Modified micro suction cup/rhizobox approach for the in-situ detection of organic acids in rhizosphere soil solution

Root–soil interactions can strongly influence the soil solution chemistry in the rhizosphere. In the present study we propose a modification of the classical rhizobox/micro suction cup system to make it suitable for the collection and analysis of organic acids in the rhizosphere. In order to show the potential of the method, we tested the modified system with Lupinus albus L. as a model plant known to exude large amounts of citrate. The suction cups were installed through the transparent front plate of the rhizoboxes just after the emergence of cluster roots in order to allow optimal localized collection of soil solution. A small dead-volume allowed almost immediate stabilisation with formaldehyde of the sampled soil solutions in the collection container to prevent microbial degradation. The concentrations of organic acids were significantly larger in the rhizosphere soil solution of active cluster roots of Lupinus albus L. than in the bulk soil solution (about 400 μM of citrate versus <0.05 μM). We were able to follow the exudation process in-situ, which occurred during 2–3 days. Also the concentrations of other organic acids and inorganic anions differed between the bulk soil and the rhizosphere of cluster roots, normal roots, and nodules.     

Response of native grasses and Cicer arietinum to soil polluted with mining wastes: Implications for the management of land adjacent to mine sites

Mine tailings are an environmental problem in Southern Spain because wind and water erosion of bare surfaces results in the dispersal of toxic metals over nearby urban or agricultural areas. Revegetation with tolerant native species may reduce this risk. We grew two grasses, Lygeum spartum and Piptatherum miliaceum, and the crop species Cicer arietinum (chickpea) under controlled conditions in pots containing a mine tailings mixed into non-polluted soil to give treatments of 0%, 25%, 50%, 75% and 100% mine tailings. We tested a neutral (pH 7.4) mine tailings which contained high concentrations of Cd, Cu, Pb and Zn. Water-extractable metal concentrations increased in proportion to the amount of tailings added. The biomass of the two grasses decreased in proportion to the rate of neutral mine-tailing addition, while the biomass of C. arietinum only decreased in relation to the control treatment. Neutron radiography revealed that root development of C. arietinum was perturbed in soil amended with the neutral tailings compared to those of the control treatment, despite a lack of toxicity symptoms in the shoots. In all treatments and for all metals, the plants accumulated higher concentrations in the roots than in shoots. The highest concentrations occurred in the roots of P. miliaceum (2500 mg kg^?1 Pb, 146 mg kg^?1 Cd, 185 mg kg^?1 Cu, 2700 mg kg^?1 Zn). C. arietinum seeds had normal concentrations of Zn (70–90 mg kg^?1) and Cu (6–9 mg kg^?1). However, the Cd concentration in this species was ～1 mg kg^?1 in the seeds and 14.5 mg kg^?1 in shoots. Consumption of these plant species by cattle and wild fauna may present a risk of toxic metals entering the food chain.     

Neutron radiography as a tool for revealing root development in soil: capabilities and limitations

Chronic atmospheric nitrogen deposition affects the cycling of carbon (C) and nitrogen (N) in forest ecosystems, and thereby alters the stable C isotopic abundance of plant and soil. Three successional stages, disturbed, rehabilitated and mature forests were studied for their responses to different nitrogen input levels. N-addition manipulative experiments were conducted at low, medium and high N levels. To study the responses of C cycling to N addition, the C concentration and ¹³C natural abundances for leaf, litter and soil were measured. Labile organic carbon fractions in mineral soils were measured to quantify the dynamics of soil organic C (SOC). Results showed that three-year continuous N addition did not significantly increase foliar C and N concentration, but decreased C/N ratio and enriched ¹³C in N-rich forests. In addition, N addition significantly decreased microbial biomass C, and increased water soluble organic C in surface soils of N-rich forests. This study suggests that N addition enhances the water consumption per unit C assimilation of dominant plant species, restricts SOC turnover in N-poor forests at early and medium successional stages (thus favored SOC sequestration), and vice versa for N-rich mature forests.     

Effects of vegetation and fertilizer on metal and Sb plant uptake in a calcareous shooting range soil

Shooting range soils frequently contain anomalous concentrations of metals (e.g. Pb, Zn, Mn) and Sb coming from bullets which may be released into the environment. In a pot experiment, we investigated metal and Sb uptake by three plant species (Plantago lanceolata, Lolium perenne and Triticum aestivum) growing on a calcareous shooting range soil (pH 7.8; 500 mg kg⁻¹ Pb, 21 mg kg⁻¹ Sb) and the uptake changes when an acidic fertilizer solution was applied to the soil. Metal and Sb solubility in the soil was determined by extraction with 0.1 M NaNO₃. In addition, we measured pH, electrical conductivity and dissolved organic carbon in drainage samples. The results showed significant increase over time of pH (from 7.8 to 8.3) and decrease of electrical conductivity and dissolved organic carbon (from 230 to ∼130 mg L⁻¹). Fertilizer application increased NaNO₃-extractable Pb and Sb and root:shoot biomass ratio but not plant metal uptake. In T. aestivum spikes accumulated more Zn, Ni and Cu than shoots and grains. Mn and Zb uptake was correlated in L. perenne shoots. P. lanceolata, a Sb-bioindicator, did not accumulate high amounts of Sb (<1 mg kg⁻¹).     

