The influence of vegetation and soil type on the speciation of iron in soil water

Soluble organic matter derived from exotic Pinus species has been shown to form stronger complexes with iron (Fe) than that derived from most native Australian species. It has also been proposed that the establishment of exotic Pinus plantations in coastal southeast Queensland may have enhanced the solubility of Fe in soils by increasing the amount of organically complexed Fe, but this remains inconclusive. In this study we test whether the concentration and speciation of Fe in soil water from Pinus plantations differs significantly from soil water from native vegetation areas. Both Fe redox speciation and the interaction between Fe and dissolved organic matter (DOM) were considered; Fe – DOM interaction was assessed using the Stockholm Humic Model. Iron concentrations (mainly Fe²⁺) were greatest in the soil waters with the greatest DOM content collected from sandy podosols (Podzols), where they are largely controlled by redox potential. Iron concentrations were small in soil waters from clay and iron oxide‐rich soils, in spite of similar redox potentials. This condition is related to stronger sorption on to the reactive clay and iron oxide mineral surfaces in these soils, which reduces the amount of DOM available for electron shuttling and microbial metabolism, restricting reductive dissolution of Fe. Vegetation type had no significant influence on the concentration and speciation of iron in soil waters, although DOM from Pinus sites had greater acidic functional group site densities than DOM from native vegetation sites. This is because Fe is mainly in the ferrous form, even in samples from the relatively well‐drained podosols. However, modelling suggests that Pinus DOM can significantly increase the amount of truly dissolved ferric iron remaining in solution in oxic conditions. Therefore, the input of ferrous iron together with Pinus DOM to surface waters may reduce precipitation of hydrous ferric oxides (ferrihydrite) and increase the flux of dissolved Fe out of the catchment. Such inputs of iron are most probably derived from podosols planted with Pinus.     

Speciation of cadmium,copper, lead,and zinc in contaminated soils

Abstract

To investigate the activity of free cadmium (Cd²⁺), copper (Cu²⁺), lead (Pb²⁺), and zinc (Zn²⁺) ions and analyze their dependence on pH and other soil properties, ten contaminated soils were sampled and analyzed for total contents of Cd, Cu, Pb, and Zn (Cd_T, Cu_T, Pb_T, and Zn_T, respectively), 0.43 MHNO₃‐extractable Cd, Cu, Pb, and Zn (Cd_N, Cu_N, Pb_N, and Zn_N, respectively), pH, dissolved organic matter (DOC), cation exchange capacity (CEC), ammonium oxalate extractable aluminum (Al) and iron (Fe), and dissolved calcium [Ca²⁺]. The activity of free Pb²⁺, Cd²⁺, Cu²⁺, and Zn²⁺ ions in soil solutions was determined using Donnan equilibrium/graphite furnace atomic absorption (DE/GFAA). The solubility of Cd in soils varied from 0.16 to 0.94 μg L^‐1, Cu from 3.43 to 7.42 μg L^‐1, Pb from 1.23 to 5.8 μg L^‐1, and Zn from 24.5 to 34.3 μg L^‐. In saturation soil extracts, the activity of free Cd²⁺ ions constituted 42 to 82% of the dissolved fraction, for Cu²⁺the range was 0.1 to 7.8%, for Pb²⁺ 0.1 to 5.1% and for Zn²⁺2 to 72%. The principal species of Cd, Cu, Pb, and Zn in the soil solution is free metal ions and hydrolyzed ions. Soil pH displayed a pronounced effect on the activity of free Cd²⁺, Cu^2t, Pb²⁺, and Zn²⁺ ions.     

