zn2+ ions. Cadmium bioaccumulation factors were inversely proportional to Zn concentration in the soil solution confirming that greater Zn availability suppressed Cd uptake by plants. Measurements of soil Zn ‘quantities’ (total, EDTA-extractable and isotopically exchangeable) and ‘intensity’ (solution concentration and free ion activity) were correlated with Zn uptake and toxicity by barley plants. Correlations using Zn intensity were much stronger than those using quantity-based measurements. The free Zn2+ ion activity appears to be a consistent driver for plant uptake and phytotoxic response for both metal-spiked soils and historically contaminated soils. Surprisingly, soil Zn accumulation of up to 100 times the current regulations for normal arable land only produced a mild toxic response suggesting that constituents in biosolids (e.g. organic matter and phosphates) strongly restrict metal bioavailability.

Iodine is an essential micronutrient for human health: insufficient intake can have multiple effects on development and growth, affecting approximately 1.9 billion people worldwide. Previous reviews have focussed on iodine analysis in environmental and biological samples, however, no such review exists for the determination of iodine fractionation and speciation in soils. This article reviews the geodynamics of both stable 127I and the long-lived isotope 129I (t1/2 ¼ 15.7 million years), alongside the analytical methods for determining iodine concentrations in soils, including consideration of sample preparation. The ability to measure total iodine concentration in soils has developed significantly from rudimentary spectrophotometric analysis methods to inductively coupled plasma mass spectrometry (ICP-MS). Analysis with ICP-MS has been reported as the best method for determining iodine concentrations in a range of environmental samples and soils due to developments in extraction procedures and sensitivity, with extremely good detection limits typically

Chemical elements that are either present naturally in the soil or introduced by pollution are more usefully estimated in terms of “availability” of the element, since it is this property that can be related to mobility and uptake by plants. A good estimate of the immediately available fraction can be achieved by measuring the concentration, or activity, of chemical species in soil pore water. Current analytical techniques enable the application of this approach to trace elements, such as plant and animal micronutrients and those defined as potentially toxic elements (PTEs) in environmental studies. A complete chemical analysis of soil pore water represents a powerful diagnostic tool for the interpretation of many soil chemical phenomena relating to soil fertility, mineralogy, and environmental fate. This chapter describes some of the current methodologies used to extract soil pore water. In particular, five laboratory-based methods are described and discussed in detail: (i) high speed centrifugation-filtration, (ii) low (negative-) pressure Rhizon samplers and passive diffusion samplers, (iii) high pressure soil squeezing, (iv) equilibration of dilute soil suspensions, and (v) diffusive gradients in thin-films (DGT). A number of operational factors are presented: pressure applicable (i.e., pore size accessed), moisture prerequisites of the soil, pore water yield, efficiency, duration of extraction, materials and possible sources of contamination for micronutrient and PTE studies. There is also consideration of the advantages and disadvantages of the methods, including costs and material availability.

Highlight Using non-destructive imaging techniques and numerical modelling, we quantify differences in hydraulic and structural properties of bulk and rhizosphere soil for sand and clay loam soils., Understanding the dynamics of water distribution in soil is crucial for enhancing our knowledge of managing soil and water resources. The application of X-ray computed tomography (CT) to the plant and soil sciences is now well established. However, few studies have utilized the technique for visualizing water in soil pore spaces. Here this method is utilized to visualize the water in soil in situ and in three-dimensions at successive reductive matric potentials in bulk and rhizosphere soil. The measurements are combined with numerical modelling to determine the unsaturated hydraulic conductivity, providing a complete picture of the hydraulic properties of the soil. The technique was performed on soil cores that were sampled adjacent to established roots (rhizosphere soil) and from soil that had not been influenced by roots (bulk soil). A water release curve was obtained for the different soil types using measurements of their pore geometries derived from CT imaging and verified using conventional methods, such as pressure plates. The water, soil, and air phases from the images were segmented and quantified using image analysis. The water release characteristics obtained for the contrasting soils showed clear differences in hydraulic properties between rhizosphere and bulk soil, especially in clay soil. The data suggest that soils influenced by roots (rhizosphere soil) are less porous due to increased aggregation when compared with bulk soil. The information and insights obtained on the hydraulic properties of rhizosphere and bulk soil will enhance our understanding of rhizosphere biophysics and improve current water uptake models.

