Effects of soil oven-drying on concentrations and speciation of trace metals and dissolved organic matter in soil solution extracts of sandy soils

Abstract

Weak salt extracts can be used to assess the availability of trace metals for leaching and uptake by soil organisms and plants in soil. Before extraction, the International Organization for Standardization recommends to dry soils in an oven at a temperature of 40 °C. Effects of soil oven-drying on different fractions of dissolved organic matter (DOM) and the effect thereof on total concentrations and speciation of trace metals in weak salt extracts have, however, not been quantified yet. In this study, free metal concentrations and DOM speciation were determined in 2 mM Ca(NO₃)₂ extracts obtained from twelve field-contaminated soils in their field-moist state and after drying at 40 °C. Free metal concentrations were measured with the Donnan Membrane Technique. DOM was fractioned into humic acid (HA), fulvic acid (FA), and hydrophilic (Hy) compounds. Soil oven-drying led to significant increases in the concentrations of total DOM and total dissolved Cu and Ni. For the measurement of total dissolved Cu and Ni concentrations, it is, therefore, better to use field-moist soils. The release of Hy compounds was mainly responsible for the increase in DOM, which accounted for 64 to 77% of the increase in total dissolved organic carbon. Soil oven-drying left the free Cd²⁺, Cr³⁺, Cu²⁺, Ni²⁺, and Zn²⁺ concentrations unchanged. Both field-moist and oven-dried soils can, therefore, be used for the measurement of free metal concentrations. Free Cd²⁺, Cu²⁺, Ni²⁺, and Zn²⁺ concentrations were predicted very well for both field-moist and oven-dried soils using ORCHESTRA, which includes the NICA-Donnan model. However, poor predictions were obtained for Cr³⁺, for which better NICA parameters need to be derived.

Introduction

Human activities like agriculture, traffic, mining, and industry have resulted in the accumulation of trace metals (e.g., As, Cd, Cr, Cu, Ni, Pb, and Zn) in many soils throughout the world. In these contaminated soils, metals can leach to ground- and surface waters and cause toxic effects when taken up in excess by soil organisms and plants (Berti and Jacobs, 1996, He et al., 2005). Consequently, metals can be a threat to the soil ecosystem and result in a loss of biodiversity (Tobor-Kapłon et al., 2005). Furthermore, uptake of metals by food crops and crops used as fodder for grazing animals can lead to their accumulation in food chains and, ultimately, the occurrence of human health effects (Brus et al., 2009, Dudka and Miller, 1999). On the other hand, trace metals like Cu and Zn are essential micronutrients for the growth of plants, and soil deficiencies of these metals can hamper the production of food crops (Alloway, 2009, He et al., 2005).

The availability of metals for leaching and uptake by soil organisms and plants is not necessarily related to total metal concentrations in soil, but rather depends on the partitioning of metals between the soil solid phase and the soil solution and on the speciation of dissolved metals (Römkens et al., 2009). In turn, metal partitioning and speciation depend on soil properties like pH, soil organic matter (SOM), amorphous Al- and Fe-(hydr)oxides, and the dissolved organic matter (DOM) concentration (Groenenberg et al., 2010b, Weng et al., 2002). To assess the transport potential of soils for metals, total metal concentrations in soil solution need to be considered (Bonten et al., 2008) while plant uptake and toxicity of metals to soil organisms generally correlate best with free metal concentrations (Parker and Pedler, 1997). The availability of metals can be assessed via the measurement of their total dissolved concentrations and speciation in neutral and non-buffered weak salt extracts like NaNO₃, CaCl₂, or Ca(NO₃)₂ (Houba et al., 2000, Peijnenburg et al., 2007, Römkens et al., 2009). Such extracts are typically employed at a high soil to solution ratio (e.g., 1:10 [w:v]) for a short period, followed by centrifugation and filtration for subsequent chemical analysis and are considered to mimic soil solution (Degryse et al., 2003). Before processing in the laboratory, soils are usually dried as soon as possible after sampling in the field to minimize microbial activity, which may lead to changes in the soil status (van Erp et al., 2001). Furthermore, soil drying ensures a more representative subsampling and storage of dried soil samples is less complicated. Soils are usually dried in the air or in an oven at a temperature not exceeding 40 °C, often in combination with forced-air ventilation. According to the International Organization for Standardization (ISO) (2006), soil drying in an oven at a temperature of 40 °C is to be preferred over air-drying, because this reduces the period of time needed for drying thereby limiting changes in the soil status resulting from microbial activity. In addition, a shorter drying period enables laboratories to optimize laboratory activities and to minimize costs (van Erp et al., 2001).