Microscale Zn and Pb distribution patterns in subsurface soil horizons: an indication for metal transport dynamics

In many studies on soil pollution, authors conclude that there is no downward migration of metal elements if no evidence for enrichment can be inferred from profiles of total metal contents. We assessed possible transfer of mobile and less mobile metal pollutants to depth in subsurface horizons of a heavy metal contaminated soil, by a study of specific pedofeatures in thin sections by optical microscopy, and their corresponding Zn and Pb distribution patterns by synchrotron‐based X‐ray microfluorescence. In the B horizon (70 cm depth), Zn accumulation was predominantly associated with clay–iron coatings. Strong correlation was found between Zn and Fe (r = 0.94), Zn and Mn (r = 0.75), Zn and Ti (r = 0.84), and Zn and K (r = 0.88), but significant correlation was absent between Zn and Pb. In the C horizon (100 cm depth), clear Pb accumulation was observed in distinct iron coatings, with large correlation coefficients found between Pb and Fe (r = 0.94–0.75), whereas correlation between Zn and Fe was absent. Detected Zn concentrations were small and attributed to the local natural geochemical background. These results were then compared with data of the composition of gravitational soil water collected in other soils from the same study area. Thus, Zn accumulation in the B horizon was ascribed to interception of dissolved Zn²⁺ by negatively charged constituents of clay–iron coatings. In contrast, Pb accumulation in C horizons was related to precipitation of Pb‐bearing iron colloids leading to neoformed, optically pure iron oxyhydroxide crystals and coatings. We demonstrate very localized accumulation of almost immobile Pb which occurs at greater depth than the more mobile Zn. The common, but strongly localized, occurrence of Pb‐bearing iron coatings in the soil groundmass explained the absence of changes in the total Pb concentrations of the C horizon compared with the concentrations in the B horizon.     

A cathodic stripping voltammetric method for nanomolar concentrations of labile and total iron and zinc in soil solutions

Abstract

Linear sweep cathodic stripping voltammetric methods were modified to measure labile and total concentrations of Fe³⁺ and Zn²⁺ in the nanomolar range in soil solutions. Labile concentrations of Fe (25–220 nmol/L) and Zn (37–208 nmol/L) were measured in 0.5 mL aliquots of filtered (0.4 μm) distilled water extracts (solution:soil ratio= 1) of four agricultural soils. After decomposition of complexed forms of the metals by evaporation of the solutions in HNO₃, total soluble Fe and Zn were measured. Labile Fe comprised approximately 1% of total soluble Fe, while labile Zn comprised 13–43% of total Zn in the four soil solutions. The methods provide a linear range of 5 ‐100 nmol/L and sufficient precision to detect concentrations of labile and total Fe and Zn likely to occur in soil solutions.     

羟基磷灰石对铅锌矿区土壤吸附Zn2+、Cd2+的影响

为探究羟基磷灰石(HAP)对矿区土壤重金属的固化效果,采用吸附试验,研究施加HAP的铅锌矿区土壤对Cd~(2+)、Zn~(2+)的动力学吸附和等温吸附效果。结果表明:土壤对Cd~(2+)、Zn~(2+)的吸附量随Cd~(2+)、Zn~(2+)初始浓度的增加而增加;在酸性条件下,其吸附量随pH上升而上升;准二级动力学方程能很好地描述两者的吸附过程,土壤吸附能力随HAP的添加量增大而增强;在Zn—Cd共存体系中,当初始浓度为20mg/L时,土壤对Zn~(2+)、Cd~(2+)的吸附无明显差异,2种金属离子竞争力度小,随着初始浓度上升,竞争明显,对Zn~(2+)的最大吸附量能达到单一体系中的79%～87%,而Cd~(2+)的最大吸附量只有单一体系中的57%～72%,Zn~(2+)的竞争力优于Cd~(2+),Zn~(2+)对Cd~(2+)吸附产生严重的抑制。综上可知,HAP能提高矿区土壤的吸附性能,在Zn、Cd污染土壤中,更能提升土壤对Zn~(2+)的吸附固持能力。     

Increased zinc supply does not inhibit cadmium accumulation by rice (Oryza sativa L.)

Rice (Oryza sativa L.) grown on cadmium (Cd)-contaminated soils has caused health problems in Asian subsistence rice farmers. For other crops, normal co-contaminant zinc (Zn) inhibits the increased uptake of Cd. We used a multi-chelator-buffered nutrient solution to characterize the interaction of Zn and Cd in uptake-translocation of Cd in “Lemont” rice. The activity of free Zn²⁺ varied from 10^?7.6 to 10^?5.2 M, while free Cd²⁺ held constant at 10^?10.7 M. Zinc activity 10^?5.6 M and higher was phytotoxic to rice, resulting in severe chlorosis, reduced growth, and increased Cd transport to shoots. In contrast to previous studies with wheat, lettuce, and spinach, free Zn²⁺ maintained at adequate to sub-phytotoxic levels (10^?7.6 to 10^?6.1) did not inhibit Cd uptake by rice. The inability of Zn to inhibit Cd uptake by rice is a key factor in Cd risk from zinc-lead mine waste contaminated soil compared with other crops.     

