Characterization of zinc in contaminated soils: complementary insights from isotopic exchange,batch extractions and XAFS spectroscopy

Isotopic exchange (IE) of trace metals is an established method for characterizing metal reactivity in soils, but it is still unclear which metal species are isotopically exchangeable. In this study, we used IE to quantify ‘labile’ zinc (Zn) in 51 contaminated soils that were previously studied by Zn K‐edge X‐ray absorption fine structure (XAFS) spectroscopy and sequential extraction (SE). All soils had been contaminated by runoff water from 17‐ to 74‐year‐old galvanized power‐line towers. They covered a wide range in pH (4.0–7.7), organic carbon (0.9–10.2%), clay (3.8–45.1%) and Zn concentrations (251–30 090 mg kg^?1). Isotopic exchange was also performed on selected Zn minerals used as references for linear combination fitting of XAFS spectra. The isotopically exchangeable fraction (%E) of Zn generally decreased with increasing pH, but small %E values were also observed for acidic soils with a large fraction of Zn in hydroxy‐interlayered minerals (Zn‐HIM). The fraction of Zn identified by XAFS spectroscopy as (tetrahedrally and octahedrally coordinated) ‘sorbed Zn’ agreed reasonably well with the isotopically exchangeable fraction but was in many cases larger than the %E, indicating that some ‘sorbed Zn’ may be isotopically non‐exchangeable, such as Zn sorbed in micropores of Fe oxyhydroxides. Zinc identified by XAFS spectroscopy as Zn precipitates (Zn phyllosilicates, Zn‐layered double hydroxide (Zn‐LDH) or hydrozincite) or as Zn‐HIM was largely isotopically non‐exchangeable (‘non‐labile’). Comparison between IE and extraction results suggested that the isotopically exchangeable Zn was mainly extracted in the first two fractions of the SE. However, non‐labile Zn was also extracted in these first two fractions for some soils, including a hydrozincite‐containing soil. Despite the presence of Zn‐LDH and/or Zn phyllosilicates in almost all soils, the Zn concentrations in solution and labile Zn increased with increasing soil total Zn at a given pH, which contradicts the concept of precipitation control by a single phase. Solution Zn was well predicted from the labile Zn following a sorption model.     

The effect of aging biosolids on the availability of cadmium and zinc in soil

A major concern with the safe re‐use of biosolids on land is the potential for release of metals from organic matter in the biosolids, due to decomposition proceeding as biosolids age. To quantify the effects of biosolid aging on Cd and Zn bioavailability, two sewage sludges (Lagoon sludge and Filtered sludge) and a garden compost were incubated at 25°C and 35°C for 100 days. Changes in availability of Cd and Zn were determined using isotope dilution principles, with the materials being labelled with carrier‐free ¹⁰⁹Cd and ⁶⁵Zn. We determined isotopically exchangeable metal pools (E values) and plant available metal pools (L values) by measuring specific activities of Cd and Zn in soil extracts and in wheat plants, respectively. Changes in carbon content over time were determined using ¹³C‐NMR spectroscopy and chemical extraction methods, and related to changes in availability of metal pools as determined by isotopic procedures. Hot‐water‐extractable carbon content, assumed to represent easily decomposable organic matter, decreased during the 100 days by 80–190 mg kg^?1. The Compost and Lagoon sludge showed no change in L values for Cd or Zn with time, but in the Filtered sludge the L values for Cd and Zn increased significantly, by 43% and 56%, respectively. The isotopically exchangeable pools of Cd and Zn did not change with incubation treatment of the biosolids. These data indicate that the potential for metal release from biosolids as organic matter decomposes depends to a large extent on the biosolid composition.     

Measuring reactive metal in soil: a comparison of multi‐element isotopic dilution and chemical extraction

The isotopically exchangeable metal pool (E‐value) of zinc (Zn), cadmium (Cd) and lead (Pb) were simultaneously measured, using stable isotope dilution, in soils contaminated by Pb/Zn mining activities and varying in properties likely to affect metal reactivity, including pH, organic matter content, metal concentration and land use. E‐values were compared with single and sequential extraction schemes. Results showed a wide range of metal reactivity (approximately 1–100% of total) depending on the extent of contamination and on the prevailing soil conditions. Across the range of soils, the E‐values showed no consistent correspondence to any single chemical extraction procedure (EDTA, DTPA and HNO₃) although there was reasonable agreement with the extractants 0.05 m EDTA and 0.43 m HNO₃ in acidic organic soils. Extraction with 0.005 m DTPA substantially under‐estimated the isotopically exchangeable metal content. E‐values corresponded reasonably well with the exchangeable metal (fraction 1 (F1) of the sequential extraction procedure) in calcareous soils but relatively poorly and inconsistently with F1–F2, F1–F3 or F1–F4 in acidic‐neutral soils. Operational aspects associated with determination of multi‐element E‐values are discussed.     

