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Citrate and phytase root exudates contribute to improved phosphorus (P) acquisition efficiency in Nicotiana tabacum (tobacco) when both exudates are produced in a P deficient soil. To test the importance of root intermingling in the interaction of citrate and phytase exudates, Nicotiana tabacum plant-lines with constitutive expression of heterologous citrate (Cit) or fungal phytase (Phy) exudation traits were grown under two root treatments (roots separated or intermingled) and in two soils with contrasting soil P availability. Complementarity of plant mixtures varying in citrate efflux rate and mobility of the expressed phytase in soil was determined based on plant biomass and P accumulation. Soil P composition was evaluated using solution (31) P NMR spectroscopy. In the soil with limited available P, positive complementarity occurred in Cit+Phy mixtures with roots intermingled. Root separation eliminated positive interactions in mixtures expressing the less mobile phytase (Aspergillus niger PhyA) whereas positive complementarity persisted in mixtures that expressed the more mobile phytase (Peniophora lycii PhyA). Soils from Cit+Phy mixtures contained less inorganic P and more organic P compared to monocultures. Exudate-specific strategies for the acquisition of soil P were most effective in P-limited soil and depended on citrate efflux rate and the relative mobility of the expressed phytase in soil. Plant growth and soil P utilization in plant systems with complementary exudation strategies are expected to be greatest where exudates persist in soil and are expressed synchronously in space and time.

Zymographic methods for the 2D distribution of phosphatase activity in soils have markedly advanced our understanding of root-soil-microbiota interactions. Robust quantitative approaches for 2D assays, which use 4-methylubelliferyl phosphate (4-MUP), are needed to advance a mechanistic understanding of enzyme behaviour and distribution in soils. We present improvements to the method for phosphatase zymography in rhizobox studies, involving (1) a systematic evaluation of 4-methylumbelliferone (4-MU) based calibration functions in relation to image exposure time and (2) the development of advanced image analysis tools for lateral and longitudinal distributions of phosphatase activity along barley roots (Hordeum vulgare L., cv Optic). Exposure time (

Aims Intercropping can improve plant yields and soil phosphorus (P) use efficiency. This study compares inter- and intra-species intercropping, and determines whether P uptake and shoot biomass accumulation in intercrops are affected by soil P availability. Methods Four barley cultivars (Hordeum vulgare L.) and three legume species (Trifolium subterreneum, Ornithopus sativus and Medicago truncatula) were selected on the basis of their contrasting root exudation and morphological responses to P deficiency. Monocultures and barley-barley and barley-legume intercrops were grown for 6 weeks in a pot trial at very limiting, slightly limiting and excess available soil P. Above-ground biomass and shoot P were measured. Results Barley-legume intercrops had 10–70% greater P accumulation and 0–40% greater biomass than monocultures, with the greatest gains occurring at or below the sub-critical P requirement for barley. No benefit of barley-barley intercropping was observed. The plant combination had no significant effect on biomass and P uptake observed in intercropped treatments. Conclusions Barley-legume intercropping shows promise for sustainable production systems, especially at low soil P. Gains in biomass and P uptake come from inter- rather than intra-species intercropping, indicating that plant diversity resulted in decreased competition between plants for P. Electronic supplementary material The online version of this article (doi:10.1007/s11104-017-3365-z) contains supplementary material, which is available to authorized users.

Phosphorus (P) and nitrogen (N) use efficiency may be improved through increased biodiversity in agroecosystems. Phenotypic variation in plants’ response to nutrient deficiency may influence positive complementarity in intercropping systems. A multicomponent screening approach was used to assess the influence of P supply and N source on the phenotypic plasticity of nutrient foraging traits in barley (H. vulgare L.) and legume species. Root morphology and exudation were determined in six plant nutrient treatments. A clear divergence in the response of barley and legumes to the nutrient treatments was observed. Root morphology varied most among legumes, whereas exudate citrate and phytase activity were most variable in barley. Changes in root morphology were minimized in plants provided with ammonium in comparison to nitrate but increased under P deficiency. Exudate phytase activity and pH varied with legume species, whereas citrate efflux, specific root length, and root diameter lengths were more variable among barley cultivars. Three legume species and four barley cultivars were identified as the most responsive to P deficiency and the most contrasting of the cultivars and species tested. Phenotypic response to nutrient availability may be a promising approach for the selection of plant combinations for minimal input cropping systems.

The buffering of phosphorus concentrations in soil solution by the soilsolid phase is an important process for providing plant root access to nutrients. Accordingly, the size of labile solid phase-bound phosphorus pool and the rate at which it can resupply phosphorous into the dissolved phase can be important variables in determining when the plant availability of the nutrient may be limited. The phosphorus labile pool (Plabile) and its desorption kinetics were simultaneously evaluated in 10 agricultural UK soils using the diffusive gradients in thin-films (DGT) technique. The DGT-induced fluxes in the soil and sediments model (DIFS) was fitted to the time series of DGT deployments (1−240 h), which allowed the estimation of Plabile, and the system response time (Tc). The Plabile concentration was then compared to that obtained by several soil P extracts including Olsen P, FeO-P, and water extractable P, in order to assess if the data from these analytical procedures can be used to represent the labile P across different soils. The Olsen P concentration, commonly used as a representation of the soil labile P pool, overestimated the desorbable P concentration by 6-fold. The use of this approach for the quantification of soil P desorption kinetic parameters found a wide range of equally valid solutions for Tc. Additionally, the performance of different DIFS model versions working in different dimensions (1D, 2D, and 3D) was compared. Although all models could provide a good fit to the experimental DGT time series data, the fitted parameters showed a poor agreement between different model versions. The limitations of the DIFS model family are associated with the assumptions taken in the modeling approach and the three-dimensional (3D) version is here considered to be the most precise among them.

The cation exchange capacity (CEC) of seven well characterized halloysite nanotubes (HNTs) in the dehydrated 7 Å form has been measured using a method based on cobalt hexammine exchange. In addition to unbuffered measurements, which varied between 2.9 and 9.3 cmol(+)kg−1, CECs were also determined over a wide pH range and proton titration measurements were conducted on two samples. The data were fitted using a constant capacitance model based on the presence of two sites: permanently charged sites and pH-dependent variable charged sites. Normalization of CEC to the average specific surface area (BET) of the halloysite samples reduces considerably the variation of CEC values for the different samples particularly over the intermediate pH range (5–9) with the average value at pH 7 equal to 8.5 cmol(+)kg−1and a standard deviation of 1.17. Overall the CEC behaviour of the seven samples appears reasonably consistent throughout the set. Calculations based on proton titrations suggest a ratio of variable charge to basal sites for the dehydrated halloysite nanotubes of ∼3:1.

