Jesús M. Peñalosa

Heavy metals distribution in soils surrounding an abandoned mine in NW Madrid (Spain) and their transference to wild flora

The present work concerns the distribution and mobility of heavy metals (Fe, Mn, Cu, Zn and Cd) in the surrounding soils of a mine site and their transfer to wild flora. Thus, soils and plants were sampled from a mining valley in NW Madrid (Spain), and total and extractable heavy metals were analysed. Soils affected by mining activities presented total Cd, Cu and Zn concentrations above toxic thresholds. The percentage of extractable element was highest for Cd and lowest for Cu. A highly significant correlation was observed between the total and extractable concentrations of metals in soils, indicating that, among the factors studied, total metals concentration is the most relevant for heavy metals extractability in these soils. (NH(4))(2)SO(4)-extractable metal concentrations in soils are correlated better with metal concentrations in several plant species than total metals in soils, and thus can be used as a suitable and robust method for the estimation of the phytoavailable fraction present in soils. Twenty-five vascular plant species (3 ferns and 22 flowering plants) were analysed, in order to identify exceptional characteristics that would be interesting for soil phytoremediation and/or reclamation. High Cd and Zn concentrations have been found in the aerial parts of Hypericum perforatum (Cd), Salix atrocinerea (Cd, Zn) and Digitalis thapsi (Cd, Zn). The present paper is, to the best of our knowledge, the first report of the metal accumulation ability of the two latter plant species. The phytoremediation ability of S. atrocinerea for Cd and Zn was estimated, obtaining intervals of time that could be considered suitable for the phytoextraction of polluted soils.

The fate of arsenic in soil-plant systems

Arsenic is a natural trace element found in the environment. In some cases and places, human activities have increased the soil concentration of As to levels that exceed hazard thresholds. Amongst the main contributing sources of As contamination of soil and water are the following: geologic origin, pyriticmining, agriculture, and coal burning. Arsenic speciation in soils occurs and is relatively complex. Soils contain both organic and inorganic arsenic species. Inorganic As species include arsenite and arsenate, which are the most abundant forms found in the environment. The majority of As in aerated soils exists as H₂AsO₄- (acid soils) or HAsO₄²- (neutral species and basic). However, HA₃sO₃ is the predomiant anaerobic soils, where arsenic availability is higher and As(III) is more weakly retained in the soil matrix than is As(V). The availability of As in soils is usually driven by multiple factors. Among these factors is the presence of Fe-oxides and/or phosphorus, (co)precipitation in salts, pH, organic matter, clay content, rainfall amount, etc. The available and most labile As fraction can potentially be taken up by plant roots, although the concentration of this fraction is usually low. Arsenic has no known biological function in plants. Once inside root cells, As(V) is quickly reduced to As(III), and, in many plant species, becomes complexed. Phosphorus nutrition influences As(V) uptake and toxicity in plants, whilst silicon has similar influences on As(III). Plants cope with As contamination in their tissues by possessing detoxification mechanisms. Such mechanisms include complexation and compartmentalization. However, once these mechanisms are saturated, symptoms of phytotoxicity appear. Phytotoxic effects commonly observed from As exposure includes growth inhibition, chlorophyll degradation, nutrient depletion and oxidative stress. Plants vary in their ability to accumulate and tolerate As (from tolerant hyperaccumulators to sensitive excluders), and some plants are useful for soil reclamation and in sustainable agriculture, The status of current scientific knowledge allows us to manage As contamination in the soil-plant system and to mitigate arsenics effects. Phytoremediation is an emerging technology suitable for reclaiming As-contaminated soils and waters. Phytoextraction has been used to clean As-contaminated soils, although its applicability has not yet reached maturity. Phytostabilization has been employed to reduce environmental risk by confining As as an inert form in soils and has shown success in both laboratory experiments and in field trials. Phytofiltration has been used to treat As-enriched waters. Such treatment removes As when it is accumulated in plants grown in or on water. In agricultural food production, appropriate soil management and plant variety/species selection can minimize As-associated human dis- eases and the transfer of As within the food chain. Selecting suitable plants for use on As-contaminated soils may also enhance alternative land use, such as for energy or raw material production.

Boron and calcium distribution in nitrogen-fixing pea plants.