Chemical changes in the rhizosphere of metal hyperaccumulator and excluder Thlaspi species

Rhizosphere processes involved in hyperaccumulation and exclusion of metals are still largely unknown. Therefore, we conducted a rhizobag experiment on contaminated and non‐contaminated soils to investigate the chemical changes in the rhizosphere of the hyperaccumulators Thlaspi goesingense and T. caerulescens, and the metal‐excluder T. arvense. Root growth was restricted to the rhizobags separated by a nylon membrane (7 μm \x 25 μm mesh size) from surrounding bulk soil. Depletion of labile Zn in rhizosphere could not explain the amount of metals accumulated in T. caerulescens, whereas the difference in EDTA‐extractable Zn even exceeded total plant uptake. Substantially increased DOC in T. arvense rhizosphere indicates alleviation of phytotoxicity by formation of metal‐organic complexes. Hyperaccumulation and depletion of labile Zn in the rhizosphere was observed for T. goesingense grown on contaminated soil. On non‐contaminated soil, Zn was accumulated but labile Zn in rhizosphere was not changed. Nickel present in background concentrations in both soils was accumulated by T. goesingense only when grown on non‐contaminated soil. In contrast, labile Ni in the rhizosphere was increased in both soils, suggesting a general tendency of Ni mobilization by T. goesingense.     

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.     

Diffusive flux of cationic micronutrients in two Oxisols as affected by low‐molecular‐weight organic acids and cover‐crop residue

Low‐molecular‐weight organic acids with one or more carboxylic groups are ubiquitous. In soils, they can originate from leaching of plants, litter decomposition, plant‐roots exudation, and microbial activity. Their presence in the soil may favor the formation of soluble organo‐metallic complexes that improve the transport of Zn, Cu, Fe, and Mn to plant‐root surfaces via diffusion. The current study sought to determine if some of the organic acids (OA) in soils and a cover‐crop residue influence the diffusive flux (DF) of Zn, Cu, Mn, and Fe. Two OA were added to two Oxisols (Typic Haplustox): a clayey Dark Red Latosol (DRL) and a sandy‐loam Red Yellow Latosol (RYL). Acetic and citric acid were added to achieve concentrations of 0, 250, 500, 1000, and 2000 mmol (m³ soil)^–1. The effect of adding plant material (pearl millet) on the soil DF of the cationic micronutrients was also determined. Soil diffusive flux was evaluated by incubating positively charged and negatively charged exchange‐resin membranes with the soil in PVC diffusion chambers for 15 d. Desorption of Zn, Cu, Fe, Mn, and OA from the resins was performed with 0.5 mol l^–1 HCl. The results demonstrated that the DF of the cationic micronutrients increased with the addition of organic acid. The DF of Zn and Mn occurred mostly towards the cationic resin, whereas the diffusive flux of Cu and Fe occurred mostly towards the anionic resin. Apparently, the dissolution of oxides and/or complexation of micronutrients adsorbed to the solid phase or in the soil solution contributed to the obtained results. Citric acid was more efficient than acetic acid in maintaining a larger DF value for Zn, Cu, and Fe. The addition of millet plant material to the soil increased the DF in the following order: Mn > Cu > Fe > Zn; Mn moved towards the cationic resin, and the other micronutrients moved towards the anionic resin. These findings suggest that organic compounds play an important role in the short‐distance transport of cationic micronutrients in highly weathered soils.     