STUDIES ON SOIL COPPER

A method based on that used by McAuliffe et al. (1948) for phosphorus was developed for determining isotopically exchangeable copper in soils using the radioisotope ⁶⁴Cu. The authors are confident that, with a few exceptions, isotopic equilibrium in soil/solution systems is attained rapidly enough to overcome possible difficulties resulting from the short half-life of this isotope. For the twenty-four soils examined, amounts of isotopically exchangeable copper were found to be between 0.19 and 12-24 μg g^-I and represented between 2 and 21 per cent of the total soil copper. A correlation test and an experiment involving fractionation of labelled soils both demonstrated that the bulk of the isotopically exchangeable copper was located in the organic-bound fraction. Not all copper specifically adsorbed by organic matter was readily exchangeable with ⁶⁴Cu : for one sample of organic material examined only 20 per cent of the adsorbed copper was isotopically exchangeable after 24 hours equilibration. The corresponding figures for clay materials and oxide material were found to be between 75 and 60 per cent.     

Methods for determining labile cadmium and zinc in soil

Isotopically exchangeable cadmium and zinc (‘E values’) were measured on soils historically contaminated by sewage sludge and ones on zinc‐rich mine spoil. The E‐value assay involves determining the distribution of an added metal isotope, e.g. ¹⁰⁹Cd, between the solid and solution phases of a soil suspension. The E values for both metals were found to be robust to changes in the position of the metal solid?solution equilibrium, even though the concentration of dissolved metal varied substantially with electrolyte composition and soil:solution ratio. Concentration of labile metal was also invariant over isotope equilibration times of 2–6 days. The use of a submicron filtration procedure, in addition to centrifuging at 2200 g , proved unnecessary if 0.1 m Ca electrolyte was used to suspend the soils. The proportion of ‘fixed’ metal, in non‐labile forms, apparently increased with increasing pH, although there was considerable variation in both sets of contaminated soil. Zinc and cadmium in the sludged soils were similarly labile. Several possible methods for the measurement of chemically reactive metal were explored for comparison with E values, including single extraction with 1 m CaCl₂ and a ‘pool depletion’ (PD) method. The latter involves comparing solid?solution metal equilibria in two electrolytes with differing degrees of (solution) complex formation, 0.1 m Ca(NO₃)₂ and CaCl₂. Both the single extraction and the PD method gave good estimates of E value for Cd, although the single extraction was more consistent. Neither technique was a useful substitute for determining labile Zn, because of weak chloro‐complexation of Zn^2+. We therefore suggest that 1 m CaCl₂ extraction of Cd alone be used as an alternative to E values to avoid the inconvenience of isotopic dilution procedures.     

Immobilization of soil copper using organic and inorganic amendments

This experiment aimed to immobilize Cu in polluted agricultural soils via the application of agrochemicals to reduce its bioavailability to plants. A greenhouse pot experiment was established using a Cu contaminated vineyard topsoil collected from a farm in Greece. The soil was mixed with inorganic [i.e., zeolite (Z), Al‐oxide (AX), Mn‐oxide (MX), and phosphate rock (PR)] as well as organic amendments [i.e., activated charcoal (AC), commercial peat soil material (CP), and compost from olive oil processing wastes (COW)] with an application rate of 2.5% and cultivated by corn (Zea maize). After plant harvesting, Cu was measured separately in the aboveground biomass and roots, respectively, whereas the soil samples were analyzed for DTPA‐extractable and geochemical fractions of Cu (soluble + exchangeable fraction, sorbed and carbonate fraction, Fe‐/Mn‐oxides fraction, and organic fraction). The immobilizing agents, except MX, reduced the soluble plus exchangeable Cu in the treated soil. The lowest concentrations of the soluble plus exchangeable Cu occurred in the soil amended with AC followed by CP, AX, COW, PR, and Z, respectively. The amendments decreased the uptake of Cu by corn. Concentrations of Cu were between 11 and 38% lower in the above ground biomass and 19 and 48% lower in the roots than the control. The organic amendments were more effective than the inorganic additives. The AC was the most effective organic additive and AX was the most effective inorganic amendment.     