An improved understanding of potential soluble phosphorus (P) loss in run-off and leachate from agriculturally managed soils presents practical and theoretical challenges. Our study aimed to discover whether modified Morgan extractable P (MMP) can be used to predict water-soluble P (WSP). We first addressed the relationship between MMP and WSP, and whether MMP is useful for predicting the WSP concentrations demanded by water quality regulations. Secondly, we applied novel soil chemical models to explain why the relationship between MMP and WSP depends upon soil properties. Thirdly, we explain how soil properties relate to potential soluble P loss in situations in which soil is subjected to a wide liquid-to-soil ratio (e.g. run-off and rivers) compared with those with a narrow ratio (e.g. soil porewater). To address these P loss scenarios, 60 agricultural topsoils (0–10 cm) were collected from a mixed-farming catchment (Lunan catchment, northeast Scotland) and chemically characterized. Theoretical understanding of P solubility was obtained with a P sorption model. The data showed variability in the relationship between MMP and WSP. Modelling shows the MMP versus WSP relationship is nonlinear, depending on several confounding factors (P sorption capacity (PSC), Ca, pH) and the liquid-to-soil ratio (L:S) employed for WSP determination. Consequently, the slope of the relationship is not unique but depends subjectively on the set of soils surveyed. MMP versus WSP at large L:S (e.g. in run-off or rivers) is positively correlated to PSC, whereas at narrow L:S (e.g. porewater) there is a negative correlation with PSC. The study provides new ideas for the interpretation and extrapolation of agronomic soil test data for soils of varied properties and highlights the need to utilize insights from soil chemistry.

The article “Organic phosphorus in the terrestrial environment: a perspective on the state of the art and future priorities”, written by Timothy S George et al., was originally published with incorrect affiliation information for one of the co-authors, E. Klumpp.

Four geographically distinct regions of Scotland are used to describe temporal and spatial trends in soil phosphorus (P) using ca. 180 000 routine advisory samples which included information on previous and next crop. Regional differences in Modified Morgan's extractable P concentrations exist primarily in response to differences in land use and particularly the greater role of livestock in the west compared to arable cropping in the east. No obvious or consistent temporal trend in extractable P was apparent, but a very wide range of soil properties and extractable P concentrations was evident. The current (since 2010) target soil P status is moderate (M) for cereal-based arable rotations and the upper half of moderate (M+) for rotations with potatoes, and more than half the samples fell into this range (with 40% M−). Some ca. 30% of samples fell below this, while 14% had high or very high P status, most noticeably where the previous crop was either potatoes, vegetables or berries. Half (ca. 13 000) of the samples with a low soil P status also had a below optimum pH. An apparent contradiction between the regularly reported long-term declines in agricultural P surplus not being matched by a drop in extractable soil P status is discussed. This ‘lag effect’ means many Scottish agricultural soils have the capability to buffer change in extractable soil P. We make the first attempt to link advisory P data with information from the National Soil Database of Scotland and explore the value of introducing ‘soil type’ into the P fertilizer recommendation system.

Made available in DSpace on 2018-12-11T16:49:52Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-06-01 Biotechnology and Biological Sciences Research Council Background: The dynamics of phosphorus (P) in the environment is important for regulating nutrient cycles in natural and managed ecosystems and an integral part in assessing biological resilience against environmental change. Organic P (Po) compounds play key roles in biological and ecosystems function in the terrestrial environment being critical to cell function, growth and reproduction. Scope: We asked a group of experts to consider the global issues associated with Po in the terrestrial environment, methodological strengths and weaknesses, benefits to be gained from understanding the Po cycle, and to set priorities for Po research. Conclusions: We identified seven key opportunities for Po research including: the need for integrated, quality controlled and functionally based methodologies; assessment of stoichiometry with other elements in organic matter; understanding the dynamics of Po in natural and managed systems; the role of microorganisms in controlling Po cycles; the implications of nanoparticles in the environment and the need for better modelling and communication of the research. Each priority is discussed and a statement of intent for the Po research community is made that highlights there are key contributions to be made toward understanding biogeochemical cycles, dynamics and function of natural ecosystems and the management of agricultural systems. The James Hutton Institute Lancaster Environment Centre Lancaster University Lincoln University, Lincoln Universidade Estadual do Norte Fluminense Darcy Ribeiro (UENF - Laboratório de Solos), Av. Alberto Lamego 2000 Plant and Soil Sciences University of Delaware, 160 Townsend Hall Faculty of Environment and Natural Resources Chair of Soil Ecology University of Freiburg, Bertoldstraße 17 Rothamsted Research West Common Harpenden Rothamsted Research West Common Harpenden, North Wyke College of Agricultural Sciences Department of Crop Science Sao Paulo State University (UNESP), 1780, Jose Barbosa de Barros st Institute of Bio- and Geosciences IBG-3: Agrosphere Forschungszentrum Jülich GmbH Oak Ridge National Laboratory, P.O.Box 2008 DISAFA Soil Biogeochemistry University of Turin, largo Braccini 2 University of Minnesota-, Cities, 1479 Gortner Ave. Saint Paul China Agricultural University Le Laboratoire des Sciences du Climat et de l’Environnement IPSL-LSCE CEA/CNRS/UVSQ Saclay Institute of Soil Science University of Hohenheim, Emil Wolff Str. 27 INRA UMR ECO&SOLS Evolutionary Biology Centre EBC, Kåbovägen 4, house 7 Centre for Agri-environmental Research School of Agriculture Policy and Development University of Reading, Whiteknights, PO Box 237 Climate Impacts Research Centre Dep. of Ecology and Environmental Science Umeå University CSIRO Agriculture & Food D-USYS ETH Zurich, Tannenstrasse 1 Department of Environmental Sciences Norwegian University of Life Sciences, Post Box 5003 University of Guelph, 50 Stone Road East National Institute of Agricultural Research of Uruguay Department of Chemical & Biological Engineering The University of Sheffield, Mappin Street, S1 3JD School of Biological Sciences University of East Anglia, Norwich Research Park, Norwich International Plant Nutrition Institute, 18 Maplewood Drive Centro Interdipartimentale di Ricerca sulla Risonanza Magnetica Nucleare per l’Ambiente l’Agro-Alimentare ed i Nuovi Materiali (CERMANU) Università di Napoli Federico II, Via Università 100 Universidad de La Frontera School of Environment and Life Sciences University of Salford School of the Environment Natural Resources and Geography Bangor University School of Biological Sciences and Institute for Global Food Security The Queen’s University of Belfast, Medical Biology Centre, 97 Lisburn Road CENA University of Sao Paulo, Avenida Centenario, 303 Teagasc Environmental Research Centre, Johnstown Castle Centre for Ecology & Hydrology, Library Avenue, Bailrigg Department of Soil Biogeochemistry Bayreuth Center of Ecology and Environmental Research (BayCEER) University Bayreuth, Dr.-Hans-Frisch-Str. 1-3 Guangdong Provincial Key Laboratory of Applied Botany South China Botanical Garden Chinese Academy of Sciences, 723 Xingke Road, Tianhe District School of Geographical Sciences & School of Chemistry University of Bristol, University Road Soil Science and Soil Protection Martin Luther University Halle-Wittenberg, von-Seckendorff-Platz 3 Centre for Ice and Climate Niels Bohr Institute University of Copenhagen NERC Isotope Geosciences Facility British Geological Survey Department of Microbiology and Ecosystem Science University of Vienna, Althanstrasse 14 University of Northern British Columbia, 3333 University Way Brigham Young University Centre for Environmental Sciences Hasselt University Building D, Agoralaan Yamagata University Department of Biology Lund University, Biology Building Sölvegatan 35 Assessment of Microbial Environment Graduate School of Biosphere Science Hiroshima University College of Agricultural Sciences Department of Crop Science Sao Paulo State University (UNESP), 1780, Jose Barbosa de Barros st Biotechnology and Biological Sciences Research Council: BB/K018167/1 Biotechnology and Biological Sciences Research Council: BB/L025671/2

