Charirat Kusonwiriyawong

Moritz Bigalke
email: moritz.bigalke@giub.unibe.ch

Sophie Cornu

David Montagne

Zuzana Fekiacova

Marina Lazarov

Wolfgang Wilcke

Response of copper concentrations and stable isotope ratios to artificial drainage in a French Retisol

Keywords: Copper isotopes, Lessivation, Redox conditions, Retisol, Soil volumes, drainage, soil water

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

trench at four distances from the drain: 0.6 m, 1.1 m, 2.1 m and 4.0 m. In the E&Bt horizon, four different soil volumes (ochre, pale brown, white-grey and black) were sampled at all four distances from the drain. Furthermore, we analyzed soil solutions sampled with piezometer, porous cups, and at the drain outlet. The Cu concentrations were lowest in the surface (Ap) horizons (6.5-8.5 µg g -1 ) and increased with depth to the clay-rich Bt horizons (10.5-12 µg g -1 ), because of clay eluviation and associated Cu transport. The δ 65 Cu values significantly decreased from the surface (Ap = -0.25±0.07 ‰) to the deeper horizons, but show no significant variation among the deeper horizons (-0.41±0.28 ‰) and no correlation with the clay content, indicating that clay eluviation does not significantly affect d 65 Cu values. The isotopically heavier d 65 Cu values in the Ap horizons can probably be explained by agricultural management practises like sludge application and fertilization. Close to the drain (position 0.6 m), Cu concentrations were depleted and the lighter Cu isotope was enriched (-0.91±0.15‰) in the uppermost part of the E&Bt horizon. We attribute this to the changing redox conditions, caused to lowering of the water level close to the drain.

Copper concentrations in black and ochre volumes were significantly higher than in pale-brown and white-grey volumes. The black volume had significantly higher δ 65 Cu values than the ochre volume indicating preferential sorption/occlusion of the heavy Cu isotope by Fe oxides. Enhanced clay eluviation in bulk soil close to the drain and in specific soil volumes did not affect d 65 Cu values. Cu concentrations (2.1 -14 µg L -1 ) and δ 65 Cu (0.04 -0.42‰) values in water samples showed no clear relation with redox changes along the trench perpendicular to the drain. The enrichment of the heavy Cu isotope in the solution samples (D 65 Cu(soil-solution)= -0.61±0.41) indicates that reductive Cu mobilization is not the main driver of Cu leaching, because this would preferentially mobilize isotopically light Cu. We conclude that the eluviation of the <2µm fraction, strongly controlled Cu concentrations, but had no discernible effect on δ 65 Cu values. The changing

Introduction

In temporal or permanently water-saturated soils, episodic anoxic redox conditions couple back to many soil chemical properties and may cause mobilization and redistribution of redoxsensitive elements like Cu. Copper is of interest because of its nutritional importance as well as pollution risk. Furthermore, the redox behavior of Cu is assumed to play an important role in colloidal mobilization of a number of toxic elements like Ag, Cd, Hg and Pb [START_REF] Abgottspon | Fast colloidal and dissolved release of trace elements in a carbonatic soil after experimental flooding[END_REF]Hofacker et al., 2013;Weber et al., 2009a). Thus, information about Cu behavior in temporarily water-saturated soils and the response of Cu to changes in the soil water regime might help to understand the release mechanisms of redox-sensitive trace elements.

When soils get waterlogged, the redox potential drops and Fe and Mn (oxyhydr)oxides are dissolved releasing associated trace elements (e.g., As, Ba, Co, Cr, V; [START_REF] Abgottspon | Fast colloidal and dissolved release of trace elements in a carbonatic soil after experimental flooding[END_REF][START_REF] Laing | Trace metal behaviour in estuarine and riverine floodplain soils and sediments: A review[END_REF][START_REF] Sipos | Accumulation of Trace Elements in Fe-Rich Nodules in a Neutral-Slightly Alkaline Floodplain Soil[END_REF]. Changes to anoxic conditions may cause microbial formation of reduced metal (Cu + and Cu[0]) colloids (Weber et al., 2009b). When the redox potential drops sufficiently, microbial sulfate reduction is initialized and the mobility of Cu can be limited by the formation of or co-precipitation with sulfides (Weber et al., 2009b;[START_REF] Borch | Biogeochemical redox processes and their impact on contaminant dynamics[END_REF]. However, sulfate reduction may also favor the release of Cu-sulfide colloids into soil solution, resulting in enhanced mobility during several days after flooding [START_REF] Abgottspon | Fast colloidal and dissolved release of trace elements in a carbonatic soil after experimental flooding[END_REF]Hofacker et al., 2013;Weber et al., 2009a). When the conditions in the soil change to oxic, Cu(0) is rapidly oxidized to Cu 2+ , while Cu + -Sorg or CuxS is only slowly oxidized limiting Cu solubility in soil (Fulda et al., 2013b). [START_REF] Balint | Leaching potential of metallic elements from contaminated soil under anoxia[END_REF] confirmed that Cu leaching decreased over four redox cycles, which they attributed to the redistribution of Cu from labile to more recalcitrant chemical fractions in soil.

