# Bioavailability and phyto-extractability of metals in a peat-amended agricultural soil under climate stress

Natalia Sánchez, Ines Merbach, Sören Drabesch, Evgenia Blagodatskaya, Floriane Jamoteau, Marco Keiluweit, Jill Bachelder, Mika Tarkka, E. Marie Muehe

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## To cite this version:

Natalia Sánchez, Ines Merbach, Sören Drabesch, Evgenia Blagodatskaya, Floriane Jamoteau, et al.. Bioavailability and phyto-extractability of metals in a peat-amended agricultural soil under climate stress. Journal of Environmental Management, 2025, 394, pp.127167. 10.1016/j.jenvman.2025.127167. hal-05301056

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Submitted on 7 Oct 2025

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Research article

# Bioavailability and phyto-extractability of metals in a peat-amended agricultural soil under climate stress

Natalia Sánchez<sup>a,b</sup>, Ines Merbach<sup>c,D</sup>, Sören Drabesch<sup>a,b,d</sup>, Evgenia Blagodatskaya<sup>e</sup>, Floriane Jamoteau<sup>f</sup>, Marco Keiluweit<sup>f</sup>, Jill Bachelder<sup>a,g,h,D</sup>, Mika Tarkka<sup>e,D</sup>, E. Marie Muehe<sup>a,b,w,D</sup>

ARTICLE INFO

Keywords:

Metal fractionation

Phytoextractability

Sequential extractions

Sustainable agriculture

Plant nutrition

ABSTRACT

Climate-induced mobilization of harmful metals in soils with a pH below 7 threatens food safety through plant uptake. While organic amendments like peat are known to immobilize metals, it remains unclear how their immobilization effectiveness changes under future climate scenarios and whether there is an optimal amendment threshold before immobilization turns into re-mobilization. This study assessed how varying peat input levels (3, 5 and 8 %) and projected climatic conditions (+4 °C, +320 ppm, CO₂) affect metal fractionation, bioavailability, and uptake by a metal-tolerant plant in historically contaminated soils. Intermediate 5 % peat levels enhanced metal immobilization via organic matter complexation, reducing exchangeable Zn and Cd ~2-fold compared to 3 % peat, despite acidification. At high 8 % peat input, a 0.65-unit pH decline and increased dissolved organic matter reversed this trend, increasing exchangeable Zn and Cd > 2-fold relative to 5 % peat. Chemical equilibrium modeling (WHAM VII) confirmed greater metal complexation with dissolved organic matter at higher 5–8 % peat levels. Under future climatic conditions—elevated temperature and CO₂—metal immobilization improved at low 3 % peat input, likely due to stable organic matter and functional group buffering. Nevertheless, metal re-mobilization occurred at higher peat inputs, likely due to enhanced peat decomposition. Despite these variations, plant Cd uptake remained low across peat and climate treatments. This emphasizes peat's protective role against Cd while maintaining the plant's nutritional status for Zn. This study highlights the dual effects of peat amendments: intermediate levels optimize metal immobilization, but excessive amendments may destabilize harmful metals, especially under future conditions.

# 1. Introduction

Metal pollution threatens ecosystems and raises concerns about harmful contaminants entering the food chain (McLaughlin et al., 2021). Metals are omnipresent in agricultural soils and originate from geogenic sources, industrial legacy contamination, and management practices involving pesticides, impure fertilizers, and irrigation (McLaughlin et al., 2021). Cadmium (Cd) is the most prominent toxic metal in

non-flooded agroecosystems, toxic even at low concentrations, easily transfers from soil to crops, and disrupts essential cellular functions without any metabolic role in plants, microbes, or animals (Suhani et al., 2021). In contrast, zinc (Zn), an essential micronutrient crucial for biota and human health (Sharma et al., 2013), is of particular concern given the increasing global Zn deficiency estimated to affect 17 % of the population (Kumssa et al., 2015). Both Zn and Cd co-occur in soils, interacting synergistically or antagonistically within soil-plant systems

<footnote><sup>a</sup> Plant Biogeochemistry, Department for Applied Microbial Ecology, Helmholtz Centre for Environmental Research - UFZ, 04318, Leipzig, Germany</footnote>

<footnote><sup>b</sup> Plant Biogeochemistry, Department of Geosciences, University of Tübingen, 72076, Tübingen, Germany</footnote>

<footnote><sup>c</sup> Bad Lauchstädt Research Station, Helmholtz Centre for Environmental Research - UFZ, 06246, Bad Lauchstädt, Germany</footnote>

<footnote><sup>d</sup> Geomicrobiology, Department of Geosciences, University of Tübingen, 72076, Tübingen, Germany</footnote>

<footnote><sup>e</sup> Soil Ecology, Department Soil Ecology, Helmholtz Centre for Environmental Research - UFZ, 06120, Halle (Saale), Germany</footnote>

<footnote><sup>f</sup> Institut des Dynamiques de la Surface Terrestre (IDYST), Université de Lausanne, 1015, Lausanne, Switzerland</footnote>

<footnote><sup>g</sup> Department of Water Resources and Drinking Water (W+T), Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600, Dübendorf, Switzerland</footnote>

<footnote><sup>h</sup> Inorganic Environmental Geochemistry, Institute of Biogeochemistry and Pollutant Dynamics, Department of Environment Systems Sciences (D-USYS), ETH Zürich, Swiss Federal Institute of Technology, 8092, Zürich, Switzerland</footnote>

<footnote>* Corresponding author. Plant Biogeochemistry Helmholtz Centre for Environmental Research - UFZ, Germany.
E-mail address: marie.muehe@ufz.de (E.M. Muehe).</footnote>
(Nan et al., 2002; Sarwar et al., 2010). Crops can take up both metals but distribute them differently across roots, leaves and reproductive organs, depending on their nutrient needs and mechanisms controlling toxic metal translocation (Gupta et al., 2019). Given the importance of Zn as a micronutrient, its uptake by crops must be ensured (Broadley et al., 2007). At the same time, the entry of toxic Cd into the food chain must be minimized. Thus, understanding the mobility of Zn and Cd in soils is crucial for managing their uptake in crops (Sarwar et al., 2010).

