Role of woody biochar and fungal-bacterial co-inoculation on enzyme activity and metal immobilization in serpentine soil

Purpose

In this study, we investigated the effect of biochar (BC) and fungal bacterial co-inoculation (FB) on soil enzymatic activity and immobilization of heavy metals in serpentine soil in Sri Lanka.

Materials and methods

A pot experiment was conducted with tomatoes (Lycopersicon esculentum L.) at 1, 2.5, and 5 % (w/w) BC ratios. Polyphenol oxidase, catalase and dehydrogenase activities were determined by idometric, potassium permanganate oxidisable, and spectrophotometric methods, respectively. Heavy metal concentrations were assessed by 0.01 M CaCl₂ and sequential extraction methods.

Results and discussion

An increase in BC application reduced polyphenol oxidase, dehydrogenase, and catalase activity. The application of FB increased soil dehydrogenase activity, with the maximum activity found in 1 % BC700?+?FB treatment. Moreover, the CaCl₂ extractable metals (Ni, Mn, and Cr) in 5 % BC700 amended soil decreased by 92, 94, and 100 %, respectively, compared to the control. Sequential extraction showed that the exchangeable concentrations of Ni, Mn, and Cr decreased by 55, 70, and 80 % in 5 % BC700, respectively.

Conclusions

Results suggest that the addition of BC to serpentine soil immobilizes heavy metals and decreases soil enzymatic activities. The addition of FB to serpentine soil improves plant growth by mitigating heavy metal toxicity and enhancing soil enzymatic activities.

     

Effect of nickel-resistant rhizosphere bacteria on the uptake of nickel by the hyperaccumulator Noccaea caerulescens under controlled conditions

Purpose

This work was conducted to determine whether Nickel-resistant rhizosphere bacteria (NiRRB) affected the Ni uptake and hyperaccumulation by Noccaea caerulescens.

Material and methods

Ni-resistant bacterial strains (10 mM Ni), Bacillus pumilus and Micrococcus spp., were originally isolated from the rhizosphere of N. caerulescens (Brassicaceae) collected from serpentine Ni-rich soil in the Vosges mountains (France). They served to inoculate axenic seeds of N. caerulescens, which were then sown aseptically into sterile soil Magenta boxes and cultured under controlled conditions.

Results and discussion

Cultures inoculated with NiRRB showed a disappearance of chlorosis signs and a significant increase of the aerial biomass compared to the axenic control (1.5-fold higher). Despite the drastic decrease of the water-extractable Ni in the soil (80 %) at the end of the growth, there was no significant effect on Ni accumulation by N. caerulescens compared to the axenic control. A highly significant promoting effect on plant growth (fourfold higher) and an increase of Ni accumulation (65-fold higher) by N. caerulescens were shown in cultures with native microorganisms (nonsterile control) compared to the axenic control.

Conclusions

The NiRRB strains used in this study reduced Ni bioavailability in the soil and did not improve the Ni uptake by N. caerulescens. Nevertheless, they induced promoting effects on the growth of this plant. They could serve as an effective metal-immobilizing and growth-promoting bioinoculant for plants in Ni-stressed soils. Optimal growth and Ni uptake by N. caerulescens requires the presence of native microbial community.     

Iron oxide mineralogy and stable iron isotope composition in a Gleysol with petrogleyic properties

Purpose  

Properties of Fe oxides are poorly understood in soils with fluctuating water tables and variable redox conditions. The objective of this research was to (a) characterize the mineralogical composition of Fe oxides and (b) determine the relationship to the stable Fe isotope ratio in a soil with temporally and spatially sharp redox gradients.     

Leaching impacts Ni toxicity differently among soils but increases its predictability according to nitrification assay

Purpose  

Deriving toxicity thresholds through bioassays has become the scientific basis for the risk assessment of metal contamination in soils, but few studies have been reported for Ni toxicity in Chinese soils. In addition, although leaching has been recommended to increase the ecological relevance of the output of soil toxicity bioassays, its impacts on Ni threshold and soil solutions in soils remained to be shown. Therefore, in this study, 17 soils were collected throughout China and were assessed for Ni toxicity using substrate-induced nitrification (SIN) assay for both un-leached and leached treatments. Meanwhile, soil solutions of all Ni treatments were extracted and analyzed. This study provides information for the development of a terrestrial biotic ligand model (TBLM) for Ni in Chinese soils.     