We demonstrate the application of a high-resolution X-ray Computed Tomography (CT) method to quantify water distribution in soil pores under successive reductive drying. We focus on the wet end of the water release characteristic (WRC) (0 to ?75 kPa) to investigate changes in soil water distribution in contrasting soil textures (sand and clay) and structures (sieved and field structured) and to determine the impact of soil structure on hydraulic behavior. The 3-D structure of each soil was obtained from the CT images (at a 10 ?m resolution). Stokes equations for flow were solved computationally for each measured structure to estimate hydraulic conductivity. The simulated values obtained compared extremely well with the measured saturated hydraulic conductivity values. By considering different sample sizes we were able to identify the smallest possible representative sample size which is required to determine a globally valid hydraulic conductivity.

Radiocaesium and radiostrontium enter the foodchain primarily via plant root uptake. Selecting varieties of crop that display low accumulation of these radionuclides has been suggested as an economically and socially acceptable remediation strategy for radiologically contaminated land. However, there is insufficient information available to assess the feasibility of this remediation approach. This paper presents a comprehensive literature-based evaluation of inter-varietal variation in accumulation of Cs and Sr in crop plants. Thirty-seven publications studying 27 plant species were identified as appropriate for these analyses. Inter-varietal variation was expressed at the ratio of the maximum to minimum observed concentrations for a given crop species and element and ranged from 1.0 to 6.3 and from 1.0 to 4.5 for Cs and Sr respectively. This variation suggests that exploitation of inter-varietal variation could be used in some crop species to reduce the transfer of these radionuclides to a similar extent to existing remediation strategies. Low-Sr accumulating varieties were also found to have lower concentrations of Ca, whereas low Cs-accumulating varieties were not shown to have low K accumulation. Concentrations of Cs and Sr in plants were not related, suggesting that finding varieties displaying low accumulation of both Sr and Cs may not be feasible. Varietal selection could be an effective remediation strategy, and could be used in combination with other existing methods, such as fertilisation and ploughing. However, a thorough investigation of species contributing the most to ingestion doses is recommended to fully determine the feasibility of varietal selection as a remediation strategy. The reproducibility of inter-varietal variation between sites and growing seasons should be the focus of future research.

In response to the comment raised by Zhang et al. (2016, doi: 10.1002/2015WR018432) we explore the differences in average velocity computed using slip and no-slip boundary conditions at the air water interface. We consider a porous medium in which the air phase acts to impede the movement of water rather than to lubricate it, a case closer to the observed distribution of water in our CT images. We find that, whilst the slip boundary condition may be a more accurate approximation, in cases where the air phase is seen to impede water movement the differences between the two approaches are negligible.

In this study, the effects of incubation time and the method of soil solution extraction and filtration on the empirical distribution coefficient (Kd) obtained by de-sorbing indigenous selenium (Se) and iodine (I) from arable and woodland soils under temperate conditions were investigated. Incubation time had a significant soil- and element- dependent effect on the Kd values, which tended to decrease with the incubation time. Generally, a four-week period was sufficient for the desorption Kd value to stabilise. Concurrent solubilisation of soil organic matter (OM) and release of organically-bound Se and I was probably responsible for the observed decrease in Kd with time. This contrasts with the conventional view of OM as a sink for Se and I in soils. Selenium and I Kd values were not significantly affected by the method of soil solution extraction and filtration. The results suggest that incubation time is a key criterion when selecting Se and I Kd values from the literature for risk assessments. Values derived from desorption of indigenous soil Se and I might be most appropriate for long-term assessments since they reflect the quasi-equilibrium state of their partitioning in soils.

