Xylem plasticity of root, stem, and branch in Cunninghamia lanceolata under drought stress: implications for whole-plant hydraulic integrity.

Introduction: A better understanding of xylem hydraulic characteristics in trees is critical to elucidate the mechanisms of forest decline and tree mortality from water deficit. As well as temperate forests and forests growing in arid regions, subtropical and tropical forests are also predicted to experience an increased frequency and intensity of climate change-induced drought in the near future. Methods: In this study, 1-year-old Cunninghamia lanceolata seedlings (a typical subtropical species in southern China) were selected for a continuous controlled drought pot experiment of 45 days duration. The experimental treatments were non-drought (control), light drought, moderate drought and severe drought stress, which were 80%, 60%, 50%, and 40%, respectively of soil field maximum moisture capacity. Results: The hydraulic conductivity, specific conductivity and water potential of roots, stems, and branches of C. lanceolata all decreased with the prolonging of drought in the different drought intensities. The relative decrease in these hydraulic values were greater in roots than in stems and branches, indicating that roots are more sensitive to drought. Root tracheid diameters normally reduce to ensure security of water transport with prolonged drought, whilst the tracheid diameters of stems and branches expand initially to ensure water transport and then decrease to reduce the risk of embolism with continuing drought duration. The pit membrane diameter of roots, stems and branches generally increased to different extents during the 15-45 days drought duration, which is conducive to enhanced radial water transport ability. The tracheid density and pit density of stems generally decreased during drought stress, which decreased water transport efficiency and increased embolism occurrence. Correlation analysis indicated that anatomical plasticity greatly influenced the hydraulic properties, whilst the relationships varied among different organs. In roots, tracheid diameter decreased and tracheid density increased to enhance water transport security; stems and branches may increase tracheid diameter and pit membrane diameter to increase hydraulic conductivity ability, but may increase the occurrence of xylem embolism. Discussion: In summary, under drought stress, the xylem anatomical characteristics of C. lanceolata organs were highly plastic to regulate water transport vertically and radially to maintain the trade-off between hydraulic conductivity efficiency and safety.

Soil microbial community, dissolved organic matter and nutrient cycling interactions change along an elevation gradient in subtropical China.

To identify possible dominating processes involved in soil microbial community assembly, dissolved organic matter (DOM) and multi-nutrient cycling (MNC) interactions and contribute to understanding of climate change effects on these important cycles, we investigated the interaction of soil chemistry, DOM components and microbial communities in five vegetation zones - ranging from evergreen broad-leaved forest to alpine meadow - along an elevation gradient of 290-1960 m in the Wuyi Mountains, Fujian Province, China. Soil DOM composition and microbial community assembly were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and Illumina MiSeq high-throughput sequencing, respectively. Sloan's neutral model and the modified stochasticity ratio were used to infer community assembly processes. Key microbial drivers of the soil MNC index were identified from partial least squares path models. Our results showed that soil DOM composition is closely related to the vegetation types along an elevation gradient, the structure and composition of the microbial community, and soil nutrient status. Overall, values of the double bond equivalent (DBE), modified aromaticity index (AImod) increased, and H/C ratio and molecular lability boundary (MLBL) percentage decreased with elevation. Lignins/CRAM-like structures compounds dominated soil DOM in each vegetation type and its relative abundance decreased with elevation. Aliphatic/protein and lipids components also decreased, but the relative abundance of aromatic structures and tannin increased with elevation. The alpha diversity index of soil bacteria gradually decreased with elevation, with deterministic processes dominating the microbial community assembly in the highest elevation zone. Bacterial communities were conducive to the decomposition of labile degradable DOM compounds (H/C ≥ 1.5) at low elevation. In the cooler and wetter conditions at higher-elevation sites the relative abundance of potentially resistant soil DOM components (H/C < 1.5) gradually increased. Microbial community diversity and composition were important predictors of potential soil nutrient cycling. Although higher elevation sites have higher nutrient cycling potential, soil DOM was assessed to be a more stable carbon store, with apparent lower lability and bioavailability than at lower elevation sites. Overall, this study increases understanding of the potential linkage between soil microbial community, multiple nutrient cycling and DOM fate in subtropical mountain ecosystems that can help predict the effect of climate change on soil carbon sequestration and thus inform ecosystem management.