Linking Ni and Cr concentrations to soil mineralogy: does it help to assess metal contamination when the natural background is high?

Purpose

In soils from serpentinitic areas the natural background of Ni and Cr is so high that the assessment of contamination by comparing metal concentrations with some fixed thresholds may give unreliable results. We therefore sought a quantitative relation between serpentines and Ni and Cr concentrations in uncontaminated soils, evaluated if the approach may help in establishing a baseline, and discussed if additional anthropogenic inputs of Ni and Cr can be realistically individuated in these areas.

Materials and methods

We analysed the total, acid-extractable and exchangeable concentrations of Ni and the total and acid-extractable concentrations of Cr in 66 soil horizons, belonging to 19 poorly developed and uncontaminated Alpine soils. The soils had different amounts of serpentines, depending on the abundance of these minerals in the parent material. We calculated an index of abundance of serpentines in the clay fraction by XRD and related total metal contents to the mineralogical index. We then tested the regressions on potentially contaminated soils, developed on the alluvial plain of the same watershed.

Results and discussion

We found extremely high total concentrations of Ni (up to 1,887 mg kg^–1) and Cr (up to 2,218 mg kg^–1) in the uncontaminated soils, but only a small proportion was extractable. Total Ni and Cr contents were significantly related to serpentine abundance (r ²?=?0.86 and 0.74, respectively). The regressions indicated that even small amounts of serpentines induced metal contents above 200 mg kg^–1, and the 95% confidence limits were 75 and 111 mg kg^–1 of Ni and Cr, respectively. When the regressions were tested on the potentially contaminated soils, a good estimate was obtained for Cr, while the Ni concentration was overestimated, probably because of some leaching of this element.

Conclusions

The concentrations of Ni and Cr that can be expected in soils because of the presence of small amounts of serpentines are comparable to the amounts accumulated in the soil because of diffuse contamination and potentially contaminated soils had metal concentrations falling in the range expected from the presence of natural sources. Only in the case of very severe contamination events, the identification of anthropogenic sources adding to the natural background would be feasible.     

Speciation and biochemical transformations of sulfur and copper in rice rhizosphere and bulk soil—XANES evidence of sulfur and copper associations

Purpose  

Contamination of heavy metals in soil and its subsequent accumulation along the food chain is a potential risk to human health. Cu speciation in soil–plant system, particularly on the availability to plant roots, has obtained great attention. X-ray absorption near-edge structure spectroscopy (XANES) provides information about the bonding of Cu soil components at the molecular scale. In paddy soils, changes of redox conditions led to microbially mediated sulfur transformation, thus affecting heavy metal behavior. The objective of this work was to investigate how sulfur transformation in a paddy soil affected Cu biogeochemical processes.     

Speciation of heavy metals in garden soils: evidences from selective and sequential chemical leaching

Purpose  

Gardening (especially food growing) in urban areas is becoming popular, but urban soils are often very contaminated for historical reasons. There is lack of sufficient information as to the bioavailability of soil heavy metals to plants and human in urban environments. This study examines the relative leachability of Cr, Ni, As, Cd, Zn, and Pb for soils with varying characteristics. The speciation and mobility of these metals can be qualitatively inferred from the leaching experiments. The goal is to use the data to shed some light on their bioavailability to plant and human, as well as the basis for soil remediation.     

Assessing long term effects of compost fertilization on soil fertility and nitrogen mineralization rate

Background

Fertilization with organic waste compost can close the nutrient cycles between urban and rural environments. However, its effect on yield and soil fertility must be investigated.

Aim

This study investigated the long-term effect of compost on soil nutrient and potentially toxic elements (PTEs) concentration, nutrient budgets, and nitrogen (N) mineralization and efficiency.