In a glasshouse experiment, plants of Pisum sativum L. cv. Argona were grown hydroponically with different B and Ca levels, in order to elucidate a specific role for B and Ca on the N(2) fixation in this temperate legume. The treatments were as follows: control (9.3 µM B and 2 mM Ca), -B (without B and 2 mM Ca), -B+Ca (without B and 3.6 mM Ca), +Ca (9.3 µM B and 3.6 mM Ca), -Ca (9.3 µM B and 0.4 mM Ca) and -Ca+B (46.5 µM B and 0.4 mM Ca). The supply of -Ca and +Ca did not affect nitrogenase activity, but the weight of old shoots and total N content increased with the Ca treatment. No symptoms of B deficiency were detected in the plants of the -B and -B+Ca treatments, apart from weight reduction in young shoots and lower nitrogenase activity. The B concentration decreased in young shoots and roots of plants grown without B (-B), but there was a sharper decrease in the roots of -B+Ca plants and the levels of B in the young shoots were similar to the control levels. The B concentration in -Ca plants was reduced in the old shoot and in the root, while plant weight and N content increased in -Ca+B plants. The cell wall and total B concentrations in the nodules were 4-fold compared with those of the roots. By contrast, the Ca root wall was 2.5 times higher than the nodule levels although total pectin was higher in the nodule than in the root. Finally, the results obtained showed that a high supply of Ca could induce B mobilisation from root to shoot. On the other hand, the high B requirement found in pea plant nodules may contribute to explain the low nitrogenase activity detected under -B conditions.

Arsenic- and mercury-induced phytotoxicity in the Mediterranean shrubs Pistacia lentiscus and Tamarix gallica grown in hydroponic culture

Hg and As resistance and bioaccumulation were studied in hydroponically grown Pistacia lentiscus and Tamarix gallica plants. Both elements caused growth inhibition in roots and shoots, with mercury showing greater phytotoxicity than arsenic. Accumulation of both elements by plants increased in response to element supply, with the greatest uptake found in T. gallica. Both elements affected P and Mn status in plants, reduced chlorophyll a concentration and increased MDA and thiol levels. These stress indices showed good correlations with As and Hg concentration in plant tissues, especially in the roots. Toxic responses to mercury were more evident than for arsenic, especially in shoot tissues. T. gallica showed higher resistance to both Hg and As than P. lentiscus, as well accumulating more As and Hg.

Phytostabilisation with Mediterranean shrubs and liming improved soil quality in a pot experiment with a pyrite mine soil

Phytoremediation can be a suitable option to manage derelict mine soils. A pot experiment was carried out under semi-controlled conditions with a mine-impacted soil. A further contamination event was mimicked by applying 5% of pyritic sludge. Four species were planted in pots (Myrtus communis, Retama sphaerocarpa, Rosmarinus officinalis and Tamarix gallica), and some pots remained unplanted as a control. The substrates were moderately to highly contaminated, mainly with arsenic and zinc. The strong acidification induced by the pyritic sludge was buffered with lime and plants survived in all the pots. Liming provoked an effective immobilisation of metals and arsenic. Plant establishment decreased labile As in the substrate by 50%, mainly M. communis, although the levels of extractable metals were not affected by the plants. R. sphaerocarpa and M. communis increased the levels of C and N in the soil by 23% and 34% respectively, and also enhanced enzymatic activities and microbial respiration to the double in some cases. The low transfer of trace elements to shoots limited the phytoextraction rate. Our results support the use of phytostabilisation in Mediterranean mine soils and show how plants of R. sphaerocarpa and M. communis may increase soil health and quality during revegetation.

Screening risk assessment tools for assessing the environmental impact in an abandoned pyritic mine in Spain

This paper describes a new methodology for assessing site-specific environmental impact of contaminants. The proposed method integrates traditional risk assessment approaches with real and variable environmental characteristics at a local scale. Environmental impact on selected receptors was classified for each environmental compartment into 5 categories derived from the whole (chronic and acute) risk assessment using 8 risk levels. Risk levels were established according to three hazard quotients (HQs) which represented the ratio of exposure to acute and chronic toxicity values. This tool allowed integrating in only one impact category all the elements involved in the standard risk assessment. The methodology was applied to an abandoned metal mine in Spain, where high levels of As, Cd, Zn and Cu were detected. Risk affecting potential receptors such as aquatic and soil organisms and terrestrial vertebrates were assessed. Whole results showed that impact to the ecosystem is likely high and further investigation or remedial actions are necessary. Some proposals to refine the risk assessment for a more realistic diagnostic are included.