Decreasing molecular diversity of soil dissolved organic matter related to microbial community along an alpine elevation gradient.

Characterization of soil dissolved organic matter (DOM) and understanding of the interactions between soil microbial communities and DOM molecules along elevation gradients in alpine ecosystems are still limited. To unravel these interactions and how they change along alpine elevation gradients, we sampled topsoil in the Sygera Mountains (Tibet, China) at elevations between 3800 and 4600 m. The molecular characteristics of soil DOM were determined using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and soil microbial composition was identified by high-throughput sequencing. Among the seven components of DOM, the lignins/CRAM (carboxyl-rich alicyclic molecules)-like structure dominated at all elevations, followed by tannins, while the relative abundance of unstable substances, including lipids, aliphatic/protein, and carbohydrates, was lower. As elevation increased, the molecular diversity, degree of oxidation, aromaticity, and unsaturation of soil DOM decreased. The abundance and diversity of soil bacteria and fungi also generally decreased with elevation. Both bacteria and fungi play an important role in the degradation of DOM molecules, but bacteria appear to have greater degradation ability. Among them, Proteobacteria and Bacteroidetes mainly promote the degradation of lignins/CRAM-like structure molecules, while Basidiomycota mainly degrade more unstable substrates. Co-occurrence network analysis revealed complex correlations between specific microbial groups and DOM molecules. Our results suggest that more active cycling of soil DOM could occur in alpine ecosystems due to climate warming, as the result of increased vegetation productivity and litter input in response to rising temperature promoting the relative abundance of microbial groups capable of degrading lignins/CRAM-like structures in soil DOM.

Role of plant growth promoting bacteria in driving speciation gradients across soil-rhizosphere-plant interfaces in zinc-contaminated soils.

Inoculation of soil or seeds with plant growth promoting bacteria ameliorates metal toxicity to plants by changing metal speciation in plant tissues but the exact location of these changes remains unknown. Knowing where the changes occur is a critical first step to establish whether metal speciation changes are driven by microbial metabolism or by plant responses. Since bacteria concentrate in the rhizosphere, we hypothesised steep changes in metal speciation across the rhizosphere. We tested this by comparing speciation of zinc (Zn) in roots of Brassica juncea plants grown in soil contaminated with 600 mg kg-1 of Zn with that of bulk and rhizospheric soil using synchrotron X-ray absorption spectroscopy (XAS). Seeds were either uninoculated or inoculated with Rhizobium leguminosarum bv. trifolii and Zn was supplied in the form of sulfide (ZnS nanoparticles) and sulfate (ZnSO4). Consistent with previous studies, Zn toxicity, as assessed by plant growth parameters, was alleviated in B. juncea inoculated with Rhizobium leguminosarum. XAS results showed that in both ZnS and ZnSO4 treatments, the most significant changes in speciation occurred between the rhizosphere and the root, and involved an increase in the proportion of organic acids and thiol complexes. In ZnS treatments, Zn phytate and Zn citrate were the dominant organic acid complexes, whilst Zn histidine also appeared in roots exposed to ZnSO4. Inoculation with bacteria was associated with the appearance of Zn cysteine and Zn formate in roots, suggesting that these two forms are driven by bacterial metabolism. In contrast, Zn complexation with phytate, citrate and histidine is attributed to plant responses, perhaps in the form of exudates, some with long range influence into the bulk soil, leading to shallower speciation gradients.

Spatiotemporal distribution of canopy litter and nutrient resorption in a chronosequence of different development stages of Cunninghamia lanceolata in southeast China.

