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Plant growth and metal uptake by a non-hyperaccumulating species
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(Lolium perenne) and a Cd-Zn hyperaccumulator (Noccaea caerulescens) in
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contaminated soils amended with biochar
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Frédéric REES, Cyril GERMAIN, Thibault STERCKEMAN, Jean-Louis MOREL
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Université de Lorraine, Laboratoire Sols et Environnement, UMR 1120, 2, avenue de la Forêt de Haye, TSA
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40602, 54518 Vandœuvre-lès-Nancy cedex, France
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INRA, Laboratoire Sols et Environnement, UMR 1120, 2 avenue de la Forêt de Haye, TSA 40602, 54518
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Vandœuvre-lès-Nancy cedex, France
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Key-words
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Biochar; Metal; Nutrient deficiency; Biomass production; Hyperaccumulation; Phytoremediation
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Number of text pages: 23
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Number of tables: 3
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Number of figures: 7
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Supporting Information document: 1 PDF document with 17 tables and 4 figures
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Short running title: Biochar controls plant growth and metal uptake
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Corresponding author: Jean-Louis Morel, Laboratoire Sols et Environnement, Université de Lorraine – INRA
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(UMR 1120), 2 avenue de la Forêt de Haye 54518 Vandœuvre-lès-Nancy, France. Tel: +33383595847.
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Fax: +33383595791. E-mail address:
[email protected]
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Abstract
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Aims. Biochar could be used as a soil amendment in metal contaminated soils, for safe crop production or soil
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remediation purposes. This work was conducted to study the effects of biochar amendments on metal uptake by
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two contrasted plants grown on metal-contaminated soils.
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Methods. A non-hyperaccumulating plant (Lolium perenne) and a Cd- and Zn-hyperaccumulator (Noccea
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caerulescens) were grown in pots on acidic (A) and alkaline (B) soil contaminated by Cd, Pb and Zn, both
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amended by a wood-derived biochar.
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Results. Biochar amendments decreased the availability of metals by increasing soil pH, but also decreased Ca, P
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and N availability. Growth of L. perenne was increased and shoot metal uptake decreased by biochar addition in
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both soils, although increasing biochar dose above 0.5% resulted in a progressive decrease of shoot production
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on soil B. Growth of N. caerulescens was not significantly affected by biochar. But an increase of Cd uptake
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with 5% biochar was recorded on both soils, and of Zn uptake on soil B.
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Conclusions. Beside immobilizing metals, biochar may decrease the availability of nutrients, leading either to
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plant deficiency or to a decreased competition with cations for metal uptake, thus enhancing extraction of metals
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by hyperaccumulators.
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Abbreviations
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WHC: Water Holding Capacity
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Introduction
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As a result of their effect on metal mobility in soil (Rees et al. 2014), biochar amendments may directly control
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plant growth and plant metal uptake. Introduction of different soil amendments may be used either to decrease or
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increase plant metal uptake (Puschenreiter et al. 2005; Meerset al. 2008). The concept of biochar as a soil
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amendment has been developed primarily to build a stable carbon sink in soils and to bring other benefits such as
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increased soil fertility (Lehmann et al. 2006). Biochar introduction in soils contaminated with metals has been
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increasingly investigated, mainly as an attempt to reduce metal uptake or to promote phytoremediation strategies
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(Beesley et al. 2011). A decrease of divalent metal uptake in the presence of biochar has been observed with
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various plant species belonging to the families Poaceae (Karami et al. 2010; Namgay et al. 2010; Cui et al. 2011;
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Zheng et al. 2012), Brassicaceae (Park et al. 2011; Houben, 2013) and Fabaceae (Gartler et al. 2012), and to
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other families (Zhang et al. 2013). Mechanisms are however poorly understood. Biochar may decrease the
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mobility of heavy metals in soils by directly sorbing heavy metals in soils or by indirect ways, e.g. an increase of
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soil pH (Rees et al. 2014), which may explain the decrease of metal transfer to the plant (Cui et al. 2011; Houben
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et al. 2013).
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Transfer of metals from soil to plant depends on numerous factors related to soil properties and plant type. pH
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and Eh are among the most important soil factors controlling metal availability. In general, metal availability is
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low when pH is high, as metals are more retained on soil particles (Bruemmer et al. 1986; Alloway 2013). Plants
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accumulate metals in their roots by different ways (Morel 1997). The movement of elements from the external
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soil solution into the intercellular spaces of root cortex and cell walls, i.e. apoplast, is a passive process
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controlled by diffusion or mass flow (Marschner 2012). Metal ions tend to accumulate in the root free space,
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being partly bound to the root cell walls. Metal can also enter the root cells, driven by the electrochemical
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gradient, and penetrate the plasma membrane through membrane transporters (Maathuis 2007). Competition
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between metals (Hart et al. 2002) and modification of the membrane potential by major cations such as Ca 2+
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(Wang et al. 2011) may affect this process. Once in the root cells, metals may be immobilized, or loaded in the
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xylem vessels and transported to the shoots (Clemens 2001). In this context, plants may be classified as
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excluders, indicators and accumulators (Baker 1981). Hyperaccumulators are plants able to accumulate metals in
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their shoots at concentrations higher than 0.1% in dry weight. In particular, Noccea caerulescens (form. Thlaspi
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caerulescens) has been shown to display high Cd and Zn concentrations in its shoots (Baker et al. 1991; Reeves
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et al. 2001; Schwartz et al. 2003).
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Biochar has been neglected so far as an amendment for phytoextraction, as reduction in metal availability could
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reduce the efficiency of the process. The only currently available results show contrasting trends. If a decrease of
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Cd uptake with the accumulator Amaranthus tricolor was observed with biochar amendments by Lu et al.
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(2014), this was not the case with the Cd-hyperaccumulator Sedum alfredii where no change of Cd uptake was
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observed with 2.5% biochar despite a decrease of available Cd (Hu et al. 2014). An increase of Cd and Pb
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concentration was even reported in shoots of accumulators Noccea rotundifolium and Anthyllis vulneraria with
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1.5% and 3% addition of wood-derived biochar (Fellet et al. 2014). As it is obvious that these experiments do
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not cover the large range of possible soil, biochar and plant properties, more investigations are needed in order to
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understand how biochar could affect the uptake of metal by both hyperaccumulating and non-hyperaccumulating
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plants.
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We hypothesize that biochar can control plant growth and uptake of metals in contaminated soils by
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immobilizing both trace metallic elements and some nutrients. Opposite responses may then be obtained
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depending on the plant type. A growth experiment was conducted on two biochar-amended contaminated soils
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with the non-hyperaccumulating plant, Lolium perenne, and the Cd- and Zn-hyperaccumulator, N. caerulescens,
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and plant growth and metal uptake were measured to verify our hypothesis.
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Material and Methods
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Materials
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Biochar and soils were the same as in a previous work (Rees et al. 2014). Biochar was produced from 80%
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coniferous and 20% hardwood chips which were pyrolysed at about 450 °C for 36 h. Before use, biochar was
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dried at room temperature for 1 week, crushed and sieved to