Several soil processes result in fractionation of Cu isotopes (Fig. 1, Bigalke et al., 2010a;c;2011;2013). Sorption of Cu to Al and Fe (oxyhydr)oxides caused an enrichment of heavy Cu on the surface of the Fe (oxyhydr)oxides [START_REF] Balistrieri | Fractionation of Cu and Zn isotopes during adsorption onto amorhous Fe(III) oxyhydroxide: experimental mixing of acid rock drainage and ambient river water[END_REF][START_REF] Pokrovsky | Copper isotope fractionation during its interaction with soil and aquatic microorganisms and metal oxy(hydr)oxides: Possible structural control[END_REF]. Sorption to organic ligands shows different fractionation factors depending on the type of organic ligand and pH (Bigalke et al., 2010b;[START_REF] Ryan | Copper isotope fractionation during equilibration with natural and synthetic ligands[END_REF]. Lighter Cu isotopes are preferentially adsorbed on clay mineral surfaces [START_REF] Li | Copper isotope fractionation during adsorption onto kaolinite: Experimental approach and applications[END_REF]. Redox reactions cause pronounced fractionation, leaving the reduced Cu species enriched in lighter Cu isotopes [START_REF] Ehrlich | Experimental study of the copper isotope fractionation between aqueous Cu(II) and covellite[END_REF][START_REF] Zhu | Mass fractionation processes of transition metal isotopes[END_REF]. [START_REF] Babcsányi | Copper stable isotopes to trace copper behavior in wetland systems[END_REF] and Bigalke et al. (2010a;2011;2013) found temporally water-saturated soil horizons and wetlands to be enriched in heavy Cu isotopes, which they attributed to the loss of light Cu by leaching of reduced colloidal Cu forms. Liu et al. (2014a) studied weathering and soil formation under different climatic conditions and attributed variations in the isotopic composition to sorption of Cu to organic carbon in soils and leaching of heavy Cu, while also different redox conditions in the soils may have caused significant fractionation. In oxic weathered soils, leaching of heavy Cu because of complexation and downward transport with humic acids was also described by [START_REF] Bigalke | Stable Cu isotope fractionation in soils during oxic weatheringand podsolization[END_REF][START_REF] Fekiacova | Tracing contamination sources in soils with Cu and Zn isotope ratios[END_REF] recently compiled data from contaminated and uncontaminated soils and found that contaminated soils tended to show heavier d 65 Cu values. In addition, fractionations associated with plant uptake of Cu [START_REF] Jouvin | Stable isotope of Cu and Zn in higher plants: Evidence for Cu reduction at the root surface and two conceptual models for isotopic fractionation processes[END_REF][START_REF] Navarrete | Copper isotope fractionation by desert shrubs[END_REF][START_REF] Ryan | Copper speciation and isotopic fractination in plants: uptake and translocation mechanisms[END_REF][START_REF] Weinstein | Isotopic fractionation of Cu in plants[END_REF] might affect Cu isotope distribution in the organic and surface horizons [START_REF] Bigalke | Stable Cu isotope fractionation in soils during oxic weatheringand podsolization[END_REF]. The literature reveals that the determination of Cu stable isotope ratios may be a valuable additional tool to mass budgeting approaches for the identification of the processes by which Cu responds to pedogenesis (e. g., clay redistribution and redox changes). To study the interaction of the latter two processes, Retisols are a model soil type.

Retisols are characterized by the eluviation of clay from the surface horizons (E horizon)

and transport and accumulation of the clay in deeper horizons (Bt horizon). The subsoil clay accumulation impedes drainage and leads to temporary water saturation in winter. In such soils, the combination of eluviation and redox processes is responsible for the morphological degradation of the soil and the formation of the E&Bt-horizon, characterized by the juxtaposition of four soil volumes differing in texture and color. To improve agricultural suitability, many

Retisols have been drained [START_REF] Fao | Lecture notes on the major soils of the world[END_REF]IUSS Working Group WRB, 2014). Artificial drainage was demonstrated to induce i) an increasing intensity of the eluviation process in the immediate vicinity of the drains and ii) the transport of dissolved Fe and Mn towards the drain lines where more oxidative conditions favored the precipitation of Fe and Mn oxides in various forms of black concretions and impregnations [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF].

We focus on the Cu isotopic composition of soil samples collected from four soil profiles located at increasing distance from a drain and the evolution of the δ 65 Cu values of four soil volumes in the E&Bt horizon as response to the drainage. We aim to answer the following questions:

1) What is the effect of clay eluviation and accumulation in the Bt horizons on Cu concentrations and d 65 Cu values?

2) What is the effect of drainage and associated changes in soil chemistry on Cu concentrations and d 65 Cu values?

3) How do redox and eluviation processes effect Cu concentrations and d 65 Cu values of soil solutions?

Materials and methods

Site description and soil sampling

The study site is located on the crest of Yonne plateau in France where Retisols developed on quaternary loamy deposits overlying an Eocene clay layer. The deposit contains 70-90% of silt and 5-20% of clay. The soil was extensively cultivated for at least 200 years. Since 1988, an artificial subsurface drainage was installed at 1 m depth. The drain spacing was 15 m between parallel drainage pipes. The soil water regime fluctuates seasonally with saturation from December or January to February or March depending on the year. The temporary water table possibly reaches to the A horizon and is lowered close to the drain (Fig. 2; [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF].

