Transport and re-entrainment of soil colloids in saturated packed column: effects of pH and ionic strength

Purpose

Colloid migration in subsurface environments has attracted considerable attention in recent years because of its suspected role in facilitating transport of strongly adsorbed contaminants to groundwater. The influence of bulk solution pH or ionic strength on model colloid (i.e., latex microsphere, amorphous silica colloids) transport is well established, while little attention has been paid to water-dispersible soil colloids. In this study, saturated packed columns were conducted to explore the mechanism of transport and fate of water-dispersible soil colloids and facilitating transport of Cu during transients in solution chemistry.

Materials and methods

Water-dispersible soil colloids were fractionated from a Cu-contaminated soil sample. Transport of soil colloidal suspensions was conducted with varying pH and ionic strengths, and then, re-entrainment of those retained colloids after completion the transport experiments was conducted by changing pore water solution transient ionic strength and pH conditions. Meanwhile, transport and fate of the Cu strongly adsorbed on the soil colloids were determined under different ionic strength conditions.

Results and discussion

The transport behavior of soil colloids in porous media was found to depend on the pH and ionic strengths of bulk solution. An increase in solution ionic strength and decrease in solution pH resulted in greater deposition which was revealed by the collision efficiency (??). It increased from 0.15 to 1.0 when solution composition changed from 0 to 50?mM NaNO₃ and decreased dramatically from 1.0 to 0.035 as the solution pH converted from 2.97 to 8.94. The results were in agreement with Derjaguin?CLandau?CVerwey?COverbeek theory. Upon stepwise reduction in ionic strength of eluting fluid or enhancement in its pH, a sharp release of colloids retained in the column occurred in each step. Meanwhile, the value of FRE _NaOH that reveals the effect of NaOH solution at pH?11 on the mobilization of retained colloids deposited in the primary minimum increased from 38.6% to 64.6% when the ionic strength of bulk solution changed from 0 to 50?mM NaNO₃ and decreased from 86.7% to 35.8% as the solution pH from 2.97 to 8.94. In addition, the transport and fate of the Cu strongly adsorbed on soil colloids were highly consistent with the results of soil colloids.

Conclusions

The colloid collision efficiency (??) decreased as the pH of bulk solution increased and increased as the ionic strength of bulk solution increased in saturated columns packed with pure quartz sand, and NaOH solution at pH?11 poses a predominant role on mobilization of the retained colloids deposited in the primary minimum. Meanwhile, the strongly adsorbed Cu on soil colloids almost cannot be detached from its carrier under the competition of coexisted cations in the bulk solution and cotransport with its carrier under different ionic strengths.     

Effect of the size of variable charge soil particles on cadmium accumulation and adsorption

Purpose

The size of soil particles strongly affects the accumulation and adsorption of heavy metals which partly controls the co-transport of heavy metals by soil colloids. However, the effect of the size of soil particles on the accumulation and adsorption of heavy metals in the colloidal dimension has seldom been studied. In this study, variable charge soils were selected and separated into five size fractions to elucidate the effect of the size of soil particles on Cd accumulation and adsorption.

Materials and methods

Five soil particle size fractions (>10, 10–1, 1–0.45, 0.45–0.2 and <0.2 μm) were obtained from Cd-contaminated soil by natural sedimentation and fractional centrifugation. The concentrations and species of Cd were measured in various sized soil particles. Batch adsorption experiments of Cd on the obtained soil particles were conducted under different pH values and concentrations of NaCl.

Results and discussion

Generally, the concentration of Cd increased with decreasing soil particle sizes, and the Cd proportion of exchangeable and carbonate fraction decreased from 43.84 to 17.75% with decreasing particle size. The soil particles with a size of 10–1 and <0.2 μm possessed a stronger adsorption ability than the other fractions in most cases. Moreover, the Cd adsorption capacities of the soil particles increased with increasing pH values and decreasing concentrations of NaCl, especially for soil particles containing more organic matter (OM) and variable charge minerals.

