Is an adjusted rhizosphere-based method valid for field assessment of metal phytoavailability? Application to non-contaminated soils Jing Fang a , Bei Wen a , Xiao-quan Shan a, * , Jin-ming Lin a , Gary Owens b a State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, China b Center for Environmental Risk Assessment and Remediation, University of South Australia, Mawson Lakes, South Australia SA 5095, Australia Received 26 July 2006; received in revised form 24 January 2007; accepted 27 January 2007 An adjusted-RHIZO method was the most promising approach for predicting metal phytoavailability to wheat under field conditions. Abstract Previously recommended rhizosphere-based method (RHIZO) applied to moist rhizosphere soils was integrated with moist bulk soils, and termed adjusted-RHIZO method (A-RHIZO). The A-RHIZO and RHIZO methods were systematically compared with EDTA, DTPA, CaCl 2 and the first step of the Community Bureau of Reference (BCR1) methods for assessing metal phytoavailability under field conditions. Results suggested that moist bulk soils are equally suited or even better than rhizosphere soils to estimate metal phytoavailability. The A-RHIZO method was preferred to other methods for predicting the phytoavailability of Ni, Cu, Zn, Cd, Pb and Mn to wheat roots with correlation coefficients of 0.730 (P < 0.001), 0.854 (P < 0.001), 0.887 (P < 0.001), 0.739 (P < 0.001), 0.725 (P < 0.001) and 0.469 (P < 0.05), respectively. When includ- ing soil properties, other extraction methods were also able to predict phytoavailability reasonably well for some metals. Soil pH, organic matter and FeeMn oxide contents, and cation-exchange capacity mostly influenced the extraction and phytoavailability of metals. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Metals; Phytoavailability; Chemical extraction; Wheat; Soil properties 1. Introduction Over the past decades much work has been directed towards the estimation of metal phytoavailability in soils. Researchers have made many attempts to predict metal phytoavailability by correlating data on metal accumulation in plants with the metal contents found in various soil pools, i.e., soluble, salt ex- changeable, oxide-bound or organic-bound, as determined by different sequential extractions or single extraction procedures (Shtangeeva, 2004). Single extraction methods using selective chemical extractants, such as strong chelating agents or mild neutral salts, are commonly used (Beckett, 1989). Among these extractants, EDTA (0.1e1 M) and DTPA (0.005 M), either as sodium or ammonium salts, have been proposed because of their ability to form very stable, water soluble and well-defined complexes with a wide range of metal cations (Haq et al., 1980; Chaignon et al., 2003). A 0.01 M CaCl 2 extractant was also pro- posed because of the similarity between this extractant and the concentration, composition and pH of soil background electro- lytes (Houba et al., 1996). Mench et al. (1997) reported that both EDTA and CaCl 2 extractable Cd was positively correlated with Cd contents in wheat roots and seeds. The Community Bureau of Reference of the European Community (now the Measurements and Testing Programme of the European Community) proposed a three-stage sequential extraction procedure (Quevauviller et al., 1993) to establish a common sequential extraction scheme. Unfortunately, conclusions obtained by many studies were inconsistent and made comparisons difficult (Qian et al., 1996; Sauve ´ et al., 1996; Shan et al., 2003). * Corresponding author. Tel.: þ86 10 62923560; fax: þ86 10 62923563. E-mail address: xiaoquan@rcees.ac.cn (X.-q. Shan). 0269-7491/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2007.01.019 Environmental Pollution 150 (2007) 209e217 www.elsevier.com/locate/envpol