Details of the soil sampling procedure are available in [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF]. Briefly, in 2004, i.e. 16 yr after installation of the drainage, soil profiles were sampled from a trench perpendicular to one drain at four different positions with increasing distance to the drain (0.6, 1.1, 2.1 and 4.0 m, respectively). At each position, bulk soil samples were collected from three soil horizons (Ap/E&Bt/Bt). The Ap horizon (0 to ~ 30-35 cm depth) has a silty texture and is enriched with organic matter (7.3±0.3 g kg -1 organic C; [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF]. The E&Bt horizon (~35 to 60 cm depth), shows pronounced eluviation and redoximorphic features resulting in the juxtaposition of four volumes differing in texture and color. The four soil volumes include whitegrey, pale-brown, ochre and black volumes [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF]. The white-grey and the palebrown volumes are most abundant in the E&Bt-horizon, while in the underlying clay-enriched Bt horizon of yellowish brown color (~55 to ~105 cm depth), the ochre soil volume is by far most abundant. Soil pH increased with depth from 7.6±0.1 in the Ap horizon to 8.0±0.8 in the Bt horizon. In addition to the bulk soil samples, soil monoliths (approximately 27x15x12 cm) were extracted from the E&Bt horizons at all four distances to the drain. In these monoliths, the whitegrey, pale-brown and ochre soil volumes were manually separated from each other, while black concretions and impregnations were sorted by wet sieving and the help of a magnetic separation technique [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF].

Piezometers were installed at three positions (at 0.7, 1.5 and 4.0 m, respectively, from the drain) in the E&Bt horizons and porous cups (1 x 2 cm-large) were placed in both, the ochre and white-grey volumes. In addition, water was collected at the outlet of the main drain of the plot with an automatic collector. Water samples were collected once a week during the years 2005 and 2006, and once every two weeks during the two following years. The Eh, pH and temperature were measured in the field. In the lab, all soil water samples were filtered through a 0.2-µm cellulose filter, acidified with suprapur HNO3 and stored at 4°C for Fe analysis. Soil water samples were bulked to obtain a sufficient mass of Cu for isotope analysis. Bulking was done for the three µg L -1 . Therefore, 40 µg L -1 Fe was used as a threshold to separate between oxic and anoxic soil solution samples.

Sample preparation and analysis

Approximately, 0.25-0.40 g of soil samples were digested in a mixture of concentrated HNO3, HF and H2O2 (ratio 3:2:1) in PFA beakers (Savillex @ MN, USA) for 24-36 h on a hotplate at 120°C. The digests were evaporated until dryness on a hot plate at 70 o C. To remove excess HF, the dried residues were redigested with a mixture of concentrated HCl and HNO3 for at least 3-4 hours, refluxed several times and evaporated to dryness on a hot plate. Samples were finally dissolved in 7 mol L -1 HCl and 0.001% H2O2. The water samples (approximately 300 mL) were evaporated yielding >300 ng of Cu for isotope analysis. The samples were refluxed in HNO3 and H2O2 (ratio 1:1) and finally dissolved in 7 mol L -1 HCl and 0.001% H2O2 for Cu purification.

All samples were purified using Poly-Prep Chromatography columns (Bio-Rad, CA, USA) filled with 2 mL of pre-cleaned 100-200 mesh AG MP-1 (Bio-Rad, CA, USA) anion exchange resin following an established method (Bigalke et al., 2010a). For soil samples, the column purification was repeated once to gain matrix-clean Cu fractions [START_REF] Bigalke | Stable Cu isotope fractionation in soils during oxic weatheringand podsolization[END_REF][START_REF] Petit | A case study of spectral and non-spectral interferences on copper isotope measurements by multi-collector ICP-MS (Wet plasma)[END_REF]. After complete separation, the purified fractions were evaporated to dryness and digested with concentrated HNO3 and H2O2. The samples were evaporated and then dissolved in 2% HNO3

for Cu isotope analysis. All samples were analyzed by ICP-MS (7700x, Agilent, CA, USA) for matrix elements and Cu recovery. Column eluates, in which Cu was not completely recovered (100±6%) or in which matrix elements were present, were discarded and sample purification was repeated.

All reagents used were of suprapur quality (Merck, Darmstadt, Germany). Hydrochloric and nitric acid were purified by sub-boiling distillation. Sample preparation and chemical purification were performed in the clean chemistry laboratory at the Institute of Geology, University of Bern. Total procedural Cu blanks averaged 1.9±0.9 ng (n=3) and 3.4±1.5 ng (n=3)

for the first and second runs of column purification, respectively. The quality of the method was evaluated by using USGS basalt BCR-2 (Basalt Columbia River 2, USGS, Reston, VA, USA) reference materials. The mean total Cu concentration we determined in BCR-2 was 18.6±0.3 µg g -1 (mean ± SD, n=11) in good agreement with the certified value of 19±2 µg g -1 .

Isotope analysis

Copper isotope ratios were analyzed by MC-ICP-MS (Thermo-Finnigan Neptune, Thermo Scientific, Waltham, MA, USA) at the Leibniz University Hannover, Germany. Instrument was operating in the low mass resolution mode. Samples and standards were diluted to 300 µg L -1 Cu with 2% HNO3 and introduced in to the MC-ICP-MS by a glass spray chamber (double pass Scott design). Nickel (NIST 986, National Institute of Standards and Technology, Gaithersburg, MD, USA) at concentration of 1000 µg L -1 was used for the instrumental mass-bias correction in combination with standard-sample bracketing. Every sample was at least analyzed twice. The average Cu isotope ratio was reported in the δ 65 Cu notation in ‰ relative to NIST 976. The accuracy of the resin purification method was validated by using spiked Cu-free matrix samples.