Experimentation on Degradation of Petroleum in Contaminated Soils in the Root Zone of Maize (Zea Mays L.) Inoculated with Piriformospora Indica

Plant-based methods such as rhizodegradation are very promising for the remediation of petroleum-contaminated soils. Associations of plants with endophytes can further enhance their phytoremediation potential. In this study, a rhizobox experiment was conducted to investigate whether inoculation with the root-colonizing fungus Piriformospora indica could further enhance the degradation of petroleum hydrocarbons in the root zone of maize (Zea mays L.). The rhizoboxes were subdivided into compartments in accordance with distance from the plants. After filling the boxes with soil from a petroleum-contaminated site, seedlings that had either been inoculated with P. indica or not were grown in the middle compartments of the rhizoboxes and grown for 64 days. A plant-free treatment was included for control. The presence of roots strongly increased the counts of total and petroleum-degrading soil bacteria, respiration, dehydrogenase activity, water-soluble phenols and petroleum degradation. All these effects were also found in the soil adjacent to the middle compartments of the rhizoboxes, but strongly decreased further away from it. Inoculation with P. indica further enhanced all the recorded parameters without changing the spatial pattern of the effects. Inoculated plants also produced around 40% more root and shoot biomass than noninoculated plants and had greener leaves. Together, the results indicate that the treatment effects on the recorded soil microbial and biochemical parameters including petroleum hydrocarbon degradation were primarily due to increased root exudation. Irrespectively of this, they show that maize can be used to accelerate the rhizodegradation of petroleum hydrocarbons in soil and that inoculation with P. indica can substantially enhance the phytoremediation performance of maize.     

Bacterial community structure and activity in different Cd-treated forest soils

In this study we compared indicators of Cd bioavailability (water extracts, Lakanen extracts, free ions) and ecotoxicity in forest soils with contrasting physico-chemical characteristics. Soil samples were treated with CdCl(2) solutions (0, 0.1, 1, 10 and 100 mM) and incubated for 30 days. Microbial activity indexes (acid phosphatase, beta-glucosidase, basal respiration) and changes in bacterial community structure using terminal restriction fragment length polymorphism (T-RFLP) fingerprinting were investigated. The Cd concentrations measured ranged from 1% to 37% of the total additions in water extracts, to higher levels in Lakanen extracts. Effects of Cd were observed at bioavailable concentrations exceeding United Nations/European Economic Commission UN/ECE guidelines for total Cd in the soil solution. Basal respiration was the most affected index, while enzymatic activities showed variable responses to the Cd treatments. We also noticed that soils with pH higher than 6.7 and clay content higher than 50% showed inhibition of basal respiration but no marked shift in bacterial community structure. Soils with lower pH (pH <5.8) with less clay content (<50%) showed in addition strong changes in the bacterial community structure. Our results provide evidence for the importance of relating the effects of Cd on the soil communities to soil properties and to bioavailability.     

Arsenic accumulation of common plants from contaminated soils

A pot experiment was conducted to investigate the relationship between soluble concentrations of arsenic (As) in soil and its accumulation by maize (Zea mays), English ryegrass (Lolium perenne), rape (Brassica napus) and sunflower (Helianthus annuus) on two different soils: a calcareous Regosol (silty loam) and a non-calcareous Regosol (sandy loam). Arsenic (Na₂HAsO₄·7H₂O) was applied to obtain comparable soluble As concentrations in the two soils. In both soils, soluble As concentrations, extracted with 0.1 M NaNO₃, were found to correlate better with As concentrations in plants after 4 month of growth than total soil concentrations, extracted with 2 M HNO₃. With all four plant species, the relationship between the soluble As concentration in the soil and As that in the plants was non- linear, following Michaelis-Menten kinetics. Similar soluble As concentrations in the two soils did not result in a similar As concentration in the plants. Except for maize, arsenic transport from roots to shoots was significant, resulting in As concentrations in the leaves and grains above the Swiss tolerance limits for fodder and food crops (4 and 0.2 mg As kg^–1, respectively). Based on these results we suggest that beside As solubility, P availability and P demand, which are plant specific, have to be taken into account to predict the uptake of As by crop plants from As contaminated soils and to predict the risk of arsenic entering into the food chain.     