Substitution of a species or cultivar with higher uptake of an element by one with lower uptake has been proposed as a remediation strategy following accidental releases of radioactivity. However, despite the importance of pasture systems for radiological dose, species/cultivar substitution has not been thoroughly investigated for forage grasses. 397 cultivars from four forage grass species; hybrid ryegrass (Lolium perenne L. x Lolium multiflorum Lam.), perennial ryegrass (Lolium perenne L.), Italian ryegrass (Lolium multiflorum Lam.) and tall fescue (Festuca arundinacea Shreb.); were sampled from 19 field-based breeding experiments in Aberystwyth and Edinburgh (UK) in spring 2013 and analysed for caesium (Cs) and strontium (Sr) concentrations. In order to calculate concentration ratios (CRs; the concentration of an element in a plant in relation to the concentration in the soil), soils from the experiments were also analysed to calculate extractable concentrations of Cs and Sr. To test if cultivars have consistently low Cs and Sr concentration ratios, 17 hybrid ryegrass cultivars were sampled from both sites again in summer 2013 and spring and summer 2014. Tall fescue cultivars had lower Cs and Sr CRs than the other species. Three of the selected 17 hybrid ryegrass cultivars had consistently low Cs CRs, two had consistently low Sr CRs and one had consistently low Cs and Sr CRs. Cultivar substitution could reduce Cs CRs by up to 14-fold and Sr CRs by 4-fold in hybrid ryegrass. The identification of species and cultivars with consistently low CRs suggests that species or cultivar substitution could be an effective remediation strategy for contaminated areas.

Highlights • We demonstrate the application of systematic model reduction to a complex crop model. • The model was manipulated under software control replacing variables with constants. • Redundancy in representation of nitrogen physiology and temperature was found. • The level of detail in Sirius is defensible if detailed prediction is required. • The approach increases the efficiency and rigour of model evaluation., An existing simulation model of wheat growth and development, Sirius, was evaluated through a systematic model reduction procedure. The model was automatically manipulated under software control to replace variables within the model structure with constants, individually and in combination. Predictions of the resultant models were compared to growth analysis observations of total biomass, grain yield, and canopy leaf area derived from 9 trials conducted in the UK and New Zealand under optimal, nitrogen limiting and drought conditions. Model performance in predicting these observations was compared in order to evaluate whether individual model variables contributed positively to the overall prediction. Of the 1 1 1 model variables considered 16 were identified as potentially redundant. Areas of the model where there was evidence of redundancy were: (a) translocation of biomass carbon to grain; (b) nitrogen physiology; (c) adjustment of air temperature for various modelled processes; (d) allowance for diurnal variation in temperature; (e) vernalisation (f) soil nitrogen mineralisation (g) soil surface evaporation. It is not suggested that these are not important processes in real crops, rather, that their representation in the model cannot be justified in the context of the analysis. The approach described is analogous to a detailed model inter-comparison although it would be better described as a model intra-comparison as it is based on the comparison of many simplified forms of the same model. The approach provides automation to increase the efficiency of the evaluation and a systematic means of increasing the rigour of the evaluation.

Arctic tundra currently stores half of the global soil carbon (C) stock(1). Climate warming in the Arctic may lead to accelerated CO2 release through enhanced decomposition and turn Arctic ecosyste ...

When assessing the radiological impacts of radioactive waste disposal, irrigation using groundwater contaminated with releases from the disposal system is a principal means of crop and soil contamination. In spite of their importance for radiological impact assessments, irrigation data are scarce and often associated with considerable uncertainty for several reasons including limited obligation to measure groundwater abstraction and differences in measuring methodologies. Further uncertainty arises from environmental (e.g. climate and landscape) change likely to occur during the assessment long time frame. In this paper, we derive irrigation data using the crop growth AquaCrop model relevant to a range of climates, soils and crops for use in radiological impact assessments. The AquaCrop estimates were compared with actual irrigation data reported in the literature and with estimates obtained from simple empirical methods proposed for use in radiological impact assessments. Further, the AquaCrop irrigation data were analysed using mixed effects modelling to investigate the effects of climate, soil and crop type on the irrigation requirement. Irrigation estimates from all models were within a reasonable range of the measured values. The AquaCrop estimates, however, were at the higher end of the range and higher than those from the empirical methods. Nevertheless, they may be more appropriate for conservative radiological assessments. The use of mixed effects modelling allowed for the characterisation of crop-specific variability in the irrigation data, and in contrast to the empirical methods, the AquaCrop and the mixed effects models accounted for the soil effect on the irrigation requirement. The approach presented in this paper is relevant for obtaining irrigation data for a specific site under different climatic conditions as well as for generic dose assessments. To the best of our knowledge, this is one of the most comprehensive analyses of irrigation data in the context of radiological impact assessment currently available.