Drying and subsequent rewetting of soils have been demonstrated to alter the extractability of organic carbon, major cations like Al and Fe, and trace metals (Bartlett and James, 1980, Haynes and Swift, 1991). For example, the DOM concentration in water and weak salt extracts obtained from dried soils increased significantly in comparison with field-moist soils (Christ and David, 1994, Courchesne et al., 1995, Kaiser et al., 2001, Klitzke and Lang, 2007, Koopmans et al., 2006, Peltovuori and Soinne, 2005). Together with an increase in the DOM concentration, total dissolved concentrations of metals like Cu, which binds strongly to humic substances (Temminghoff et al., 1997), have been demonstrated to increase as well (Klitzke and Lang, 2007).

DOM is operationally defined as an ensemble of organic molecules of different sizes and structures able to pass through a filter with a pore size of 0.45 μm. It consists of complex, high molecular-weight compounds collectively termed humic substances and more simple, low-molecular weight hydrophilic (Hy) compounds (Stevenson, 1994). Humic substances represent the hydrophobic DOM fraction, and consist of humic acids (HA) and fulvic acids (FA). Each of these DOM fractions exhibits a different capacity and affinity to bind metals (Groenenberg et al., 2010a, Milne et al., 2003). The ability of DOM to increase the total dissolved metal concentrations after soil drying may, therefore, depend very well on the changes in the concentrations and fractions of the DOM. To our knowledge, however, effects of soil oven-drying on the concentrations of the different fractions of DOM and the effect thereof on the total concentrations and speciation of trace metals in weak salt extracts have not been quantified yet. It is important, amongst others, to investigate changes in the DOM speciation as induced by soil oven-drying, because FA and HA concentrations are required as input to models, which can be used to predict metal binding to humic substances, like the Non Ideal Competitive Adsorption-Donnan (NICA-Donnan) model (Kinniburgh et al., 1999). Nevertheless, it is important to be aware of differences between the results of different methods which can be applied to measure DOM concentrations. The use of water or weak salt solutions to extract DOM from soils can lead to differences in the quantity and quality of DOM as compared to soil solutions collected with other techniques like centrifugation, lysimeters, and suction cups. For example, the amount of DOM in water and weak salt extracts has been demonstrated to be larger than in soil solutions obtained by centrifugation (Amery et al., 2007, Amery et al., 2009). Furthermore, the extracted additional DOM was found to have a higher aromaticity and a more hydrophobic character than the DOM present in centrifuged soil solutions (Amery et al., 2007, Amery et al., 2009).

Our first objective was to investigate the effects of soil oven-drying at 40 °C on total dissolved concentrations and speciation of trace metals and DOM extractable with a weak salt solution. Free metal concentrations were determined with the Donnan Membrane Technique (DMT). With this technique, the solution of interest (donor), which contains the free and complexed metals, is separated from an acceptor solution by a negatively charged cation exchange membrane. Positively charged cations can pass the membrane, while transport of negatively charged ions and neutral complexes is restricted. When Donnan equilibrium is reached, concentrations of free cations in the acceptor solution are either equal to the free cations concentrations in the donor solution or can be calculated using simple correction factors (Temminghoff et al., 2000). DOM was fractioned into HA, FA, and Hy according to a rapid batch technique developed by van Zomeren and Comans (2007). Our second objective was to obtain insight in the relation between changes in the concentrations of the different DOM fractions and total dissolved concentrations and speciation of metals. Therefore, we compared measured free metal concentrations with those predicted by the chemical speciation program ORCHESTRA (Meeussen, 2003), which includes the NICA-Donnan model. The results from this study provide insight in the effects of soil oven-drying on the speciation of trace metals in weak salt extracts which is of interest with respect to their availability for leaching and uptake by soil organisms and plants. Our results are also relevant for the field situation where natural drying and rewetting cycles may influence the solubility of metals.