Methods

After 21 years of annual compost application (100/400 kg N ha^–1 year^–1 [100BC/400BC]) alone and combined with mineral fertilization, soil was analyzed for pH, organic carbon (SOC), nutrient (total N and P, N_min, extractable CAL-P, CAL-K, and Mg), and PTE (Cu, Ni, Zn) concentrations. Yields were recorded and nutrient/PTE budgets and apparent net mineralization (ANM, only 2019) were calculated.

Results

N efficiency was the highest in maize and for mineral fertilization. Compost application led to lower N efficiencies, but increased ANM, SOC, pH, and soil N, and surpluses of N, P, and all PTEs. Higher PTE concentrations were only found in 400BC for Cu. Nutrient budgets correlated with soil nutrient concentration. A surplus of 16.1 kg P ha^–1 year^–1 and 19.5 kg K ha^–1 year^–1 resulted in 1 mg kg^–1 increase in CAL-P and CAL-K over 21 years.

Conclusion

Compost application supplies nutrients to crops with a minor risk of soil-accumulation of PTEs. However, the nutrient stoichiometry provided by compost does not match crop offtakes causing imbalances. Synchronization of compost N mineralization and plant N demand does not match and limits the yield effect. In winter wheat only 65–70% of N mineralization occurred during the growth period.     

Soil structure amelioration with quicklime and irrigation experiments in earth graves

Purpose

Air supply and soil moisture have significant impact on the decay time necessary for complete decomposition of an interred body. Concerning the general structure and hydraulic as well as pneumatic conditions, in many cases, a permeable refilled soil material surrounded by the undisturbed and less permeable soil outside the grave results in water ponding, less aerated conditions, and lower redox potential values within the grave. This reduces the decomposition speed or even leads to preservation of the entire body.

Materials and methods

In order to ascertain soil structural processes and hydraulic properties in an earth grave within the first year after burial, a monitoring of soil redox and matric potentials was realized in newly refilled artificial (empty) graves. We surveyed four variations: undisturbed reference soil, soil backfill in artificial grave, soil backfill in artificial grave amended with 20 kg CaO m^?3, and grave base and walls strewed with CaO. In the fourth artificial grave (soil backfill only), irrigation experiments were conducted in order to simulate the effects of grave maintenance on soil water budget. Pore size distribution, air conductivity, and saturated hydraulic conductivity were measured on soil core samples from the variations. The monitoring was realized with redox sensors and tensiometers in 50- and 130-cm depth in all four variations.

Results and discussion

Soil structure disruption increased soil porosity but also favored saturation of the soil in context with precipitation events. Compared with the graves without amendment, the addition of quicklime resulted in higher air capacity and air permeability, saturated hydraulic conductivity, and a better-aerated (higher redox potentials) and less water-saturated soil. Non-recurring irrigation with 2.2, 4.4, and 8.9 mm did not affect the soil moisture in the 50- and 130-cm depth. Repeated irrigation with 8.9 mm on consecutive days led to persistent water saturation in the soil, especially in the 130-cm depth.

Conclusions

The disturbed soil structure in the cover layer of an earth grave is sensitive to settlement and, together with a tendency to the development of stagnic conditions, this can have negative impact on soil aeration in the grave. Addition of quicklime to the soil enhances crack development in the base and walls of the grave, stabilizes the soil fragments in the backfill, and prevents intensive settlement processes. This reduces water ponding and leads to a better aeration of the soil. Irrigation of earth graves should be reduced to a minimum.

     

An Improved Procedure for Determining Magnesium Fertilizer Dissolution in Field Soils

Abstract

The sequential extraction procedure currently used to measure magnesium (Mg) fertilizer dissolution in soils consists of removing dissolved Mg (step 1), and partially dissolved Mg (step 2), followed by an 18‐h extraction with 2 M HCl at room temperature to determine undissolved Mg (step 3). This procedure is satisfactory for soluble and moderately soluble Mg fertilizers but is not an accurate procedure for slightly soluble fertilizers, such as serpentine. When step 3 is replaced by a digestion procedure using 2 M HCl for 4 h at 90–95°C (improved step 3), the total serpentine Mg recovery (dissolved and undissolved Mg) from soil samples, either immediately after serpentine was added to soil or after a 21‐day incubation with moist soil, was about 100% compared to 40–50% by the original procedure. The improved procedure also increased the recovery of serpentine Mg applied to field soils. Therefore, this study recommends that the third step of the sequential extraction procedure be replaced by a 4 h digestion using 2 M HCl (90–95°C).     