The rate of reactions between humic acid (HA) and iodide (I(-)) and iodate (IO3(-)) have been investigated in suspensions spiked with (129)I at concentrations of 22, 44 and 88 μg L(-1) and stored at 10 °C. Changes in the speciation of (129)I(-), (129)IO3(-) and mixed ((129)I(-) + (129)IO3(-)) spikes were monitored over 77 days using liquid chromatography inductively coupled plasma mass spectrometry (LC-ICP-MS). In suspensions spiked with (129)I(-) 25% of the added I(-) was transformed into organic iodine (Org-(129)I) within 77 days and there was no evidence of (129)IO3(-) formation. By contrast, rapid loss of (129)IO3(-) and increase in both (129)I(-) and Org-(129)I was observed in (129)IO3(-)-spiked suspensions. However, the rate of Org-(129)I production was greater in mixed systems compared to (129)IO3(-)-spiked suspensions with the same total (129)I concentration, possibly indicating IO3(-)I(-) redox coupling. Size exclusion chromatography (SEC) demonstrated that Org-(129)I was present in both high and low molecular weight fractions of the HA although a slight preference to bond with the lower molecular weight fractions was observed indicating that, after 77 days, the spiked isotope had not fully mixed with the native (127)I pool. Iodine transformations were modelled using first order rate equations and fitted rate coefficients determined. However, extrapolation of the model to 250 days indicated that a pseudo-steady state would be attained after ∼200 days but that the proportion of (129)I incorporated into HA was less than that of (127)I indicating the presence of a recalcitrant pool of (127)I that was unavailable for isotopic mixing.

SUMMARYSimulation models for many crops are often simple, empirical equations that lack generality across different locations or different levels of abiotic stress making them difficult to apply by independent researchers. The objective of this paper is to describe and demonstrate a user-oriented model ‘BAMGRO’ for an underutilized crop, bambara groundnut with enough detail to be general across locations and genotypes but not so complex that independent users cannot apply it to their own situations. The model consists of different sub-modules that deal specifically with weather (thermal time), crop growth (canopy development, biomass production and yield), temperature (heat and cold stress), photoperiod (daylength control of phenology and reproductive development) and soil water (drought stress). The model predictions are achieved by defining genotype (i.e. cultivar) input files, daily weather parameters and soil characteristics. The model achieves a good fit between observed and predicted data for leaf area index (Nash and Sutcliffe (N-S), 0.80–0.84; mean absolute error (MAE) with maximum less than ± 0.50) for tested genotypes. Pod yield simulation correlated well with measured values (N-S 0.73–0.87; MAE ± 16 g m−2). Available soil moisture content correlated well with the observed data for a field site in Botswana indicating successful performance of the soil water module.