Phosphorus-induced zinc deficiency in wheat pot-grown on noncalcareous and calcareous soils of different properties

High levels of phosphorus (P) often induce zinc (Zn) deficiency in plants grown on Zn-poor soils. We investigated P-induced Zn deficiency in durum wheat (Triticum durum L. ‘Carpio’) grown on 16 noncalcareous and 31 calcareous soils differing in levels of available (Olsen) P and available (diethylenetriaminepentaacetic acid (DTPA)-extractable) Zn using micropots. A completely randomized factorial design with two levels of P (0 and 40 mg P kg^?1 soil) and Zn (0 and 3 mg Zn kg^?1 soil), i.e. four treatments (‘control’, + P, + Zn, and + PZn), were used. Grain yield of control plants depended mainly on the Olsen P level. Phosphorus had a negative effect on yield in 6 soils with Olsen P/Zn_DTPA > 25, and Zn a positive one in 5 soils with Olsen P/Zn_DTPA > 50; and the + PZn treatment generally resulted in the highest yield. Grain Zn concentration of control plants was negatively correlated with growth and Olsen P. Calcareous soils were less sensitive to P-induced Zn deficiency than noncalcareous soils because phosphate is sorbed by calcite rather than being co-adsorbed with Zn on the Fe oxides. Co-application of P and Zn to soil at low and application of Zn at high Olsen P ensured both maximum yield and grain Zn bioavailability.     

Dissolved organic matter leaching in some contrasting New Zealand pasture soils

Leaching of dissolved organic matter (DOM) from pastoral soils is increasingly seen as an important but poorly understood process. This paper examined the relationship between soil chemical properties, microbial activity and the losses of dissolved organic carbon (DOC) and nitrogen (DON) through leaching from six pasture soils. These soils differed in carbon (C) (4.6–14.9%) and nitrogen (N) (0.4–1.4%) contents and in the amount of organic C and N that had accumulated or been lost in the preceding 20+ years (i.e. −5131 to +1624 kg C ha⁻¹ year⁻¹ and −263 to +220 kg N ha⁻¹ year⁻¹, respectively). The paper also examined whether between‐soil‐type differences in DOC and DON leaching was a major explanatory factor in the observed range of soil organic matter (SOM) changes in these soils. Between 280 and 1690 kg C ha⁻¹ year⁻¹ and 28–117 kg N ha⁻¹ year⁻¹ leached as DOC and DON, respectively, from the six soils in a lysimeter study, with losses being greater from two poorly drained gley soils. Losses of C and N of this magnitude, while at the upper end relative to published data, could not fully explain the losses at Rawerawe, Bruntwood and Lepperton sites reported by Schipper et al. (2007) . The study highlights the leaching of DOM as a significant pathway of loss of C and N in pasture soils that is often ignored or given little attention in predictive models and nutrient budgeting. Leaching losses of DOC and DON alone, or in combination with slightly increased respiration losses of SOM given a 0.2°C increase in the mean annual soil temperature, do not fully explain long‐term changes in the SOM observed at these sites. When soils examined in the present study were separated on the basis of drainage class, the losses of DOC by leaching were correlated with both total and hot‐water extractable C (HWC), the latter being a measure of the labile SOM fraction. Basal microbial CO₂ respiration rates, which varied between 1 and 3.5 µg CO₂‐C g⁻¹ soil hour⁻¹ in surface soils (0–75‐mm depth), was also linked to HWC and the quantities of C lost as DOC. Adoption of the HWC method as an approach that could be used as a proxy for the direct measurement of the soil organic C lost by leaching as DOC or respired needs to be examined further with a greater number of soils. In comparison, a poor relationship was found between the hot‐water extractable N (HWN) and loss of DON by leaching, despite HWN previously being shown to be a measure of the mineralizable pool of N in soils, possibly reflecting the greater competition for N than C in these soils.     

Zinc exposure has differential effects on uptake and metabolism of sulfur and nitrogen in Chinese cabbage