Temperature-dependent oxide removal from manganese- and iron oxide-coated soil redox bars

Purpose

Soil temperature is a fundamental parameter affecting not only microbial activity but also manganese (Mn^III,IV) and iron (Fe^III) oxide reduction rates. The relationship between Mn^III,IV oxide removal from oxide-coated redox bars is missing at present. This study investigated the effect of variable soil temperatures on oxide removal by Mn^III,IV and Fe^III oxide-coated redox bars in water-saturated soil columns in the laboratory.

Materials and methods

The Mn coatings contained the mineral birnessite, whereas the Fe coatings contained a mixture of ferrihydrite and goethite. Additionally, platinum (Pt) electrodes designed to measure the redox potential (E_H) were installed in the soil columns, which were filled with either a humic topsoil with an organic carbon (C_org) content of 85 g kg^?1 (pH 5.8) or a subsoil containing 2 g C_org kg^?1 (pH 7.5). Experiments were performed at 5, 15, and 25 °C.

Results and discussion

Although elevated soil temperatures accelerated the decrease in E_H after water saturation in the topsoil, no E_H decreases regardless of soil temperature occurred in the subsoil. Besides soil temperature, the importance of soil organic matter as an electron donor is highlighted in this case. Complete removal of the Mn^III,IV oxide coating was observed after 28, 14, and 7 days in the soil columns filled with topsoil at 5, 15, and 25 °C, respectively. Along the Fe redox bars, Fe^III reducing conditions first appeared at 15 °C and oxide removal was enhanced at 25 °C because of lower E_H, with the preferential dissolution of ferrihydrite over goethite as revealed by visual differences in the Fe^III oxide coating. Oxide removal along redox bars followed the thermodynamics of the applied minerals in the order birnessite > ferrihydrite > goethite.

Conclusions

In line with Van’t Hoff’s rule, turnover rates of Mn^III,IV and Fe^III oxide reduction increased as a result of increased soil temperatures. Taking into account the stability lines of the designated minerals, E_H-pH conditions were in accordance with oxide removal. Soil temperature must therefore be considered a master variable when evaluating the oxide removal of redox bars employed for the monitoring of soil redox status.

     

Effects of artificial aeration and iron inputs on the transformation of carbon and phosphorus in a typical wetland soil

Purpose

Artificial aeration changes the redox conditions at the soil surface. The introduction of iron (Fe) into wetlands can influence carbon (C) and phosphorus (P) cycling under the fluctuating redox conditions. However, artificial Fe introduced into wetlands is uncommon, and there are no Fe dose guidelines. We compared aerobic and anaerobic conditions to test the hypothesis that Fe addition can, although redox-dependent, affect P forms and the coupling of organic C.

Materials and methods

Twenty-four intact soil cores were collected randomly from a lacustrine wetland of Lake Xiaoxingkai. And representative and homogeneous seedlings of Glyceria spiculosa were collected. The incubation was designed with two treatment factors: Fe/P ratio (5 or 10) and high and low dissolved oxygen (DO) concentrations (>?6 and <?2 mg L^?1, respectively). Four groups with three replicates were separated randomly and labeled as aerobic + plant treatment, anaerobic + plant treatment, and aerobic or anaerobic treatment (control).

Results and discussion

The DO concentrations were stratified, decreasing with soil depth and increasing with time, especially under aerobic conditions. The Eh values generally increased with fluctuations under aerobic conditions. The artificial aeration substantially changed the redox environment at the water–soil interface. Of the total P, 45% was in the reactive Fe-bound P, indicating that Lake Xiaoxingkai had high internal P loading. No significant differences were observed in total Fe, amorphous Fe, and organic C at the soil surface between the two Fe/P ratios; however, a significant difference in free Fe was observed. And soil amorphous Fe was found to be a significant correlation with soil organic C, indicating that iron oxides were related with the soil chemical properties.