Conclusions

Smaller soil particles are more capable of accumulating Cd and make Cd more stable. The adsorption capability of Cd is negatively related to the particle size and NaCl concentration and is positively related to the pH. The effects of the size of variable charge soil particles on Cd accumulation and adsorption are attributed to the differences in the physicochemical properties among various soil particle size fractions. This study contributes to the understanding of the co-transport of heavy metals in soil by soil colloids.

     

病原菌在红壤胶体上的吸附机制研究

研究了pH和KCl离子强度对猪链球菌和大肠杆菌在红壤胶体表面吸附的影响,结合表面物化性质和Derjaguin-Landau-Verwey-Overbeek(DLVO)理论分析互作机制。结果表明,细菌在红壤胶体表面的吸附等温线能较好拟合Freundlich方程(R20.97),猪链球菌在红壤胶体表面吸附的分配系数(Kf)是大肠杆菌的4.5倍～6.1倍,细菌在去有机质胶体表面吸附的Kf值为含有机质胶体的2.4倍～3.2倍。比表面积越大或zeta电位绝对值越小,细菌吸附能力越强,吸附态细菌位于距红壤胶体表面90～100 nm处的次级小能位置。随着体系pH降低(9.0～4.0)或离子强度增大(1～10 mmol L-1),细菌与红壤胶体互作能障降低,细菌吸附量增大,吸附机制符合DLVO理论;而在高离子强度下(50～100 mmol L-1),猪链球菌吸附量降低了3.4%～5.6%,表明除DLVO作用力外,非DLVO作用力如空间位阻排斥和疏水作用对吸附也有贡献。     

Enhanced bioremediation of PAH-contaminated soil by immobilized bacteria with plant residue and biochar as carriers

Purpose

Polycyclic aromatic hydrocarbons (PAHs) are largely accumulated in soils in China. The immobilized-microorganism technique (IMT) is a potential approach for abating soil contamination with PAHs. However, few studies about the application of IMT to contaminated soil remediation were reported. Due to recalcitrance to decomposition, biochar application to soil may enhance soil carbon sequestration, but few studies on the application of biochars to remediation of contaminated soil were reported. In this study, we illustrated enhanced bioremediation of soil having a long history of PAH contamination by IMT using plant residues and biochars as carriers.

Materials and methods

Two PAH-degrading bacteria, Pseudomonas putida and an unidentified indigenous bacterium, were selected for IMT. The extractability and biodegradation of 15 PAHs in solution and an actual PAH-contaminated soil amended with immobilized-bacteria materials were investigated under different incubation periods. The effects of carriers and the molecular weight of PAHs on bioremediation efficiency were determined to illustrate their different bio-dissipation mechanisms of PAHs in soil.

Results and discussion

The IMT can considerably enhance the removal of PAHs. Carriers impose different effects on PAH bio-dissipation by amended soil with immobilized-bacteria, which can directly degrade the carrier-associated PAHs. The removal of PAHs from soil depended on PAH molecular weight and carrier types. Enhanced bio-dissipation by IMT was much stronger for 4- and 5-ring PAHs than for 3- and 6-ring ones in soil. Only P400 biochar-immobilized bacteria enhanced bio-dissipation of all PAHs in contaminated soil after a 90-day incubation.

Conclusions

Biochar can promote bioremediation of contaminated soil as microbial carriers of IMT. It is vital to select an appropriate biochar as an immobilized carrier to stimulate biodegradation. It is feasible to use adsorption carriers with high sorptive capabilities to concentrate PAHs as well as microorganisms and thereby enhance dissipation of PAHs and mitigate soil pollution.     

Retention and release of diethyl phthalate in biochar-amended vegetable garden soils

Purpose

Diethyl phthalate (DEP) is one of the most commonly used plasticizers as well as a soil contaminant. Using biochar to remediate soils contaminated with DEP can potentially reduce the bioavailability of DEP and improve soil properties. Therefore, a laboratory study was conducted to evaluate the effect of biochar on soil adsorption and desorption of DEP.