Root exudation of phytase could improve the ability of plants to access organic forms of soil phosphorus (P), thereby minimizing fertilizer requirements and improving P use efficiency in agroecosystems. After 75 days growth in a high available P soil, shoot biomass and P accumulation, soil pH, and rhizosphere P depletion were investigated in Nicotiana tabacum wild-type and transgenic plant-lines expressing and exuding Aspergillus niger phytase (ex::phyA), or a null-vector control. Solution 31P NMR analysis revealed a 7% to 11% increase in orthophosphate and a comparable depletion of undefined monoester P compounds (-13 to -18%) in the rhizosphere of tobacco plants relative to the unplanted soil control. Wild-type plants had the greatest impact on the composition of rhizosphere P based on the depletion of other monoester P, polyphosphate, and phosphonate species. The depletion of phytase-labile P by ex::phyA plants was associated with decreased proportions of other monoester P, rather than myo-InsP6 as expected. Rhizosphere pH increased from 6.0 to 6.5–6.7 in transgenic plant soils, beyond the pH optimum for A. niger phyA activity (pH=5), and may explain the limited specificity of ex::phyA plants for phytate in this soil. The efficacy of single exudation traits (e.g., phytase) therefore appear to be limited in P-replete soil conditions and may be improved where soil pH matches the functional requirements of the enzyme or trait of interest.

By use of a multi-site surface complexation model, Duputel et al. (2013) showed that citrate can decrease the solubility of phosphorus in soils, in contrast to what is commonly expected. We have identified several major errors in their model, which put the conclusions in doubt. We argue that major re-evaluation of their modelling approach is needed. (c) 2014 Elsevier Ltd. All rights reserved.

Background and Aims Plant acquisition of endogenous forms of soil phosphorus (P) could reduce external P requirements in agricultural systems. This study investigated the interaction of citrate and phytase exudation in controlling the accumulation of P and depletion of soil organic P by transgenic Nicotiana tabacum plants. Methods N. tabacum plant lines including wild-type, vector controls, transgenic plants with single-trait expression of a citrate transporter (A. thaliana frd3) or fungal phytases (phyA: A. niger, P. lycii) and crossed plant lines expressing both traits, were characterized for citrate efflux and phytase exudation. Monocultures and intercropped combinations of single-trait plants were grown in a low available P soil (12 weeks). Plant biomass, shoot P accumulation, rhizosphere soil pH and citrate-extractable-P fractions were determined. Land Equivalent Ratio and complementarity effect was determined in intercropped treatments and multiple-linear-regression was used to predict shoot P accumulation based on plant exudation and soil P depletion. Results Crossed plant lines with co-expression of citrate and phytase accumulated more shoot P than single-trait and intercropped plant treatments. Shoot P accumulation was predicted based on phytase-labile soil P, citrate efflux, and phytase activity (Rsq=0.58, P < .0001). Positive complementarity occurred between intercropped citrate- and phytase-exuding plants, with the greatest gains in shoot P occurring in plant treatments with A. niger phyA expression. Conclusions We show for the first time that trait synergism associated with the exudation of citrate and phytase by tobacco can be linked to the improved acquisition of P and the depletion of soil organic P.

In a recent article by Devau et al. (2011) a mechanistic model based on a surface complexation theory is presented which aims to predict the solubility of inorganic phosphorus (P) in soils. Compared with many empirical soil studies, the surface complexation approach as applied to soils is to be encouraged since it provides a coherent and mechanistically based foundation for the interpretation of a wide range of soil chemical phenomena. Soil chemists have attempted to explain P solubility for decades, so Devau et al. (2011) have attempted a difficult task but unfortunately they have introduced a number of errors that need to be corrected. The purpose of the present contribution is to suggest some corrections to their proposed model and provide commentary on the errors and inconsistencies concerning the application and citation of the sources of thermodynamic data. Hopefully this will provide some insights for others wishing to progress this subject.

Moorland carbon reserves in organo-mineral soils may be crucial to predicting landscape-scale variability in soil carbon losses, an important component of which is dissolved organic carbon (DOC). Surface water DOC trends are subject to a range of scaling, transport and biotic processes that disconnect them from signals in the catchment's soils. Long-term soil datasets are vital to identify changes in DOC release at source and soil C depletion. Here we show, that moorland soil solution DOC concentrations at three key UK Environmental Change Network sites increased between 1993–2007 in both surface- and sub- soil of a freely-draining Podzol (48 % and 215 % increases in O and Bs horizons, respectively), declined in a gleyed Podzol and showed no change in a Peat. Our principal findings were that: (1) considerable heterogeneity in DOC response appears to exist between different soils that is not apparent from the more consistent observed trends for streamwaters, and (2) freely-draining organo-mineral Podzol showed increasing DOC concentrations, countering the current scientific focus on soil C destabilization in peats. We discuss how the key solubility controls on DOC associated with coupled physico-chemical factors of ionic strength, acid deposition recovery, soil hydrology and temperature cannot readily be separated. Yet, despite evidence that all sites are recovering from acidification the soil-specific responses to environmental change have caused divergence in soil DOC concentration trends. The study shows that the properties of soils govern their specific response to an approximately common set of broad environmental drivers. Key soil properties are indicated to be drainage, sulphate and DOC sorption capacity. Soil properties need representation in process-models to understand and predict the role of soils in catchment to global C budgets. Catchment hydrological (i.e. transport) controls may, at present, be governing the more ubiquitous rises in river DOC concentration trends, but soil (i.e. source) controls provide the key to prediction of future C loss to waters and the atmosphere.

Rapid organic matter turnover is driven by advective transport processes in sandy permeable sediments, allowing them to act as biocatalytic filters. This filtering capacity is largely defined by the permeability of the sediment, which describes the flow of water through a porous medium. However, little is known about the temporal variability of sandflat permeability under natural conditions. Therefore, the changes in sediment permeability in an intertidal sandflat were measured over a year. The results demonstrated temporal variation in permeability related to the sediment water content and concentration of extracellular polymeric substances (EPS) in the pore space. The refractory bound fraction of EPS was one of the main contributors to this variation. Proteins associated with the bound EPS fraction appeared to covary with the effect of EPS on permeability. Levels of EPS proteins were as high (415 to 2170 μg g−1 bovine serum albumin equivalents) as carbohydrate levels (560 to 1400 μg g−1 D-glucose equivalents) in this sediment. Elevated levels of EPS were observed in the summer corresponding to low permeability, which also corresponded with microphytobenthic blooms and resulting production of exudates. A powerful storm event during the experiment with associated wind-generated waves resulted in a reworking of the sediment. This cleared pore spaces, resuspended material and reduced sediment compaction, effecting an overall change in permeability from 3.6 × 10−11 to 4.8 × 10−11 m2. The hypotheses that sandy sediment permeability varies with time and is influenced by EPS production were supported, and the effect of an episodic storm event was recorded.