Conclusions

After short-term incubation, Fe addition can affect the cycling of major elements in wetlands, although this effect is redox dependent. Excessive Fe doses may result in regional environmental risks, such as eutrophication and C sinks of wetland ecosystems. Large-scale controlled experiments are needed to fully understand the behaviors of soil elements in wetlands.

     

Effect of biosolid application to Mollisol Chilean soils on the bioavailability of heavy metals (Cu,Cr, Ni,and Zn) as assessed by bioassays with sunflower (Helianthus annuus) and DGT measurements

Purpose

This study assessed the effect of biosolid application on the bioavailable fraction of some trace elements (Cu, Cr, Ni, and Zn) using a bioassay with sunflower (Helianthus annuus) and a chemical assay, diffusion gradient in thin films (DGT).

Materials and methods

Five surface soil samples (0–20 cm) were collected from an agricultural zone in Central Chile where biosolids are likely to be applied. Municipal biosolids were mixed with the soil at concentrations of 0, 30, 90, and 200 Mg ha^?1. The experiment to determine the bioavailability of metals in the soil using the bioassay was performed using sunflower. The DGT technique and Community Bureau of Reference (BCR) sequential extraction were used to determine the bioavailable fractions of the metals.

Results and discussion

The application of biosolids increased the phytoavailability of Zn, Ni, and Cr in most of the soils, as indicated by the increasing concentrations in sunflower plants as the biosolid application rate increased. In two of the soils, Codigua and Pelvín, this increase peaked at an application rate of 90 Mg ha^?1. Decreases in the bioavailable fractions of Zn, Ni, and Cr were observed with higher biosolid application rates. The bioavailability of metals was estimated through multiple linear regression models between the metals in the sunflower plants and the different chemical fractions of metals in the soils treated with different biosolid rates, which displayed a positive contribution of the labile (water soluble, carbonate, and exchangeable), oxide, and organic metal forms in the soil, particularly with respect to Ni and Zn at application rates of 30 and 90 Mg ha^?1. The bioavailable fraction of metals was determined in soils using the DGT technique. The effective concentration (C _E) results were compared with those in sunflower plants. The DGT technique could effectively predict the bioavailable fractions of Cr, Ni, and Zn in the Taqueral soil but only that of Zn in the Polpaico soil.

Conclusions

The application of biosolids significantly increased the labile fraction of most of the metals in the studied soils, particularly at the highest biosolid application rate. C _E increased as the concentration of biosolids increased for most of the metals. The effectiveness of the DGT technique for predicting the bioavailability of metals was dependent on the soil type and the metal. However, the C _E for soil Cu was not related to plant Cu for all soils studied.     

Spatial distribution and source apportionment of the heavy metals in the agricultural soil in a regional scale

Purpose

Understanding the spatial distribution and sources of soil heavy metals (HMs) in a large city helps prevent and control soil pollution. This study aimed to investigate the spatial patterns of soil HMs and identify their main sources in a regional scale.

Materials and methods

A total of 110 topsoil samples were collected from Tai’an City, China. Cd, Cr, Cu, Hg, Ni, Pb, and Zn concentrations in each soil sample were determined. Geostatistics, geographic information system (GIS), and positive matrix factorization (PMF) were used to explore the spatial distribution of seven soil HMs and to reveal the main sources of soil HMs in Tai’an City, respectively.