Materials and methods

Two surface soils (0–20 cm) with contrasting organic carbon (OC) contents were collected from a vegetable garden. Biochars were derived from bamboo (BB) and rice straw (SB) that were pyrolyzed at 350 and 650 °C. Biochars were added to two types of soil at rates of 0.1 and 0.5 % (w/w). A batch equilibration method was used to measure DEP adsorption-desorption in biochar treated and untreated soils at 25 °C. The adsorption and desorption isotherms of DEP in the soils with or without biochar were evaluated using the Freundlich model.

Results and discussion

The biochar treatments significantly enhanced the soil adsorption of DEP. Compared to the untreated low organic matter soil, the soils treated with 0.5 % 650BB increased the adsorption by more than 19,000 times. For the straw biochar treated soils, the increase of DEP adsorption followed the order 350SB?>?650SB. However, for the bamboo biochars, the order was 650BB?>?350BB. Bamboo biochars were more effective than the straw biochars in improving soils’ adsorption capacity and reducing the desorption ability of DEP.

Conclusions

Adding biochar to soil can significantly enhance soil’s adsorption capacity on DEP. The 650BB amended soil showed the highest adsorption capacity for DEP. The native soil OC contents had significant effects on the soils’ sorption capacity treated with 650BB, whereas they had negligible effects on the other biochar treatments. The sorption capacity was affected by many factors such as the feedstock materials and pyrolysis temperature of biochars, the pH value of biochar, and the soil organic carbon levels.     

Quantifying effects of primary parameters on adsorption–desorption of atrazine in soils

Purpose

Adsorption and desorption are important processes that influence the transport, transformation, and bioavailability of atrazine in soils. Equilibrium batch experiments were carried out to investigate the adsorption–desorption characteristics of atrazine. The objectives of this study were to (1) determine and quantify the main soil parameters governing atrazine adsorption and desorption phenomena; (2) find the correlativity between the identified soil parameters; and (3) investigate the universal desorption hysteresis traits.

Materials and methods

Fifteen soils with contrasting physico-chemical characteristics were collected from 11 provinces in eastern China. The equilibrium time was 24 h both for adsorption and desorption experiments. Atrazine was detected by Waters 2695/UV HPLC.

Results and discussion

Adsorption isotherms of atrazine could be well described by the Freundlich equation (r?≥?0.994, p?<?0.01). The total organic carbon (TOC) was the first independent variable that described 53.0 % of the total variability of K _f, followed by the pH (9.9 %), and the clay (4.0 %) and silt (1.2 %) contents, separately; while the primary soil properties that affect desorption parameters included the TOC, pH, free Fe₂O₃ (Fe_d) and the sand content, with the biggest contribution achieved by the TOC (ranged from 48.5–78.1 %). The results showed that when the content ratio of clay to TOC (RCO) was less than 40, the atrazine adsorption was largely influenced by the organic matrix, while when the RCO was greater than 40, they were vital affected by the clay content.

Conclusions

Adsorption–desorption isotherms of atrazine in soils were nonlinear. The content of TOC, clay, and iron oxides, as well as the pH value were the key soil parameters affecting the adsorption–desorption of atrazine in soil, among which the RCO especially exhibited relevance. Additionally, the desorption hysteresis existed for atrazine retention in all 15 tested soils, and the hysteretic effect enhanced with the increasing time for desorption. This would be ascribed to the heterogeneity physical–chemical properties of these soils.     

Effect of agricultural land use change on community composition of bacteria and ammonia oxidizers

Purpose

Soil microbial communities can be strongly influenced by agricultural practices, but little is known about bacterial community successions as land use changes. The objective of this study was to determine microbial community shifts following major land use changes in order to improve our understanding of land use impacts on microbial community composition and functions.

Materials and methods

Four agricultural land use patterns were selected for the study, including old rice paddy fields (ORP), Magnolia nursery planting (MNP), short-term vegetable (STV), and long-term vegetable (LTV) cultivation. All four systems are located in the same region with same soil parent material (alluvium), and the MNP, STV, and LTV systems had been converted from ORP for 10, 3, and 30 years, respectively. Soil bacteria and ammonia oxidizer community compositions were analyzed by 454 pyrosequencing and terminal restriction fragment length polymorphism, respectively. Quantitative PCR was used to determine 16S rRNA and amoA gene copy numbers.