Multi‐element stable isotopic dilution and multi‐surface modelling to assess the speciation and reactivity of cadmium and copper in soil

Chemical extraction, multi‐element stable isotopic dilution (ID) and multi‐surface modelling were used to investigate the lability of cadmium (Cd) and copper (Cu) in nine types of soil with different properties and contaminated or not with Cd and Cu. The chemical extraction and ID analyses both showed that Cd was more labile than Cu in all the soil types studied. From the ID results, 32.8–93.3% of total Cd and 14.7–71.8% of total Cu were isotopically exchangeable after 3 days of equilibration. A single extraction in 0.43 m HNO₃ gave similar results to the 3‐day ID assay for Cu in most of the soils and for Cd in the non‐calcareous soils. However, an eight‐step selective sequential extraction (SSE) procedure gave different results from the ID assay for both metals. Predictions of the multi‐surface model for the amounts of Cd and Cu adsorbed, based on measured metal ion activities in the soil solution and the concentrations of reactive surfaces in the soil, agreed with the ID results. The model predicted that soil organic matter was the predominant sorbent for Cd and Cu in the soils and that manganese oxide was the least important sorbent. The contributions of iron oxides to sorption were predicted to be small except in soil with a high pH and little organic matter. The predicted sorption on different soil components did not match SSE measurements.     

Partitioning of metals (Cd, Co, Cu, Ni, Pb, Zn) in soils: concepts, methodologies, prediction and applications – a review

Prediction of the fate of metals in soil requires knowledge of their solid–liquid partitioning. This paper reviews analytical methods and models for measuring or predicting the solid–liquid partitioning of metals in aerobic soils, and collates experimental data. The partitioning is often expressed with an empirical distribution coefficient or K_d, which gives the ratio of the concentration in the solid phase to that in the solution phase. The K_d value of a metal reflects the net effect of various reactions in the solid and liquid phases and varies by orders of magnitude among soils. The K_d value can be derived from the solid–liquid distribution of added metal or that of the soil‐borne metal. Only part of the solid‐phase metal is rapidly exchangeable with the solution phase. Various methods have been developed to quantify this ‘labile’ phase, and K_d values based on this phase often correlate better with soil properties than K_d values based on total concentration, and are more appropriate to express metal ion buffering in solute transport models. The in situ soil solution is the preferred solution phase for K_d determinations. Alternatively, water or dilute‐salt extracts can be used, but these may underestimate in situ concentrations of dissolved metals because of dilution of metal‐complexing ligands such as dissolved organic matter. Multi‐surface models and empirical models have been proposed to predict metal partitioning from soil properties. Though soil pH is the most important soil property determining the retention of the free metal ion, K_d values based on total dissolved metal in solution may show little pH dependence for metal ions that have strong affinity for dissolved organic matter. The K_d coefficient is used as an equilibrium constant in risk assessment models. However, slow dissociation of metal complexes in solution and slow exchange of metals between labile and non‐labile pools in the solid phase may invalidate this equilibrium assumption.     

Evaluation of the BCR sequential extraction for trace elements in European reference volcanic soils

Trace metal behaviour in volcanic ash soils displays distinctive features related to the soils’ large contents of metal‐binding phases and to the rapid release of trace metals from glasses and weatherable minerals. In this work, the BCR (Community Bureau of Reference) sequential extraction scheme (exchangeable + weak acid soluble, reducible, oxidizable, and non‐extractable metal fractions) was applied to selected COST‐622 European reference volcanic soils to determine partitioning of zinc and copper between various solid‐phase constituents, along with the major elements Al, Fe and Mn. The total extracted Al (ΣAl) was strongly correlated with acid ammonium oxalate extractable Al (Al_o) (ΣAl = 0.985Al_o+ 0.11, R²= 0.98), while the total extracted Fe clearly underestimated the amorphous fraction. Large values for the non‐extractable Al fraction were associated with the presence of gibbsite and phyllosilicates. Although the Zn and Cu contents of the soils were generally large, total amounts extracted (the potentially mobilizable fraction) were small, especially for Zn and for soils with crystalline secondary minerals. The fraction of the total Cu which was potentially mobilizable generally exceeded that of Zn. In the potentially mobilizable Cu the oxidizable fraction was generally dominant. Biocycling appears to play an important role in the surface enrichment of potentially mobilizable Zn and Mn. Although further methodological research seems necessary, the BCR sequential extraction appears to be a valuable tool for studies on metal dynamics in soils with andic properties.     