Canopy litter is an important component of coarse woody debris (CWD), which affects nutrient and carbon cycling in forest ecosystems. For marcescent plant species (characterized by dead branches and leaves remaining in the canopy for several years before abscission), nutrient resorption from senescing leaves is an important nutrient conservation strategy. However, investigating the ecological function of canopy litter is challenging due to its limited accessibility and also the heterogeneous canopy microclimate in terms of light transmission, temperature and moisture. We studied the spatiotemporal distribution of canopy litter mass and seasonal dynamics of leaf nutrients and nutrient resorption during senescence in the canopy along a chronosequence of Chinese fir [Cunninghamia lanceolata (Lamb.) Hook] plantations in southeast China. The dry mass weight of dead branches and dead leaves in the canopy significantly increased with stand stage (14.6, 14.2, and 17.4 t ha-1 for young, middle-aged, and mature stands respectively), accounting for high proportions of total aboveground litter of 85.7%, 79.1% and 80.0%, respectively, along with annual litterfall production (2.44, 3.75, and 4.34 t ha-1, respectively). The canopy height distribution of dead branches and leaves also increased with stand age, ranging from 0 to 4 m in young stands, 3-8 m in middle-aged stands, to 4-10 m in mature stands. The seasonal pattern of canopy litter mass was the inverse of litterfall production: canopy litter mass peaked, while litterfall production was lowest in winter. Mean N, P, K, and Mg nutrient resorption efficiencies across stands at each stage were 53.8-58.9%, 64.0-68.9%, 85.0-90.2%, and 46.5-56.6%, respectively, while Ca was not retranslocated from senescing leaves. In summary, Chinese fir plantations retain large amounts of dead branches and leaves in the canopy from which at least ~50% of the nutrients N, P, K and Mg are recycled, representing an important nutrient conservation strategy that has evolved to adapt to nutrient-limited habitats. Canopy litter therefore plays an important role in these forest plantation ecosystems and should be protected instead of being removed from the canopy to the forest floor.

Biochar Phosphorus Release Is Limited by High pH and Excess Calcium.

Aside from its use for improving soil properties, biochar is increasingly promoted as a direct nutrient provider for sustainable recycling of waste materials. However, incomplete understanding of the interacting factors that determine P release from biochar may limit the efficiency of P recycling from biochar to soil. In particular, the contrasting pH of biochar and soil need to be considered. In this study, soil-free biochar (rice [ L.] husk, 700°C) extractions were performed under different pH (4.6-9.9) and extractant conditions to test how solution composition affects biochar P release. When solution pH was in the range of 7.6 to 8.6 and excess Ca was present in the solution or in biochar, P release was low-only 1 to 7% of the total P was released compared with ∼20% under most other conditions. Importantly, we demonstrate that biochar total Ca concentration is closely related to P availability ( = 0.76) and could be used to predict biochar P release. The results suggest that for maximum P release, low Ca concentrations in biochar and (soil) solution are needed and/or a pH <7.5 at the soil-biochar interface. This novel understanding will help engineer sustainable biochar fertilizers optimized for P provision.

Soil Bacteria Override Speciation Effects on Zinc Phytotoxicity in Zinc-Contaminated Soils.

The effects of zinc (Zn) speciation on plant growth in Zn-contaminated soil in the presence of bacteria are unknown but are critical to our understanding of metal biodynamics in the rhizosphere where bacteria are abundant. A 6-week pot experiment investigated the effects of two plant growth promoting bacteria (PGPB), Rhizobium leguminosarum and Pseudomonas brassicacearum, on Zn accumulation and speciation in Brassica juncea grown in soil amended with 600 mg kg-1 elemental Zn as three Zn species: soluble ZnSO4 and nanoparticles of ZnO and ZnS. Measures of plant growth were higher across all Zn treatments inoculated with PGPB compared to uninoculated controls, but Zn species effects were not significant. Transmission electron microscopy identified dense particles in the epidermis and intracellular spaces in roots, suggesting Zn uptake in both dissolved and particulate forms. X-ray absorption near-edge structure (XANES) analysis of roots revealed differences in Zn speciation between treatments. Uninoculated plants exposed to ZnSO4 contained Zn predominantly in the form of Zn phytate (35%) and Zn polygalacturonate (30%), whereas Zn cysteine (57%) and Zn polygalacturonate (37%) dominated in roots exposed to ZnO nanoparticles. Inoculation with PGPB increased (>50%) the proportion of Zn cysteine under all Zn treatments, suggesting Zn coordination with cysteine as the predominant mechanism of Zn toxicity reduction by PGPB. Using this approach, we show, for the first time, that although speciation is important, the presence of rhizospheric bacteria completely overrides speciation effects such that most of the Zn in plant tissue exists as complexes other than the original form.

Assessing the role of bed sediments in the persistence of red mud pollution in a shallow lake (Kinghorn Loch, UK).

