No abstract

Soil quality is related to soil characteristics such as fertility and contamination. The aim of this study is to assess the effect of land use on these soil characteristics and to confirm the following anthropisation gradient: (i) forest, (ii) grassland, (iii) cultivated, (iv) orchard and vineyard, (v) urban vegetable garden, and (vi) SUITMA (urban, industrial, traffic, mining and military areas). A database comprising the characteristics of 2451 soils has been constituted. In order to compare the topsoils from six contrasting land uses, a principal components analysis (PCA) was performed on nine geochemical variables (C, N, pH, POlsen, total Cd, Cu, Ni, Pb, Zn). The first axis of the PCA is interpreted as a global increase of topsoil metallic elements along the anthropisation gradient. Axis 2 reflects the variability of fertility levels. Human activity increases the pressure on soils along the proposed gradient according to six different distribution patterns. This better knowledge of topsoil quality and its dependence on current land use should therefore help to manage and preserve the soil mantle.

Ultramafic soils are typically enriched in nickel (Ni), chromium (Cr), and cobalt (Co) and deficient in essential nutrients, making them unattractive for traditional agriculture. Implementing agromining systems in ultramafic agricultural soils represent an ecological option for the sustainable management and re-valorisation of these low-productivity landscapes. These novel agroecosystems cultivate Ni-hyperaccumulating plants which are able to bioaccumulate this metal in their aerial plant parts; harvested biomass can be incinerated to produce Ni-enriched ash or "bio-ore" from which Ni metal, Ni ecocatalysts or pure Ni salts can be recovered. Nickel hyperaccumulation has been documented in ∼450 species, and in temperate latitudes these mainly belong to the family Brassicaceae and particularly to the genus Odontarrhena (syn. Alyssum pro parte). Agromining allows for sustainable metal recovery without causing the environmental impacts associated with conventional mining activities, and at the same time, can improve soil fertility and quality and provide essential ecosystem services. Parallel reductions in Ni phytotoxicity over time would also permit cultivation of conventional agricultural crops. Field studies in Europe have been restricted to Mediterranean areas and these only evaluated the Ni-hyperaccumulator Odontarrhena muralis s.l. Two recent EU projects (Agronickel and LIFE-Agromine) have established a network of agromining field sites in ultramafic regions with different edapho-climatic characteristics across Albania, Austria, Greece and Spain. Soil and crop management practices are being developed so as to Kidd et al. Sustainable Agromining Systems for Nickel Recovery optimize the Ni agromining process; field studies are evaluating the potential benefits of fertilization regimes, crop selection and cropping patterns, and bioaugmentation with plant-associated microorganisms. Hydrometallurgical processes are being up-scaled to produce nickel compounds and energy from hyperaccumulator biomass. Exploratory techno-economic assessment of Ni metal recovery by pyrometallurgical conversion of O. muralis s.l. shows promising results under the condition that heat released during incineration can be valorized in the vicinity of the processing facility.

Root exudates were collected from maize plants (Zea mays L.), grown under sterile and hydroponic conditions. Exudates of low molecular weight were isolated by filtration and dialysis (MW 1000) of the nutrient solution and analysed. Metal binding properties of the exudates were investigated at 25°C in 0.1 M NaCIO, ionic medium, using a potentiometric method. The total acidity of the exudates and the successive stability constants of complexes formed with Cu2+, Pb2+ and Zn2+ were determined.Sugars and organic acids were the major components of the exudates. Measured total acidity was resolved into strong (5.85meq g-' C), weak (2.10meq g-'C) and very weak (2.20 meq g-' C) acidities. Mono-complexes were formed with Cu, Pb and Zn and bis-complexes with Cu. At pH 5, stability constants followed the order Cu > Pb > Zn. I N T R O D U C T I O NTo understand food-chain contamination by toxic metals, as well as the uptake ofmicronutrients by plants, requires knowledge of the processes by which metals are transferred to plant roots, including phenomena which take place in the rhizosphere (Nye & Tinker, 1977; Cushman, 1982). Besides microbial activity, pH and ionic concentrations changes, the release of organic exudates by the plant root may modify the flow of nutrients in the rhizosphere. The exudates include soluble compounds ranging from simple sugars to complex vitamins and mucilages (Rovira et al., 1979). Maize root mucilages and soluble exudates of high molecular weight have been shown to form complexes with metal ions (Morel et al., Mench et al., 1987). Moreover, there is increasing evidence that soluble root exudates promote the solubility of metals (Bromfield, 1958; Godo & Reisenauer, 1980;Olsen & Brown, 1980;Uren, 1984), possibly because of the formation of soluble metal complexes (Merckx et al., 1986).In this work the complexing properties of root exudates of low molecular weight were studied to complete previous measurements made on other fractions. The exudates were isolated from maize plants grown under sterile conditions, and their ability to bind metals was measured using a potentiometric method. M A T E R I A L S A N D M E T H O D SCollection of root exudates Root exudates were collected from maize plants (Zea mays L., var LGll) grown under sterile hydroponic conditions as described previously (Morel et al., 1986). The device consisted of a series of 35 x 480 mm glass tubes containing 50 ml of a Hoagland plant nutrient solution and glass beads. The tubes were autoclaved (120°C for 30 min), and maize seeds were surface sterilized by washing with 95% ethanol and immersing for 45 min in 10% H202 solution. Six seeds were introduced 52 1

Sewage sludges have been used for many years as sources of P for agricultural crops, but there is a lack of information regarding the proportion of sludge P that can be used by crops. The aim of this work was to assess the importance of soil available P and sludge origin on the utilization of sludge P by plants. First, the changes in soil P isotopically exchangeable within 1 min (E1min) were measured in incubated soil‐sludge mixtures using two soils and four sludges. Then, the uptake of sludge P by ryegrass (Lolium perenne L.) was measured on the same sludge‐amended soils. The application of sludges increased E1min to values lower or equal to those obtained following the application monocalcium phosphate. Similarly the utilization of sludge P by ryegrass was systematically lower than the utilization of P derived from a water‐soluble fertilizer. In both soils, the lowest utilization of sludge P was observed for the two FeSO4 flocculated and anaerobically digested sludges, while the primary sludge and the aerobically digested sludge released somewhat higher quantities of P to ryegrass. In the clayey soil, the amount of sludge P taken up by the crop was significantly related both to the sludge and soil available P content, whereas no such relation was observed in the loamy soil because of its high available P content. The origin of the sludge and the soil available P content must therefore be taken into account when advising sludge application to crops to adjust P inputs to plant needs.