Complexation of copper with dissolved organic matter in soil solution: role of organic matter characteristics and effect on mobility of copper in the soil profile

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    The copper (Cu) concentrations in agricultural soils exceed natural background values due to long-term applications of animal manure, fungicides and sewage sludge. Current Cu emissions to agricultural soils have decreased from historic ones, however the topsoil Cu concentrations still gradually increase in areas with intensive agriculture. On the long-term, this may lead to adverse effects on soil quality and, through leaching, on the quality of the aquatic system. This study is devoted to the processes controlling leaching of Cu in soil. It is well established that the mobility of Cu in soils is intimately related to the concentration of Dissolved Organic Matter (DOM) in soil solution that forms strong soluble complexes with Cu2+. The composition of DOM is extremely variable and it is unclear to what extent Cu mobilization is related to DOM structural variability (hereafter DOM quality). The goal of this study is to quantify the variability in Cu affinity of DOM and to relate this variability to DOM source and characteristics. In addition, the importance of the dissociation kinetics of Cu-DOM complexes for Cu leaching in soils is addressed.
    A method was developed to measure the Cu Mobilizing Potential (CuMP) of DOM which measures the Cu that is bound to DOM under constant, environmentally relevant composition (pCu 11.3, pH 7.0, [Ca] = 1.5 mM). The CuMP, expressed per unit dissolved organic carbon (DOC), varied tenfold for DOM isolated from a range of agricultural soils and using different extraction methods. A significant positive correlation between the specific UV-absorption at 254 nm of DOM (SUVA, indicating aromaticity) and CuMP was found across all DOM samples (r = 0.76). Soil extractions isolate additional, and structurally different, DOM than that present in soil solutions isolated by soil centrifugation, therefore soil extractions should be discouraged to characterize soil DOM. Drying/wetting cycles or long dry storage of soil sharply increase DOM concentrations but this additional DOM has low SUVA and CuMP values and is easily degradable because it is less humified. In the field, leachates were collected during 5 subsequent months at 45 cm depth in one arable land. The dissolved Cu and DOC concentrations varied 7-fold and were only weakly correlated (r = 0.56). The [Cu]/[DOC] ratio varied 5-fold and exhibited a significant positive correlation (r = 0.77) with SUVA of DOM, again indicating that the affinity of Cu to DOM increases with increasing aromaticity of DOM. Highest [Cu]/[DOC] ratios were observed at lowest DOC concentrations. Predictions of Cu concentrations with the assemblage model WHAM6 were consistently improved if the DOM characteristics, i.e. SUVA values, were taken into account. From this lab and field experiment it is concluded that the DOM quality should be considered when estimating Cu mobility in soils.
    Relating Cu affinity to DOM structure and source requires adapted methods for soil DOM characterization. An automated column based DOM fractionation method was set up for 10 mL DOM samples dividing DOM into hydrophilic (HPI), hydrophobic acid (HPOA) and hydrophobic neutral (HPON) fractions. Distributions of DOM from 8 soils ranged 31–72% HPI, 25–46% HPOA and 2–28% HPON of total DOC. For each sample, the SUVA of the HPI DOM was consistently lower compared to the HPOA DOM. Fractionation did not result in smaller variability in SUVA values within the fractions compared to the unfractionated samples, which casts some doubts on the assumption that fractionation results in more homogeneous fractions.
    The survey of DOM quality was extended with a number of soil treatments, solution isolation methods and some non-agricultural soils, including wetlands and forest soils. Here, DOC concentrations ranged from 4 to 175 mg L 1 with the lowest values for the arable field soils. The SUVA of DOM varied from 18 to 54 L g 1 cm 1, percentage humic acid (%HA) ranged 17–76%, %HPI 4%–34%, %HPOA 11%–64% and %HPON 7%–33%. There were no clear effects of land use or soil properties on DOM concentrations and qualities. Somewhat lower DOC concentrations, SUVA and %HPI values were observed in column and lysimeter leachates than in corresponding soil solutions isolated by centrifugation. Plant residue incorporation (straw) largely increased %HPI and decreased SUVA and %HA. The SUVA of DOM significantly correlated with %HA (positive; r = 0.81) and %HPI (negative; r = 0.88). The variability in DOM quality was smaller than the variation in DOC concentration, e.g. the SUVA of DOM varied only a factor 3 compared to the almost 40-fold variation in DOC concentration. This suggests that, at a larger scale, DOM quantity is overall more important for Cu mobilization than DOM quality.
    Copper-DOM complexes that dissociate very slowly may facilitate Cu leaching through subsurface soil horizons when transport rates exceed dissociation and soil Cu2+ adsorption rates. The dissociation kinetics of different Cu-DOM complexes from soil, wastewater, pig manure and sewage sludge was measured with the Competitive Ligand Exchange Method (CLEM) and Diffusive Gradient in Thin films (DGT) technique. The average dissociation rate constant kd of the complexes, as measured by CLEM, was about 10-3 s-1 and the fractions of dissolved Cu that were undissociated after 8 hours ranged from <1 to 25%. These fractions determined by CLEM were significantly correlated with the non-labile fractions determined in the DGT tests (0–83%) and data analysis show that DGT data can be predicted from CLEM data. The dissociation rates decreased when Cu-DOM complexes had been equilibrated at lower Cu2+ activities and at increasing Cu-DOM contact times (7–297 days). The non-labile fractions were positively correlated with SUVA suggesting that aromatic moieties in DOM hold non-labile Cu. However, inert Cu fractions were too small to confirm their contribution to Cu leaching in soil column experiments.
    In conclusion, it was shown that structural and functional properties of soil DOM can vary up to tenfold among different soils, soil conditions and even within one season at field scale. The weight of evidence suggests that the affinity of Cu to DOM increases with increasing extent of humification of DOM as revealed by the positive correlations between CuMP or [Cu]/[DOC] and SUVA, which, in turn, increases with the humic acid fraction and decreases with the hydrophilic fraction of DOM. At a wide scale of soil and land uses, it appears that DOM quantity is overall more important for Cu mobilization than DOM quality. Kinetically non-labile Cu fractions were less than 25% of Cu-DOM and previous claims that such complexes explain unretarded Cu transport in soil were not confirmed in experiments of this study.
    StatusGepubliceerd - 2010

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