Germund Tyler

Rare earth elements in soil and plant systems - A review

The rare earth elements (REEs) form a chemically uniform group and include yttrium (Y), lanthanum (La) and the lanthanides cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Their average abundance in the Earth’s crust range from 66µg g−1 in Ce to 0.5µg g−1 in Tm and «0.1µg g−1 in Pm. Recent great improvements in more routine analytical technique, the use of REEs as fertilisers, at least in East Asian agriculture, and the importance of these elements as indicators in both pedological and physiological processes and reactions have contributed to an increased interest in these previously less considered elements in environmental sciences. This review of recent and current literature deals with REEs in primary and secondary soil minerals, concentrations in surface soils, factors influencing adsorption, solubility and transport in soils, including weathering and transformations of REE minerals, and vertical distribution in soil profiles. Reviewed and discussed are also concentrations, distribution and localisation of REEs in plants and plant organs, soil-plant relationships and interactions, effects on plant growth and crop production and their importance in plant physiology and biochemistry. The REEs are found, usually several elements together, as phosphates, carbonates and silicate minerals finely dispersed especially in magmatic and metamorphic rocks. REE concentrations in surface soils of humid climates, such as the A(E)-horizons of Podzols and Laterites, are usually lower than in the parent material, due to higher weathering and leaching rates than of the average soil constituents. Some fractionation may occur due to the formation of more element-specific secondary minerals. Transfer from soil to plant is usually low, but extreme accumulators are found, e.g., among several species of ferns. Roots have generally higher concentrations than shoots. Possible uptakemechanisms of REEs are discussed. Uptake is positively, though often weakly, correlated with soil acidity and easily soluble concentrations of the elements, but rarely well related to their total concentrations in the soil. Under certain conditions, low concentrations of at least some REEs seem to favour plant growth and productivity, but the physiological mechanisms are still not well understood. Some considerations concerning the boundary between essential and non-essential micro nutrients are discussed.

Sorption and retention of heavy metals in the woodland moss Hylocomium splendens (Hebw.) Br. et Sch.

The capacity of Hylocomium splendens to sorb heavy metal ions from dilute solutions was studied. The sorption and retention generally followed the order: Cu, Pb > Ni > Co > Zn, Mn. This order proved valid within a wide range of concentrations and independently whether the ions were supplied in pure or mixed solutions. The capacity of the moss tissues to sorb traces of Cu and Pb in the presence of comparatively large amounts of Ca, K, Mg, and Na was found to be very great. This makes it probable that these ions, supplied with the precipitation, are almost quantitatively sorbed by the moss carpets. A large share of Ni when present in precipitation, will also possibly be sorbed. A natural carpet of Hylocomium splendens showed a continuous uptake of Mn, Fe, and Ca from young to old tissues, whereas the increase in the concentrations of minor heavy metals was balanced by the dry matter decrease through decomposition. The developing and mature mor layer below the moss carpet had not enriched heavy metals above the concentrations of the old moss tissues.

Heavy-metal ecology of terrestrial plants, microorganisms and invertebrates

Ecological and physiological effects of heavy metals on terrestrial organisms are reviewed, considering evidence from both laboratory and field studies. Problems concerning how to define heavy-metal exposure and to assess the sensitivity of field biota to heavy metals are discussed. Mechanisms of heavy-metal tolerance are considered including avoidance, exclusion, immobilization and excretion, as well biochemical mechanisms including enzymatic change. The taxonomy of heavy-metal tolerance and problems concerning tolerance and ecological performance are discussed briefly. Efforts are made to compare the relative sensitivity of various groups, including bacteria, fungi, bryophytes, lichens, vascular plants and soil invertebrates. An emphasis is placed on organisms of temperate forest ecosystems, particularly coniferous forests.

Heavy metal deposition in Scandinavia

Maps showing the regional differences in the atmospheric deposition of Pb, Cd, Hg, Cr, Ni, Cu, Zn, and Fe have been prepared for Finland, Norway and Sweden from data obtained by means of moss analysis, a method previously shown to be a sensitive and reliable tool in measuring heavy metal deposition. In all elements the minima were recorded in northern Norway, the maxima in south-central or south-western Sweden and southernmost Norway. The greatest regional differences were measured in Pb and Cd.

Heavy metal pollution and decomposition of spruce needle litter

Comparisons were made on the decomposition rates of different fractions of spruce needle litter from numerous sites around two metal processing industries in central and southeastern Sweden, emitting Cu, Zn, Cd and Ni, and Pb, Ni and Cd, respectively. All measurements were performed under controlled laboratory conditions. At water capacity and 15?C a C02 evolution rate in partly disintegrated litter of ca 0.23 mg C/g dry matter, 24 hr was measured in samples with > 10 prmol Zn + Cu + Cd + Ni per g, compared with ca 0.50 mg C in samples with < 5 jimol/g. Highly significant negative correlations were obtained in all materials between heavy metal concentration and C02 evolution rate or dehydrogenase activity. It is concluded that, in acid forest sites, the general decomposition rate will be depressed even by moderate concentrations of heavy metal ions, at least during those parts of the year when the water supply is adequate.