The transfer of Zn and Cd from soil to plant is influenced by the speciation of these elements in the soil (Aiqing et al., 2022; Li et al., 2021). Speciation refers to the chemical form a metal is present in a soil (Michalke, 2003). For example, metals adsorb to surfaces of clay minerals, silicates, (hydr)oxides, and carbonates, depending on the number and type of exposed surface sites (Li et al., 2022; McLean and Bledsoe, 1992). They can also be complexed by diverse components of organic matter, either particulate, bridged to mineral phases, colloidal, or dissolved in soil solution (Sharma et al., 2011). Chemical forms of metals that are bioavailable can be taken up by biota. Bioavailable forms of metals include those present as free ions in soil solution (e.g. Zn²⁺, Cd²⁺), weakly adsorbed to soil surfaces and weakly complexed by organic matter (Rao et al., 2008; Uchimiya et al., 2020). The partitioning of metals into bioavailable forms is governed by soil biogeochemical properties and environmental conditions (Li et al., 2022). Soil pH is a key factor affecting metal bioavailability, with Zn and Cd solubility generally increasing in soils as pH decreases (McLean and Bledsoe, 1992; Uchimiya et al., 2020). Future climatic conditions – specifically elevated temperature, atmospheric CO₂ and evaporation – can also alter pH, and thus enhance Cd mobility (Drabesch et al., 2024). These conditions promote organic acid exudation by plants and ammonium oxidation by microorganisms (Drabesch et al., 2024; Zeng et al., 2024), leading to soil acidification, which displaces Cd from mineral surfaces into soil solution. This climate-induced metal mobilization raises the risk of plant uptake (Wang et al., 2020a).

The relationship explored between bioavailable metals in soil and their uptake by plants is not always consistent (Rao et al., 2008; Xu et al., 2022). While various extractants have been widely used to study the speciation of metals and assess the phytoextractable fraction of metals, there lacks a standard method for extractants evaluating the portion of bioavailable metals in the soil that plants absorb during a given growth period, i.e. reflecting the actual uptake by plants. The concentrations resulting from single extractions are specific to plant species and soils, especially in agricultural soils (Rao et al., 2008). Plant bioassays, involving living plants to measure the phytoextractable metals in soil, are therefore needed to study the bioavailability of elements (Basta et al., 2005; Zhang et al., 2016). While regular plants may not be ideal for accurately tracking phytoavailability due to sensitivity to metal toxicity, metal-hyper accumulator plants can tolerate and accumulate high concentrations of metals in shoots by evolving different physiological strategies to deal with metals (Bert et al., 2000; Clemens, 2006; Manara et al., 2020; Zhao et al., 2006). Consequently, these metal-hyper accumulator plants provide a more reliable indication of phytoextractable metals in soils, especially in soils where the complexation of metals has been modified by organic amendments.

The application of organic amendments is a widely practiced strategy for managing metal contamination in agricultural soils (Park et al., 2011). Organic amendments immobilize metals primarily through adsorption and complexation, offering charged surfaces and functional groups that facilitate metal binding (Hamid et al., 2019a; Uchimiya et al., 2011). The composition of the organic matter is crucial in providing suitable binding sites for metals (Mohamed et al., 2010). Organic amendments typically contain functional groups, such as carboxyl, phenol and hydroxyl groups, which are particularly effective in complexing metals and enhancing metal-organic matter interactions (Gao et al., 2023; Uchimiya et al., 2011). Additionally, organic amendments can alter physical soil properties, influencing the dispersion of metal-contaminated particles (Park et al., 2011). Peat, a

commonly used low-cost organic amendment in agriculture, is known to be rich in functional groups that have a high binding capacity for metal ions, while also improving soil porosity (Klavins and Purmalis, 2013; Zhang et al., 2022). Despite these benefits, the effects of organic peat-like amendments on metal fractionation and extractability in agricultural soils under climate change – particularly elevated temperatures and high atmospheric CO₂ levels – remain largely unexplored.

This study aimed to evaluate the effectiveness of peat in reducing the bioavailability and phytoextractability of Zn and Cd under different climatic conditions, similar to the ones projected for the year 2100 under SSP3 7.0 scenario of the Intergovernmental Panel on Climate Change (IPCC, 2021). A greenhouse pot experiment was conducted using the metal-hyperaccumulating plant *Arabidopsis halleri* (L.) O'Kane & Al-Shehbaz. Metal mobility, bioavailability, and phytoextractability were assessed through sequential extractions and quantification of plant metal accumulation, alongside analysis of metal interactions with soil organic matter. This research is expected to provide insight into the potential of peat, as a representative organic amendment to reduce bioavailable metals in an anticipated future climate scenario.

## 2. Methods

### 2.1. Soil and plant characterization

Agricultural top-soil with a pH of 7.4 was obtained from Schladebach, Germany (51°18'30.6"N 12°06'16.4"E) in September 2020. Soil was air-dried and sieved to 2 mm. Peat was chosen as the organic amendment due to its common use in agriculture (Bamdad et al., 2022; Gruda et al., 2019), and its richness in exposed functional groups capable of complexing metals (e.g., carboxyl and phenols) (Klavins and Purmalis, 2013). Peat (Plantop Gärtnertorf H3-H5) was ground for lower heterogeneity in the soil, and 3, 5 and 8 %w were mixed thoroughly and as evenly as possible into the soil using a concrete mixer. Peat additions between 3 and 5 % are common in agriculture and have been used before to evaluate the effect of peat on soil elemental cycling (Aguilar-Garrido et al., 2020; Egene et al., 2018; Rijk et al., 2024; Zhang et al., 2005). The 8 % treatment was additionally chosen as an amount that would exceed the realistic level of application and exaggerate geochemical impacts. The pH of all the soils amended with peat ranged from 5.6 to 6.2 at which metals such as Cd and Zn are mobile (Martínez and Motto, 2000) and may become more mobile and more easily taken up by plants due to future climatic conditions (Drabesch et al., 2024). Given this large pH range, comparing data of peat-amended and non-amended soils was not possible and this study refers to Drabesch et al. (2024), for comparison of unamended soils with a similar pH range. The soil with 3 % of amended peat was geochemically