Quantifying the amount and distribution of soil organic carbon (SOC) within natural soils is important for sample design, C budgeting/pool estimation, and understanding SOC turnover at a process level. We examined the distribution of SOC across a typical UK upland, moorland catchment to establish the amount and spatial structure of variability in key soil properties affecting SOC stocks, namely O horizon C content, bulk density (DB) and horizon depth. Organic horizons of Histosols and Gleysols had greater SOC contents but smaller bulk densities than Podzols and Leptosols. Consequently, SOC density differences between soils were minimized and horizon depth variation became crucial to the measurement of SOC stocks. However, individual Podzol profiles stored appreciable amounts of SOC in O horizons (up to 50 kg m–2). Geostatistical analyses showed spatially structured variance in many properties relating to SOC storage at both plot (variograms reaching sills at ranges 3–8 m) and catchment scales (ranges 437–529 m). The increase in variance from plot to catchment scales was large for O horizon depth. However, DB showed complex scale and soil type inter-relationships, with similar variance at different scales. We show that detailed soil investigations spanning multiple spatial scales are necessary to quantify soil C storage properties for purposes of hydro-ecological modeling and C budgeting at small catchment scales. This has implications for upscaling to regional or national soil C databases.

This study assesses a simple sediment source tracing method using major- (Al, Ca, Fe, K, Mg, Mn, Na, P, Si, Ti) and trace-element (Ba, Be, Ce, Co, Cr, Mo, Nd, Pb, Sr, Th, V, Y, Zn) signatures of stream suspended particulate matter (SPM), bed sediments and soils in a small agricultural catchment in NE Scotland. Whilst most erosion studies characterise the large amounts of material mobilised at the highest flows, this study aimed to assess properties of sediments during moderate to low flow periods. These occur more frequent than intense storms and are important in linking stream sediments, near-channel sources and aquatic ecosystem impacts. Data were transformed by multivariate statistical methods to compare elemental signatures of SPM (ranging from 3 to 53 mg L −1 in the stream) and stream bed sediments with a limited number of near-channel source soils. Increased concentrations of Ce, Nd, Th and Y in subsoils contributed to the ability to discriminate between surface fieldslope and stream bank erosion sources. Stream bed sediments showed close matches with compositions of stream bank and arable surface soils, but signatures of SPM differed greatly from any of the sources. Large concentrations of Cr, Pb and Zn in SPM, particularly during summer (677, 177 and 661 mg kg −1 , respectively) exceeded water quality standards and were linked to an accumulation of trace elements associated with biological material. The potential for within-stream alteration of SPM in relation to erosion sources was confirmed by changes in the nature of the SPM organic matter observed by IR spectroscopy. Thus the potential is shown for multi-element signatures to give information on catchment sediment sources to aid land management decisions, given careful consideration of the effects of in-stream alteration of eroded material. However, this combined information may be beneficial to process understanding linking land use and stream ecosystems at critical ecological periods.

Establishing vegetated buffer strips (VBS) between cropland and watercourses is currently promoted as a principal control of diffuse pollution transport. However, we lackthe mechanistic understanding to evaluate P retention in VBS and predict risks of P transport to aquatic ecosystems. We observed that VBS establishment led to enhanced rates of soil P cycling, increasing soil P solubility and the potential amount leached to watercourses. Soil in VBS, relative to adjacentfields, had increased inorganic P solubility indices, dissolved organic P, phosphatase enzyme activity, microbial diversity, and biomass P. Small relative increases in the pool of soil P rendered labile had disproportionate effects on the P available for leaching. We propose a mechanism whereby the establishment of VBS on previous agricultural land causes a diversifying plant-microbial system which can access previous immobilized soil P from past fertilization or trapped sediment P. Laboratory experiments suggested that sediment-P inputs to VBS were insufficient alone to increase P solubility without biological cycling. Results showthat VBS management may require strategies, for example, harvesting vegetation, to offset biochemical processes that can increase the susceptibility of VBS soil P to move to adjoining streams.

Respirable forms of crystalline silica including the mineral cristobalite are known carcinogens and the subject of increasing regulation and codes of good practice. Some bentonites may contain cristobalite, but more commonly they contain opal-CT, or opal-C, which can be confused with cristobalite. In the light of increasing regulation there is a clear need for simple and reliable methods that can distinguish cristobalite from the opals in bentonites and other clay materials. Here we propose the use of a simple NaOH method that is easily applied to bulk samples of bentonites. The method consists of boiling the clay in 0.5 M NaOH for 10 min and measuring the weight of undissolved solid. The method is shown to be accurate for both the identification and quantification of opal in bentonites since these phases are soluble when thus treated. It is also a straightforward method to distinguish the opals from cristobalite as the latter is essentially insoluble. Adoption by industry of a simple operationally-defined method such as this would be preferable to ad hoc reliance on direct instrumental methods alone such as X-ray powder diffraction, as the latter almost always require a large element of judgement and experience on the part of the analyst, although direct methods do need to be used in a supporting role.

Summary Organic upland soils store large amounts of humified organic matter. The mechanisms controlling the leaching of this C pool are not completely understood. To examine the effects of temperature and microbial cycling on C leaching, we incubated five unvegetated soil cores from a Podzol O horizon (from NE Scotland), over a simulated natural temperature cycle for 1 year, whilst maintaining a constant soil moisture content. Soil cores were leached with artificial rain (177 mm each, monthly) and the leachates analysed for dissolved organic carbon (DOC) and their specific C-normalized UV absorbance determined (SUVA, 285 nm). Monthly values of respiration of the incubated soils were determined as CO2 efflux. To examine the effects of vegetation C inputs and soil moisture, in addition to temperature, we sampled O horizon pore waters in situ and collected five additional field soil cores every month. The field cores were leached under controlled laboratory conditions. Hysteresis in the monthly amount of DOC leached from field cores resulted in greater DOC on the rising, than falling temperature phases. This hysteresis suggested that photosynthetic C stimulated greater DOC losses in early summer, whereas limitations in the availability of soil moisture in late summer suppressed microbial decomposition and DOC loss. Greater DOC concentrations of in-situ pore waters than for any core leachates were attributed to the effects of soil drying and physico-chemical processes in the field. Variation in the respiration rates for the incubated soils was related to temperature, and respiration provided a greater pathway of C loss (44 g C m−2 year−1) than DOC (7.2 g C m−2 year−1). Changes in SUVA over spring and summer observed in all experimental systems were related to the period of increased temperature. During this time, DOC became less aromatic, which suggests that lower molecular weight labile compounds were not completely mineralized. The ultimate DOC source appears to be the incomplete microbial decomposition of recalcitrant humified C. In warmer periods, any labile C that is not respired is leached, but in autumn either labile C production ceases, or it is sequestered in soil biomass.