Results and discussion

Soil Cd, Cr, Pb, and Zn generally showed slight pollution levels in the study area. However, soil Hg and Cu contents reached moderate to heavy pollution levels in some areas. Soil Hg content increased from north to south across the city, and the highest Hg concentration was detected in Ningyang County. Soil Cd, Cu, and Zn distributions exhibited a similar pattern, and their contents increased from west to east; the highest Cd, Cu, and Zn concentrations were found in Xintai County. The highest soil Ni concentration was obtained in the northeast of Feicheng and Xintai counties. PMF analysis revealed the following four potential sources of agricultural soil HMs in Tai’an City: industrial and mining activities, agricultural activities, residential living activities, and business activities. Soil Hg mainly originated from residential living activities, which accounted for 75.3% of the total source. The main sources of soil Ni were residential living activities, agricultural activities, and industrial and mining activities, which account for 38.2, 27.50, and 25.1% of the total source, respectively. Soil Cu was mainly produced by agricultural activities (36.6%), followed by residential living activities (29.8%) and industrial and mining activities (25.8%).

Conclusions

PMF combined with GIS could be effectively applied to determine the main sources of HMs in agricultural soils in a regional scale.

     

Inorganic and organic pollution in agricultural soil from an emerging e-waste recycling town in Taizhou area, China

Purpose

The emerging recycling of electronic and electric waste (e-waste) is causing critical levels of soil pollution in those relatively poor towns surrounding the central cities, which have been involved in recycling activities for quite some time. Agricultural soil is of great importance due to its direct impact on food and human health. The objective of this study was to provide a systematic investigation of the contamination in agricultural soil for a range of inorganic compounds (Cr, Cd, Pb, Zn, Cu, and Ni) and organic compounds (PAHs and PCBs) in town A, an emerging e-waste recycling town in China.

Materials and methods

A total of 20 agricultural soil samples were collected from three sampling locations throughout town A. Levels of inorganic compounds (Cr, Cd, Pb, Zn, Cu, and Ni) and organic compounds (PAHs and PCBs) were determined by AAS, GC/MS, and GC/electron capture detector, respectively. Data was processed with SPSS 13 and Arcview 3.3 GIS software.

Results and discussion

The findings demonstrate that agricultural soil was contaminated to various extents by inorganic and/or organic pollutants. Comparison among the three sampling areas indicated that the soil was highly contaminated in the agricultural area near e-waste recycling workshops. Moreover, the contaminants (Cu, Pb, PAHs, and PCBs) may be connected through a common source as found in the Pearson correlations and cluster analysis.

Conclusions

There exists a heightened sense of awareness concerning the hazardous implications of current emerging e-waste recycling issues in the agricultural soil of those areas close to the central city in Taizhou.     

Influence of pH on the redox chemistry of metal (hydr)oxides and organic matter in paddy soils

Purpose

The primary purpose of this study was to determine how flooding and draining cycles affect the redox chemistry of metal (hydr)oxides and organic matter in paddy soils and how the pH influences these processes. Our secondary purpose was to determine to what extent a geochemical thermodynamic equilibrium model can be used to predict the solubility of Mn and Fe during flooding and draining cycles in paddy soils.

Material and methods

We performed a carefully designed column experiment with two paddy soils with similar soil properties but contrasting pH. We monitored the redox potential (Eh) continuously and took soil solution samples regularly at four depths along the soil profile during two successive flooding and drainage cycles. To determine dominant mineral phases of Mn and Fe under equilibrium conditions, stability diagrams of Mn and Fe were constructed as a function of Eh and pH. Geochemical equilibrium model calculations were performed to identify Mn and Fe solubility-controlling minerals and to compare predicted total dissolved concentrations with their measured values.

Results and discussion

Flooding led to strong Eh gradients in the columns of both soils. In the acidic soil, pH increased with decreasing Eh and vice versa, whereas pH in the alkaline soil was buffered by CaCO₃. In the acidic soil, Mn and Fe solubility increased during flooding due to reductive dissolution of their (hydr)oxides and decreased during drainage because of re-oxidation. In the alkaline soil, Mn and Fe solubility did not increase during flooding due to Mn(II) and Fe(II) precipitation as MnCO₃, FeCO₃, and FeS. The predicted levels of soluble Mn and Fe in the acidic soil were much higher than their measured values, but predictions and measurements were rather similar in the alkaline soil. This difference is likely due to kinetically limited reductive dissolution of Mn and Fe (hydr)oxides in the acidic soil. During flooding, the solubility of dissolved organic matter increased in both soils, probably because of reductive dissolution of Fe (hydr)oxides and the observed increase in pH.

