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Culivated land, grassland or forest: metal pollution follows different paths in different soils

Concerns about soil status lie at the crossroads between public health issues, protection of the environment and the sustainable use of space. INRA researchers have demonstrated the crucial role of land use in the fate of metal pollutants by studying the distribution of metals in soils near an old zinc metallurgy factory. They were thus able to show that lead and zinc levels, and their distribution in the soil, differed depending on whether the land was used for cultivation, grassland or forest. These findings will reinforce the interest of stakeholders in agriculture regarding the management of polluted soils.

Bureau de cartogéo-statistique. Ici carte présentant la pollution en plomb des sols.. © INRA, MAITRE Christophe
Updated on 05/18/2015
Published on 10/18/2011

Concerns about soil status lie at the crossroads between public health issues, protection of the environment and the sustainable use of space.

INRA scientists from the Versailles-Grignon Research Centre carried out their study in an agricultural region in northern France, about 3 km distant from an old zinc metallurgy factory.
The activities of this industrial complex, which ceased production in 1963, caused major metal pollution (zinc, lead, cadmium) which is still present today on this site in the form of buried waste (about 20,000 tonnes), as well as in the surrounding soils and sediments (approximately 12,000 tonnes).
The INRA researchers compared the soil pollution of three neighbouring plots, all at equal distance from the old factory: two plots of agricultural land that have remained unchanged for the past century (one used for permanent grassland and the other for conventional cultivation) and a forest (beech and pine). The focused in particular on two metals that were present
in the atmospheric dust emitted over time by the factory: lead (Pb) and zinc (Zn), because these two elements generally display contrasted transfer profiles in soils.

Cultivated land or grassland: identical quantities of pollutants, but distributed differently

The amounts of metallic elements of industrial origin measured at a depth of 1 metre were comparable in the two soil samples: 17.3 and 19.1 g Pb per m3; 44.6 and 46.3 g Zn per m3 under the cultivated land and grassland, respectively. These amounts were supplementary to the natural metal content of these soils (14.5 g Pb per m3 and 45 g Zn per m3). Thus, in both the soils studied, about half of the levels of metallic elements were made up of metals arising from the historical industrial site.
By contrast, their vertical distribution differed markedly. The concentrations fell as a function of soil depth, but they were lower near the surface under grassland. Lead was present at greater depths in the grassland soil when compared with the cultivated soil.

A vertical distribution of pollutants linked to the biological activities of soils

The scientists studied both the distribution of Zn, Pb and organic matter using chemical analysis, and the morphological characteristics of the soils using optical microscopy: on the surface of both soils, large quantities of Zn and Pb were found to be present in aggregates of very small size (between 2 and 20 μm). These aggregates mainly resulted from microbial and root activities in the soils.
However, in the soil under the grassland, a large proportion of these metals was also evidenced in larger aggregates (50-100 μm) at depths of up to 80 cm. These aggregates were characteristic of the activities of soil fauna, and notably earthworms, which were more numerous under the grassland (about 400 individuals per m²) than in cultivated soil (about 50 individuals per m²).

Different mechanisms of metal incorporation in the soils

In both soils, Zn migrated in the soil solution in the form of free ions; it was intercepted and retained at depth by specific constituents (clays and iron oxides). Furthermore, in the soil under the grassland, earthworms ingested metal-containing soil they found on the surface, expelling their droppings at depth and thus contributing to the incorporation of Pb and Zn at depth, resulting in a reduction of metal concentrations near the surface.

In the forest, more lead and less zinc

In the forest soil, the amount of Pb attributed to pollution was larger than that found in agricultural soils (approximately 25 g per m3). By contrast, the total quantity of Zn represented only about 20% of that seen in agricultural land. This could be explained by the fact that the upper parts (canopy) of the trees subject to the dominant winds filtered the atmosphere and concentrated the metallic elements. These reached the soil in rainwater or as a result of leaf fall. But under the acid conditions of forest soils, Zn was carried in the rainwater towards the water table, unlike Pb which accumulated on the surface.

Measuring the burden of the past in order to prepare the future

Thus, in 2003, or 40 years after the metallurgy factory ceased operation, the cultivated soil still contained on the surface 92% of Pb and 60% of Zn supplied in the form of dusts, which might be mobilized during cultivation. In the soil under grassland, 80% of Pb and only 40% of Zn were still found on the surface. Finally, under the forest soil, only 20% of Zn remained,
the rest having been carried towards the water table.

Thus in order to obtain a clearer understanding of the dynamics of metal pollutants in soils, it is necessary to take account of the occupancy, and more generally the management, of soils, depending on whether they have been allocated to agricultural or forest use

Références

F. van Oort et al. Pollutions métalliques : distributions hétérogènes du Zn, Pb, Cd, et Cu et relations avec l'usage des sols. In : Contaminations métalliques des agrosystèmes et écosystèmes péri-urbains.Éditions Quae. 2009.  P. Cambier, C. Schwartz & F. van Oort (coord.), Versailles, France, p. 15.

C. Fernandez et al.  2010. Fate of airborne metal pollution in soils as related to agricultural management: 2. Assessing the role of biological activity in micro-scale Zn and Pb distributions in A, B and C horizons. European Journal of Soil Science 61: 514.