Summary We investigated the importance of physico-chemical mechanisms responsible for the release of dissolved organic matter (DOM) from a peaty soil. Columns containing peat aggregates (embedded within a sand matrix) provided an experimental system in which both convective and diffusive processes contributed to DOM leaching. The use of aggregated peat avoided the problems associated with traditional batch equilibration experiments in which soil structure is destroyed. Biotic and abiotic processes operating in the columns were manipulated by working with two unsterilized columns (at 5°C and 22°C) and one gamma irradiation-sterilized column (5°C). Continuous solute flows (< 80 hours) and periods of flow interruption (five interruptions of 6 hours to 384 hours) were applied to the columns (using a 1-mm NaCl electrolyte) to investigate mechanisms of diffusion-controlled release of DOM. For all columns, dissolved organic carbon and nitrogen (DOC and DON) effluent concentrations increased after resumption of flow and the maximum concentrations increased with increased flow-interruption duration. Measurements of effluent UV absorbance (λ= 285 nm) showed that the DOM leached immediately after the flow interruptions contained fewer aromatic moieties of lower molecular weight than the DOM leached after periods of steady flow. The sterilized column had larger DOC and DON effluent concentration spikes than those from the unsterilized column at 5°C (38 mg C dm−3 and 6.5 mg N dm−3 versus 13 mg C dm−3 and 6.5 mg N dm−3 after the 384 hours flow interruption). This result suggested that the concentrations of DOM resulting from physico-chemical release mechanisms (sterilized column) were attenuated by biological activity (unsterilized columns). Our results indicate that the peat’s microporous structure provides reservoirs of DOM that interact with solute in transport pores via abiotic, rate-controlled mass transport. Hence, diffusion can influence the quantity and composition of DOM leached from peat in the field depending on intensity and duration of rainfall.

Past disposal of high-lime chromite ore processing residue (COPR) from a chemical works in S.E. Glasgow, UK, has led to continuing release of toxic and carcinogenic hexavalent chromium (Cr(VI)) to groundwaters which are highly contaminated with Cr(VI)O4(2-). Traditional methods of remediating Cr(VI)-contaminated land, e.g. using ferrous sulfate and organic matter, have had limited success in converting Cr(VI) to less harmful and insoluble Cr(III). This paper describes the first application of calcium polysulfide (CaS(x)) to the remediation of contaminated groundwater and high-lime COPR in a series of laboratory experiments, which have demonstrated the effectiveness of the treatment in quantitatively and rapidly reducing Cr(VI) to Cr(III) over the pH range (8-12.5) typically found at the sites. Cr(III)-organic complexes, present in groundwater at one location, were also effectively precipitated upon treatment with CaS(x). The potential for large-scale use of CaS(x) in the remediation of Cr(VI) from COPR is also discussed.

A chemical model (constructed in the ORCHESTRA modeling framework) of an organic soil horizon was used to describe soil solution data (10 cm depth) and assess if seasonal variations in soil solution dissolved organic carbon (DOC) could be explained by purely abiotic (geochemical controls) mechanisms or whether factors related to biological activity are needed. The NICA-Donnan equation is used to describe the competitive binding of protons and cations and the charge on soil organic matter. Controls on organic matter solubility are surface charge and a parameter, gamma, that accounts for the distribution of humic molecules between hydrophobic and hydrophilic fractions. Calculations show that the variations in solute chemistry alone are not sufficient to account for the observed variations of DOC, but factors that alter gamma, such as biological activity, are. Assuming that DOC in organic soils is derived from soluble humic material and that gamma is modified seasonally due to biological activity (with monthly soil temperature used as a surrogate for biological activity) we are able to model the observed seasonality of soil solution DOC over a 10-year period. Furthermore, with modeled DOC coupled to other geochemical processes we also model soil solution pH and Al concentrations.

Summary The partitioning of chemical elements between the solid and solution phases in soil is fundamental in understanding processes such as leaching and bioavailability. Here I present a model in which the partitioning of Cd, Al and carbon in both mineral and organic soils can be simulated in the pH range 2–8. A two-phase additivity approach simulates ion adsorption by the soils using a hydrous ferric oxide and humic type surface. A model for the partitioning of soil humic matter has also been developed in which the NICA–Donnan model calculates humic surface charge. Other key processes represented include mineral solubilization and solution speciation. Methods for deriving model input parameters either from analytical data or by parameter optimization were used. Acid ammonium-oxalate-extractable Fe was used to estimate the amount of hydrous ferric oxide, and reactive humic substances were estimated by a scaled down version of the International Humic Substances Society method for the extraction of humic and fulvic acid. For initial calculation the 0.1 m HCl-extractable Al was used to estimate reactive Al. Optimization of reactive Al improved the fit of both the total dissolved Al data and the adsorbed Cd. The model for the solid–solution partitioning of humic substances could simulate reasonably well the release of carbon in the pH range 4–8 for both the organic and mineral soils.

The retention of Cd by an organic soil was investigated as a function of pH and ionic strength. The adsorption of Cd at pH values from 2 to 11 at two ionic strengths (0.053 M and 0.017 M LiNO3) wer...

Summary Gustafsson et al. in a recent paper in this Journal reported the effects of adding HCl, AlCl3 and Si(OH)4 on the pH and concentrations of Al and Si in 1:1 soil:solution systems at three different temperatures, using samples of soil from an allophanic Bs horizon. Contrary to their conclusions, their observations are compatible with Al in the soil solution being in equilibrium with a proto-imogolite allophane; it is neither necessary nor even plausible to postulate a hypothetical Al hydroxide. Concentrations of 0.2–0.4 mm Si in the equilibrated solutions at pH 5 could arise from an amorphous silica, probably phytoliths. They cannot come from the allophane.