International audience; This letter details the collective views of a number of independent researchers on the technical assessment and evaluation of environmental models and software. The purpose is to stimulate debate and initiate action that leads to an improved quality of model development and evaluation, so increasing the capacity for models to have positive outcomes from their use. As such, we emphasize the relationship between the model evaluation process and credibility with stakeholders (including funding agencies) with a view to ensure continued support for modelling efforts. Many journals, including EM&S, publish the results of environmental modelling studies and must judge the work and the submitted papers based solely on the material that the authors have chosen to present and on how they present it. There is considerable variation in how this is done with the consequent risk of considerable variation in the quality and usefulness of the resulting publication. Part of the problem is that the review process is reactive, responding to the submitted manuscript. In this letter, we attempt to be proactive and give guidelines for researchers, authors and reviewers as to what constitutes best practice in presenting environmental modelling results. This is a unique contribution to the organisation and practice of model-based research and the communication of its results that will benefit the entire environmental modelling community. For a start, our view is that the community of environmental modellers should have a common vision of minimum standards that an environmental model must meet. A common vision of what a good model should be is expressed in various guidelines on Good Modelling Practice. The guidelines prompt modellers to codify their practice and to be more rigorous in their model testing. Our statement within this letter deals with another aspect of the issue it prompts professional journals to codify the peer-review process. Introducing a more formalized approach to peer-review may discourage reviewers from accepting invitations to review given the additional time and labour requirements. The burden of proving model credibility is thus shifted to the authors. Here we discuss how to reduce this burden by selecting realistic evaluation criteria and conclude by advocating the use of standardized evaluation tools as this is a key issue that needs to be tackled. (C) 2010 Elsevier Ltd. All rights reserved.

This research investigates the long-term kinetics of Cd(2+) sorption and desorption by calcium-exchanged clinoptilolite (CaCpt) and Y-type (CaY) zeolite using isotopic exchange with (109)Cd while maintaining pH at circumneutral values. The effects of Si/Al ratio and crystal structure of these zeolitic materials on intracrystalline transport of Cd are discussed. A first-order kinetic model was developed to describe the progressive transfer of Cd(2+) to a less reactive form within the zeolite structure, following initial sorption and subsequent desorption of Cd subject to different initial contact times. The kinetic model differentiates between two forms of sorbed Cd(2+) designated 'labile' and 'non-labile' in which the labile form is in immediate equilibrium with the free Cd(2+) ion activity in solution. A model combining diffusion and first-order kinetics for cation exchange was also employed to determine Cd(2+) diffusivity and intracrystalline exchange rates in CaY and CaCpt. The efficiency of Permeable Reactive Barriers (PRBs) containing zeolitic materials in protecting water systems against lateral flow of metal-contaminated leachate was simulated for three contrasting zeolites. The slow transfer of Cd between labile and non-labile forms was particularly important in moderating high concentration pulses of Cd traversing the PRB. In addition, the reversibility of Cd fixation effectively restored the sorption capability of the zeolite through slow leakage to drainage water.

Canopy development of bambara groundnut (Vigna subterranea (L.) Verdc) is affected by temperature stress, drought stress and photoperiod. The quantification of these documented effects by means of a suitable crop model, BAMGRO is presented in this paper. Data on canopy development from five growth chamber, four glasshouse and three field experiments were analyzed to calibrate and validate the BAMGRO model to produce simulations for temperature stress, drought stress and photoperiodic effect on two contrasting landraces; Uniswa Red (Swaziland) and S19-3 (Namibia). The daily initiation rate of new leaves is calculated by means of a Gaussian function and is altered by temperature stress, drought stress, photoperiod and plant density. The rate in dead leaf number is dependent upon the maximum senescence fraction which can be explained by physiological maturity, mutual shading, temperature stress and drought stress. The leaf area expansion is calculated as a function of leaf number and individual leaf size. The canopy development of bambara groundnut is modelled (and tested) in BAMGRO model by means of leaf initiation and leaf area expansion and branching and stem production are not considered. Nash and Sutcliffe value (N–S) is used as the measure of the mean square error to the observed variance. The simulated leaf number per plant correlated well with the glasshouse measurements for Uniswa Red for the two considered years (2007 and 2008) with high N–S value (Nash and Sutcliffe; from 0.76 to 0.92), low mean absolute error (MAE; from ±2.91 to 3.98) and Botswana field grown in January 18 and February 1, 2007 (N–S, 0.88 and 0.74; MAE, ±0.59 and ±6.18). Overall, simulated LAI correlated well with measured values for the two tested landraces; S19-3 (N–S value, 0.84) and Uniswa Red (N–S value, 0.80) with maximum MAE less than ±0.50 thus explaining canopy development under temperature stress, drought stress and variable photoperiods.