<table><caption>**Table 1**

Geochemical characterization of the agricultural soil with 3 % of peat.</caption><thead><tr><th>Property</th><th>Mean ± sd</th></tr></thead><tbody><tr><td>pH<sup>a</sup></td><td>6.18 ± 0.19</td></tr><tr><td>Total organic C (%)<sup>b</sup></td><td>1.14 ± 0.29</td></tr><tr><td>Total N (%)<sup>b</sup></td><td>0.17 ± 0.01</td></tr><tr><td>Sand (%)<sup>c</sup></td><td>80.0 ± 5.8</td></tr><tr><td>Silt (%)<sup>c</sup></td><td>11.6 ± 4.4</td></tr><tr><td>Clay (%)<sup>c</sup></td><td>8.3 ± 1.7</td></tr><tr><td>Cation exchange capacity (cmol kg<sup>-1</sup>)<sup>c</sup></td><td>10.00 ± 1.00</td></tr><tr><td>Total Cd (µg kg<sup>-1</sup> dry soil)<sup>d</sup></td><td>87.30 ± 0.02</td></tr><tr><td>Total Zn (mg kg<sup>-1</sup> dry soil)<sup>d</sup></td><td>33.07 ± 0.80</td></tr></tbody></table>

<footnote>a Determined with Milli-Q water at a 1:5 w/v ratio, triplicates.</footnote>

<footnote>b Determined by EA-IRMS, triplicates.</footnote>

<footnote>c Determined with hydrometer using sodium hexametaphosphate (50 g L⁻¹ (NaPO₃)₆), six replicates.</footnote>

<footnote>d Extracted with aqua regia and determined by inductively coupled plasma mass spectrometry, triplicates.</footnote>

<footnote>e Extracted with 0.1 M BaCl₂ and determined by inductively coupled plasma mass spectrometry, triplicates.</footnote>
characterized in triplicates given that 3 % is most commonly applied (Table 1).

The metal-hyperaccumulating plant *Arabidopsis halleri* was chosen here for its ability to assess the total capacity for Zn and Cd phytoextraction without being affected by the phytotoxicity of climate-induced increases in bioavailable metals. Mother plants of *A. halleri* subsp. *halleri* (accession Langelsheim) were provided by the Department of Plant Physiology, Ruhr University Bochum, and maintained by greenhouse staff at Bad Lauchstädt research station. Offshoot clones of similar size and approximately equal leaf number were used for the experiment.

## 2.2. Pot microcosm study

The pot study was conducted in greenhouse facilities in Bad Lauchstädt, Germany. One greenhouse with 20 °C day-time average and atmospheric CO₂ of 490 ppm_v represented today's climate while another greenhouse with 24 °C day-time average and atmospheric CO₂ of 810 ppm_v (Supplementary Fig. S1, IPCC SSP3-7.0 (IPCC et al., 2021)). Black plastic pots (8x8x8 cm) were filled by weight with the peat amended soil. The pots were placed randomly in a tray (70 × 40 cm) and irrigated by weight between 48 and 50 % of the maximum water holding capacity under today's climate and between 46 and 50 % under future climatic conditions. The pots were irrigated by capillarity in the beginning of the experiment. Offshoot *A. halleri* clones were washed and

singly planted in the middle of each pot, with triplicates per climate and peat amended treatment (Fig. 1). Thus, *A. halleri* was grown in soils amended with three levels of peat, 3 %, 5 % and 8 % under simulated today's climatic conditions typical of regions where *A. halleri* thrives (Murawska-Wlodarczyk et al., 2022; Szopiński et al., 2020) and future conditions as outlined above (IPCC et al., 2021). After six weeks, 5 cm long soil cores of 1 cm diameter were taken from each pot, mixed, and aliquoted for (1) immediate determination of water content, (2) quantification of metal content by first flash freezing soil samples on dry ice and storing at -20 °C until extraction and analysis (see soil elemental analysis), (3) 2 g of soil for immediate pH measurements directly in the greenhouse with the implemented climate. Plant shoots were cut, washed in VE water, dab-dried with tissue, weighed, flash-frozen on dry ice and later stored at -80 °C until sample treatment (see plant analysis section). A portion of the shoots was not frozen but oven dried to estimate the dry weight. Roots were washed in VE water, dabbed-dry with tissue, weighed, flash frozen on dry ice and stored at -80 °C for further analysis (see plant analysis section).

To confirm the results of the 3 % peat amended soil, a second study was run with 12 replicates and native and spiked in Cd and Zn contents. For this second study, a spike of metals was included to confirm that the observed data not only hold true at low but also at elevated soil metal concentrations. The spiked soil was prepared by saturating soil to field capacity with metal solutions aiming at final concentrations of 0.32 mg

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Schematic representation of the experiment and analysis of soils with three peat amendments. (A) Experimental design, (B) Flowchart of plant and soil analysis. TOC: total organic carbon. SEC-UV-ICP-MS/MS: size exclusion chromatography coupled to ultraviolet detection and inductively coupled plasma tandem mass spectrometry. ICP-MS: inductively coupled plasma mass spectrometry. 0.01 M CaCl₂ targeted soluble and exchangeable metals, 1 M Na acetate and 0.1 M HCl targeted the next layer of more recalcitrant metals, 0.1 M pyrophosphate targeted organic-bound metals, WHAM VII differentiated free metals and complexed to DOM metals.</figcaption></figure>
$kg^{-1}$ CdCl$_2$ and 342 mg $kg^{-1}$ ZnCl$_2$ (analytical grade salts) followed by mixing. The soil was placed in boxes and located in the greenhouse facilities to be air-dried, remaining there for six months to acclimate to the amended metals. To prove that metal mobility was consistently lower under future climatic conditions, time-resolved 0.01 M CaCl$_2$-extractions were performed. To prove that metal extraction by a metal-hyper accumulator plant was not affected by climate, shoot Cd contents of *A. halleri* were determined at the end of the experiment.