Red mud is a by-product of alumina production. Little is known about the long-term fate of red mud constituents in fresh waters or of the processes regulating recovery of fresh waters following pollution control. In 1983, red mud leachate was diverted away from Kinghorn Loch, UK, after many years of polluting this shallow and monomictic lake. We hypothesised that the redox-sensitive constituents of red mud leachate, phosphorus (P), arsenic (As) and vanadium (V), would persist in the Kinghorn Loch for many years following pollution control as a result of cycling between the lake bed sediment and the overlying water column. To test this hypothesis, we conducted a 12-month field campaign in Kinghorn Loch between May 2012 and April 2013 to quantify the seasonal cycling of P, As, and V in relation to environmental conditions (e.g., dissolved oxygen (DO) concentration, pH, redox chemistry and temperature) in the lake surface and bottom waters. To confirm the mechanisms for P, As and V release, a sediment core incubation experiment was conducted using lake sediment sampled in July 2012, in which DO concentrations were manipulated to create either oxic or anoxic conditions similar to the bed conditions found in the lake. The effects on P, As, and V concentrations and species in the water column were measured daily over an eight-day incubation period. Phosphate (PO4-P) and dissolved As concentrations were significantly higher in the bottom waters (75.9 ± 30.2 µg L-1 and 23.5 ± 1.83 µg L-1, respectively) than in the surface waters (12.9 ± 1.50 µg L-1 and 14.1 ± 2.20 µg L-1, respectively) in Kinghorn Loch. Sediment release of As and P under anoxic conditions was confirmed by the incubation experiment and by the significant negative correlations between DO and P and As concentrations in the bottom waters of the lake. In contrast, the highest dissolved V concentrations occurred in the bottom waters of Kinghorn Loch under oxic conditions (15.0 ± 3.35 µg L-1), with the release from the bed sediment apparently being controlled by a combination of competitive ion concentrations, pH and redox conditions.

Biochar and enhanced phosphate capture: Mapping mechanisms to functional properties.

A multi-technique analysis was performed on a range of biochar materials derived from secondary organic resources and aimed at sustainable recovery and re-use of wastewater phosphorus (P). Our purpose was to identify mechanisms of P capture in biochar and thereby inform its future optimisation as a sustainable P fertiliser. The biochar feedstock comprised pellets of anaerobically digested sewage sludge (PAD) or pellets of the same blended in the ratio 9:1 with ochre sourced from minewater treatment (POCAD), components which have limited alternative economic value. In the present study the feedstocks were pyrolysed at two highest treatment temperatures of 450 and 550 °C. Each of the resulting biochars were repeatedly exposed to a 20 mg l-1 PO4-P solution, to produce a parallel set of P-exposed biochars. Biochar exterior and/or interior surfaces were quantitatively characterised using laser-ablation (LA)-ICP-MS, X-ray diffraction, X-ray photo-electron spectroscopy (XPS) and scanning electron microscopy coupled with energy dispersive X-ray. The results highlighted the general importance of Fe minerals in P capture. XPS analysis of POCAD550 indicated lower oxidation state Fe2p3 bonding compared to POCAD450, and LA-ICP-MS indicated stronger covariation of Fe and S, even after P exposure. This suggests that low-solubility Fe/S compounds are formed during pyrolysis, are affected by process parameters and impact on P capture. Other data suggested capture roles for aluminium, calcium and silicon. Overall, our analyses suggest that a range of mechanisms for P capture are concurrently active in biochar. We highlighted the potential to manipulate these through choice of form and composition of feedstock as well as pyrolysis processing, so that biochar may be increasingly tailored towards specific functionality.

Bioavailability of phosphorus, other nutrients and potentially toxic elements from marginal biomass-derived biochar assessed in barley (Hordeum vulgare) growth experiments.