Results and discussion

The results showed that when land use was changed from rice paddy to upland systems, the relative abundance of Chloroflexi increased whereas Acidobacteria decreased significantly. While LTV induced significant shifts of bacterial composition, MNP had the highest relative abundance of genera GP1, GP2, and GP3, which were mainly related to the development of soil acidity. The community composition of ammonia-oxidizing bacteria (AOB) but not ammonia-oxidizing archaea was strongly impacted by the agricultural land use patterns, with LTV inducing the growth of a single super predominant AOB group. The land use changes also induced significant shifts in the abundance of 16S rRNA and bacterial amoA genes, but no significant differences in the abundance of archaea amoA was detected among the four land use patterns. Soil total phosphorous, available phosphorous, NO₃ ^?, and soil organic carbon contents and pH were the main determinants in driving the composition of both bacteria and AOB communities.

Conclusions

These results clearly show the significant impact of land use change on soil microbial community composition and abundance and this will have major implications on the microbial ecology and nutrient cycling in these systems, some of which is unknown. Further research should be directed to studying the impacts of these microbial community shifts on nutrient dynamics in these agroecosystems so that improved nutrient management systems can be developed.     

Arsenic Chemistry in Soils: An Overview of Thermodynamic Predictions and Field Observations

Published information, both theoretical and experimental, on As chemical behavior in soils is reviewed. Because of many emission sources, As is ubiquitous. Thermodynamic calculations revealed that As(V) species (HAsO ₄ ^2- >H₂AsO ₄ ^- at pH 7) are more abundant in soil solutions that are oxidized more than pe+pH>9. Arsenic is expected to be in As(III) form (HAsO ₂ ⁰ =H₃AsO ₃ ⁰ >AsO ₂ ^- =H₂AsO ₃ ^- at pH 7) in relatively anoxic soil solutions with pe+pH<7. Adsorption on soil colloids is an important As scavenging mechanism. The adsorption capacity and behavior of these colloids (clay, oxides or hydroxides surfaces of Al, Fe and Mn, calcium carbonates, and/or organic matter) are dependent on ever-changing factors, such as hydration, soil pH, specific adsorption, changes in cation coordination, isomorphous replacement, crystallinity, etc. Because of the altering tendencies of soil colloids properties, adsorption of As has become a complex, empirical, ambiguous, and often a self contradicting process in soils. In general, Fe oxides/hydroxides are the most commonly involved in the adsorption of As in both acidic and alkaline soils. The surfaces of Al oxides/hydroxides and clay may play a role in As adsorption, but only in acidic soils. The carbonate minerals are expected to adsorb As in calcareous soils. The role of Mn oxides and biogenic particles in the As adsorption in soils appears to be limited to acidic soils. Kinetically, As adsorption may reach over 90% completion in terms of hours. Precipitation of a solid phase is another mechanism of As removal from soil solutions. Thermodynamic calculations showed that in the acidic oxic and suboxic soils, Fe-arsenate (Fe₃(AsO₄)₄)₂) may control As solubility, whereas in the anoxic soils, sulfides of As(III) may control the concentrations of the dissolved As in soil solutions. In alkaline acidic oxic and suboxic soils, precipitation of both Fe- and Ca-arsenate may limit As concentrations in soil solutions. Field observations suggest that direct precipitation of discrete As solid phases may not occur, except in contaminated soils. Chemisorption of As oxyanions on soil colloid surfaces, especially those of Fe oxide/hydroxides and carbonates, is believed to a common mechanisms for As solid phase formation in soils. It is suggested that As oxyanions gradually concentrate on colloid surfaces to a level high enough to precipitate a discrete or mixed As solid phase. Arsenic volatilization is another As scavenging mechanism operating in soils. Many soil organisms are capable of converting arsenate and arsenite to several reduced forms, largely methylated arsines which are volatile. These organisms may generate different or similar biochemical products. Methylation and volatilization of As can be affected by several biotic (such as type of organisms, ability of organism for methylation, etc.) and abiotic factors (soil pH, temperature, redox conditions, methyl donor, presence of other ions, etc.) factors. Information on the rate of As biotransformations in soils is limited. In comparison to the biologically assisted volatilization, the chemical volatilization of As in soils is negligible.     