## 2.3. Soil analysis

To evaluate impacts of climatic conditions on Zn and Cd mobility in soils, a sequential extraction was performed with CaCl$_2$, Na-acetate and HCl solutions. 0.01 M CaCl$_2$ extracts the exchangeable fraction of metals, by displacing cations from soil surfaces (referred to as exchangeable metal in the manuscript) (Houba et al., 1986). 1 M acetate is a stronger extractant, capable of tackling the next layer of strongly adsorbed and potentially trapped metals (Shannon and White, 1991). It also extracts carbonates, which do not play a role in such low pH soils (Tessier et al., 1979). The third extraction with 0.1 M HCl slightly dissolves mineral phases around their rims so that recalcitrant metals will be extracted (Shuman, 1982). It is also used to approximate the metal fraction taken up by metal-hyperaccumulating plants (Muehe et al., 2015). Most often 0.5 or 1 M HCl extractions are used. These stronger extractions were not selected since they fully dissolve short range ordered iron minerals (Heron et al., 1994) and this study targets surface processes. 1.2 g of frozen soil samples were suspended in 12 mL of 0.01 M CaCl$_2$, the samples were shaken at room temperature for 15 min and filtered with 0.22 µm PES filters. After this, 12 mL of 1 M Na acetate, with a pH of 4.5, was added to the soil, the samples were shaken at room temperature for 1 h and filtered with 0.22 µm PES filters. Lastly, 12 mL of 0.1 M HCl was added before shaking at room temperature for 30 min and filtering with Whatman filter paper No. 1.

Soils were extracted with 0.1 M Na-pyrophosphate, which targets organically bound metals by solubilizing solid-phase organic matter (Jones et al., 2020). Organically-bound Zn and Cd were approximated by extracting 1 g of soil with 10 mL of 0.1 M Na-pyrophosphate at room temperature in the dark for 1 h and filtered with 0.22 µm PES filters (Jones et al., 2020; Nico and Zasoski, 2001). Na-pyrophosphate extractions were characterized quantifying organic C (referred to as total extractable organic carbon in the manuscript) and specific UV absorption at 254 nm, 260 nm and 280 nm to determine the aromaticity, hydrophobicity and molecular weight of extracted organic matter molecules. The fluorescence index was calculated as the ratio of emission wavelengths at 450 nm and 500 nm for an excitation wavelength of 370 nm (Cory and McKnight, 2005) with Tecan Spark Multimode reader, CH. All samples were analyzed with ICP-MS (Agilent 7900 ICP-MS, USA).

## 2.4. Plant analysis

Shoots and roots were assessed for wet and dry biomass by drying at 40 °C until constant weight. For elemental analysis, plant tissues were ground in liquid nitrogen to a fine powder. 0.1 g of plant tissue or rye grass reference material (ERMCD281) were weighed into Teflon digestion vessels, adding 5 mL of a mixture containing 30 % H$_2$O$_2$ and 67 % HNO$_3$ at a 2:3 ratio, and digesting the samples in a microwave digester (Mars6, CEM Corporation, USA). During digestion, samples reached 180 °C after 10 min and remained at that temperature for 20 min. Samples were filtered with Whatman No. 1 filter papers and analyzed using ICP-MS (Agilent 7900 ICP-MS, USA) and microwave plasma atomic emission spectroscopy (4200 MP-AES, Agilent, USA).

## 2.5. Windermere Humic Aqueous Model (WHAM)

To estimate the concentrations of free ions and the proportion of metals bound to dissolved organic matter (DOM) in 0.01 M CaCl$_2$ soil

extracts, the chemical equilibrium Windermere Humic Aqueous Model (WHAM version VII) was used (Tipping, 1994). Parameter input into the model included pH, the partial pressure of atmospheric CO$_2$ set to 0.004 atm, as done in previous studies of soil extracts (Mossa et al., 2021), and total concentrations of Ca, Mn, Fe, Co, Ni, Cu, Zn, and Cd in the extracts (Supplementary Table S1). DOM was calculated as total dissolved organic C multiplied by a factor of two (Lofts et al., 2008), of which 65 % was considered as "reactive DOM" able to bind metals. WHAM modeling was performed as it is an established method for predicting aqueous-phase Zn and Cd speciation in soil. The model was not validated with direct measurements of Zn and Cd speciation (e.g. free Zn$^{2+}$ or Cd$^{2+}$) because the chemical properties between treatments were very similar, and these models are based on established thermodynamic principles and calculations. The main parameter which is a source of error is the reactive DOM parameter (Djae et al., 2017). Yet for Zn and Cd, slight variations in this parameter between treatments should not greatly affect the outcome of the model, and Mueller et al. even showed that the assumption that 65 % of DOM is reactive led to results that aligned well with direct measurements; furthermore, this was not improved by considering DOM quality, though this was in aquatic samples and not soil (Mueller et al., 2012). Ultimately, the goal of this WHAM modelling was to compare inorganic vs. organic-bound Zn and Cd species in the CaCl$_2$ extracts. The similarity in chemical properties of the samples and the established nature of the chemical equilibrium model are sufficient for this goal.

## 2.6. Size exclusion chromatography

To characterize the organic matter in the solid phase with a high metal binding affinity, evaluating a potential role in immobilizing metals, 0.1 M Na-pyrophosphate extracts were analyzed with size exclusion chromatography (SEC) (Tolu et al., 2022). Details of this method and verifications are provided in Supplementary Method 2; and Figs. S2, S3, S4.

## 2.7. Data analysis

Data were analyzed with linear or generalized linear models to determine the effects of climatic conditions, peat amendment level, and their interaction (Supplementary Tables S2 and S3). Estimated marginal means were implemented to determine statistical differences. Correlations between 0.01 M CaCl$_2$-extractable Zn and Cd and 1 M Na-acetate-extractable Zn and Cd were assessed with Pearson coefficient (Supplementary Table S4). Statistical analyses and visualization were performed with R software version 4.3.0 (Lenth, 2024; R Core Team, 2024; Xu et al., 2021).

# 3. Results

## 3.1. Exchangeable and mineral associated metals in soil amended with peat

Sequential extraction of Zn and Cd revealed a different fate of these elements in soils amended with 3 % peat compared to soils amended with 5 or 8 % peat under both climatic conditions (Fig. 2). The extraction with 0.01 M CaCl$_2$ approximated the exchangeable elements. The CaCl$_2$-extractable Zn in soil amended with 3 % peat was $1.22 \pm 0.15$ mg kg$^{-1}$ under today's climate (Fig. 2A), which decreased significantly by 56 % under future climate conditions. In the 5 % peat-amended soil, CaCl$_2$-extractable Zn was $0.49 \pm 0.07$ mg kg$^{-1}$ and 2.49-fold less compared to 3 % under today's climate, which increased by 67 % under future conditions. In the soil with 8 % peat, CaCl$_2$-extractable Zn was $1.32 \pm 0.23$ mg kg$^{-1}$ and 1.08-fold more compared to 3 % under today's climate and increased by 21 % under future conditions.