Biochars produced from marginal biomass feedstocks are a potential source of recycled nutrients for agriculture, but may also contain potentially toxic elements (PTEs) which can cause phytotoxicity. We assessed the potential for nutrient recycling from such materials against potential environmental risks in 17 biochars containing high concentrations of various PTEs and nutrients. Methods for investigating the risk of biochar-derived PTEs were developed and assessed. Short-term (21days) growth experiments with barley (Hordeum vulgare) in 5% biochar/sand mixtures were used to present the 'worst-case scenario' of high dose and low pH buffering. We compared plant nutrient and PTE concentrations with amounts extracted from the same biochars using 1M NH4NO3 or 0.01M CaCl2 (buffered and unbuffered, respectively) and Mehlich 3 to analyse whether such extractions could be used to predict bioavailability. The yields of barley grown with biochars "EPOCAD550", and "WLB550" were significantly higher than the control (p<0.05). Total phosphorus (P) concentration in above-ground biomass was higher than the control for the EPOCAD550 treatment (p<0.01). Both buffered and unbuffered 0.01M CaCl2 biochar extractions were significantly positively correlated with plant leaf concentration for six of the 18 elements investigated, more than any of the other extractions. CaCl2 extractions provided the most representative assessment of element bioavailability from marginal biochars compared to more resource-intensive growth experiments. Our results provide new insights into the bioavailability of elements in biochar and the standardisation of methods which accurately assess this attribute, which is necessary for promoting use of biochars from marginal biomass for recycling nutrients from wastewater and to agricultural production.

The use of red mud as an immobiliser for metal/metalloid-contaminated soil: A review.

This review focuses on the applicability of red mud as an amendment for metal/metalloid-contaminated soil. The varying properties of red muds from different sources are presented as they influence the potentially toxic element (PTE) concentration in amended soil. Experiments conducted worldwide from the laboratory to the field scale are screened and the influencing parameters and processes in soils are highlighted. Overall red mud amendment is likely to contribute to lowering the PTE availability in contaminated soil. This is attributed to the high pH, Fe and Al oxide/oxyhydroxide content of red mud, especially hematite, boehmite, gibbsite and cancrinite phases involved in immobilising metals/metalloids. In most cases red mud amendment resulted in a lowering of metal concentrations in plants. Bacterial activity was intensified in red mud-amended contaminated soil, suggesting the toxicity from PTEs was reduced by red mud, as well as indirect effects due to changes in soil properties. Besides positive effects of red mud amendment, negative effects may also appear (e.g. increased mobility of As, Cu) which require site-specific risk assessments. Red mud remediation of metal/metalloid contaminated sites has the potential benefit of reducing red mud storage and associated problems.

Assessing the Legacy of Red Mud Pollution in a Shallow Freshwater Lake: Arsenic Accumulation and Speciation in Macrophytes.

Little is known about long-term ecological responses in lakes following red mud pollution. Among red mud contaminants, arsenic (As) is of considerable concern. Determination of the species of As accumulated in aquatic organisms provides important information about the biogeochemical cycling of the element and transfer through the aquatic food-web to higher organisms. We used coupled ion chromatography and inductively coupled plasma mass spectrometry (ICP-MS) to assess As speciation in tissues of five macrophyte taxa in Kinghorn Loch, U.K., 30 years following the diversion of red mud pollution from the lake. Toxic inorganic As was the dominant species in the studied macrophytes, with As species concentrations varying with macrophyte taxon and tissue type. The highest As content measured in roots of Persicaria amphibia (L.) Gray (87.2 mg kg(-1)) greatly exceeded the 3-10 mg kg(-1) range suggested as a potential phytotoxic level. Accumulation of toxic As species by plants suggested toxicological risk to higher organisms known to utilize macrophytes as a food source.

Optimising the recovery and re-use of phosphorus from wastewater effluent for sustainable fertiliser development.

Recovery and re-use of phosphorus (P) from wastewater treatment systems as agricultural fertiliser presents an important and viable target for P waste reduction and recycling. In this study novel biochar materials for P filtration of wastewater were designed and produced using waste feedstocks, with consideration of the plant accessibility of the P captured by the biochars. The biochars were produced using batch slow pyrolysis at 450 °C and 550 °C from a) AD: anaerobically digested sewage sludge and b) OCAD: a 1:1 mixture of anaerobically digested sewage sludge and ochre, a mineral product from mine drainage treatment. A set of experiments was designed using pH buffering to provide a robust framework for assessing the P recovery capacity and affinity of the biochars compared to other potential P recovery materials (unprocessed ochre, activated carbon and zeolite). After 5 days of repeated exposure to a P solution at a wastewater-relevant concentration (0.02 g P l(-1)) replenished each 24 h, relatively high masses of P were recovered by ochre (1.73 ± 8.93×10(-3) mg P g(-1)) and the biochars OCAD550 (1.26 ± 4.66×10(-3) mg P g(-1)), OCAD450 (1.24 ± 2.10×10(-3) mg P g(-1)), AD450 (1.06 ± 3.84×10(-3) mg P g(-1)), and AD550 (0.986 ± 9.31×10(-3) mg P g(-1)). The biochar materials had higher removal rates than both activated carbon (0.884 ± 1.69×10(-2) mg P g(-1)) and zeolite (0.130 ± 1.05×10(-2) mg P g(-1)). To assess the extractability of recovered P, P exposure was followed by repeated extraction for 4 days with pH 7-buffered deionised water. The AD biochars retained 55% of the P recovered, OCAD biochars 78% and ochre 100%. Assessment of potentially toxic element concentrations in the biochars against guideline values indicated low risk associated with their use in the environment. Our successful demonstration of biochar materials highlights the potential for further development of P filters for wastewater treatment systems from anaerobic digestate produced and pyrolysed on-site with energy recovery.