Radio-labile cadmium and zinc in soils as affected by pH and source of contamination

The determination of radio‐labile metals in soil has gained renewed interest for predicting metal availability. There is little information on to what extent the fraction of labile metal is affected by the soil properties and the source of metal contamination. The radio‐labile content (E value) of Cd and Zn was measured in field‐collected soils with Cd and Zn originating from different sources. The E values were erratic and sometimes even exceeded total metal content when the concentration in the soil extract was less than 8 μg Zn l^?1 or less than 3 μg Cd l^?1. Addition of EDTA (0.1 mm ) to the radio‐labelled soil suspension resulted in larger concentrations of Cd and Zn in solution and smaller E values for these soils. The E values were, however, unaffected by the presence of EDTA (0.1 mm ) in soils with larger concentrations of Cd and Zn in solution. The %E values (E value relative to metal soluble in aqua regia) ranged from 9% to 92% (mean 61%) for Cd and from 3% to 72% (mean 33%) for Zn. No correlation between soil properties and %E was observed for Cd, and the %E of Zn was negatively correlated with soil pH (r = ?0.65). There was a strong negative correlation between pH and %E in soils enriched with metals in soluble form (e.g. metal salts, corrosion of galvanized structures). In soils where Cd or Zn were added in a less soluble form, no such correlation was found, and %E values were generally less than in soils spiked with metal salts, suggesting that the source of the contamination controls mainly the labile fractions of Cd and Zn.     

Labile lead in polluted soils measured by stable isotope dilution

It is well known that lead (Pb) is strongly immobilized in soil by adsorption or precipitation. However, the reversibility of these reactions is poorly documented. In this study, the isotopically exchangeable Pb concentration in soils (E‐value) was measured using a stable isotope (²⁰⁸Pb). Soils were collected at three industrialized sites where historical Pb emissions have resulted in elevated Pb concentrations in the surrounding soil. Lead concentrations ranged from background values, in the control soils collected far from the emission source, to highly elevated concentrations (5460–14440 mg Pb kg^?1). The control soil of each site was amended in the laboratory with Pb(NO₃)₂ to the same total Pb concentrations as the field‐contaminated soils. The %E values (E‐value relative to total Pb content) were greater than 84% in the laboratory‐amended soils, and ranged from 45% to 78% (mean 58%) in the field‐contaminated soils. The relatively large labile fractions of Pb in the field‐contaminated soils show that the majority of Pb is reversibly bound despite the fact that the binding strength is large. The Pb concentrations in soil solution were up to 3500‐fold larger for the laboratory‐amended soils than for field‐contaminated soils at corresponding total Pb concentrations. These differences cannot be explained by differences in labile fractions of Pb but are attributed to the decrease in soil solution pH upon addition of Pb²⁺‐salt.     

Distribution and Availability of Heavy Metals in Raw and Acidulated Phosphate Rock–Amended Soils

Abstract

The effectiveness of the application of raw (PR‐1), and partially acidulated phosphate rock (PR), at 25% (PR‐25) and at 50% (PR‐50), was investigated to reduce extractability and plant uptake of Pb, Cd, Cu, Ni, and Zn in three calciorthids soils.Furthermore, the effects of soil treatments on metal extractability were evaluated by sequential extraction. Similarly, such effects were assessed on the phytoavailability of metals of maize (Zea mays L.) through a pot experiment. Water‐soluble and exchangeable metal fractions (the bioavailable fractions) were influenced distinctively by PR treatments and soil properties. In addition, decrease of soluble and exchangeable metal fractions was compensated by an increase in metal extracted from other fractions. Most bioavailable soil metals correlated significantly with their associated level in plant tissue. Finally, plant metal uptake decreased with PR treatments, suggesting that PR application was likely to be effective in controlling metal immobilization in these soils.     