The trend of the 0.01 M CaCl$_2$-extractable Cd fraction was similar to that of Zn (Fig. 2B), with decreasing CaCl$_2$-extractable Cd in soil with 3
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Extractable metals in an agricultural soil amended with 3 %, 5 % or 8 % peat and exposed to today's and future (+4 °C, +320 ppm CO₂) conditions. (A,C,E) Zn and (B,D,F) Cd were sequentially extracted with (A,B) 0.01 M CaCl₂, (C,D) 1 M Na-acetate and (E,F) 0.1 M HCl. Error bars represent means ± standard error of the mean (n = 3) extraction replicates. Asterisks (*) indicate significant differences between climatic conditions according to estimated marginal means (*p* < 0.05, **p** < 0.01, ***p** < 0.001). Note the different y-axis scales.</figcaption></figure>

% peat under future conditions and the opposite for soils with 5 % and 8 % of peat. CaCl₂-extractable Cd in 3 % peat-amended soils was 4.6 ± 0.6 µg kg⁻¹, which significantly decreased by 54 % under future conditions. CaCl₂-extractable Cd in soils with 5 % and 8 % peat decreased 2.3-fold and increased 1.02-fold compared to 3 %, respectively. Both increased by 56 % and 16 % under future conditions, respectively.

The observed decreasing trend of CaCl₂-extractable Cd in the 3 % peat-amended soil with a native metal content was confirmed by an additional experiment with 6–12 biological replicates (Supplementary Fig. S5). Acetate extracted more metals than CaCl₂ (Fig. 2C and D). It represents a stronger extractant and mobilizes strongly adsorbed and potentially trapped metals. Neither amendment nor imposed climatic condition significantly affected the acetate-extractable Zn and Cd, though a trend toward reduced acetate-extractable Zn and Cd was observed under future conditions. Overall, the Pearson correlation coefficient confirmed that Zn and Cd correlated significantly within the CaCl₂-extractable fraction and the acetate-extractable fraction but not between the two extractions. This suggests that there was no direct evidence of competitive interactions between the metals, and that acetate tackled a different, most likely more particle-associated, fraction of

metals than CaCl₂ within the soil (Supplementary Table S4).

Similar amounts of Zn but less Cd were removed with the following 0.1 M HCl extraction compared to the acetate extraction. HCl can extract more recalcitrant metals. Different amounts of amended peat nor imposed climatic conditions affected 0.1 M HCl-extractable Zn (Fig. 2E). In contrast, Cd extracted with 0.1 M HCl was only quantifiable in the soils with 3 % and 5 % peat under today's climate, while in the soil with 8 % peat, it was two-times higher under today's climate compared to future conditions (Fig. 2F).

## 3.2. Equilibrium modelling of metal speciation in soil amended with peat

According to the WHAM model, free Zn²⁺ in the soil amended with 3 % peat was 1.0 ± 0.1 mg kg⁻¹ under today's climate (Fig. 3A). Addition of 5 % peat reduced free Zn²⁺ by 62 % which subsequently increased again 2.9-fold under 8 %. Future conditions reduced free Zn²⁺ by 55 % in the 3 % peat soil compared to today's conditions, but increased free Zn²⁺ non-significantly by 89 % and 33 % in 5 % and 8 % peat-amended soils, respectively.

The concentration of free Cd²⁺ in the soil amended with 3 % peat was
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Modelled free and dissolved organic matter (DOM)-associated Zn and Cd in CaCl₂ extractions from an agricultural soil amended with 3 %, 5 % or 8 % peat and exposed to today's and future (+4 °C, +320 ppm CO₂) climate conditions. The Windermere Humic Aqueous Model (WHAM VII) was used to estimate the concentrations of (A) free Zn²⁺ and (B) free Cd²⁺, and the percent of (C) Zn bound to DOM and (D) Cd bound to DOM. Error bars represent means ± standard error of n = 3. Asterisks (*) indicate significant differences between climatic conditions according to estimated marginal means pairwise comparisons (*p* < 0.05, **p** < 0.01, ***p* < 0.001). Note the different y-axis scales. Data used in the WHAM model is given in Supplementary Table S1.</figcaption></figure>

4.3 ± 0.6 µg kg⁻¹ under today's climate (Fig. 3B). Adding 5 % peat to the soil reduced free Cd²⁺ 2.4-fold. With 8 % peat, free Cd was 2.4-fold higher again compared to 5 % peat. Under future conditions, free Cd²⁺ decreased by 55 % in 3 % peat-amended soils. Similar to Zn, in the soils with 5 % and 8 % of peat, there was a 66 % and 21 % increasing trend of free Cd²⁺ under future conditions, respectively.

WHAM-calculated binding of metals to DOM revealed significant effects of both climate and peat treatment. This followed the input parameter CaCl₂-extractable organic matter, as CaCl₂-extractable organic carbon and nitrogen increased with increasing peat content in the soil and decreased with future climatic conditions (Supplementary Fig. S6). Metal binding to DOM as determined by WHAM followed a similar trend. The percentage of Zn bound to DOM in CaCl₂-extracts of the soil with 3 % peat was approximately 13.5 % under both climatic conditions (Fig. 3C). In the soil with 5 % peat, DOM-bound Zn was 21.7 % under today's climate, which significantly decreased to 11.8 % under future climatic conditions. At 8 % peat amendment, DOM-bound Zn significantly decreased from 15.7 % under today's climate to 8.1 % under future conditions.

The trend of the percentage of Cd bound to DOM in soils amended with peat and under different climatic conditions was similar to Zn bound to DOM (Fig. 3D). Approximately 6.7 % of Cd was bound to DOM in soil amended with 3 % of peat under both climatic conditions. In the 5 % peat-amended soil, 12.2 % of Cd was bound to DOM, which

significantly decreased by 50 % under future climatic conditions. At 8 % peat amendment, 8.47 % of Cd was bound to DOM and significantly decreased by 52 % under future conditions. In addition, the amount of peat added to the soil affected the binding of Cd to DOM. Similar to Zn, Cd bound to DOM was significantly affected by peat amendment and climate interactively.