Sorption of humic acid on Fe oxides,bacteria, and Fe oxide-bacteria composites

Purpose

Sorption of humic substances on other soil components plays an important role in controlling their function and fate in soil. Sorption of humic substances by individual soil components has been studied extensively. However, few studies reported the sorption characteristic of humic substances on composites of soil components. This study aimed to investigate the sorption characteristics of humic acid on Fe oxide-bacteria composites and improve the understanding on the interaction among humic substance Fe oxide bacteria in soil.

Materials and methods

Humic acid was purchased from Sigma-Aldrich and was purified. Hematite and ferrihydrite were synthesized in the lab. Bacillus subtilis and Pseudomonas putida were cultivated in Luria-Broth medium and harvested at stationary growth phase. Batch sorption experiments were carried out at pH 5.0. Various amounts of humic acid were mixed with 20 mg of Fe oxide, bacteria, or Fe oxide-bacteria composite (oxide to bacteria of 1:1) in 10 mL of KCl (0.02 mol L^?1) to construct sorption isotherms. The effects of phosphate concentration and addition order among humic acid, Fe oxide, bacteria on the sorption of humic acid were also studied. The sorption of humic acid was calculated by the difference between the amount of humic acid added initially and that remained in the supernatant.

Results and discussion

The maximum sorption of humic acid on hematite, ferrihydrite, B. subtilis and P. putida was 73.2, 153.5, 69.1, and 56.7 mg C g^?1, respectively. The maximum sorption of humic acid on examined Fe oxide-bacteria composite was 28.2–57.2 % less than the predicted values, implying that the sorption of humic acid was reduced by the interaction between Fe oxides and bacteria. The presence of phosphate exerted negligible influence on the sorption of humic acid on bacteria while it inhibited the sorption of humic acid on Fe oxides. On Fe oxide-bacteria composites, inhibiting influences followed by promoting or weak inhibiting effects of phosphate with increasing concentration on the sorption of humic acid were found.

Conclusions

The interaction between Fe oxides and bacteria reduced the sorption of humic acid; moreover, the reduction was greater by the interaction of bacteria with ferrihydrite than that with hematite. Phosphate exerted negligible and inhibiting influence on the sorption of humic acid by bacteria and Fe oxides, respectively. On Fe oxide-bacteria composites, humic acid sorption was initially inhibited and then promoted or weakly inhibited by phosphate with increasing concentration.     

Combined bioremediation of soil co-contaminated with cadmium and endosulfan by <Emphasis Type="Italic">Pleurotus eryngii</Emphasis> and <Emphasis Type="Italic">Coprinus comatus</Emphasis>

Purpose

The subjects of this study were to investigate the remediating potential of the co-cultivation of Pleurotus eryngii and Coprinus comatus on soil that is co-contaminated with heavy metal (cadmium (Cd)) and organic pollutant (endosulfan), and the effects of the co-cultivated mushrooms on soil biochemical indicators, such as laccase enzyme activity and bacterial counts.

Materials and methods

A pot experiment was conducted to investigate the combined bioremediation effects on co-contaminated soil. After the mature fruiting bodies were harvested from each pot, the biomass of mushrooms was recorded. In addition, bacterial counts and laccase enzyme activity in soil were determined. The content of Cd in mushrooms and soil was detected by the flame atomic absorption spectrometry (FAAS), and the variations of Cd fractions in soil were determined following the modified BCR sequential extraction procedure. Besides, the residual endosulfan in soil was detected by gas chromatography-mass spectrometry (GC-MS).

Results and discussion

The results indicated that co-cultivation of P. eryngii and C. comatus exerted the best remediation effect on the co-contaminated soil. The biomass of mushroom in the co-cultivated group (T group) was 1.57–13.20 and 19.75–56.64% higher than the group individually cultivated with P. eryngii (P group) or C. comatus (C group), respectively. The concentrations of Cd in the fruiting bodies of mushrooms were 1.83–3.06, 1.04–2.28, and 0.67–2.60 mg/kg in T, P, and C groups, respectively. Besides, the removal rates of endosulfan in all treatments exceeded 87%. The best bioremediation effect in T group might be caused by the mutual promotion of these two kinds of mushrooms.