The Cu-free matrix samples were prepared from the matrix fraction derived from the purification of the original samples and spiked with the ERM @ -AE633 Cu isotope standards (Institute for Reference Materials and Measurements, Geel, Belgium), which is isotopically identical with NIST 976 [START_REF] Moeller | Calibration of the new certified reference materials ERM-AE633 and ERM-AE647 for copper and IRMM-3702 for zinc isotope amount ratio determinations[END_REF]. The spiked matrices were treated and purified in the same manner as the original samples. The δ 65 Cu value of the matrix samples was -0.03±0.04‰ (mean±2SD, n=5)

and undistinguishable from ERM @ -AE633 (-0.01±0.05‰, [START_REF] Moeller | Calibration of the new certified reference materials ERM-AE633 and ERM-AE647 for copper and IRMM-3702 for zinc isotope amount ratio determinations[END_REF]. Reproducibility and accuracy of δ 65 Cu measurements were monitored with the help of certified reference materials BCR-2 and NBS C 125-2 (SRM C1252, National Institute of Standards and Technology, Gaithersburg, MD, USA). The NBS C 125-2 was used as an in-house Cu standard to check the MC-ICP-MS stability yielding a δ 65 Cu value of 0.36±0.06‰ (mean±2SD, n=10). BCR-2 yielded a δ 65 CuNIST976 = 0.15±0.08‰ (mean±2SD, n=11) comparable to the previously published data ranging from 0.14±0.05‰ to 0.22±0.06‰ (e.g., Bigalke et al., 2010a;2013;Liu et al., 2014b;[START_REF] Moeller | Calibration of the new certified reference materials ERM-AE633 and ERM-AE647 for copper and IRMM-3702 for zinc isotope amount ratio determinations[END_REF].

Calculations and statistics

The overall mass flux for any soil volume mj;flux in g cm -2 was then calculated for each element j using Eq. ( 1) proposed by Brimhall et al. (1991) and modified by [START_REF] Egli | Formulation of pedologic mass balance based on immobile elements: A revision[END_REF]:

! ";$%&' = ) )** × , -./ ×0 1;-./ ×23 ×5 1.7 8 9;7 :) (1)
in which ρ is the bulk density, Cj is the concentration of j in weight percent, Th (cm) is the thickness of the considered soil horizon. The subscripts ref and w referto the soil taken as a reference and to the weathered product, respectively. We used positions 60 and 110 m as representing the weathered product (because drainage changes the soil composition at these distances) and positions 210 and 400 m as reference (because here the effect of drainage is very low, [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF]. This is different to the classical approach of comparing soil horizons with parent material. The εi;w and τj;w values are the strain and the open-system mass-transport functions, respectively, calculated according to Eqs. 2 and 3 (Brimhall et al., 1991). The εi;w is a measure for the change of the soil volume over time using an immobile element i and τj;w; is the mass fraction of element j gained or lost from the weathered product with respect to the mass originally present in the reference material (i.e. the soil at positions 210 and 400 m). We used quartz as an immobile compound.

; <;= = , -./ 0 9;-./ , 7 0 9;7 -1

(2)

@ <;" = , 7 0 9;1 , -./ 0 1;-./ A; <;= + 1C -1 (3)
After checking the data for homoscedasticity with the Levené test, an analysis of variance (ANOVA) followed by a Tukey's Honestly Significant Difference (HSD) post hoc test was conducted to compare the mean Cu concentrations and isotopic compositions among soil volumes.

Normal distribution of residuals was checked by visual inspection. Significance was set at p < 0.05.

3.

Results

Copper concentrations in the bulk horizons increased with depth at three of the four positions (1.1, 2.1 and 4.0 m). Copper concentrations were closely related with those of the clay fraction (Fig. 3a). The Cu concentrations in the surface (Ap) horizon and clay-rich Bt horizons varied only little along the trench. In contrast, in the E&Bt-horizon, there was a large lateral variation in the Cu concentrations, with the lowest value at position 0.6 m (Tab. 1, Fig. 4a). The

Response of Cu concentrations and d 65 Cu values to drainage

The low Cu concentration in the E&Bt at position 0.6 m, suggests that the drainage induced Cu leaching (Table 1, Fig. 4a). Mass flux calculations indicated that Cu mass flux (mCu flux) at position 0.6 m was, on average, three times higher than at positions 2.1 and 4.0 m (43.8 and 15.3 mg cm -2 , respectively). This loss of Cu from the E&Bt horizon at position 0.6 m is linked with substantial loss of Fe and clay at positions 0.6 m and 1.1 m, (6.5 and 2.3 kg m -2 Fe and 75.8 and 25.6 kg m -2 clay, respectively; [START_REF] Montagne | Do we need to include soil evolution module in models for prediction of future climate change[END_REF]. The loss has been explained by strongly enhanced eluviation caused by drainage-induced higher water fluxes, and is most pronounced in the upper part of the E&Bt horizon at position 0.6 m [START_REF] Montagne | Do we need to include soil evolution module in models for prediction of future climate change[END_REF]. These findings agree with the close correlation between the clay and the Cu concentrations (Fig. 3a), which furthermore suggests that clay is the dominant Cu pool in this soil. In contrast, the observed eluviation had no significant effect on the d 65 Cu value of the drained soil, as illustrated by the lack of a correlation between d 65 Cu values and clay concentrations (Fig. 3b).