### 3.3. Characterization of 0.1 M Na-pyrophosphate-extractable organic matter

The 0.1 M Na-pyrophosphate extracted approximately 8-times more Zn and Cd than 0.01 M CaCl₂ (Supplementary Fig. S7). While total amounts of pyrophosphate-extracted metals did not differ among peat and climate treatments (Supplementary Fig. S7C and D), the ratio of CaCl₂-extracted to pyrophosphate-extracted followed the same trend as CaCl₂-extractable metals (Supplementary Fig. S7A and B versus Fig. 2A and B).

Pyrophosphate extracted more than 10-fold higher organic carbon compared to 0.01 M CaCl₂ (Supplementary Fig. S6 versus Supplementary Fig. S8A) though no climate impact was observed for pyrophosphate-extractable carbon (Supplementary Fig. S8). Pyrophosphate extracted less organic carbon in 3 % peat-amended soils compared to 5 or 8 % (Supplementary Fig. S8A), and differences due to climate were not perceived. The quality of the carbon was similar among peat
and climate treatments in terms of aromaticity, hydrophobicity, and
molecular weight of the extracted C (Supplementary Fig. S8B-E).

For SEC, organic matter size and chemical composition were evalu-
ated (Fig. 4, Supplementary Fig. S8, Supplemental Method 2). Three size
fractions were identified from SEC analyses. Fraction F0 was identified
as mineral nanoparticles which can be present in soil, mobilized during
extraction, or precipitated due to high extractant pH (Yang et al., 2018)
(Supplementary Fig. S4A). This fraction was defined by large peaks of
Fe, Zn, Al, and the absence of UV (i.e., no organic matter in this fraction).
Fraction F1 represents higher-molecular-weight organic matter
(Supplementary Fig. S4B). Fraction F2 represents
lower-molecular-weight organic matter (Supplementary Fig. S4B).
Further discussion of the SEC fractions is provided in Supplementary
Note 1.

In contrast to the specific UV absorbance characterization (Supplementary Fig. S8), SEC data revealed differences in organic matter chemical properties between treatments. With increasing peat addition, the peak area of UV also increased, which was used as a proxy for aromatic organic matter contributions (Fig. 4). This was observed for total peak area (summed F1 and F2), and individually in the dominant organic matter fraction F1 and the lower-abundance organic matter fraction F2. With 5 % and 8 % addition of peat, summed UV peak area increased by 44 % and 66 % compared to soil with 3 % peat, respectively.

3.4. *Phytoextractability of metals from peat-amended soils*

Arabidopsis *halleri* accumulated approximately 4.2 g Zn kg⁻¹ dry shoot under today's climate in the soil amended with 3 % of peat (Fig. 5A), with a potential increasing trend of 42 and 35 % of Zn in the presence of 5 % and 8 % of peat, respectively. Future climatic conditions did not significantly impact the concentrations of extracted Zn in shoots of *A. halleri*. Presenting the data as total plant Zn concentration (shoots + roots, Fig. 5C) revealed similar trends as for concentration in shoots and overall no difference due to climatic conditions. Total Zn per plant increased only with 5 % peat (Supplementary Fig. S9, Supplementary Table S2).

Similar data was obtained for Cd extracted by *A. halleri* (Fig. 5B-D). In the shoots, less than 20 mg kg⁻¹ of Cd was found, which was not affected by either the level of peat added or by imposed climatic conditions (Fig. 5B). No differences due to climate were found for total plant Cd concentrations and total extracted Cd per plant (Fig. 5D-S9B). Plant biomass was not affected by peat amendment nor climatic conditions (Supplementary Fig. S10), but there was an increasing trend in *A. halleri* biomass in the soil with 5 % peat and with a tendency to reduce plant biomass with 8 % peat amendment to soil compared to *A. halleri* grown in 3 % peated soil. The missing climate impact on *A. halleri* extracted Cd

is also confirmed in the 3 % peat-amended soil with the additional
experiment with 12 biological replicates (Supplementary Fig. S5E).

4. Discussion

4.1. Peat addition impacts on metal fractionation and phytoextractability

The addition of increasing amounts of 3, 5 and 8 % peat to the soil increased total extractable (approximated with pyrophosphate extraction) and exchangeable (approximated with CaCl₂ extraction) organic carbon content proportionally and significantly (Supplementary Figs. S6 and S7) and resulted in a decreasing soil pH from 6.2 to 5.6 (Supplementary Table S1). These two soil parameters are important determinants of metal bioavailability in soils (Cataldo and Wildung, 1978) and act in different directions: Decreasing soil pH increases overall metal mobility (Bradl, 2004), while increasing organic matter content removes metals from the bioavailable fraction by complexing metals to functional groups such as alkyls, carboxyl, phenolic hydroxyl and nitrogen compounds (Hamer and Marschner, 2002; Klavins and Purmalis, 2013). Therefore, the combined effects of peat amendments on soil pH and organic carbon content are likely to have significantly impacted the distribution of metals across different pools.

In all three peat-amended soils, Zn and Cd existed as free Zn²⁺ and Cd²⁺ (Fig. 3A and B), mineral surface-associated exchangeable ions (Fig. 2A and B), partially locked or incorporated in mineral aggregates (Fig. 2C–F), or complexed with organic matter (Fig. 3C,D, Supplemen- tary Fig. S7C and D). The dominance of organically bound metals over exchangeable forms (Supplementary Fig. S7A and B) reflects their in- teractions with peat’s metal-binding functional groups and was expected from the literature (Kyziol, 2002). In parallel, the fractions of 0.1 M Na-pyrophosphate-extractable organic matter with high metal-binding affinity increased with increasing peat content. This increase was evident from the relative rise in UV absorption in the summed peak area and fractions F1 and F2, as determined by size exclusion chromatog- raphy (Fig. 4). Conversely, Cd association with 0.1 M HCl-extractable mineral phases decreased with increasing peat content (Fig. 2E and F). This trend was only evident for Cd, not Zn, due to its lower soil abun- dance (nearly 1000 times less than Zn, Table 1), increasing analytical sensitivity. The large hydrated ionic radii of both elements (97 p.m. pure ion, 426 p.m. hydrated Cd²⁺ vs. 74 p.m. pure ion and hydrated 430 p.m. for Zn²⁺) limiting their trapping into mineral lattices and aggregate structures (Alam and Elzinga, 2023). Collectively, these patterns for Zn and Cd highlight the competitive binding of metals to organic matter, whether small and mobile (CaCl₂-extractable) or larger and stationary (pyrophosphate-extractable), emphasizing the role of peat in shifting metal speciation from mineral associations to organic complexation.