Research questions at the regional, national and global scales frequently require the upscaling of existing models. At large scales, simple model aggregation may have a prohibitive computational cost and lead to over-detailed problem representation. Methods that guide model simplification and revision have the potential to support the choice of the appropriate level of detail or heterogeneity within upscaled models. Efficient upscaling will retain only the heterogeneity that contributes to accurate aggregated results. This approach to model revision is challenging, because automatic generation of alternative models is difficult and the set of possible revised models is very large. In the case where simplification alone is considered, there are at least 2 n − 1 possible simplified models where n is the number of model variables. Even with the availability of High Performance Computing, it is not possible to evaluate every possible simplified model if the number of model variables is greater than roughly 35. To address these issues, we propose a method that extends an existing procedure for simplifying and aggregating mechanistic models based on replacing model variables with constants. The method generates simplified models by selectively aggregating existing model variables, retaining existing model structure while reducing the size of the set of possible models and ordering them into a search tree. The tree is then searched selectively. We illustrate the method using a catchment scale optimization model with c. 50,000 variables (Farm-adapt) in the context of adaptation to climatic change. The method was successful in identifying redundant model variables and an adequate model 10% smaller than the original model. We discuss how the procedure can be extended to other large models and compare the method to those proposed by others. We conclude by urging model developers to regard their models as a starting point and to consider the need for alternative models during model development.

The effect of prolonged contact time (up to 130 days) on the immobilization of Cd by sorption to calcium exchanged zeolite-X (CaX), under environmentally relevant conditions, was studied using both isotopic exchange and extended X-ray absorption fine structure spectroscopy (EXAFS). Sorption and isotopic exchange measurements revealed time-dependent Cd sorption and indicated the movement of Cd2+ ions into less accessible sites due to ageing. EXAFS suggested progressive fixation of Cd in the double six-ring (D6R) unit of the CaX structure. Proportional allocation of the apparent Cd–Si bond distance to two ‘end-members’, across all contact times, indicated that the bond distance for labile Cd was 3.41 A and for non-labile (or fixed) Cd was 3.47 A.

While mechanistic models tend to be detailed, they are less detailed than the real systems they seek to describe, so judgements are being made about the appropriate level of detail within the process of model development. These judgements are difficult to test, consequently it is easy for models to become over-parameterised, potentially increasing uncertainty in predictions. The work we describe is a step towards addressing these difficulties. We propose and implement a method which explores a family of simpler models obtained by replacing model variables with constants (model reduction by variable replacement). The procedure iteratively searches the simpler model formulations and compares models in terms of their ability to predict observed data, evaluated within a Bayesian framework. The results can be summarised as posterior model probabilities and replacement probabilities for individual variables which lend themselves to mechanistic interpretation. This provides powerful diagnostic information to support model development, and can identify areas of model over-parameterisation with implications for interpretation of model results. We present the application of the method to 3 example models. In each case reduced models are identified which outperform the original full model in terms of comparisons to observations, suggesting some over-parameterisation has occurred during model development. We argue that the proposed approach is relevant to anyone involved in the development or use of process based mathematical models, especially those where understanding is encoded via empirically based relationships.

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 (14)C 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 (14)C and (3)H 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.