Bacteria-zinc co-localization implicates enhanced synthesis of cysteine-rich peptides in zinc detoxification when Brassica juncea is inoculated with Rhizobium leguminosarum.

Some plant growth promoting bacteria (PGPB) are enigmatic in enhancing plant growth in the face of increased metal accumulation in plants. Since most PGPB colonize the plant root epidermis, we hypothesized that PGPB confer tolerance to metals through changes in speciation at the root epidermis. We employed a novel combination of fluorophore-based confocal laser scanning microscopic imaging and synchrotron based microscopic X-ray fluorescence mapping with X-ray absorption spectroscopy to characterize bacterial localization, zinc (Zn) distribution and speciation in the roots of Brassica juncea grown in Zn contaminated media (400 mg kg(-1) Zn) with the endophytic Pseudomonas brassicacearum and rhizospheric Rhizobium leguminosarum. PGPB enhanced epidermal Zn sequestration relative to PGBP-free controls while the extent of endophytic accumulation depended on the colonization mode of each PGBP. Increased root accumulation of Zn and increased tolerance to Zn was associated predominantly with R. leguminosarum and was likely due to the coordination of Zn with cysteine-rich peptides in the root endodermis, suggesting enhanced synthesis of phytochelatins or glutathione. Our mechanistic model of enhanced Zn accumulation and detoxification in plants inoculated with R. leguminosarum has particular relevance to PGPB enhanced phytoremediation of soils contaminated through mining and oxidation of sulphur-bearing Zn minerals or engineered nanomaterials such as ZnS.

Mixed planting with a leguminous plant outperforms bacteria in promoting growth of a metal remediating plant through histidine synthesis.

The effectiveness of plant growth promoting bacteria (PGPB) in improving metal phytoremediation is still limited by stunted plant growth under high soil metal concentrations. Meanwhile, mixed planting with leguminous plants is known to improve yield in nutrient deficient soils but the use of a metal tolerant legume to enhance metal tolerance of a phytoremediator has not been explored. We compared the use of Pseudomonas brassicacearum, Rhizobium leguminosarum, and the metal tolerant leguminous plant Vicia sativa to promote the growth of Brassica juncea in soil contaminated with 400 mg Zn kg(-1), and used synchrotron based microfocus X-ray absorption spectroscopy to probe Zn speciation in plant roots. B. juncea grew better when planted with V. sativa than when inoculated with PGPB. By combining PGPB with mixed planting, B. juncea recovered full growth while also achieving soil remediation efficiency of >75%, the maximum ever demonstrated for B. juncea. µXANES analysis of V. sativa suggested possible root exudation of the Zn chelates histidine and cysteine were responsible for reducing Zn toxicity. We propose the exploration of a legume-assisted-phytoremediation system as a more effective alternative to PGPB for Zn bioremediation.

Mechanisms behind bacteria induced plant growth promotion and Zn accumulation in Brassica juncea.