Interestingly, Zn and Cd exhibited distinct changes in their

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Aromatic organic matter as measured by SEC-UV-ICP-MS in 0.1 M Na-pyrophosphate extracts obtained from an agricultural soil amended with 3 %, 5 % and 8 % peat and exposed to today's and future (+4 °C, +320 ppm CO₂) climate conditions. The peak area of UV signal is presented as (A) total summed value, (F1, B) fraction 1 represents higher-molecular-weight organic matter, and (F2, C) fraction 2 represents lower-molecular-weight organic matter. Error bars represent means ± standard error of n = 3 replicates.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Metal extraction by *Arabidopsis halleri* grown on an agricultural soil amended with 3 %, 5 % or 8 % peat under today's and future (+4 °C, +320 ppm CO₂) climate conditions. (A,C) Zn and (B,D) Cd concentrations in (A,B) shoots and (C,D) shoots and roots of *A. halleri*. Take note of unit differences between accumulated Zn and Cd. Error bars represent means ± standard error of the mean (n = 3). No statistical differences were found according to post hoc comparisons.</figcaption></figure>

exchangeable fractions in all three peat-amended soils – assessed through 0.01 M CaCl₂ extractions which target metals non-specifically adsorbed at binding sites of clay minerals and oxides (Kim et al., 2015). Less Zn and Cd were extracted in soils amended with 5 % peat compared to 3 %, but extraction increased with 8 % peat (Fig. 2A and B; Supplementary Table S3). This pattern reflects a balance between the availability of peat's metal-binding functional groups and its impact on soil properties, especially pH (Li et al., 2004). At 5 % peat, an approximate 0.6-unit pH decrease from 3 % peat was estimated to release approximately 1.9 µM of protons (Supplementary Table S1). Protons likely displaced metals from soil surfaces through cation exchange (Fest et al., 2005). However, the increased presence of peat's metal-binding functional groups offset this pH shift by complexing mobilized metals or scavenging additional protons so that overall CaCl₂ extracted less metals (Fig. 2A and B) and specifically supported by a lower mineral association for Cd in reduced 0.1 M HCl (Fig. 2F). The additional metal-binding functional groups provided by 5 % peat likely overrode the metal-liberating effects of lower soil pH, with more solid-phase, metal-complexing organic matter observed with increasing amended peat (Fig. 4) rather than exchangeable organic forms (see ratio of exchangeable vs. pyrophosphate extractable metals in Supplementary Fig. S7A and B). For metals remaining in the exchangeable fraction, WHAM modeling indicated that in the CaCl₂-extractable metal pool, dissolved organic matter could significantly scavenge Zn and Cd ions more effectively at 5 % peat compared to 3 % (Fig. 3C and D), reducing free ions (Fig. 3A and B). This aligns with previous studies that showed

mobile peat fractions bind positively charged metals through cation exchange (Lima et al., 2022).

In 8 % peat-amended soils, total-extractable organic carbon concentrations were comparable to those with 5 % peat (Supplementary Fig. S6), likely due to the extraction capacity of pyrophosphate being reached or excess peat preventing effective extraction. While specific UV absorbance analysis of bulk 0.1 M Na-pyrophosphate extracts did not reveal an effect of peat addition on organic matter quality (Supplementary Fig. S8), using SEC to target specific organic matter fractions revealed an effect of peat addition on solid-phase organic matter likely to have high affinity for metal binding. Overall, increased organic matter with high metal-binding capacity was observed at higher peat levels (Fig. 4), which likely lead to the observation of low exchangeable metal fractions which decreased with increasing peat. However, exchangeable Zn and Cd fractions increased at 8 % compared to 5 % peat (Fig. 2A and B). As shown by others, organic amendments can mobilize metals through increased organic carbon in pore water (Clemente et al., 2010), and dissolved organic matter-metal-complexes (Egene et al., 2018). Metal mobilization was attributed to a drop in pH by 0.65 units from 3 to 8 % peat addition, introducing 1.99 µM more protons into the soil solution. These protons most likely facilitated cation exchange with metals on soil surfaces (Vermeer et al., 1999), and protonated exchangeable peat functional groups (Villaverde et al., 2009) to an extent at which its metal scavenging power was reduced. WHAM modelling of the exchangeable fraction confirmed that in 8 % peat, fewer metals were bound by dissolved organic matter, leaving
more free metal ion compared to 5 % peat (Fig. 3).

Though metal speciation does not always align with plant metal contents (Rao et al., 2008), the bioavailability of metals in soils often correlates with uptake by plants (Liu et al., 2019). Studies have shown that organic amendments can variably affect soil metal speciation and metal concentrations in plant tissues (Al Chami et al., 2013; Hamid et al., 2019b). For the greenhouse experiment, a metal-hyperaccumulator plant was chosen, A. halleri, as it is not physiologically affected by increased exposure to toxic metals, thus ensuring toxicity-independent results (Zhao et al., 2006). Despite the species' known capacity to accumulate 10-fold higher concentrations, Zn and Cd levels in A. halleri leaves were consistently low across treatments, likely due to the metal scavenging ability of organic amendments (Ahmad et al., 2015). Metal concentration in plants did not significantly differ among peat amendments (Fig. 5), even though differences in exchangeable metals were observed for all peat levels (Fig. 2A and B). This highlights the well-known ability of high-molecular-weight, negatively charged, aromatic organic matter in soils (Saar and Weber, 1982). Thus, peat metal-binding functional groups effectively counteracted lower pH-induced metal mobilization, maintaining low plant uptake.