At 0.6 m distance, the E&Bt horizon showed a strong negative δ 65 Cu value in its upper part (35-45cm), while its lower part (45-55cm) with similar properties (clay and Cu concentrations)

did not show differences in d 65 Cu values from the soil at other distances (Fig. 3b). This strongly negative value was ascertained by three replicate analyses including separate digestion, purification and analysis of each replicate. We suggest that the light d 65 Cu value in the upper part of the E&Bt horizon at position 0.6 m might be attributable to the change in redox conditions following drainage. Redox changes can cause a comparatively large fractionation of d 65 Cu values with the reduced Cu(I) enriched in the light isotopes (Fig. 1; [START_REF] Zhu | Mass fractionation processes of transition metal isotopes[END_REF]. Under anoxic conditions, the reduced Cu fraction may account for a major part of total soil Cu and may carry a heavy isotope signal to balance that of a Cu-isotopically light residual fraction (Kusonwiriyawong et al., 2015). This Cu-isotopically heavy reduced fraction might be lost by oxidation (Fulda et al., 2013b) [START_REF] Fekiacova | Tracing contamination sources in soils with Cu and Zn isotope ratios[END_REF] and ours must have different reasons.

Additionally to analyzing the bulk soil samples, we partitioned the soil in the E&Bt horizon into four different soil volumes. The differentiation starts from the ochre volume, developing successive pale-brown and white-grey soil volumes by increasing eluviation and redox-induced bleaching [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF]. Within the ochre volume, the black volume forms because of the precipitation of Mn oxides. With increasing proximity to the drain the ochre volumes decreased and the black, pale-brown and white-grey volumes increased [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF]. The black volumes always had the highest Cu concentrations, probably because of precipitation with and sorption of Cu on Mn oxides [START_REF] Negra | Soil manganese oxides and trace metals: Competitive sorption and microfocused synchrotron X-ray fluorescence mapping[END_REF]. The ochre volume always showed higher Cu concentrations than the pale-brown and white grey volumes because of eluviation and reductive leaching of clay minerals and (oxyhydr)oxides in the latter two volumes (Fig. 5a, [START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF]. Because of the drainage, Cu concentrations in the black and ochre volumes decreased by approx. 50% in the direct vicinity (position 0.6 m) of the drain, which is consistent with the decrease in the bulk soil. The decrease in the Cu concentrations of the ochre and black volumes at position 0.6 m is driven by the strong clay loss by eluviation. In contrast, the more oxidizing conditions near the drain caused an increase in the abundance of the ochre and black volumes, attributable to the precipitation of Mn and Fe oxi(hydr)oxides. Because ochre and black volumes formed in a soil, which was already depleted in Cu, they showed lower Cu concentrations.. Independent of the distance to the drain, the contributions of the Cu stocks in the black, pale-brown and white-grey volumes to the total Cu stock of the bulk E&Bt horizon did not change, while the contribution of the Cu stock in the ochre volume to the total Cu stock of the bulk horizon decreased (Fig. 5b).

There were no clear variations in the d 65 Cu values of the individual volumes with distance to the drain (p < 0.05) indicating that the drainage-induced morphological changes at position 0.6 m did not cause a Cu isotope fractionation among the soil volumes. At position 0.6 m, the soil volumes were taken from the lower depth layer (45-55 cm), which had a similar d 65 Cu value as all other bulk soil samples (Fig. 2). The black volumes showed always significantly higher d 65 Cu values than the ochre volumes they develop from. This might be attributable to variable redox at the small spatial scale at which the differentiation into the four soil volumes occurred and related Cu isotope fractionation or by the sorption on Fe and Mn oxy(hydr)oxides in the black volumes.

As redox variation usually causes a strong isotope fractionation (Fig. 1) we consider more likely that the limited changes observed in the d 65 Cu values of the different soil volumes depended on the adsorption to Fe and Mn (oxyhydr)oxides (which preferentially adsorb heavy isotopes, Fig. 1b, [START_REF] Pokrovsky | Copper isotope fractionation during its interaction with soil and aquatic microorganisms and metal oxy(hydr)oxides: Possible structural control[END_REF][START_REF] Balistrieri | Fractionation of Cu and Zn isotopes during adsorption onto amorhous Fe(III) oxyhydroxide: experimental mixing of acid rock drainage and ambient river water[END_REF].

The overall lack of a correlation between the clay concentration and d 65 Cu values, the differences in d 65 Cu values between the 35-45 and 45-55cm depth layers despite a similar degree of eluviation and the absence of significant variations between the ochre and the white-grey soil volumes (Fig. 5, the white grey volume is clay-depleted) imply limited importance of clay eluviation for the d 65 Cu values of the soil, despite the marked effect of lessivation on Cu concentrations in bulk horizons (Fig. 3).

Cu in soil water

The Cu concentrations in our soil water samples were similar to the previously published range of Cu concentrations in soil pore water during weathering of black shale of 1-16 µg L -1 [START_REF] Mathur | Cu isotopes and concentrations during weathering of black shale of the Marcellus Formation, Huntingdon County, Pennsylvania (USA)[END_REF], dissolved Cu in river of 0-3 µg L -1 [START_REF] Vance | The copper isotope geochemistry of rivers and the oceans[END_REF] and dissolved Cu in wetlands 1-12 µg L -1 [START_REF] Babcsányi | Copper stable isotopes to trace copper behavior in wetland systems[END_REF]. The low concentrations in the drain water may be caused by co-precipitation with or sorption to Mn and/or Fe (oxyhydr)oxides precipitating near the drain pipe where the reduced Mn and Fe comes into contact with oxygen. Samples with low (<40 µg L -1 ) and high (>40 µg L -1 ) Fe concentrations (indicative for oxic and reducing conditions, respectively) did not show systematically different Cu concentrations, indicating that the redox potential was not sufficiently low to reduce Cu.

