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TECHNICAL REPORTS

Heavy Metals in the Environment

White Clover Nodulation Index in Heavy Metal Contaminated Soils– A Potential Bioindicator

^a Institut Lillois d'Ingénierie de la Santé (ILIS), EA 2690, 42, rue Ambroise Paré, 59120 Loos, France
^b Département de Botanique, Université Droit et Santé de Lille, EA 2690, 42, rue Ambroise Paré, 59120 Loos, France
^c VITO, Flemish Institute for Technological Research, Boeretang 200, B-2400 Mol, Belgium

^* Corresponding author (nmanier{at}gmail.com).

Received for publication January 10, 2008. The morphological effects of heavy metal stress on the nodulation ability of Rhizobium spp. and growth of white clover (Trifolium repens L.) were studied in the laboratory under controlled conditions. Fourteen topsoils were collected from an area with elevated metal concentrations (Cd, Zn, and Pb). White clover was cultivated using a specialized "rhizotron" method to observe the development of root and nodule characteristics. Results show effects of increasing heavy metal concentrations on nodulation development, especially the nodulation index (i.e., the number of nodules per gram of the total fresh biomass). A significant decrease in nodulation index was observed at about 2.64 mg Cd kg^–1, 300 mg Zn kg^–1, and 130 mg Pb kg^–1 in these soils. The sensitivity of the nodulation index in relation to other morphological characteristics is discussed further. It is proposed that the nodulation index of white clover is a suitable bioindicator of increased heavy metal concentrations in soil.

Abbreviations: AFNOR, French Association for Normalization • CEC, cation exchange capacity • ICP AES, Inductively Coupled Plasma–Atom Emission Spectrometry • ICP MS, Inductively Coupled Plasma– Mass Spectrometry • PAR, photosynthetically active radiation • RI, robustness index