Conclusions

The biomass of mushroom, laccase activity, bacterial counts, and Cd content in mushrooms were significantly enhanced, and the dissipation effect of endosulfan was slightly higher in the co-cultivated group than in the individually cultivated groups. In this study, the effect of co-cultivated macro fungi P. eryngii and C. comatus on the remediation of Cd and endosulfan co-contaminated soil was firstly reported, and the results are important for a better understanding of the co-remediation for co-contaminated soil.

     

The impact of adsorption on the exposure assessment of <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> in the water-sediment system: a mathematical model approach

A model to assess the environmental fate of bacteria to be used as plant protection products or biocides is presented. The model, based on a black-box approach, takes into account the bacterial cell adsorption on the sediment phase and the degradation kinetics and allows the calculation of the expected environmental density of bacterial cells and the predicted environmental concentration of bacterial toxins in the water-sediment system. An example of calculation results is reported for the study cases of Bacillus thuringiensis subsp. aizawai strain GC-91, B. thuringiensis subsp. kurstaki strain ABTS-351, and B. thuringiensis subsp. tenebrionis strain NB 176, used as plant protection products, and B. thuringiensis subsp. israelensis strains AM 65–52 and SA3A, used as biocides. The calculation of expected environmental density and predicted environmental concentration for bacteria in soil-water systems allows to achieve an appropriate risk assessment of the environmental fate and of the impact of bacterial pest control agents or bacterial biocides on non-target organisms.     

Evolution and phosphorus fractionation in saline Spolic Technosols flooded with eutrophic water

Purpose

The aim of this study was to evaluate the behaviour of P in saline Spolic Technosols flooded with eutrophic water, with and without plant rhizosphere, in order to assess the role of these soils as sinks or sources of this nutrient.

Materials and methods

Samples were taken from basic (pH?~7.8), carbonated and acidic (pH?~6.2), de-carbonated soils of salt marshes polluted by mine wastes. Three treatments were assayed: pots with Sarcocornia fruticosa, pots with Phragmites australis and pots without plants (bare soil). The pots were flooded for 15?weeks with eutrophic water (PO ₄ ^3? ~6.92?mg?L^?1) and pH, Eh and water-soluble organic carbon and PO ₄ ^3? concentrations were monitored in the soil solution. A soil P fractionation was applied before and after the flooding period.

Results and discussion

The PO ₄ ^3? concentration in the soil solution decreased rapidly in both soils, with and without plant, being diminished by 80?C90?% after 3?h of flooding. The Fe/Mn/Al oxides and the Ca/Mg compounds played an important role in soil P retention. In pots with S. fruticosa, the reductive conditions due to flooding induced P release from metal oxides and P retention to Ca/Mg compounds. In turn, P. australis may have favoured the release of P from carbonates, which was transferred to Fe/Mn/Al compounds.

Conclusions

The retention of P by the soil was the main mechanism involved in the removal of PO ₄ ^3? from the eutrophic flooding water but to evaluate the capacity of these systems as long-term P sinks, the combined effect of metals, Ca/Mg compounds and specific plant species should be considered.     

Competitive adsorption and desorption of oxytetracycline and cadmium with different input loadings on cinnamon soil

Purpose

The binary competitive effect could obviously influence the fate and transport behavior of oxytetracycline (OTC) and cadmium (Cd²⁺) in cinnamon soil. However, two pollutants loading into soil usually are different, perhaps because of the three reasons including occurrence of OTC before Cd²⁺, simultaneous occurrence of OTC and Cd²⁺, or occurrence of Cd²⁺ before OTC. The purpose of the study was to predict the competitive adsorption and desorption of OTC and Cd²⁺ as a function of above input loadings on cinnamon soil.