The d 65 Cu values in our water samples are well within the range reported for soil, river, and wetland water ranging from 0.02-1.45‰ [START_REF] Vance | The copper isotope geochemistry of rivers and the oceans[END_REF][START_REF] Mathur | Cu isotopes and concentrations during weathering of black shale of the Marcellus Formation, Huntingdon County, Pennsylvania (USA)[END_REF][START_REF] Babcsányi | Copper stable isotopes to trace copper behavior in wetland systems[END_REF] with the findings from the bulk soils, where we also did not observe an indication for redox mobilization of Cu. The δ 65 Cu value of dissolved Cu was heavier than that of the bulk solid soil (D 65 Cu(soil-solution)= -0.61±0.41), but fractionation was less pronounced than reported for redoxinduced fractionations in field and laboratory experiments (Fig. 1). The pattern of Cu isotopically light solid soils and heavy dissolved Cu fits well into the findings of a weathering experiments with basalts at pH 5 [START_REF] Li | Copper isotope fractionation during basalt weathering at pH = 0[END_REF]) and results of the analysis of soil solutions from oxic weathering of black shales, which both always showed an enrichment of the isotopoically heavy Cu in the dissolved phase [START_REF] Mathur | Cu isotopes and concentrations during weathering of black shale of the Marcellus Formation, Huntingdon County, Pennsylvania (USA)[END_REF].

Independent of the redox conditions, the d 65 Cu values of the dissolved fraction in rivers, wetlands and soils always showed a heavy δ 65 Cu value, while the particulate and colloidal fraction showed light d 65 Cu values and a strong response to redox changes [START_REF] Babcsányi | Copper stable isotopes to trace copper behavior in wetland systems[END_REF][START_REF] Vance | The copper isotope geochemistry of rivers and the oceans[END_REF]. The lack of a relationship of the d 65 Cu values of dissolved Cu with the redox potential might be explained by the fact that the Cu isotope ratio of dissolved Cu in soils and rivers is more strongly controlled by complexation with strong dissolved ligands [START_REF] Vance | The copper isotope geochemistry of rivers and the oceans[END_REF][START_REF] Vance | The behaviour of Cu and Zn isotopes during soil development: Controls on the dissolved load of rivers[END_REF] than by redox changes. The responsible ligands were identified by cathode-stripping voltametry and are subdivided in the ligand classes L1 and L2 [START_REF] Muller | Chemical speciation of copper and zinc in surface waters of the western Black Sea[END_REF]. Both ligand classes have high stability constants up to 10 16 and often occur in excess compared to Cu concentrations in solution [START_REF] Vance | The copper isotope geochemistry of rivers and the oceans[END_REF]. Therefore, it can be assumed that almost all dissolved Cu occurs in complexed form in environmental solutions [START_REF] Muller | Chemical speciation of copper and zinc in surface waters of the western Black Sea[END_REF].

Conclusions

1) The slightly decreasing d 65 Cu values with increasing depth in the bulk soils might be caused by addition of heavy Cu (e.g., fertilizer and sewage sludge) to the surface soil. [START_REF] Abgottspon | Fast colloidal and dissolved release of trace elements in a carbonatic soil after experimental flooding[END_REF]Hofacker et al., 2013b;Weber et al., 2009b).

  different water types separately (piezometer, porous cup, and drain water samples) for two different time periods (2005/06 and 2007/08) resulting in seven different soil water samples. The water samples were classified according to their Fe concentrations as indicator of redox conditions in the soil. The Fe concentrations under oxic condition (Eh > 300 mV) were always lower than 40