Conclusions

Under alternating flooding and draining conditions, the pH greatly affected Mn and Fe solubility via influencing either reductive dissolution or carbonate formation. Comparison between measurements and geochemical equilibrium model predictions revealed that reductive dissolution of Mn and Fe (hydr)oxides was kinetically limited in the acidic soil. Therefore, when applying such models to systems with changing redox conditions, such rate-limiting reactions should be parameterized and implemented to enable more accurate predictions of Mn and Fe solubility.     

Characterization of soil physico-chemical parameters and limitations for revegetation in serpentine quarry soils (NW Spain)

Purpose

Quarrying activities in areas with serpentinized rocks may have a negative impact on plant growth. Quarry soils generally offer hostile environments for plant growth due to their low-nutrient availability, low organic matter, and high-trace metal content.

Materials and methods

In order to determine the factors that can limit plant revegetation, this study was carried out in two serpentine quarries in Galicia (NW Spain): one abandoned in 1999 and the other still active.

Results and discussion

The results show that in soils developed in the abandoned quarry, the limitations for revegetation were: moderate alkaline pH (7.87–8.05), strong Ca/Mg (<1) imbalance, low N (<0.42 mg kg^?1) and P (<2 mg kg^?1) content, and high total heavy-metal content (Co 76–147 mg kg^?1; Cr 1370–2600 mg kg^?1; and Ni 1340–2040 mg kg^?1). The limitations were much less intense in the soils developed in the substratum in the active quarry, which were incipient soils poorly developed and permanently affected by the quarrying activity.

Conclusions

Restoration work should be geared toward establishing a stable diverse vegetation cover, including serpentinophile species, which would provide the necessary modifications to correct nutritive imbalances and improve soil quality.

     

Heavy metal distribution in different soil aggregate size classes from restored brackish marsh,oil exploitation zone,and tidal mud flat of the Yellow River Delta

Purpose

Heavy metal distribution in soils is affected by soil aggregate fractionation. This study aimed to demons trate the aggregate-associated heavy metal concentrations and fractionations in “sandy,” “normal,” and “mud” soils from the restored brackish tidal marsh, oil exploitation zone, and tidal mudflat of the Yellow River Delta (YRD), China.

Materials and methods

Soil samples were sieved into the aggregates of >2, 0.25–2, 0.053–0.25, and <0.053 mm to determine the concentrations of exchangeable (F1), carbonate-bound (F2), reducible (F3), organic-bound (F4), and residual fraction (F5) of Cd, Cr, Cu, Ni, Pb, and Zn.

Results and discussion

The 0.25–2 mm aggregates presented the highest concentrations but the lowest mass loadings (4.23–12.18 %) for most metal fractions due to low percentages of 0.25–2 mm aggregates (1.85–3.12 %) in soils. Aggregates <0.053 mm took majority mass loadings of metals in sandy and normal soils (62.04–86.95 %). Most soil aggregates had residual Cr, Cu, Ni, Zn, and reducible Cd, Pb dominated in the total Cd, Cr, Cu, Ni, Pb, and Zn concentrations. Sandy soil contained relatively high F4, especially of Cu (F4) in 0.25–2 mm aggregates (10.22 mg kg^?1), which may relate to significantly high organic carbon contents (23.92 g kg^?1, P?<?0.05). Normal soil had the highest total concentrations of metals, especially of Cu, Ni, and Pb, which was attributed to the high F3 and F5 in the <0.053 mm aggregates. Although mud soil showed low total concentrations of heavy metals, the relatively high concentrations of bioavailable Cd and Cu resulted from the relatively high Cd (F2) and Cu (F2) in the >2 mm aggregates indicated contribution of carbonates to soil aggregation and metal adsorption in tidal mud flat.