Chromite ore processing residue (COPR) contains very high levels of chromium as Cr(III) and Cr(VI) and has a pH of ∼11.5 to 12. Millions of tonnes of COPR have in the past been deposited in urban areas. We have studied the factors that control leaching of Cr(VI), Ca, Al, Si, and Mg from COPR by means of batch experiments, mineralogical characterization of COPR via X-ray powder diffraction and scanning electron microscopy, and chemical equilibrium modeling. Batch experiments at a range of pH values and two liquid:solid ratios showed that mineral solubility control exists for aqueous concentrations of Cr(VI) above pH 10. Calculations indicate that the solid phases that control the solubility of Cr(VI) at pH values above 11 are Cr(VI)-substituted hydrogarnet (Ca3Al2(H4O4,CrO4)3) and Cr(VI)-hydrocalumite (Ca4Al2(OH)12CrO4·6 H2O), a layered double-hydroxide clay with chromate anions held in the interlayers. In the pH range 9.5 to 11, the description of the Cr(VI) concentration in solution was strongly improved by the incorporation in the model of Cr(VI)-ettringite (Ca6Al2(OH)12(CrO4)3·26 H2O), which precipitates as a secondary phase when hydrocalumite dissolves. The proposed model for leaching of COPR at high pH includes Cr(VI)-bearing hydrogarnet, Cr(VI)-hydrocalumite, Cr(VI)-ettringite, brucite, calcite, Ca2Al2(OH)10·3 H2O, CaH2SiO4, and gehlenite hydrate (Ca2Al2(OH)6SiO8H8·H2O). The model accurately predicts the concentrations of Cr(VI), Ca, Al, Si, and Mg in solution in the pH range 10 to 12 as well as the pH-buffering behavior. Below pH 8, a decrease in the Cr(VI) concentration in solution is observed, which may be attributed to sorption of chromate onto freshly precipitated Al and Fe hydroxide surfaces. Sulfate and carbonate show the same type of behavior as chromate. The chemistry of COPR shows similarities with cement and high-pH municipal waste incinerator bottom ash.

Chromium concentrations of up to 91 mg l(-1) were found by ICP-OES for ground water from nine boreholes at four landfill sites in an area of S.E. Glasgow/S. Lanarkshire where high-lime chromite ore processing residue (COPR) from a local chemical works had been deposited from 1830 to 1968. Surface water concentrations of up to 6.7 mg l(-1) in a local tributary stream fell to 0.11 mg l(-1) in the River Clyde. Two independent techniques of complexation/colorimetry and speciated isotope dilution mass spectrometry (SIDMS) showed that Cr was predominantly (90%) in hexavalent form (CrVI) as CrO4(2-), as anticipated at the high pH (7.5-12.5) of the sites. Some differences between the implied and directly determined concentrations of dissolved CrIII, however, appeared related to the total organic carbon (TOC) content. This was most significant for the ground water from one borehole that had the highest TOC concentration of 300 mg l(-1) and at which3% of Cr was in the form of CrVI. Subsequent ultrafiltration produced significant decreases in Cr concentration with decreasing size fractions, e.g.0.45 microm,100 kDa,30 kDa and1 kDa by the tangential-flow method. As this appeared related more to concentrations of humic substances than of TOC per se, horizontal bed gel electrophoresis of freeze-dried ultrafilter retentates was carried out to further characterise the CrIII-organic complex. This showed for the main Cr-containing fraction, 100 kDa-0.45 microm, that the Cr was associated with a dark brown band characteristic of organic (humic) matter. Comparison of gel electrophoresis and FTIR results for ultrafilter retentates of ground water from this borehole with those for a borehole at another site where CrVI predominated suggested the influence of carboxylate groups, both in reducing CrVI and in forming soluble CrIII-humic complexes. The implications of this for remediation strategies (especially those based on the addition of organic matter) designed to reduce highly mobile and carcinogenic Cr(VI)O4(2-) to the much less harmful CrIII as insoluble Cr(OH)3 are discussed.

Summary The podzolization process is examined in the light of measurements of the solubility characteristics of aluminium fulvate, the extent of dissolution of a proto-imogolite sol by fulvic acid, the adsorption capacity of proto-imogolite for fulvic acid and aluminium fulvate, and published evidence. Fulvic acid at 500 mg l−1 acting on a proto-imogolite (PI) preparation containing 0.95 mmol l−1 Al as PI did not bring enough Al into solution at pH 4.5–5.0 over 4–15 months to cause significant precipitation of the fulvic acid. As allophanic Bs horizons of podzols typically have pH ≥ 4.8, fulvic acids entering them in drainage water cannot be quantitatively precipitated by dissolution of Al from the allophane. They are, however, strongly absorbed on the allophane, and this must be the mechanism that removes most of the fulvic acid at the top of the Bs horizon, and which contributes, along with colloidal humus and root decomposition, to the formation of a Bh horizon. We conclude that fulvic acid plays no active role in podzolization, but only recycles Al and Fe, that have been transferred by biological processes to the O horizon, back to the Bh horizon. The podzolization process, which leads to the formation of an allophanic Bs horizon underlying a progressively deepening E horizon, requires the dissolution of Al-humate and allophanic precipitates at the Bh–Bs interface as well as progressive attack on the more readily weatherable minerals. Inorganic acids, particularly episodic fluxes of nitric acid, could play a major role in this, as well as attack by readily metabolized complexing acids such as oxalic and citric acids released by roots and fungi. In addition to throwing light on the podzolization process, the experimental results provide an explanation of the lower limit to C:Al ratios reported in natural waters, and a check on the applicability of the WHAM chemical equilibrium model to Al–fulvate–proto-imogolite equilibria. In Ca-containing fulvate solutions, Al-fulvate begins to precipitate when C:Al falls below 50, which is also the limiting ratio observed in natural waters. WHAM calculations overestimate by 70–85% the amount of Al-fulvate formed over 4 months at pH 4.5–5.0 in Ca-containing fulvate–imogolite systems.

A soil, containing waste material from an industrially contaminated site, was found to be heavily contaminated with several heavy metals and As. A risk assessment for As leaching from this material has been carried out in several stages, collation and examination of historical records, solid-phase characterization and chemical modelling. The historical record indicates that the most probable source of As was arsenopyrite. However, the solid phase characterization of the soil, using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive microanalysis (EDAX), did not yield any direct evidence for pyritic phases, although there was clear evidence of known pyrite-weathering products, such as jarosite. The relative stability of pyrite and arsenopyrite have been modelled for the range of acidity and redox potentials likely to be encountered on the site. For adsorption modelling, a surface complexation model was used to predict arsenate desorption as a function of pH. It was assumed that the principal reactive adsorbent for As was hydrous ferric oxide (HFO) and this assumption was supported by the results of direct and indirect measurements and by the mineral stability calculations. This approach was successful at predicting the increased mobility of As at increasingly alkaline conditions. The modelling predictions were supported by results from batch equilibration experiments. Thus, it was possible to link direct observations of mineralogy, mineral stability calculations and adsorption models in order to predict the mobility of As. The success of this approach was dependent on identifying the reactive phase in this particular soil and having the appropriate data required for the adsorption modelling.

The hypothesis, that metal toxicity is dominated by free ion activity, was tested by comparing calculated metal activities with measured toxic responses to a genetically modified, luminescent bacterium, Escherichia coli. The toxicity of Cd, Cu, and Zn sulphate salts in the presence of EDTA and fulvic acid in well-defined solutions was measured. Good agreement between free metal activity and toxicity was found for Cu but not for Zn and Cd. The toxicity relationships were altered by glucose addition to the organism. Stable chloride complexes may have contributed to the toxicity of Cd under the test conditions. The results suggest that there is not always a simple relationship between toxicity and free-ion metal concentration and that further account should be taken of competitive interactions between living cells and ligands and the physiological status of the organism.

