The rare earth elements (REE) comprise a group of 16 chemically very similar elements that occur widespread in rocks, soils, and water bodies, share similar ionic radii to the essential element Ca²⁺, and consequently also occur in biota. Given that REE form mainly trivalent cations, they also share similarities to Al³⁺. Compared to their chemical cognate Ca, they have a higher reactivity. Thus, their accumulation in soils may constitute a severe environmental threat. Over the last decades, the increasing use of REE in modern technology and fertilizers raised concerns about the pollution of soils and water bodies, which led to a rapidly increasing number of publications dealing with REE toxicity to plants, animals and humans, the fate of REE in soil–plant systems, REE cycling in ecosystems and impacts of REE pollution on food security. This review aims to give an overview of the current knowledge on the occurrence of REE in the total environment, including relevant environmental processes governing their mobility, chemical speciation and transfer from abiotic compartments into biota. Beginning with an overview of analytical approaches, we summarize the current knowledge on the ecology of REE in the lithosphere, pedosphere, hydrosphere and biosphere, including impacts of soil pollution on food security and public health.

Urban soils have been changed much by human impacts in terms of structure, composition and use. This review paper gives a general introduction into changes from compaction, mixing, water retention, nutrient inputs, sealing, gardening, and pollution. Because pollutions in particular have caused concerns in the past, metal pollutions and platinum group metal inputs have been treated in more detail. Though it is not possible to cover the entire literature done on this field, it has been tried to give examples from all continents, regarding geochemical background levels. Urban metal soil pollution depends on the age of the settlement, current emissions from traffic and industry, and washout. It seems that in regions of high precipitation, pollutants are swept away to the watershed, leaving the soils less polluted than in Europe. Health hazards, however, are caused by ingestion and inhalation, which are higher in 3rd world countries, and not by concentrations met in urban soils as such; these are not treated within this paper in detail. With respect to pollutants, this paper is focused on metals. Contrary to many reviews of the past, which mix all data into one column, like sampling depth, sieved grain sizes, digestion and determination methods, these have been considered, because this might lead to considerable interpretation changes. Because many datasets are not Gaussian distributed, medians and concentration ranges are given, wherever possible. Urban dust contains about two to three fold the hazardous metal concentrations met in urban soils. Some data about metal mobilities obtained from selective and sequential leaching procedures, are also added. Soil compaction, pollution, sealings and run-offs cause stress situations for green plants growing at roadside locations, which is discussed in the Section 5. Environmental protection measures have led to decrease metal pollutions within the last decade in many places.

In the case of Se, the concentration range between essentiality and toxicity for terrestric animals and humans is rather narrow, while aquatic organisms are much less affected, and no essentiality to green plants and aquatic macrophytes has been established yet. This review focuses on the situation in Europe, where Se levels are generally low. Apart from industrial and mining activities, the main Se sources are the burning of coal and selenite additions to animal feedstuffs. Reduction processes in sediments, soils, and feedstuffs to yield elemental Se act as sinks for available Se forms. In soils and crops, Se levels get enhanced from recycling of manure, dung, and sewage sludge, which is beneficial for Europe. New data from Austria have been added to the detailed discussions. In human nutrition, Se is supplied via pork, liver and kidneys, seafood, and cereals, but main sources as well as blood Se levels vary between different countries and nutritional habits. Food processing, like boiling, baking, or grilling, results in Se losses.