The growth and metal-extraction efficiency of plants exposed to toxic metals has been reported to be enhanced by inoculating plants with certain bacteria but the mechanisms behind this process remain unclear. We report results from glasshouse experiments on Brassica juncea plants exposed to 400mgZnkg(-1) that investigated the abilities of Pseudomonas brassicacearum and Rhizobium leguminosarum to promote growth, coupled with synchrotron based µXANES analysis to probe Zn speciation in the plant roots. P. brassicacearum exhibited the poorest plant growth promoting ability, while R. leguminosarum alone and in combination with P. brassicacearum enhanced plant growth and Zn phytoextraction. Reduced growth in un-inoculated plants was attributed to accumulation of Zn oxalate and Zn sulfate in roots. In plants inoculated with P. brassicacearum the high concentration of Zn polygalacturonic acid in the root may be responsible for the stunted growth and reduced Zn phytoextraction. The improved growth and increased metal accumulation observed in plants inoculated with R. leguminosarum and in combination with P. brassicacearum was attributed to the storage of Zn in the form of Zn phytate and Zn cysteine in the root. When combined with the observation that both bacteria do not statistically improve B. juncea growth in the absence of Zn, this work suggests that bacteria-induced metal chelation is the key mechanism of plant growth promoting bacteria in toxicity attenuation and microbial-assisted phytoremediation.

The effect of broadleaf woodland on aluminium speciation in stream water in an acid-sensitive area in the UK.

Acidification can result in the mobilisation and release of toxic inorganic monomeric aluminium (Al) species from soils into aquatic ecosystems. Although it is well-established that conifer trees enhance acidic atmospheric deposition and exacerbate soil and water acidification, the effect of broad-leaved woodland on soil and water acidification is less clear. This study investigated the effect of broadleaf woodland cover on the acid-base chemistry and Al species present in stream water, and processes controlling these in the acid-sensitive area around Loch Katrine, in the central Highlands, Scotland, UK, where broadleaf woodland expansion is occurring. A nested sampling approach was used to identify 22 stream sampling locations, in sub-catchments of 3.2-61 ha area and 0-45% broadleaf woodland cover. In addition, soils sampled from 68 locations were analysed to assess the influence of: (i) broadleaf woodland cover on soil characteristics and (ii) soil characteristics on stream water chemistry. Stream water pH was negatively correlated with sub-catchment % woodland cover, indicating that woodland cover is enhancing stream water acidification. Concentrations of all stream water Al species (monomeric total, organic and inorganic) were positively correlated with % woodland cover, although not significantly, but were below levels that are toxic to fish. Soil depth, O horizon depth and soil chemistry, particularly of the A horizon, appeared to be the dominant controls on stream water chemistry rather than woodland cover. There were significant differences in soil acid-base chemistry, with significantly lower O horizon pH and A horizon base saturation and higher A horizon exchangeable Al in the wooded catchments compared to the control. This is evidence that the mobile anion effect is already occurring in the study catchments and suggests that stream water acidification arising from broadleaf woodland expansion could occur, especially where tree density is high and acid deposition is predominantly in dry or occult forms.

Manganese concentrations in Scottish groundwater.

Groundwater is increasingly being used for public and private water supplies in Scotland, but there is growing evidence that manganese (Mn) concentrations in many groundwater supplies exceed the national drinking water limit of 0.05 mg l(-1). This study examines the extent and magnitude of high Mn concentrations in groundwater in Scotland and investigates the factors controlling Mn concentrations. A dataset containing 475 high quality groundwater samples was compiled using new data from Baseline Scotland supplemented with additional high quality data where available. Concentrations ranged up to 1.9 mg l(-1); median Mn concentration was 0.013 mg l(-1) with 25th and 75th percentiles 0.0014 and 0.072 mg l(-1) respectively. The Scottish drinking water limit (0.05 mg l(-1)) was exceeded for 30% of samples and the WHO health guideline (0.4 mg l(-1)) by 9%; concentrations were highest in the Carboniferous sedimentary aquifer in central Scotland, the Devonian sedimentary aquifer of Morayshire, and superficial aquifers. Further analysis using 137 samples from the Devonian aquifers indicated strong redox and pH controls (pH, Eh and dissolved oxygen accounted for 58% of variance in Mn concentrations). In addition, an independent relationship between Fe and Mn was observed, suggesting that Fe behaviour in groundwater may affect Mn solubility. Given the redox status and pH of Scottish groundwaters the most likely explanation is sorption of Mn to Fe oxides, which are released into solution when Fe is reduced. Since the occurrence of elevated Mn concentrations is widespread in groundwaters from all aquifer types, consideration should be given to monitoring Mn more widely in both public and private groundwater supplies in Scotland and by implication elsewhere.