## 4.2. Impact of climatic conditions on metal fractionation and extractability

Climatic conditions affected the exchangeable fraction (approximated with 0.01 M CaCl₂), but not the total extractable carbon fraction (approximated with 0.1 M pyrophosphate). Exposure to future conditions may influence soil metal speciation by altering organic matter decomposition, microbial elemental turnover, and plant growth dynamics (Jansson and Hofmockel, 2020; Poorter and Navas, 2003; Wang et al., 2020b). Under all peat levels, less Cd became recalcitrant under future climatic conditions (Fig. 2E and F), suggesting that higher temperature reduced metal incorporation into newly forming mineral lattices, leaving more metals in bioavailable forms. This aligns with Drabesch et al., who reported increased Cd mobility in soils below pH 7 under future conditions, attributed to (1) selective use of organic carbon, leaving residual carbon to scavenge metals, and (2) stimulated microbial ammonium oxidation generating protons. Their soil with a pH of 6.3 experienced a decrease in pH of 0.2 units, which supposedly liberated Cd through cation exchange. In our study, 3 % peat maintained a similar soil pH of 6.2 as for Drabesch et al., with no pH drop (Supplementary Table S1) and less exchangeable Zn and Cd (Fig. 2A and B) under future conditions. Climate-liberated protons were likely adsorbed by peat functional groups, reducing metal exchange on minerals and supported by an unaltered amount of Zn and Cd being scavenged by exchangeable organic matter (Fig. 3C and D). Additionally, warming likely altered soil organic matter composition (Guan et al., 2018), enhancing metal complexation (Cornu et al., 2016), which in our system may have favored immobilization by total extractable organic carbon over exchangeable metal forms (Supplementary Fig. S7A and B). Findings for 3 % peat were confirmed in repeated experiments with native and spiked metals (Supplementary Fig. S5). Overall, 3 % peat was most likely able to reverse climate-associated metal mobilization in soils.

For soils amended with 5 % and 8 % peat, the observed increase in exchangeable metals (Fig. 2A and B) coincided with a reduction in exchangeable organic carbon (Supplementary Fig. S6A) and weaker metal associations with organic matter (Fig. 3C and D), resulting in more free ions (Fig. 3A and B). This could be due to the enhanced microbial decomposition of peat at elevated temperatures and CO₂ (Ofiti et al., 2023), releasing metals while simultaneously breaking down organic matter into less complex forms. The reduced presence of metal-binding functional groups in exchangeable organic matter under these conditions diminishes the metal-binding capacity, thereby increasing free ion concentrations and shifting the metal speciation away from organic complexation. Collectively, this data indicates that at higher peat levels future conditions diminish the ability of peat to immobilize metals.

Zn and Cd extractability by A. halleri confirms the potential of peat to reduce metal accumulation in plants under future climatic conditions. Shoot metal concentrations and total plant metal accumulation remained constant across climatic conditions for each peat regime (Fig. 5, Supplementary Fig. S9). The increase in metals bound to DOM under 5 % and 8 % peat were not reflected in A. halleri metal concentrations in biomass. This may be due to the reduced Cd concentration in the soil, stimulation for metal phytoextraction highlighting the capacity of peat to suppress plant metal uptake.

## 4.3. Implications and limitations of applying peat

Despite the potential of peat to reduce the mobility of potentially harmful elements in future climates, some limitations should be considered when implementing it as an amendment. While peat should be used in moderation due to its crucial role in carbon storage and its origin from climate-vulnerable lands (FAO, 2020), this analysis paves the way for exploring other organic amendments with similar chemical compositions and innovative, solutions for managing pollution. Additionally, the degree of decomposition and pH of peat amendments vary (Sharma et al., 2024), affecting the effectiveness of metal immobilization and the C and N input into the soil. These properties must therefore be evaluated prior to application. Lastly, maintaining the balance of nutrients in agricultural soils is essential for plant performance (Fageria, 2001), so it is important to assess the impact of amendments on soil fertility. The selection of alternative amendments would also depend on their capacity to improve or maintain plant growth (Martenot et al., 2018) and to maintain reduced concentrations of toxic elements such as Cd, while ensuring sufficient concentrations of other elements such as Zn, which is an essential micronutrient and only toxic to plants at high concentrations in the soil.

## 5. Conclusion

Peat effectively mitigated metal mobility and accumulation in soils and plants under future climatic conditions, but its efficacy depends on the amendment level. At the low peat level of 3 %, exchangeable Zn and Cd decreased under future climatic conditions, even reversing a potential climate-associated effect on metal mobility. However, increased metal mobility was observed at 5 % and 8 % peat levels, due to the microbial decomposition of peat and the reduction of metal-binding functional groups, leading to weaker metal-organic matter associations. Despite differences in soil metal speciation, plant metal accumulation remained low across all peat levels, confirming peat's ability to suppress metal uptake by plants. This study highlights peat's effectiveness in limiting the phytoextractability of metals and reducing pollution. Further research on amended agricultural soils that contain high concentrations of toxic elements under future climatic scenarios will provide insight on the type of organic amendments that make this strategy feasible.

### CRediT authorship contribution statement

**Natalia Sánchez:** Writing – review & editing, Writing – original draft, Visualization, Validation, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Ines Merbach:** Writing – review & editing, Resources, Methodology. **Sören Drabesch:** Writing – review & editing, Formal analysis. **Evgenia Blagodatskaya:** Writing – review & editing, Resources, Formal analysis. **Floriane Jamoteau:** Writing – review & editing, Formal analysis. **Marco Keiluweit:** Writing – review & editing, Resources, Formal analysis. **Jill Bachelder:** Writing – review & editing, Validation, Methodology, Investigation, Formal analysis, Data curation. **Mika Tarkka:** Writing – review & editing, Supervision. **E. Marie Muehe:** Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.
Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank Paul-Georg Richter, Carolina Vergara Cid, Mara Breit, Shirley Yang, Sabine Straßenburg, Birgit Sawall and Renate Hintz for support in lab and greenhouse. Ute Krämer and Andreas Auermann provided A. halleri and advised on plant propagation. Anja Miltner, Felicitas Ehme, Jacqueline Rose, the Elemental and Inorganic Analysis group at the Helmholtz Centre for Environmental Research, and the Soil Biogeochemistry Group at the University of Lausanne supported us with elemental analysis. Many thanks to Julie Tolu and Lenny Winkel for support with SEC-UV-ICP-MS/MS analyses at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and the Swiss Federal Institute of Technology (ETH Zürich). This work was financed through the Helmholtz Young Investigator Grant RhizoThreats.

Appendix B. Supplementary data

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jenvman.2025.127167.

**Appendix A. Supplementary material**

The following is the Supplementary data to this article.

Data availability

Data will be made available on request.

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