Distribution of potassium fractions in sweet potato (Ipomoea batatas) garden soils in the Central Highlands of Papua New Guinea and management implications

Previous studies indicated that potassium (K) deficiency is an important soil‐related factor for yield decline of the sweet potato gardens in the Central Highlands of Papua New Guinea, where sweet potato is an important staple food crop. An effort was made to characterize various fractions of K in the diverse soils of this region under sweet potato, to ascertain the probable reasons behind the observed K deficiency and its relationship to decreasing yield trends. Soils from two depths (0–10 cm) and (10–20 cm) in two types of gardens (old and new gardens) were assessed for different fractions of soil potassium in volcanic and non‐volcanic soil groups. Volcanic soils (Hydrandepts and Andaquepts) were significantly lower (P < 0.05) in exchangeable K than the non‐volcanic soils (Dystropepts, Tropoqualfs and Eutropepts). Mean exchangeable K content of the non‐volcanic soils was 95.5 mg/kg, whereas that of volcanic soils was 72.4 mg/kg. Similarly, new gardens had an average exchangeable K content of 94.1 mg/kg, which was significantly greater than 71.6 mg/kg soil of older gardens. Non‐exchangeable K content differed significantly (P < 0.001) between the soil types; mean K content was 85.9 mg/kg for the volcanic soils, whereas in non‐volcanic soils, it was 184.9 mg/kg. Garden types also differed significantly (P < 0.05) with respect to non‐exchangeable K content; new gardens registering higher average values (by almost 20%) than the older gardens. Multiple regression analysis showed that variability in the tuber yield was as a result of variability of water soluble and exchangeable K (up to 22%), non‐exchangeable K (2%), mineral K (4%) and leaf K concentrations (10%). Older gardens, which are in volcanic soil groupings, are more susceptible to the K depletion problem because of continuous sweet potato cultivation, possibly owing to their lower K reserves. Such gardens should be managed either with sufficient fallow periods for regeneration of soil fertility or with suitable application of mineral K fertilizers to enhance productivity.     

Effect of two soil moisture levels and wetting‐drying cycles on manganese release in NaCl‐amended soils

Abstract

Effect of two moisture levels (22.5 and 13.5%, w/w) and wetting‐drying cycles on manganese solubility was studied in NaCl‐amended soil. During 6 d incubation, higher moisture level released 40‐fold more water‐soluble Mn and 60‐fold more NH₄OAc‐exchangeable‐Mn in non‐salinized soil. In NaCl‐treated soil, 50 to over 200% greater soluble and exchangeable Mn was recovered from samples incubated at 22.5% compared to 13.5% water levels. Wetting‐drying cycles significantly (P≤0.05) decreased water‐soluble Mn, which accounted for 50 to 60% increases in the exchange‐able Mn. Since other non‐oxidizing/reducing cations (Ca, Mg, Na, K) also demonstrated similar behavior, it is proposed that in addition to oxidation upon drying and reduction upon wetting, the increases in exchangeable Mn and simultaneous decreases in soluble Mn concentration are due to sorption processes. These results suggest that under field conditions, the insolubility of Mn due to continued wetting‐drying cycles may eventually lead to Mn deficiency in soils low in Mn.     

Role of soil constituents in fixation of soluble Zn, Cu, Ni and Cd added to soils

Slow immobilization of trace metals in soil, termed ‘fixation’, affects their natural attenuation but it is still unclear which reactions occur. Twenty‐eight soils were selected to assess the role of Fe oxides and carbonates on fixation of Cu, Cd, Zn and Ni. Soils included samples from 2 toposequences (Vietnam, Spain) and 13 European topsoils with different soil characteristics (pH 3.4–7.7). Samples were amended with 250 mg Zn kg⁻¹, 100 mg Cu kg⁻¹, 80 mg Ni kg⁻¹ and 2.5 mg Cd kg⁻¹ as metal salts and incubated for 850 days. Fixation was measured as the increase of the fraction of added metals that were not isotopically exchangeable. Fixation increased with time and was, averaged over all the soils, 43% (Cu), 41% (Zn), 41% (Ni) and 28% (Cd) after 850 days. Metal fixation within samples from each toposequence was generally positively related to total Fe oxide concentration (Fe_d) for Zn, Ni and Cd. However, the fixation of Cd, Zn and Ni was mainly explained by pH and not by Fe_d when considering all soils. Fixation of Zn and Cd in soils with pH >7.0 increased with increasing concentrations of carbonates at initial ageing times. Fixed fractions of Zn, Ni and Cd were significantly released when experimentally removing 50% of carbonates by acidification. Fixation of Cu was most poorly related to soil properties. Our data suggest that fixation of Cd, Zn and Ni is related to a pH‐dependent diffusion into oxides and that of Cd and Zn also to diffusion and/or coprecipitation in carbonates. Fixation of Ni at neutral pH may also be related to stabilization of precipitates that form readily in soil.     