Hydroponics as a valid tool to assess arsenic availability in mine soils

The low solubility of As in mine soils limits its phytoavailability. This makes the extrapolation of data obtained under hydroponic conditions unrealistic because the concentration in nutrient solution frequently overexposes plants to this metalloid. This work evaluates whether As supply in hydroponics resembles, to some extent, the As phytoavailable fraction in soils and the implications for phytoremediation. Phytotoxicity of As, in terms of biomass production, chlorophyll levels, and As concentrations in plants, was estimated and compared in both soils and hydroponics. In order for hydroponic conditions to be compared to soil conditions, plant exposure levels were measured in both cultures. Hydroponic As concentration ranging from 2-8microM equated to the same plant organ concentrations from soils with 700-3000mgkg(-1). Total and extractable As fractions exceeded those values, but As concentrations in pore water were bellow them. According to our results (i) hydroponics should include doses in the range 0-10microM As to allow the extrapolation of the results to As-polluted soils, and (ii) phytoextraction of As in mining sites will be limited by low As phytoavailability.

Feasibility of arsenic phytostabilisation using Mediterranean shrubs: impact of root mineralisation on As availability in soils

An incubation experiment was carried out to evaluate the rate of arsenic release from roots to soil during root mineralisation, in order to determine the viability of phytostabilisation of metal-polluted soil. Arsenic (As)-loaded roots were mixed with the soil, monitoring arsenic, phosphorus, copper and zinc solubility and pH. Arsenic dynamics in soil during root mineralisation depend on various factors such as mineralisation rate, pH and soil chemistry. After 56 days incubation, only the soil with T. gallica roots showed higher NH4(SO4)2-extractable As than soil without roots. Phosphorus release was positively correlated with extractable arsenic. Extractable Cu depended on root mineralisation but Zn solubility was controlled by soil pH. On the whole, species with high As accumulation in roots and a high mineralisation rate may release arsenic in soluble forms in the soil immediately after root addition, but the soil can partly retain arsenic subsequently. In this respect, the risk of As mobilisation due to root decomposition was found to be low, confirming the prospects for long-term phytomanagement and phytostabilisation of arsenic.

Natural attenuation of residual heavy metal contamination in soils affected by the Aznalcóllar mine spill, SW Spain

Non-amended soils affected by pyritic sludge residues were monitored for 7 years to assess the long-term natural attenuation ability of these soils. The decrease in both the total concentration of elements (particularly As) and (NH(4))(2)SO(4)-extractable fractions of Mn, and Zn, below the maximum permissible levels indicate a successful natural ability to attenuate soil pollution. Soil acidification by pyrite oxidation and rainfall-enhanced leaching were the largest contributors to the reduction of metals of high (Mn, Cu, Zn and Cd) and low (Fe, Al, and As) availability. Periodic use of correlation and spatial distribution analysis was useful in monitoring elemental dispersion and soil property/element relationships.

Pre-plant slow-release fertilization of strawberry plants before fertigation

The advantages of pre-plant fertilization were studied by using a slow-release fertilizer (nitrophoska permanent) with strawberry plants (Fragaria ananassa, cv Chandler) before fertigation. A sandy soil was used in the experiment in conditions of abundant rain. When the slow-release NPK was mixed with soil, the leachate analysis of a glasshouse crop showed a lower loss of N and therefore a lower degree of ground water contamination compared with the traditional NPK fertilizer. However, when the fertilizer in question was placed at 10 cm from the surface, as in the case of ornamental plants, the results were less favourable. Consequently, the slow-release fertilizer mixed with the soil not only increases the N uptake by the plant as well as the leaf and root weight, but it also produces higher yields.ResumenSe estudian las ventajas de una fertilización de fondo del cultivo de fresón, con un fertilizante de liberación lenta (nitrophoska permanent), previa a la fertilización. Se utilizó un suelo arenoso y se reprodujo una alta pluviosidad en los ensayos. En cultivo de invernadero se comprobó, mediante el análisis de drenajes, una menor pérdida de N y por lo tanto menor contaminación de acuíferos, para el NPK de liberación lenta mezclado con el suelo. Sin embargo, cuando este fertilizante se localiza a 10 cm de la superficie como en el caso de plantas ornamentales los resultados son menos positivos. Por tanto, el fertilizante de liberación lenta, mezclado con el suelo, proporciona mayor exportación de N por la planta y mayor peso de hoja y raiz, así como mejores rendimientos.