The adsorption of fulvic acid by goethite was determined experimentally as a function of concentration, pH, and ionic strength. The data were described with the CD-MUSIC model of Hiemstra and Van Riemsdijk (1996), which allows the distribution of charge of the bound fulvate molecule over a surface region. Simultaneously, the concentration, pH, and salt dependency of the binding of fulvic acid can be described. Using the same parameters, the basic charging behavior of the goethite in the absence of fulvic acid could be described well. The surface species used in the model indicate that inner sphere coordination of carboxylic groups of the fulvate molecule is important at low pH, whereas at high pH the outer sphere coordination with reactive groups of the fulvate molecule with high proton affinity is important.

Fifteen Swedish and Finnish soil samples from spodic B horizons containing imogolite-type materials were equilibrated with dilute NaCl and HC1 at 8°C. For the NaCl extracts, apparent equilibrium with respect to an AI(OH)3 phase was reached in one to two weeks. Equilibrium with respect to an imogolite-type phase was slower, especially at large soil:solution ratios. The results show that log *Ks (gibbsite), i.e. the logarithm of the solubility constant for reactive AI(OH)3 at 8°C was ~9.40 (8.29 at 25°C) while log *Ks (imogolite) was variable; for the soil studied in most detail it was 7.66 (6.64 at 25°C) As well-ordered gibbsite rarely forms in spodic B horizons, it is suggested that Al(OH)3 and poorly-ordered allophanic materials may be slowly converted to less soluble and less reactive imogolite-type materials.

Summary The amounts of aluminium liberated from a proto-imogolite (P-I) sol by 1–3 mM oxalate at pH 4·8–7·1 are found to be in excellent agreement with those calculated on the basis of the solubility product of the P-I sol, and the stability constants for aluminium oxalate complexes established earlier. Calculated solubility curves are presented for proto-imogolite sols in the presence of 10−3, 10−4 and 10−5 M total oxalate over the pH range 4–8, and also for the case where oxalate concentrations are controlled by solid calcium oxalate in the presence of 0·05, 0·5 and 5 mM calcium ion. We conclude that the presence of a proto-imogolite allophane in podzol B horizons of pH 5·0 requires oxalate concentrations less than 10−5 M. For fertile agricultural soils with typical calcium concentrations (approximately 5 mM) in the soil solution, aluminium oxalate concentrations would be limited by calcium oxalate solubility to less than 10−6 M in the presence of proto-imogolite allophane at pH values exceeding 5·5. In leached podzolic soils calcium concentrations are generally less than 0·05 mM, and would not limit the formation of aluminium oxalate complexes.

The binding of toxic metals by soil is usually highly pH dependent. Soil acidification resulting in small changes of pH may lead to the mobilization and increased bioavailability of toxic metals. To predict the binding of toxic metals to soil particles, an understanding of physico-chemical retention mechanisms of soil particles as a function of pH is necessary. This study describes the effects of pH and soluble Al on Cd retention by different soil horizons, using a simple binary surface complexation modelling approach. In this approach the importance of metal binding sites associated with both mineral and soil humic components is included. The metal binding sites associated with the mineral component, are described using a constant-capacitance model and those associated with soil humic substances by a diprotic acid discrete functional group binding site. In order to explain Cd retention in acid soil solutions the model assumes that soluble Al species will compete with Cd and protons for metal binding sites. Results comparing model calculations with experimental data suggest that for Cd the competitive effects of Al will be most marked in the pH range 4–5 due to suppression of Cd binding to the soil humic component.

Proto-imogolite sols can be considered as highly dispersed forms of proto-imogolite allophane, the most widespread type of allophane in volcanic and non-volcanic soils world-wide. The solubility characteristics of such sols define the conditions of precipitation of allophanes in soils, and the maximum concentrations of aluminium released during acidic episodes from soils, such as podzols, that contain allophane. Direct measurement of Al, Si and pH values in equilibrium with proto-imogolite sols, approached from higher and lower pH, indicated a solubility equation: log{Al 3+ } + 0.5 log{H 4 SiO 4 } + 3pH = log * K so where log *K so lay in the range 7.14 to 7.23 after equilibration for 4-24 weeks at 22 ± 2°C in 17 of the 20 systems studied. The mean value of log *K so at 298 K was calculated as 7.02. This value indicates that proto-imogolite will be more stable than amorphous aluminium hydroxides at H 4 SiO 4 concentrations above 5 x 10 -6 M, but less stable than bayerite below 10 -3 M H 4 SiO 4 , and than gibbsite below 10 -2 M. Proto-imogolite is more stable than micro-crystalline gibbsite in 10 -4 M H 4 SiO 4 , a typical minimum concentration in soil solutions and streams in landscapes where podzols are present. The rapid formation of proto-imogolite effectively prevents the formation of gibbsite seeds in soil, except in highly leached and warm environments, i.e. in older landscapes in the tropics. Although the presence of 10 -4 M silicic acid has been found to eliminate the acute toxicity to fish exhibited by solutions containing 6-7 μM Al at pH 4.96, little or no proto-imogolite would form under these conditions. Silicic acid would, however, prevent the precipitation of aluminium hydroxides, and could inhibit the formation of the Al 13 polycation. These polymeric species are a likely cause of the increased toxicity exhibited by partially neutralized aluminium solutions.

The extent of contamination of soils by toxic heavy metals not only depends on the rate of loading of the metal but also on the nature of the adsorbing surfaces, the degree of alkalinity or acidity of the soil and the presence of aqueous complexant ligands. This work reports on the role of pH on the retention of Cd, Hg, Pb and Zn by two soils and on the influence of the chloride, Cl‐, ion on the chemical speciation and retention of the four metals. Batch adsorption experiments were conducted from pH 3 to 7 in the presence of either 0.1 M LiCl or LiClO4. The results of the study showed that high concentrations of Cl‐ ions can greatly decrease the retention of Hg and have an increasingly lesser effect on Cd, Pb and Zn retention. The effect of the Cl‐ons was directly related to the metal‐Cl formation constants. The results of computer modeling of Cd and Hg retention by goethite and humic acid fractions indicated the relative importance of aqueous vs. surface complexation on metal retention. For orga...

