Comparison of a rhizosphere-based method with other one-step extraction methods for assessing the bioavailability of soil metals to wheat

Abstract

There is no method recognized as a universal approach for evaluation of bioavailability of heavy metals in soil. Based on the simulation of the rhizosphere soil conditions and integration of the combined effects of root-soil interactions as a whole, a rhizosphere-based method has been proposed. Wet fresh rhizosphere soil was extracted by lowmolecular-weight organic acids (LMWOAs) to fractionate metal fractions of soil pools, which were then correlated with the metal contents of wheat roots and shoots. The rhizosphere-based method was compared with other one-step extraction methods using DTPA, EDTA, CaCl 2 , and NaNO 3 as extractants and the first step of the Community Bureau of Reference (BCR) method. Simple correlation and stepwise multiple regression analysis were used for the comparison. Simple correlation indicated that the extractable Cu, Zn, Cr, and Cd of soils by the rhizosphere-based method were significantly correlated with the metal contents of wheat roots. For DTPA, BCR1 and EDTA methods there was a relatively poor correlation between the extractable Cu, Zn and Cd of soil and metal contents of wheat roots. Stepwise multiple regression analysis revealed that the equation of the rhizosphere-based method was the simplest one, and no soil properties variables needed to be added. In contrast, the equations of other one-step extraction methods were more complicated, and soil properties variables needed to be entered. The most distinct feature of the rhizosphere-based method was that the recommended method was suitable for acidic, neutral and near alkaline soils. However, the DTPA and EDTA extraction methods were suitable for calcareous soils only-or-only for acidic soils. The CaCl 2 , and NaNO 3 extraction methods were only suitable for exchangeable metals. In short, the rhizosphere-based method was the most robust approach for evaluation of bioavailability of heavy metals in soils to wheat.

The use of differential pulse anodic stripping voltammetry and diffusive gradient in thin films for heavy metals speciation in soil solution

“…This fact appears to have a big impact of plant to soil solution pH values. This is in accordance with the opinion that root activity is the driving force in HM uptake and phytoextraction [38]. The plant efficiency of HM extraction (the HM´s bioavailability) is also affected by soil sorption parameters, cation exchange capacity (CEC) [39], and organic matter content [40,41].…”

Cobalt, chromium and nickel contents in soils and plants from a serpentinite quarry

“…Extractions with DTPA (0.005 M DTPA + 0.1 M TEA + 0.01 M CaCl 2 ) were done in accordance with Lindsay and Norvell (1978). Bidistilled water (BDW) was used following Pueyo et al (2004) and a 10 mM mixture of five different low molecular weight organic acids (LMWOA) was used according to the instructions of Feng et al (2005). The LMWOA composition was acetic, lactic, citric, malic and formic acids with molar concentration ratio of 4 : 2 : 1 : 1 : 1, respectively.…”

“…This is a rhizosphere-based extraction method that simulates the rhizosphere conditions and takes into account the effect of soil-root interactions as a whole, at least to some extent (Feng et al, 2005).…”

Is an adjusted rhizosphere-based method valid for field assessment of metal phytoavailability? Application to non-contaminated soils

“…A-RHIZO and RHIZO: Bulk or rhizosphere soil (2 g) was extracted with an aqueous solution of LMWOAs (20 ml) in a 50 ml plastic centrifuge tube with continual shaking for 16 h. The total concentration of acetic, lactic, citric, malic, and formic acids was 0.01 M, and their molar ratio was 4:2:1:1:1 (c/c) (Feng et al, 2005a); EDTA: Soil (2 g) was extracted with 0.05 M EDTA (20 ml) adjusted by ammonia solution to pH 7.0 in a 50 ml plastic centrifuge tube with shaking for 1 h (Wear and Evans, 1968); DTPA: Soil (10 g) was extracted with 0.005 M DTPA þ 0.01 M CaCl 2 þ 0.01 M TEA adjusted to pH 7.3 (20 ml) with shaking for 2 h (Lindsay and Norvell, 1978); CaCl 2 : Soil (2 g) was extracted with 0.01 M CaCl 2 (20 ml) in a 50 ml plastic centrifuge tube with shaking for 3 h (Novozamsky et al, 1993); BCR1: Soil (1 g) was extracted with 0.11 M acetic acid (20 ml) in a 50 ml plastic centrifuge tube with shaking for 16 h (Ure et al, 1993).…”