Zinc (Zn) is a plant nutrient; however, at elevated levels it rapidly becomes phytotoxic. In order to obtain insight into the physiological background of its toxicity, the impact of elevated Zn²⁺ concentrations (1 to 10 μM) in the root environment on physiological functioning of Chinese cabbage was studied. Exposure of Chinese cabbage (Brassica pekinensis) to elevated Zn²⁺ concentrations (≥ 5 μM) in the root environment resulted in leaf chlorosis and decreased biomass production. The Zn concentrations of the root and shoot increased with the Zn²⁺ concentration up to 68‐fold and 14‐fold, respectively, at 10 μM compared to the control. The concentrations of the other mineral nutrients of the shoot were hardly affected by elevated Zn²⁺ exposure, although in the root both the Cu and Fe concentrations were increased at ≥ 5 µM, whereas the Mn concentration was decreased and the Ca concentration strongly decreased at 10 µM Zn²⁺. The uptake and metabolism of sulfur and nitrogen were differentially affected at ≥ 5 µM Zn²⁺. Zn²⁺ exposure resulted in an increase of sulfate uptake and the activity of the sulfate transporters in the root, and in enhanced total sulfur concentration of the shoot, which could be ascribed partially to an accumulation of sulfate. Moreover, Zn²⁺ exposure resulted in an up to 6.5‐fold increase in water‐soluble non‐protein thiol (and cysteine) concentration of the root. However, nitrate uptake by the root and the nitrate and total nitrogen concentrations of the shoot were decreased upon Zn²⁺ exposure, demonstrating the absence of a mutual regulation of the uptake and metabolism of sulfur and nitrogen at toxic Zn levels. Evidently, elevated Zn²⁺ concentrations in the root environment did not only disturb the uptake, distribution and assimilation of sulfate, it also affected the uptake and metabolism of nitrate in Chinese cabbage.     

Zinc,copper, and nickel availabilities as determined by soil solution and DTPA extraction of a sludge‐amended soil

Abstract

Extracting sludge‐amended soil with DTPA does not always give a reliable measure of plant‐available heavy metals. The major purpose of this greenhouse pot study was to help explain why. Two anaerobically digested sludges from sewages treated with either Ca(OH)₂or FeCl₃were applied to 3‐kg samples of a Mollic Albaqualf previously limed with Ca(OH)₂rates of 0, 2.5, and 10g/pot that resulted in pHs in the check pots of 5.4, 6.2, or 7.7 after the first harvest. Sludge rates provided 0, 200, 40, 800, and 1600 mg Zn kg^‐1of soil. Two consecutive crops of soybeans (Glycine MaxL.) were grown for 42 d each in the greenhouse. DTPA‐extractable, soil‐solution, and plant concentrations of Cu²⁺, Ni²⁺, and Zn²⁺were measured.

Dry matter yields were depressed due to salt toxicity, while DTPA‐extracted Cu²⁺correlated with plant uptake of Cu²⁺for both sludges. DTPA‐extracted Ni²⁺also correlated with plant Ni²⁺from the Ca(OH)₂‐sludge‐amended soil, although DTPA‐extracted Ni²⁺did not correlate with plant uptake of Ni²⁺from the FeCl₃‐sludge‐amended soil, DTPA‐extracted Zn did not correlate with plant uptake of Zn²⁺from any sludge‐amended soil. Soil‐solution composition correlated with plant uptake of Cu²⁺and Ni²⁺in both sludges; it also correlated with plant uptake of Zn²⁺from FeCl₃‐sludge‐amended soil but not from Ca(OH)₂‐sludge‐amended soil. DTPA extraction probably failed with Ni²⁺and Zn²⁺because of (i) its ineffectiveness at low pH, (ii) the inability of DTPA to buffer each soil extract near pH 7.3, and (iii) increased amounts of soluble chelated micronutrients at higher sludge rates and higher soil pHs. Soil‐solution composition seemed to fail only where micronutrient cations in solution probably were present largely as organic chelates     

Effect of the physicochemical parameters of soils on the biological availability of natural and radioactive zinc