Materials and methods

Adsorption and desorption were determined using the batch equilibrium method in a single or binary system. The Freundlich equation was applied to describe the adsorption/desorption data of OTC and Cd²⁺ in order to obtain adsorption/desorption isotherms for each tested compound and the respective adsorption/desorption coefficients.

Results and discussion

The results indicated that cinnamon soil could strongly adsorb OTC with the adsorption affinity (K _f value) of more than 718 and Cd²⁺ with K _f value of more than 536 in the competitive and non-competitive system, and all adsorption and desorption isotherms of OTC and Cd²⁺ on cinnamon soil were well fitted by the Freundlich equation with r value of more than 0.99 (p?<?0.01). The coexistence of OTC and Cd²⁺ on cinnamon soil promoted significantly Cd²⁺ adsorption when Cd²⁺ firstly or simultaneously occurred on soil, but their coexistence did not affect adsorption of OTC when OTC firstly or simultaneously occurred on soil. Among the three input loadings, the pollutant with later occurring mode had lower K _f and hysteresis coefficient (HI) than the other two input loadings. According to the adsorption intensity parameter (1/n), the presence of Cd²⁺ or OTC with different input loadings could decrease the adsorption intensity of OTC or Cd²⁺ when compared with single occurrence of OTC or Cd²⁺.

Conclusions

The binary competitive effect influenced the adsorption/desorption of OTC and Cd²⁺ differently. The presence of OTC had a stronger influence on the adsorption/desorption of Cd²⁺ than the presence of Cd²⁺ on the adsorption of OTC. The later occurring pollutant on soil had stronger ecological risk than the former occurring pollutant in the binary competitive system. The physical adsorption in the single or binary system could be identified as the dominant mechanisms of OTC and Cd²⁺ adsorption.     

Pig manure vermicompost (PMVC) can improve phytoremediation of Cd and PAHs co-contaminated soil by Sedum alfredii

Purpose

A major challenge to phytoremediation of co-contaminated soils is developing strategies for efficient and simultaneous removal of multiple pollutants. A pot experiment was conducted to investigate the potential for enhanced phytoextraction of cadmium (Cd) by Sedum alfredii and dissipation of polycyclic aromatic hydrocarbons (PAHs) in co-contaminated soil by application of pig manure vermicompost (PMVC).

Materials and methods

Soil contaminated by Cd (5.53?mg?kg^?1 DW) was spiked with phenanthrene, anthracene, and pyrene together (250?mg?kg^?1 DW for each PAH). A pot experiment was conducted in a greenhouse with four treatments: (1) soil without plants and PMVC (Control), (2) soil planted with S. alfredii (Plant), (3) soil amended with PMVC at 5?% (w/w) (PMVC), and (4) treatment 2?+?3 (Plant?+?PMVC). After 90?days, shoot and root biomass of plants, Cd concentrations in plant and soil, and PAH concentrations in soil were determined. Abundance of PAH degraders in soil, soil bacterial community structure and diversity, and soil enzyme activities and microbial biomass carbon were measured.

Results and discussion

Application of PMVC to co-contaminated soil increased the shoot and root dry biomass of S. alfredii by 2.27- and 3.93-fold, respectively, and simultaneously increased Cd phytoextraction without inhibiting soil microbial population and enzyme activities. The highest dissipation rate of PAHs was observed in Plant?+?PMVC treatment. However, neither S. alfredii nor PMVC enhanced PAH dissipation when applied separately. Abundance of PAH degraders in soil was not significantly related to PAH dissipation rate. Plant?+?PMVC treatment significantly influenced the bacterial community structure. Enhanced PAH dissipation in the Plant?+?PMVC treatment could be due to the improvement of plant root growth, which may result in increased root exudates, and subsequently change bacterial community structure to be favorable for PAH dissipation.

Conclusions

This study demonstrated that remediation of Cd and PAHs co-contaminated soil by S. alfredii can be enhanced by simultaneous application of PMVC. Long-term evaluation of this strategy in co-contaminated field sites is needed.     