Model averaging is a group of methods for combining predictions from several models which have the benefit of considering model uncertainty in addition to parameter uncertainty. The aim of this paper is to introduce these methods in the context of mechanistic model development. In model averaging predictions are combined, by weighting with factors related to model performance, resulting in ensemble predictions. Bayesian Model Averaging (BMA) is model averaging in a Bayesian framework where the model weights are Posterior model probabilities (PMPs). We describe three approximation methods (AIC, BIC and Laplace) for calculating PMPs and to compare with a full Bayesian approach implemented using a Markov Chain Monte Carlo (MCMC) method (Metropolis-Hastings). We also describe a simplified BMA approach which is readily implemented, as it only requires the maximum likelihood parameter estimates and Laplace approximation of the marginal likelihoods. We illustrate the application of BMA using a mechanistic model for predicting the plant uptake of radiocaesium from contaminated soils (the 'Absalom Model'). Ten models were selected for averaging, these comprised the full Absalom model and nine reduced models each derived from the full model. To assess performance model predictions and ensemble predictions were compared using an independent data set. The PMPs estimated using the MCMC approach and the Laplace approximation were similar and strongly weighted the models with fewer parameters. The AIC- and BIC-based estimates of the PMPs were correlated but differed considerably from the Laplace and MCMC-based PMP methods. For our example the simplified BMA approach was performed as well as the full approach. Individual predictions differed among models and the prediction ensembles resulting from all the approaches captured this uncertainty. We conclude that BMA is a valuable approach, relevant to mechanistic model development, and suggest a framework for incorporating BMA into model development.

Urban geochemical maps of Wolverhampton and Nottingham, based on multielement analysis of surface soils, have shown distribution patterns of “total” metals concentrations relating to past and present industrial and domestic land use and transport systems. Several methods have been used to estimate the solubility and potential bioavailability of metals, their mineral forms and potential risks to urban population groups. These include sequential chemical extraction, soil pore water extraction and analysis, mineralogical analysis by scanning electron microscopy, source apportionment by lead isotope analysis and the development of models to predict metal uptake by homegrown vegetables to provide an estimate of risk from metal consumption and exposure. The results from these research strategies have been integrated with a geographical information system (GIS) to provide data for future land-use planning.

Time-dependent sorption and desorption of Cd on calcite was studied over 210 days utilizing 109 Cd as a tracer to distinguish between ‘labile’ and ‘non-labile’ forms of sorbed Cd. Stabilizing the calcite suspensions for 12 months under atmospheric PCO2 and controlled temperature was necessary to reliably follow Cd dynamics following initial sorption. Results revealed time-dependant Cd sorption and marked desorption hysteresis by calcite under environmentally relevant conditions. Data obtained were fitted to a first-order kinetic model and a concentric shell diffusion model. Both models described the progressive transfer of Cd 2+ to a less reactive form within calcite and subsequent desorption of Cd subject to different initial contact times. The kinetic model provided a better fit to the combined sorption and desorption data (R 2 = 0.992). It differentiates between two ‘pools’ of sorbed Cd 2+ on calcite, ‘labile’ and ‘non-labile’, in which labile sorbed Cd is in immediate equilibrium with the free Cd 2+ ion activity in solution whereas non-labile Cd is kinetically restricted. For the diffusion model (R 2 = 0.959), the rate constants describing Cd dynamics in calcite produced a half-life for Cd desorption of � 175 d, for release to a ‘zerosink’ solution. Results from this study allow comment on the likely mechanisms occurring at the calcite surface following long-term Cd sorption. 2008 Published by Elsevier Ltd.

Antarctic lakes with simple plankton ecosystems are believed to be sensitive biological indicators of climate change. Models of the physical environment, in particular the ice layer, support understanding of how the ecosystems respond to meteorological variables. This paper describes how data from a previously reported automatic measuring probe and meteorological data from Davis station were used to develop a detailed thermodynamic model of the ice layer on Crooked Lake, one of the largest and deepest freshwater lakes in Antarctica. The general model structure is similar to a previously reported model of sea ice but with modifications specific to the Antarctic freshwater lake case informed by the data. The model inputs are atmospheric variables as well as water temperature, ice albedo and the radiation extinction coefficient for the ice. Heat and radiation fluxes at the ice–air and ice–water boundaries are calculated using equations chosen for their suitability for the Antarctic. In the case of shortwave radiation, equations were fitted to data from the automatic probe. Using the heat fluxes to establish boundary conditions, and incorporating the known thermodynamic properties of ice, the temperature profile within the ice and the resulting growth and melt of the ice can be calculated. The model uses a largely mechanistic approach, with most equations taken from established thermodynamic theories or empirical studies and only one adjustable parameter related to the sensible heat flux from the water, which is not easily calculated from the available data. It was found to accurately reproduce ice temperature and ice thickness data for the year 2003, with r 2 = 0.89, n = 2005. Finally, the model was simplified to run with air temperature as the only input variable and was shown to perform well—this suggests that freshwater lake ice is affected more by air temperature than any other variable, and is therefore a useful indicator of climate change in its own right.