δ

  65 Cu values tended to decrease from the Ap horizon to the deeper horizons (Fig.4b), but showed no relation to Cu concentrations or the clay fraction (Fig.3b). However, the δ 65 Cu values at different depths in the E&Bt and Bt horizons and at the different positions along the trench were not different. We only detected a single much lower δ 65 Cu value compared to all other samples in the 35-45 cm depth layer (E&Bt horizon) at position 0.6 m (Tab. 1, Fig.3b, 4b). The Cu concentrations were significantly higher in the black and ochre volumes than in the pale-brown and white-grey volumes, respectively. Copper concentrations were not related with distance to the drain in pale-brown and white-grey volumes but were lower in the black and ochre volumes at position 0.6 m than at all other positions (Fig.5a, Tukeys HSD test, p < 0.05). Overall the black volumes had the significantly highest and the ochre volumes the significantly lowest d 65 Cu values, while the δ 65 Cu values of the pale-brown and white-grey volumes were not significantly different from those of the ochre and black volumes (Fig.6). The bulk δ 65 Cu value calculated from the mass-balanced sum of the individual soil volumes (ranging from -0.36±0.04‰ to -0.41±0.04‰) showed good agreement with the δ 65 Cu value of the bulk soil in the E&Bt horizon at the different distances from the drain (ranging from -0.38±0.03‰ to -0.41±0.02). At position 0.6 m, this is true for the lower bulk sample (45-55 cm depth), which overlaps with the depth where soil volumes were sampled (Fig.2), while for the upper 35-45 cm depth layer of the E&Bt horizon with the low d 65 Cu value (-0.91±0.15‰) we did not have samples of individual soil volumes for comparison. The dissolved Cu concentrations in the porous cup sample were highest of all analyzed soil solutions. The Cu concentrations of drain water were consistently lower than those of the piezometer sample in all three studied samples (Tab. 2). While in the hydrological year 2005/2006 the δ 65 Cu values in the piezometer and drain waters seemed to be lower in the anoxic samples (Fe > 40 µg L -1 ); compared to the oxic samples, the δ 65 Cu values of the anoxic samples were similar to those in the oxic samples from the Piezometer in the following hydrological year 2007/08. Consequently, the variations in d 65 Cu values among the various solution types and sampling dates could neither be clearly assigned to redox conditions nor to the way of sampling. There was no clear difference in Cu concentrations in waters taken under anoxic conditions (Fe > 40µg L -1 ) compared with oxic conditions (Fe < 40µg L -1 ). There were no clear differences in δ 65 Cu values among the water samples from the piezometers and the drain in 2005/2006, but small variations in 2007/08. The single porous cup sample showed the lowest δ 65 Cu value. Overall, the water samples showed higher δ 65 Cu values than the solid soil samples, with D 65 Cu(soil-solution)= -0.61±0.41. Cu concentrations and d 65 Cu valuesThe vertical distribution of Cu in the study soil is influenced by (1) the amendment of limed sludge from 1998 to 2001 resulting in a Cu input of approx. 0.9 g m -2[START_REF] Montagne | Effect of agricultural practices on trace-element distribution in soil[END_REF], (2) regular fertilization e.g., with mineral fertilizer (no manure application) and (3) pedogenetic processes including lessivation and hydromorphy[START_REF] Montagne | Impact of drainage on soil-forming mechanisms in a French Albeluvisol: Input of mineralogical data in mass-balance modeling[END_REF]. The Cu input with sludge and fertilizer has increased Cu concentrations in the Ap horizons and also might have changed the δ 65 Cu value. In the deeper horizons, clay eluviated from the Ap horizons which accumulated in the Bt horizons likely explains the increase in Cu concentrations because the clay fraction usually contains higher Cu concentrations than the coarser particle sizes[START_REF] Minkina | Effect of the particle-size distribution on the adsorption of Copper, Lead and Zinc by Chernozemic soil of Rostov Oblast[END_REF]. The latter is also confirmed by the close correlation between the clay and the Cu concentrations (r = 0.80; p < 0.001). This correlation even became closer, when Ap horizons (with anthropogenic Cu input) were removed (Fig.3a). No d 65 Cu values for agriculturally used sludge or mineral fertilizers have up to now been reported. However, in case that these additions carry a heavier d 65 Cu value than the soilthey might be responsible for the higher d 65 Cu values in the Ap horizons. The different soil depths in E&Bt and Bt horizons show no significant d 65 Cu changes, despite the significant changes in Cu concentrations linked to the clay eluviation. We explain this finding by the fact that Cu bound to clay controls the concentration and the d 65 Cu value of total soil Cu. Our findings suggests that lessivation does not change d 65 Cu values of the bulk soils, because the eluviated and illuviated horizons have the same Cu isotopic composition. The lacking influence of clay concentrations on d 65 Cu values is reflected by the absence of a correlation between these two variables (r < 0.001, p = 0.95). Furthermore, soil volumes with different clay concentrations (Montagne et al, 2008) did not show a significant difference in d 65 Cu values, again indicating that other soil processes than the clay concentration (e.g. sludge application, weathering; Fig. 7) controlled Cu isotope ratios.

  . The d 65 Cu values during anoxic conditions in 2005/06 overlapped with those during oxic conditions in 2007/08, showing no clear relationship with the redox potential in the piezometer. In the drain water, the d 65 Cu values seemed to be lower under anoxic conditions, but were similar to the oxic sample from the piezometer in 2007/08. The uniform Cu concentrations and d 65 Cu values indicate that there was no redox-induced change in Cu mobility, agreeing well

2)

  Drainage did not change d 65 Cu values in bulk soil and soil volumes, despite Cu redistribution by enhanced clay eluviation, with the exception of one point. A low δ 65 Cu value and Cu concentration in the upper E&Bt horizon near to the drain may indicate oxidative weathering and leaching of heavy Cu isotopes formerly stored in the reduced Cu pool and is the only d 65 Cu value which we could link to redox changes. The drainage caused changes in the Cu distribution among the soil volumes indicative of locally changed pedogenetic processes. The δ 65 Cu values showed significant differences among the soil volumes but did not change with distance to the drain, indicating that the Cu isotope signals are dominated by sorption processes but little by redox changes. 3) The Cu concentrations and δ 65 Cu values in the solution samples did not respond to changes in soil redox conditions, indicating that short-term changes in redox conditions in the soil have a small or no effect on the isotope signals of dissolved Cu. The overall δ 65 Cu value of dissolved Cu was heavier than that of bulk solid soil (D 65 Cu(soil-solution)= -0.61±0.41), which we attribute to weathering and sorption of dissolved Cu to strong ligands in solution in line with several reports of soils solutions and river waters in the literature. A conceptual model of the effect of the different processes on Cu distribution and d 65 Cu values in the soil is displayed in Fig. 7. In summary, the two dominant pedogenetic processes in the study soils (lessivation and hydromorphy) seem to have limited influence on the Cu stable isotope ratios although lessivation strongly controls Cu concentrations. The effect of drainage on

Fig. 2

 2 Fig. 2 Schematic diagram of the study design showing the drain, the disturbed zoned caused by

Fig. 3 .

 3 Fig. 3. Relationship between a) the ,clay and Cu concentrations among all soil samples (solid line,

Fig. 4 .

 4 Fig. 4. Vertical distribution of a) Cu concentrations, b) δ 65 Cu values of the different bulk soil

Fig. 5 .