Root-zone modeling of heavy metal uptake and leaching in the presence of organic ligands

We present a mechanistic model which describes root uptake and leaching of heavy metals in the plant root zone, accounting for solution- and surface-complexation, (kinetic) mineral dissolution, heavy metal diffusion towards the root, root uptake, root exudation, ligand degradation and convective-dispersive transport of the soluble species. The model was used to simulate the influence of EDTA addition on Cu transport and plant uptake and the effect of oxalate exudation by roots on Cu transport and bioavailability using parameter values from the literature. In the simulations we assumed that free Cu²⁺ is the bioavailable form. Under slightly acidic conditions (pH 6) the model predicted that EDTA stabilizes Cu while at a slightly alkaline pH (pH 7.5), EDTA mobilizes Cu. The addition of EDTA approximately halved the cumulative Cu uptake after 360 days at pH 4.5, and reduced the uptake by a factor of 100 and 1000 at pH 6 and 7.5, respectively. Although the total dissolved concentration was increased, plant uptake was reduced by the formation of bio-inavailable complexes. The exudation of oxalate resulted in a decrease of the Cu concentration breaking through below the root zone, due to sorption of Cu-oxalate. In the presence of dissolved organic carbon (DOC), the exudation of oxalate increased Cu leaching considerably at pH 6 and 7.5. In the absence of DOC, the exudation of oxalate reduced Cu uptake due to the formation and adsorption of Cu-oxalate on goethite surface sites. Exudation of oxalate in the presence of DOC resulted in a further decrease of Cu uptake. Oxalate gradually takes over from DOC in binding Cu due to simultaneous production of oxalate and leaching of DOC. The simulations show that addition or exudation of ligands does not necessarily increase the solubility, transport and bioavailability of metals. Depending on the conditions (mainly the pH), also reduced transport and uptake can be observed, either by formation of ternary surface complexes or reduction of free metal concentration. The model can be easily extended to include further processes.     

The Phytomanagement of Trace Elements in Soil

Trace elements (TEs) occur at low concentrations (<1000 mg kg ^?1) in organisms, yet they have a large biological effect, both as essential nutrients and environmental contaminants. Phytomanagement describes the manipulation of soil-plant systems to affect the fluxes of TEs in the environment with the goal of remediating contaminated soils, recovering valuable metals, or increasing micronutrient concentrations in crops. Phytomanagement includes all biological, chemical, and physical technologies employed on a vegetated site. Successful phytomanagement should either cost less than other remediation or fortification technologies, or be a profitable operation, by producing valuable plant biomass products. This may include bioenergy or timber production on contaminated land, a practice that does not reduce food production. We review the components of phytomanagement and the underlying biogeochemical processes, with a view to elucidating situations where this technology may be successfully applied and identifying future research needs. Many full-scale operations have proved the efficacy of plants to reduce contaminant mobility in soils (phytostabilization), particularly when used in combination with other technologies. As a stand-alone technology, the oft-touted use of plants to extract TEs from contaminated soils (phytoextraction) or low-grade ore bodies (phytomining) is unsuitable for most, if not all, sites due to low-extraction rates and problems caused by site heterogeneity, the limited rooting depth of plants and the presence of contaminant mixtures. Unsubstantiated claims about phytoextraction have tarnished the reputation of all “phyto” technologies. Nevertheless, phytoextraction, as part of a larger environmental toolkit, has a role in phytomanagement. The growth, or lack thereof, of profitable companies that provide phytomanagement will indicate its value. A critical knowledge gap in phytomanagement is the integration of the processes that affect plant–TE interactions and the biophysical processes affecting TE fluxes in the root zone, especially the effect of roots on contaminant fluxes.     

Effects of heavy metal soil pollution and acid rain on growth and water use efficiency of a young model forest ecosystem

In a 4-year lysimeter experiment, we investigated the effects of topsoil heavy metal pollution (3,000 mg kg⁻¹ Zn, 640 mg kg⁻¹ Cu, 90 mg kg⁻¹ Pb and 10 mg kg⁻¹ Cd) and (synthetic) acid rain (pH 3.5) on tree growth and water use efficiency of young forest ecosystems consisting of Norway spruce (Picea abies), willow (Salix viminalis), poplar (Populus tremula) and birch (Betula pendula) trees and a variety of understorey plants. The treatments were applied in a Latin square factorial design (contaminated vs uncontaminated topsoil, acidified rain vs ambient rain) to 16 open-top chambers, with 4 replicates each. Each open-top chamber contained two lysimeters, one with a calcareous, and the other with acidic subsoil. The four tree species responded quite differently to heavy metal pollution and type of subsoil. The fine root mass, which was only sampled at the end of the experiment in 2003, was significantly reduced by heavy metal pollution in P. abies, P. tremula and B. pendula, but not in S. viminalis. The metal treatment responses of above-ground biomass and leaf area varied between years. In 2002, the heavy metal treatment reduced above-ground biomass and leaf area in P. tremula, but not in the other species. In 2003, metals did not reduce above-ground growth in P. tremula, but did so in the other species. It appears that the responses in above-ground biomass and leaf area, which paralleled each other, were related to changes in the relative competitive strength of the various species in the two experimental years. S. viminalis gained relative to P. tremula in absence of metal stress, in particular on calcareous subsoil, while P. abies showed the largest increases in growth rates in all treatments. Above- and below-ground growth was strongly inhibited by acidic subsoil in S. viminalis and to a lesser degree also in P. abies. In P. abies, this subsoil effect was enhanced by metal stress. Acid rain was not found to have any substantial effect. Whole-system water use efficiency was reduced by metal stress and higher on calcareous than on acidic subsoil.