Following the Chernobyl accident in 1986, restrictions were placed on the movement and slaughter of sheep within upland areas of the UK because radiocaesium activity concentrations in their meat exceeded 1000 Bq kg(-1) fresh weight. Some farms remain under restriction in 2007. From 1991 to 1993 detailed studies were conducted on three sheep farms within the restricted area of west Cumbria to systematically assess the various parameters which may contribute to the observed variability in radiocaesium activity concentrations within sheep flocks. This paper reports the spatial variation in soil and vegetation activity concentrations across the grazed areas at these farms and determines the influence of grazing behaviour on variability in (137)Cs activity concentrations between individual sheep within the flocks. Together with previously reported results, these new data are used to draw conclusions on the factors determining variability within the three flocks. However, the factors are too site specific to be able to generalise the findings to other farms within the restricted areas of the UK.

The solubility, lability and fractionation of zinc in a range of calcareous soils from Peshawar, Pakistan were studied (18 topsoils and 18 subsoils). The lability (E-value) of Zn was assessed as the fraction isotopically exchangeable with 70Zn2+; comparative extractions included 0.005M DTPA, 0.43M HNO3 and a Tessier-style sequential extraction procedure (SEP). Because of the extremely low concentration of labile Zn the E-value was determined in soils suspended in 0.0001M Na2-EDTA which provided reliable analytical conditions in which approximately 20% of the labile Zn was dissolved. On average, only 2.4% of soil Zn was isotopically exchangeable. This corresponded closely to Zn solubilised by extraction with 0.005 DTPA and by the carbonate extraction step (F1+F2) of the Tessier-style SEP. Crucially, although the majority of the soil CaCO3 was dissolved in F2 of the SEP, the DTPA dissolved only a very small proportion of the soil CaCO3. This suggests a superficial carbonate-bound form of labile Zn, accessible to extraction with DTPA and to isotopic exchange. Zinc solubility from soil suspended in 0.01M Ca(NO3)2 (PCO2 controlled at 0.03) was measured over three days. Following solution speciation using WHAM(VII) two simple solubility models were parameterised: a pH dependent 'adsorption' model based on the labile (isotopically exchangeable) Zn distribution coefficient (Kd) and an apparent solubility product (Ks) for ZnCO3. The distribution coefficient showed no pH-dependence and the solubility model provided the best fit to the free ion activity (Zn2+) data, although the apparent value of log10 Ks (5.1) was 2.8log units lower than that of the mineral smithsonite (ZnCO3). Refereed/Peer-reviewed

Short rotation coppice (SRC) such as Salix spp. can be grown as an energy crop and offers some potential for economic and practical phytoextraction of marginally contaminated arable soil. This study tested various soil amendments intended to increase soil metal availability to Salix, investigated the distribution of metal between different tree fractions and assessed the viability of phytoextraction using SRC on arable soils. Several Salix genotypes were grown in field trials over 4 years. Cd and Zn concentrations were generally ranked in the order leaves > bark > wood. Metal concentrations in wood increased towards the top of the willow stems, whereas concentrations in leaves showed the opposite trend. None of the amendments significantly increased uptake of Zn by willow. However, in response to a range of soil HCl treatments, mean Cd concentrations in stems and leaves were 112% and 130% of control values. Data from the current experiment, and previous studies, were combined to develop a predictive model of Cd and Zn stem uptake by Salix. The minimum biological concentration factor (BCF) required to achieve a prescribed soil metal target was also calculated based on typical proportions of bioavailable Cd in sludge-amended soils for a 25-year Salix rotation. The best Salix genotypes investigated achieved less than 20% of the uptake rate required to remove one third of the soil Cd content (equivalent to the average isotopically exchangeable Cd fraction in soils at the study site).