Heavy metal pollution and soil enzymatic activity

SummaryThe activity of hydrolytic soil enzymes was studied on spruce mor, polluted with Cu and Zn from a brass foundry in Sweden. Approximately straight regression lines were obtained between enzymatic activity or respiration rate and log Cu+Zn concentration, with highly significant negative regression coefficients for urease and acid phosphatase activity as well as respiration rate, whereas β-glucosidase activity was not measurably lower at high concentrations of Cu+Zn.

Changes in acidity and cation pools of south Swedish soils between 1949 and 1985

Abstract The pH of south Swedish soils have decreased considerably during the last 15–35 years. The decrease has occurred throughout the soil profile, not only in the rhizosphere, and is particularly marked in the originally less acid soils. The pH decrease was accompanied by considerable losses of exchangeable Na, K, Mg and Ca, as well as of Zn and (in the originally less acid soils) of Mn. The changes can be prognosticated from the current relationship between soil pH and base saturation and from budget calculations based on lysimeter data.

Herb layer vegetation of south Swedish beech and oak forests—effects of management and soil acidity during one decade

Abstract Effects of forest management and soil acidity on herb layer vegetation were studied after 10 years on 190 permanent plots in south Swedish beech (Fagus sylvatica) and oak (Quercus robur) forests. Species richness generally increased with management intensity, mainly due to establishment of ruderal species from the seed bank. Species richness of the typical forest flora was unaffected by management. Moderate management of oak stands favoured several species which are commonly found in semi-natural pastures. Classification and ordination of the data showed that the main floristic gradient within Swedish beech and oak forest vegetation is related to soil acidity. Species richness of the typical forest flora was strongly positively correlated with soil pH in beech forests, but this correlation was weaker in oak forests. The number of herbaceous plants with a broader habitat range increased with pH only in the oak forest plots. Long term changes in the forest environment, which may affect the vegetation, are the decline of grazing 150-50 years ago and soil acidification mainly caused by atmospheric pollutants during the last 50 years. In the one-decade perspective of this study, however, we did not find a general trend towards a more acid-tolerant flora. Neither could we find a general decrease of pasture species in currently ungrazed oak stands. The results indicate that most typical forest plants are well adapted to and partly depend on occurrence of canopy gaps and soil disturbance. If canopy thinning is followed by periods of canopy closure the characteristic shade tolerant flora of Swedish beech and oak forests may be able to persist as long as soil chemical limits of existence are not exceeded.

Leaching rates of heavy metal ions in forest soil

The leachability of Mn, Zn, Cd, Ni, V, Cu, Cr, and Pb was studied in two purely organic spruce forest soils: one control soil and one similar soil heavily polluted by Cu and Zn from a brass foundry in southern Sweden. Artificial rainwater, acidified to pH 4.2, 3.2, and 2.8 was used in the experiment. The 10% residence times, estimated from the experimental data, varied from 3 yr (Mn) to 70 to 90 yr (Pb) in the control soil and from 2 yr (V) to > 200 yr (Ph) in the polluted soil with a precipitation water of pH 4.2. Residence times for most elements studied (except V and Cr) decreased with pH of precipitation water.

Impact of Atmospheric Long Range Transport of Lead, Mercury and Cadmium on the Swedish Forest Environment

Emissions of Hg, Pb, and Cd to air aretransported over wide areas in Europe and deposited far awayfrom their sources. About 80% of the atmospheric depositionof these metals in south Sweden originate from emissions inother countries. As a result of the increased anthropogenicdeposition the concentrations of Hg, Pb, and Cd in the morlayer of forest soils have increased considerably, mainlyduring the 20th century. Although the atmosphericdeposition of these elements has declined during the mostrecent decades, the reduction of the input of Hg and Pb isnot sufficient to prevent a further accumulation. Theconcentrations of Hg and Pb are still increasing by ca. 0.5and ca. 0.2% annually in the surface layer of forest soils.In contrast, the Cd concentration is currentlydecreasing in a large part of Sweden as a result of bothdeposition decreases and enhanced leaching induced by soilacidification. The accumulation factors of Hg and Pb,especially in the forest topsoils of south Sweden, arealready above those at which adverse effects on soilbiological processes and organisms have been demonstrated instudies of gradients from local emission sources andlaboratory assessment. There are also indications of sucheffects at the current regional concentrations of Hg and Pbin mor layers from south Sweden, judging from observationsin field and laboratory studies. There is an apparent riskof Pb induced reduction in microbial activity over parts ofsouth Sweden. This might cause increased accumulation oforganic matter and a reduced availability of soil nutrients.At current concentrations of Hg in Swedish forest soils,effects similar to those of Pb are likely. Increasedconcentrations of these elements in organs of mammals andbirds have also been measured, though decreases have beendemonstrated in recent years, related to changes inatmospheric deposition rates. As a result of current andpast deposition in south Sweden, concentrations of Hg infish have increased about fivefold during the 20thcentury. This implies risks for human health, when fish frominland waters are used for food. Although the concentrationof Hg in fish has decreased ca. 20% during the last decade,probably as a result of the reduced deposition, the levelstill exceeds the general limit (0.5 mg kg-1) in about half(ca. 40 000) of the Swedish lakes. In order to reduceconcentrations in fish to the level recommended, and avoidfurther accumulation of Hg in soils, the atmosphericdeposition has to be reduced to ca. 20% of the current deposition rate. This can only be achieved by international co-operation.