The relationship between the main physicochemical properties of soils and the accumulation of natural Zn and ⁶⁵Zn radionuclide has been studied, and the capacity of soils to limit the mobility of the element in the soil–plant system has been assessed. The contribution of each of the selected soil state parameters to the accumulation of zinc by barley has been determined, and the soil state parameters have been ranked. It has been found that the largest contributions to the variation of the resulting parameter (⁶⁵Zn accumulation coefficient, K_a) are made by mobile Fe (25%), free carbonates (21%), and acid-soluble Zn (18%). The largest contributions to the Zn_acK_a are made by free carbonates (13%) and mobile Fe (8%). The contributions of physical clay and organic carbon in soils and qualitative composition of humic substances are almost similar (4% for each). No differences in the inactivating capacity of different soils (soddy-podzolic soils, gray forest soils, and chernozems) for ⁶⁵Zn are observed. This is related to the fact that the transfer of ⁶⁵Zn to plants is statistically significantly controlled by the contents of free carbonates, mobile iron, and potentially plantavailable forms of stable natural Zn (carrier of ⁶⁵Zn) rather than the quantitative and qualitative composition of organic matter and the degree of dispersion of mineral particles. The analysis of the Zn_acK_a/⁶⁵Zn K_a ratios has shown that the share of plant-available Zn in the acid-soluble form of the metal (1 M HCl) is 0.61 on the average for the studied soils, and its share in the total Zn content in the soils is only 0.14.     

Tracking litter-derived dissolved organic matter along a soil chronosequence using 14C imaging: Biodegradation,physico-chemical retention or preferential flow?

The cycling of dissolved organic matter (DOM) in soils is controversial. While DOM is believed to be a C source for soil microorganisms, DOM sorption to the mineral phase is regarded as a key stabilization mechanism of soil organic matter (SOM). In this study, we added ¹⁴C-labelled DOM derived from Leucanthemopsis alpina to undisturbed soil columns of a chronosequence ranging from initial unweathered soils of a glacier forefield to alpine soils with thick organic layers. We traced the ¹⁴C label in mineralized and leached DOM and quantified the spatial distribution of DO¹⁴C retained in soils using a new autoradiographic technique. Leaching of DO¹⁴C through the 10 cm-long soil columns amounted up to 28% of the added DO¹⁴C in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-DO¹⁴C as only 2–9% were mineralized, with the highest rates in mature soils. In line with the mass balance of ¹⁴C fluxes, measured ¹⁴C activities in soils indicated that the major part of litter DO¹⁴C was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all DO¹⁴C in the upper 3 cm of the soil columns. In the deeper soil, the ¹⁴C label was concentrated along soil pores and textural discontinuities with similarly high ¹⁴C activities than in the uppermost soil. These findings indicate DOM transport via preferential flow, although this was quantitatively less important than DOM retention in soils. The leaching of DO¹⁴C correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of DO¹⁴C immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization.     

Association of soil properties and soil and plant iron to iron deficiency response in rice (Oryza Sativa L.)

Abstract

Problems are invariably encountered when attempts are made to explain the variability in Bray percent yields or plant response in terms of soil or plant iron (Fe). To resolve this inconsistency, the present investigation was initiated to identify a combination of soil extractable Fe, soil properties and form of plant Fe that may be used as a measure of Fe deficiency. The study involved 16 diverse soils, using upland rice (Oryza sativa L.) as the test crop and Fe‐EDDHA [ferric ethylenediamine di (o‐hydroxyl‐phenyl acetic acid)] as source of Fe. The results showed that Bray percent yields were neither related to DTPA (diethylenetriamine pentaacetic acid) or EDTA (ethylenediamine tetraacetic acid) extractable Fe nor with total plant Fe. Even the inclusion of pH, lime, organic carbon and clay data in the regression equations was of no value. However, Bray percent yields were significantly and positively (r = 0.57* ) associated with ferrous Fe (Fe²⁺) in 40‐day‐old rice plants. The explanation concerning variability in Bray percent yields obtained on diverse soils could be increased about one and half 2 times (R²= 0.59*) if the contribution of lime and soil pH was also incorporated in the stepwise regression analysis. The individual contribution to R of lime, pi respectively. Thus, it appears that Fe²⁺ concentration in plants (along with soil pH) may identify Fe deficiency. The critical limit to separate Fe deficient from green rice plants was set at 45 ug Fe²⁺/g in the leaves.     