Modelling the effects of ageing on Cd,Zn, Ni and Cu solubility in soils using an assemblage model

Ageing reactions can reduce trace metal solubility and can explain natural attenuation of contaminated soils. We modelled ageing reactions in soil with an assemblage model that considers slow reactions in Fe‐oxyhydroxides and reversible sorption on organic matter and clay minerals. Metal adsorption kinetics on Fe‐oxyhydroxides was obtained from data with synthetic oxyhydroxides. Metal solubility and isotopic exchangeability data were obtained from 28 soils amended with Ni, Zn, Cu and Cd metal salts and monitored for 850 days. The assemblage model was constructed in WHAM 6.0 and used soil properties and dissolved organic matter as input data. The model was first validated to predict dissolved metal concentrations, based on the concentration of isotopic exchangeable metals. The model overestimated metal solubility without parameter adjustment by mean factors of 4–7, and successful fits were obtained by increasing the specific surface area of Fe‐oxyhydroxides from measured values of synthetic systems to a value of 600 m² g^?1 recommended by other authors. The effect of ageing on the isotopic exchangeable metal fraction was subsequently modelled starting from the predicted fraction of metals present on Fe‐oxyhydroxides immediately after soil spiking. The observed isotopic exchangeable metal fractions of Ni, Zn and Cd agreed reasonably well with predicted values. The model predicts that ageing reactions are more pronounced at higher pH because metal sorption is increasingly directed to oxyhydroxide surfaces with increasing soil pH. Modelling fixation of Cu requires more information on fixation of that metal in organic matter.     

Exchangeability of phosphate extracted by four chemical methods

Isotopically exchangeable phosphate (P) is a major source of P for plants. In practice, however, plant‐available P is assessed by chemical extractions solubilizing a mixture of P forms the availability of which is ill defined. We undertook an isotopic approach to assess the exchangeability of P extracted by (1) CO₂‐saturated water (P‐CO₂), (2) ammonium acetate EDTA (P‐AAEDTA), and (3) sodium bicarbonate (P‐NaHCO₃) compared to the exchangeability of P extracted by water. Five topsoils with similar P‐fertilization histories but different soil properties were studied. Phosphorus was extracted from soils labeled with carrier‐free ³³P after 1 week of incubation, and the specific activity (SA = ³³P / ³¹P) of the extracts was compared with the SA of P extracted by water to calculate the amount of P isotopically exchangeable that had been solubilized during the extraction. P‐CO₂ extracted between 20 and 100 times less P than P‐AAEDTA and P‐NaHCO₃. The SA of P‐CO₂ was not different from the SA of water‐extractable P, showing that P‐CO₂ solubilized similar forms of P as water and that these forms can be considered as available. The SA of P extracted by the two other methods ranged between 25% and 63% for P‐AAEDTA and 66% and 92% for P‐NaHCO₃ of the SA of water‐extractable P. The fraction of exchangeable P extracted by AAEDTA decreased linearly with increasing soil pH, suggesting that this method dissolves slowly or non‐exchangeable P from calcium phosphates.     

The role of earthworms (<Emphasis Type="Italic">Eisenia fetida</Emphasis>) in influencing bioavailability of heavy metals in soils

The effects of earthworm (Eisenia fetida) activity on soil pH, dissolved organic carbon (DOC), microbial populations, fraction distribution and bioavailability of heavy metals (Zn, Cu, Cr, Cd, Co, Ni, and Pb) in five Chinese soils were investigated using pot experiments. A three-step extraction procedure recommended by the European Community Bureau of Reference (BCR; now Standards, Measurements and Testing Programme of the European Community) was used to fractionate the metals in soils into water soluble, exchangeable and carbonate bound (B1), Fe-oxides and Mn-oxides bound (B2) and organic matter and sulfide bound (B3). After the soils were treated with earthworms, the soil pH, water-soluble metal fraction and DOC increased. A significant correlation was obtained between the increased DOC and the increased metals in the water-soluble fraction. The heavy metals in fraction B1 increased after earthworm treatments, while those in fraction B3 decreased. No significant differences were observed for heavy metals in fraction B2. The microbial populations in soil were enumerated with the dilution plate method using several media in the presence of earthworms. The microbial populations increased due to earthworm activity. The biomass of wheat shoots and roots, and the heavy metal concentrations in wheat roots and shoots, were also increased due to the earthworm activity. The present results demonstrated that earthworm activity increases the mobility and bioavailability of heavy metals in soils.     