 5 Fig. 5. Copper concentrations in the soil volumes at different distances from the drain (a) and Cu

Fig. 6 .

 6 Fig. 6. Mean δ 65 Cu values of the four soil volumes indicative of the dominating pedogenetic

Fig. 7 .

 7 Fig. 7. Mechanistic model of the fate of Cu in the drained Retisol. The size of the arrows indicate

  Figure 1

  δ 65 Cu is visible only at one position close to the drain, which probably showed the strongest change in redox conditions. Our results illustrate that redox induced Cu leaching is only visible where the soil is most oxidized. Instead, in the Retisol clay eluviation and leaching of organically complexed Cu, drive Cu mobility. In general the d 65 Cu approach on bulk soils is helpful to investigate the influence of redox and sorption processes on Cu mobility in the soil system, but does not help for clay eluviation were two pools (e.g. clay and soil) are isotopically not discernible. The application of d 65 Cu values to investigate into redox controlled Cu mobility might be of high importance as

	reductive Cu mobilisation is driving the mobilisation of a number highly relevant pollutant
	elements (Ag, Cd, Hg, Pb;

Table 1 601

 1 Copper concentrations and δ 65 Cu values of the different soil horizons as a function of distance to

	602
	603	the drain.
	604
		Horizon	Depth (cm)	Cu (µg g -1 )	SD	δ 65 Cu (‰)	2SD	n a
			position 0.6 m
		Ap	10-20	7.3	0.1	-0.25	0.01	1
		E&Bt	35-45	5.7	0.1	-0.91	0.15	3
			45-55	5.6	0.1	-0.40	0.02	1
		Bt	55-65	9.1	0.2	-0.39	0.06	2
			65-75	10.8	0.4	-0.40	0.09	2
			position 1.1 m
		Ap	10-20	8.1	0.6	-0.28	0.01	1
		E&Bt	40-55	8.2	0.7	-0.39	0.03	1
		Bt	55-65	10.7	0.2	-0.40	0.06	2
			65-80	10.5	0.3	-0.31	0.03	1
			position 2.1 m
		Ap	10-20	6.5	0.1	-0.27	0.01	1
		E&Bt	40-50	10.6	0.9	-0.41	0.02	2
			50-60	10.8	0.7	-0.39	0.01	2
		Bt	70-83	11.4	0.3	-0.37	0.04	2
			position 4.0 m
		Ap	10-20	8.5	0.2	-0.20	0.01	2
		E&Bt	35-45	8.4	0.2	-0.38	0.03	1
			45-55	10.2	0.6	-0.39	0.03	1
		Bt	55-65	11.7	0.8	-0.33	0.02	3
			65-75	12.0	1.1	-0.35	0.01	3
	605	a n is the number of independent digestions and purifications for isotope analysis
	606

Table 2

 2 Copper concentration and δ 65 Cu values of piezometer, drain water and porous cup samples in the hydrological years 2005/06 and 2007/08. Compilation of a) d 65 Cu values found in soils, soil and river waters and b) D 65 Cu values reported for different processes, which might be of relevance in the Retisol. 1 Bigalke et al. Fekiacova et al. (2015), 7 Mathur et al. (2012), 8 Ilina et al. (2013), 9 Petit et al. 2013), 10 Vance et al. (2008),11 [START_REF] Balistrieri | Fractionation of Cu and Zn isotopes during adsorption onto amorhous Fe(III) oxyhydroxide: experimental mixing of acid rock drainage and ambient river water[END_REF],12 [START_REF] Clayton | Isotopic effects during Cu sorption onto goethite[END_REF],13 [START_REF] Pokrovsky | Copper isotope fractionation during its interaction with soil and aquatic microorganisms and metal oxy(hydr)oxides: Possible structural control[END_REF],14 Li et al. (2015),15 Bigalke et al. 2010b),16 [START_REF] Ryan | Copper isotope fractionation during equilibration with natural and synthetic ligands[END_REF]),17 [START_REF] Ehrlich | Experimental study of the copper isotope fractionation between aqueous Cu(II) and covellite[END_REF],18 Zhu et al. (2002), 19 Asael (2006),20 Mathur et al. (2005),21 [START_REF] Mathur | Copper Isotopic Perspectives on Supergene Processes: Implications for the Global Cu Cycle[END_REF],22 Mathur et al.

				2005/06				2007/08
	Sample	Fe < 40 µg L -1	Fe > 40 µg L -1	Fe < 40 µg L -1
	name	Cu	δ 65 Cu 2SD	Cu	δ 65 Cu 2SD	Cu	δ 65 Cu 2SD
		µg L -1	(‰)		µg L -1 (‰)		µg L -1	(‰)
	Piezometer	5.5	0.40	0.08	5.3	0.10	0.09	6.5	0.11 0.03
	Drain water	2.5	0.42	0.18	2.9	0.17	0.01	2.1	0.36 0.02
	Porous cup	14.1	0.04	0.05

(2012).

Acknowledgments

We thank the group of Isotope Geology of the University of Berne, Klaus Mezger, Thomas Nägler, Igor Villa and Gabriela Balzer for access to the clean room and support. The authors are grateful to L. Prud'Homme, B. Renault, G. Giot, N. Chigot from URSol (INRA, Orléans, France) for field sampling and monitoring. We thank the AE Edward Nater, and Ryan Mathur as well as two anonymous reviewers for their constructive comments, which significantly improved the manuscript. We thank the Agricultural Research Development Agency (Public Organization), ARDA, Thailand, for funding Charirat Kusonwiriyawong.