Extraction of water‐soluble organic matter from mineral horizons of forest soils

Dissolved organic matter (DOM) is involved in many important biogeochemical processes in soil. As its collection is laborious, very often water‐soluble organic matter (WSOM) obtained by extracting organic or mineral soil horizons with a dilute salt solution has been used as a substitute of DOM. We extracted WSOM (measured as water‐soluble organic C, WSOC) from seven mineral horizons of three forest soils from North‐Rhine Westphalia, Germany, with demineralized H₂O, 0.01 M CaCl₂, and 0.5 M K₂SO₄. We investigated the quantitative and qualitative effects of the extractants on WSOM and compared it with DOM collected with ceramic suction cups from the same horizons. The amounts of WSOC extracted differed significantly between both the extractants and the horizons. With two exceptions, K₂SO₄ extracted the largest amounts of WSOC (up to 126 mg C kg^–1) followed by H₂O followed by CaCl₂. The H₂O extracts revealed by far the highest molar UV absorptivities at 254 nm (up to 5834 L mol^–1 cm^–1) compared to the salt solutions which is attributed to solubilization of highly aromatic compounds. The amounts of WSOC extracted did not depend on the amounts of Fe and Al oxides as well as on soil organic C and pH. Water‐soluble organic matter extracted by K₂SO₄ bore the largest similarity to DOM due to relatively analogue molar absorptivities. Therefore, we recommend to use this extractant when trying to obtain a substitute for DOM, but as WSOM extraction is a rate‐limited process, the suitability of extraction procedures to obtain a surrogate of DOM remains ambiguous.     

Soil solution concentration of Cd and Zn canbe predicted with a CaCl2 soil extract

Risk assessment of heavy metals in soil requires an estimate of the concentrations in the soil solution. In spite of the numerous studies on the distribution of Cd and Zn in soil, few measurements of the distribution coefficient in situ, K_d, have been reported. We determined the K_d of soils contaminated with Cd and Zn by measuring metal concentrations in the soil and in the soil solution and attempted to predict them from other soil variables by regression. Soil pH explained most of the variation in logK_d (R² = 0.55 for Cd and 0.70 for Zn). Introducing organic carbon content or cation exchange capacity (CEC) as second explanatory variable improved the prediction (R² = 0.67 for Cd and 0.72 for Zn), but these regression models, however, left more than a factor of 10 of uncertainty in the predicted K_d. This large degree of uncertainty may partly be due to the variable degree of metal fixation in contaminated soils. The labile metal content was measured by isotopic dilution (E value). The E value ranged from 18 to 92% of the total metal content for Cd and from 5 to 68% for Zn. The prediction of K_d improved when metals in solution were assumed to be in equilibrium with the labile metal pool instead of the total metal pool. It seems necessary therefore to discriminate between ‘labile’ and ‘fixed’ pools to predict K_d for Cd and Zn in field contaminated soils accurately. Dilute salt extracts (e.g. 0.01 m CaCl₂) can mimic soil solution and are unlikely to extract metals from the fixed pool. Concentrations of Cd and Zn in the soil solution were predicted from the concentrations of Cd and Zn in a 0.01 m CaCl₂ extract. These predictions were better correlated with the observations for field contaminated soils than the predictions based on the regression equations relating logK_d to soil properties (pH, CEC and organic C).     

Microbial biomass and activity in urban soils contaminated with Zn and Pb

Summary The effects of heavy metals on microbial biomass and activity were investigated in 30 urban soils, contaminated mainly with Zn and Pb to different extents, in terms of the physicochemical and biological characteristics of the soils. Evaluated by simple and multiple regression analyses, the microbial biomass was not affected significantly by easily soluble Zn + Pb (extractable with 0.1 NHCI). The biomass was accounted for as a function of cation exchange capacity (CEC), total organic C and the numbers of fungal colonies present (R ² = 0.692). Carbon dioxide evolution from soils, which reflected microbial activity, was studied on soils incubated with microbial-promoting substrates (glucose and ammonium sulfate) or without. Carbon dioxide evolution was negatively related to Zn+Pb, and this inhibitory effect of the metals was greater in the soils incubated with substrates. Carbon dioxide evolution in soils with substrates was closely related to Zn+Pb, bacterial numbers and the numbers of fungal colonies (R ² = 0.718). Carbon dioxide evolution in soils without substrates was accounted for as a function of Zn + Pb, biomass and the C/N ratio (R ² = 0.511). Using these relationships, the effects of heavy metals on soil microorganisms are discussed in terms of metabolically activated and dormant populations.