Conclusions

Soil type and aggregate distribution were important factors controlling heavy metal concentration and fractionation in YRD wetland soil. Compared with mud soil, normal soil contained increased concentrations of F5 and F3 of metals in the 0.053–0.25 mm aggregate, and sandy soil contained increased concentrations of bioavailable and total Cr, Ni, and Zn with great contribution of mass loadings in the <0.053 mm aggregate. The results of this study suggested that oil exploitation and wetland restoration activities may influence the retention characteristics of heavy metals in tidal soils through variation of soil type and aggregate fractions.

     

The transformation and migration of selenium in soil under different Eh conditions

Purpose

Soil selenium (Se) sequestration and transformation, which are strongly controlled by soil redox conditions, are critical for understanding the mobility and bioavailability in the environment. Thus, the effect of redox potential on Se transformation was investigated for exploring the release mechanism of Se in soil.

Materials and methods

Soils were incubated under anoxic condition in four treatments at room temperature over 56 days, and the soil solution pH, Eh, and Fe and Se concentrations were measured at given reaction time. The sequential extraction and X-ray photoelectron spectroscopy (XPS) were used to obtain the species distribution of Se in soil. High-resolution transmission electron microscopy (HR-TEM) was employed to observe morphology characteristic of soil.

Results and discussion

Parts of soil Se can be released into solution, and Se speciation in soil changed during the incubation period. XPS and sequential extraction analyses revealed that the primary speciation of Se in soil was elemental Se, and metallic selenides were formed under aerobic condition. Moreover, XPS and HR-TEM data revealed the crystalline state of iron oxides in soil changed after anoxic incubation, and certain amorphous iron oxides were formed.

Conclusions

Se release is activated by short-term incubation, whereas Se can be transformed into less soluble state after long-term incubation. Organic matter takes extremely an important role in Fe oxide reductive dissolution and Se transformation. This study is useful to understand the environmental behaviors of Se and enhance the application of Se fertilizers effectively and safely in Se deficiency area.

     

Effects of 15-year application of municipal wastewater on microbial biomass,fecal pollution indicators,and heavy metals in a Tunisian calcareous soil

Purpose

Water shortage in most countries of the southern Mediterranean basin has led to the reuse of municipal wastewater for irrigation. Despite numerous advantages for soil fertility and crop productivity, recycling wastewater in the soil also has several ecotoxicological and sanitary problems. To evaluate the chronic soil contamination and the cumulative impact of wastewater, we compared seven plot sites irrigated with treated wastewater 1, 2, 7, 9, 13, and 15 years and one nonirrigated taken as control, and these were sampled for soil analysis.

Materials and methods

Soil samples were analyzed for pH, electrical conductivity (EC), total Kjeldahl nitrogen (TKN), total organic matter, and total concentrations of Cu, Zn, Fe, Ni, Pb, and Cd. Microbial biomass and enteric bacteria (fecal coliforms and fecal streptococci) were determined in all soil samples.

Results and discussion

The soil pH values were not consistently affected. Soil salinity, measured as EC, appeared significantly high and proportional to the duration of wastewater irrigation. Also, concentrations of total Ni, Zn, Cu, Pb, and Cd increased significantly (P?≤?0.05) according to the number of irrigation years but are usually under Tunisian standards. The concentration of heavy metals (Ni, Zn, Cu, Pb, and Cd) showed a significant decrease in the soil profile. The microbial biomass carbon (MB_C) is 1.5 times larger in the soil irrigated for 15 years with treated wastewater as compared to the one taken as control. The growth of microorganisms might be explained by the ready source of easily degradable compounds in the oligotrophic soil environment brought about by wastewater irrigation. Soil bacteriological analysis showed that the number of fecal coliforms (FC) and that of fecal streptococci (FS) were affected appreciably (P?≤?0.05) by the duration of wastewater application (number of years) and by the soil depth (0–20, 20–40, and 40–60 cm).

Conclusions

Treated wastewater irrigation led to changes in physicochemical and microbiological soil properties. The magnitude and specificity of these changes significantly correlated with the duration of such practice. It can be concluded, based on these results, that the proper management of wastewater irrigation and periodic monitoring of soil fertility and quality parameters are required to ensure successful, safe, and long-term reuse of wastewater irrigation.