Potassium isotherm partitioning based on modified quantity‐intensity relation and potassium buffering characterization of soils of North India

Background : Potassium (K) availability in soil and plant uptake is restrained by the dynamic interactions among the different pools of K. Aims : To understand these interactions, a study was undertaken to assess the quantity–intensity (Q/I) and buffering characteristics of rainfed maize (Zea mays L.) growing soils. Ten contrasting soils were evaluated for K partitioning changes in exchangeable K (ΔEK) and non‐exchangeable K (ΔNEK) pools in the soil‐solution phase and buffering characteristics using a modified version of Q/I approach. Results : The partitioned Q/I isotherms showed strong adsorption with the increase in K concentration ratio (CR^K) and the changes due to ΔEK were higher than changes due to ΔNEK. Total buffering capacity (PBC^K) significantly correlated (r = 0.92, p <0.01) with clay content with a major share contributed by buffering capacity owing to non‐exchangeable K ( ${\mathrm{PBC}}_{\Delta \mathrm{NEK}}^K$) rather than exchangeable K ( ${\mathrm{PBC}}_{\Delta \mathrm{EK}}^K$). The fixation capacity (β) factor, the magnitude of added K converted into a non‐exchangeable pool, ranged from 41 to 63%, whereas release (α) factor, the magnitude of added K converted to the exchangeable pool, ranged from 19 to 36%. Both threshold solution K (CK_r) and threshold exchangeable K (EK_r) values were found to be high in Satran clay loam (S2) and lower in Doon silty clay loam (S3) soils. The equilibrium exchangeable K (EK_o) was found close to minimum exchangeable K (E_min) in Doon silty clay loam (S3) and Babaweyl sandy clay loam (S1) soils and overall E_min constituted about 8.94 to 0.57% of the EK_o. Conclusion : It may be concluded that K Q/I isotherm partitioning provides a valuable insight to assess the dynamic relations. The ratio of α/β (K recharge index) could be used to evaluate the K enrichment capacity of soil to K additions while EK_r and E_min can be potentially useful in the elucidation of exchangeable K as K fertility index especially in soils with poor K fertilizer management.     

Determination of the Bioavailable Fraction of Cu and Zn in Soils Amended with Biosolids as Determined by Diffusive Gradients in Thin Films (DGT), BCR Sequential Extraction, and Ryegrass Plant

This study assessed the effect of biosolids applied at rates, 0, 30, 45, and 60?Mg?ha^?1 on the chemical associations and bioavailability of Cu and Zn in soils from an important agricultural zone of the Metropolitan Region in Central Chile. Three methods were used to determine the bioavailability of Cu and Zn in soils: ryegrass (Lolium perenne) plants, diffusive gradients in thin films (DGT) technique, and Community Bureau of Reference (BCR) sequential extraction. The DGT effective concentration (C _E) and sequential extract acid soluble fraction of the BCR extraction (most labile fraction of the soils, normally associated with bioavailability) were compared with total metal concentration in ryegrass plants as a means to compare the chemical and biological measures of bioavailability. Total Zn was higher in comparison to Cu for all treatments. Concentrations were within the limits set by the Chilean regulations for land-applied biosolids. Metals in the control soil were primarily found in the residual fraction of soils. Biosolids application generally decreased this fraction, with a subsequent increase observed mainly in the acid soluble fraction. The contents of Cu and Zn in ryegrass plants increased with increasing rates of biosolids. Comparison of the Cu and Zn content in ryegrass plants with C _E, showed a good correlation for Zn. However, the C _E for soil Cu was only related to plant Cu for some of the soils studied. Correlation between Zn in ryegrass plants and the labile fraction of Zn as measured by the sequential extraction was excellent, with correlation coefficients >0.9, while for Cu, correlation coefficients were lower.