Better knowledge of spatial and temporal delivery of dissolved organic carbon (DOC) in small catchments is required to understand the mechanisms behind reported long-term changes in C fluxes from some peatlands. We monitored two storms with contrasting seasons and antecedent conditions in a small upland UK moorland catchment. We examined DOC concentrations and specific UV absorbance (SUVA at 285 nm), together with solute concentrations required to undertake end-member mixing analyses to define dominant flow paths contributing to streamflow. This was combined with laboratory soil-solution equilibrations. We aimed to resolve how seasonal biogeochemical processing of DOC and flowpath changes in organo-mineral soils combine to affect DOC exported via the stream. An August storm following a dry period gave maximum DOC concentration of 10 mg l−1. Small DOC:DON ratios (16–28) and SUVA (2.7–3.6 l mg−1 m−1) was attributed to filtration of aromatic compounds associated with up to 53% B horizon flow contributions. This selective filtration of high SUVA DOC was reproduced in the experimental batch equilibration system. For a November storm, wetter antecedent soil conditions led to enhanced soil connectivity with the stream and seven times greater DOC stream-load (maximum concentration 16 mg l−1). This storm had a 63% O horizon flow contribution at its peak, limited B horizon buffering and consequently more aromatic DOC (SUVA 3.9–4.5 l mg−1 m−1 and DOC:DON ratio 35–43). We suggest that simple mixing of waters from different flow paths cannot alone explain the differences in DOC compositions between August and November and biogeochemical processing of DOC is required to fully explain the observed stream DOC dynamics. This preliminary evidence is in contrast to other studies proposing hydrological controls on the nature of DOC delivered to streams. Although our study is based only on two storms of very different hydrological and biogeochemical periods, this should promote wider study of DOC biogeochemical alteration in headwaters so that this be better incorporated in modelling to predict the impacts of changes in DOC delivery to, and fate in, aquatic systems.

The extent of the reaction A1 3+ + Si(OH) 4 → AlOSi(OH) 3 2+ + H + has been assessed by measuring the shift in log[H + ] between solutions containing A1(ClO 4 ) 3 , HClO 4 , and NaClO 4 with and without added silicic acid. The observed shifts of 0.018–0.027 units are about one tenth of those predicted from the formation constant ( pK 110 = 1.07 ± 0.06) proposed for the monomeric aluminosilicate species by Browne and Driscoli (1992). An estimate of this formation constant on the basis of the shifts in log[H + ] ( pK 110 = 2.50 ± 0.05) indicates that the concentration of aluminosilicate species can be neglected in many natural waters.

Within-river cycling of P is a crucial link between catchment pollution sources and the resulting ecological impacts and integrates the biogeochemistry and hydrodynamics of river systems. This study investigates benthic sediment P sorption in relation to river soluble reactive phosphorus (SRP) concentrations during high- to low-flow changes in a major mixed land use river system in NE Scotland. We hypothesised that sediments comprised P sinks during moderate to higher flows but became P saturated with loss of buffering function during prolonged baseflow. Sediment characteristics were evaluated and equilibrium P concentrations (EPC(0)) calculated using a standardised batch adsorption method (EPC(0) values 0.04-1.75 micromol Pl(-1)). Pollution-impacted tributaries (32-69% catchment agricultural land cover) had increased SRP concentrations (0.19-2.62 micromol Pl(-1)) and maintained EPC(0)SRP values during changing flow conditions. Moorland-dominated tributaries and main stem sites had small SRP concentrations (0.03-0.19 micromol Pl(-1)) but showed EPC(0)SRP values during summer baseflow so that sediments were indicated as P sources. This deviation from a geochemical sediment-water P equilibrium was attributed to biological accumulation of P from the water column into the sediments. In particular, large stores of sediment P accumulated in main stem reaches below agricultural tributaries and this may be consequential for sensitive downstream ecosystems. Hence, biogeochemical processes at the river bed may strongly influence river SRP cycling between geochemical and biotic pools. The nature of this internal reservoir of river P and its ecosystem interactions needs better understanding to enable best results to be attained from catchment mitigation actions designed to maintain/improve ecological status under the Water Framework Directive.

For understanding both the environmental behavior and developing remediation treatments for chromium ore processing residue (COPR) it is important to identify all the potentially soluble sources of Cr(VI). Hydrogarnet has been identified as a major phase in COPR and it has been previously speculated that it has a capacity to host Cr(VI). Here we provide direct evidence of this capacity by demonstrating the incorporation of Cr(VI) into laboratory synthesized hydrogarnet. Electron microscopy and energy dispersive X-ray microanalysis show incorporation of approximately 17000-22000 mg Cr(VI) kg(-1) hydrogarnet. X-ray powder diffraction data show that peak intensities are altered by chromium substitution and that chromium substituted hydrogarnets have a smaller unit cell than the pure Ca-Al end member. This is consistent with substitution of hydroxyl tetrahedra by smaller chromate tetrahedra. Electron energy loss spectroscopy confirms the tetrahedral coordination and hexavalent oxidation state of chromium in the hydrogarnets. The maximum amount of hexavalent chromium that can be introduced synthetically corresponds to a replacement of about one out of every eight hydroxyl tetrahedral per unit cell by a CrO4(2-) tetrahedra and tallies closely with the amount of chromium measured in hydrogarnets from COPR. Chromium-bearing hydrogarnet is the most abundant crystalline phase in millions of tons of COPR contaminating land around Glasgow, Scotland, and was recently identified in COPR from sites in North America. Calculations based on its abundance and its Cr(VI) content indicate that hydrogarnet can host as much as 50% of the Cr(VI) found in some COPR samples.

We present results from modelling studies, which suggest that, at most, only about 10–20% of recently observed soil carbon losses in England and Wales could possibly be attributable to climate warming. Further, we present reasons why the actual losses of SOC from organic soils in England and Wales might be lower than those reported.

Infrared spectroscopy was used to corroborate predictions made by newly developed heterogeneous site binding models for humic substances. Experimental conditions to acquire the spectra of soil humic substances (humic and fulvic acid and a polysaccharide fraction) in an aqueous state using horizontal attenuated total reflectance Fourier transform infrared spectroscopy (HATR-FTIR) were established. Elimination of the water spectrum from that of the sample was achieved by spectral subtraction of the water peak at 2020 cm(-1). A KSCN internal standard with an absorption band at 2067 cm(-1) was used to verify the efficacy of the subtraction procedure. Spectral artifacts produced by the water spectrum subtraction and from contaminants within the humic materials have been identified. Three fulvic and one humic acid solution were examined in solutions of varying pH. Results show that the observed proportion of ionized carboxylate in relation to pH is consistent with models that assume electrostatic effects and a continuous distribution of proton association constants (log KH). The spectroscopic data were in accordance with calculations made using the generic humic and fulvic acid NICA-Donnan model parameters.

The sensitivity of mineral soils to anthropogenically-induced acidification may be assessed using the Skokloster classification or by considering the short-term acid buffering capacity (STABC). The Skokloster classification is based on the assumption that the majority of acidification neutralization is due to base cation release from mineral weathering. It therefore considers the long-term neutralizing capacity of a soil on a scale of decades. The STABC of a soil is due largely to the adsorption and exchange of H+ ions by soil components, and acts on the scale of single years. The United Kingdom maps generated to identify soils that are sensitive to acidification use the Skokloster classification. A comparison ofthe laboratory-determined STABC ofthe different horizons from soil profiles with the Skokloster classes of soils shows that there is generally no correlation between the two. Maps showing soil sensitivity to acidification based on measured STABC and Skokloster classes are markedly different. The STABC measure has the potential for quantitatively assessing a soil’s sensitivity to acidification, while the Skokloster approach is empirical. It maybe possible to combine the two approaches to produce a dynamic method of assessing soil recovery from acidification.