A hollow bacterial diversity pattern with elevation in Wolong Nature Reserve,Western Sichuan Plateau

Purpose

The pattern of eukaryotic macroorganisms varies with altitude is well-documented; by contrast, very little is known of how a bacterial pattern in soils varies with the elevation in a montane ecosystem. Mostly, previous studies on soil bacteria have either found a diversity decline, no trend, or a hump-back trend with increasing elevation. The aim of this study was to investigate the bacterial community composition and diversity patterns of Mount Nadu in Wolong Nature Reserve, Western Sichuan Plateau (3000–3945 masl).

Materials and methods

In total, 30 soil samples from the mountain at 10 sampling elevational zones (every 100 m from the baseband to the summit) were collected. High-throughput pyrosequencing approach was performed of soil bacterial 16S rRNA targeting V3?+?V4 region by MiSeq PE300 and taxonomically classified based on Silva database. Bacterial community composition and diversity patterns were detected, and bacterial data were correlated with environmental factors to determine which factors influenced bacterial community composition.

Results and discussion

We obtained an average of 30,172 sequences per soil and found that the relative abundance of Acidobacteria and Proteobacteria count more than 70 % of the whole bacteria. Cooperative network analysis also revealed that Acidobacteria and Proteobacteria were important hubs in the community. Bacterial diversity pattern was found to be a significant hollow trend along altitudinal gradients and diversity of the dominant phyla (e.g., Acidobacteria, Proteobacteria) followed the results of the whole bacterial diversity. Moreover, distance-based linear model identified that soil pH and TN significantly provided 7.40 and 6.01 % of the total variation.

Conclusions

The hollow trend of bacterial diversity has rarely been observed in nature. It indicated that no unifying bacterial diversity pattern can be expected along elevational gradients among the mountain system, and our result suggested the importance of environmental factors in structuring bacterial communities in this montane ecosystem.

     

Bacterial community incorporating carbon derived from plant residue in an anoxic non-rhizosphere soil estimated by DNA-SIP analysis

Purpose

Plant residues are one of the main sources of soil organic matter in paddy fields, and elucidation of the bacterial communities decomposing plant residues was important to understand their function and roles, as the microbial decomposition of plant residues is linked to soil fertility. We conducted a DNA stable isotope probing (SIP) experiment to elucidate the bacterial community assimilating 13-carbon (¹³C) derived from plant residue under an anoxic soil condition. In addition, we compared the bacterial community with that under the oxic soil condition, which was elucidated in our previous study (Lee et al. in Soil Biol Biochem 43:814–822, 2011).

Materials and methods

We used the ¹³C-labeled dried rice callus cells as a model of rice plant residue. A paddy field soil was incubated with unlabeled and ¹³C-labeled callus cells. DNA extracted from the soils was subjected to buoyant density gradient centrifugation to fractionate ¹³C-enriched DNA. Then, polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) analysis of bacterial 16S rDNA band patterns and band sequencing method were used to evaluate bacterial community.

Results and discussion

DGGE analysis showed that the band patterns in the ¹³C-enriched fractions were distinctly changed over time, while the changes in the community structure before fractionation were minor. Sequencing of the ¹³C-labeled DGGE bands revealed that Clostridia were a major group in the bacterial communities incorporating the callus-derived carbon although Gram-negative bacteria, and Actinobacteria also participated in the carbon flow from the callus under the anoxic condition. The proportion of Gram-negative bacteria and Actinobacteria increased on 14 days after the onset of incubation, suggesting that the callus was decomposed by diverse bacterial members on this phase. When the bacterial groups incorporating the ¹³C were compared between under anoxic and oxic soil conditions, the composition was largely different under the two opposite conditions. However, some members of Gram-negative bacteria were commonly found under the anoxic and oxic soil conditions.

Conclusions

The majority of bacterial members assimilating the callus carbon was Clostridia in the soil under anoxic conditions. However, several Gram-negative bacterial members, such as Acidobacteria, Bacteroidetes, and Proteobacteria, also participated in the decomposition of callus under anoxic soil conditions. Our study showed that carbon flow into the diverse bacterial members during the callus decomposition and the distinctiveness of the bacterial communities was formed under the anoxic and oxic soil conditions.

     

Evaluation of soil analytical methods for the characterization of alkaline Technosols: I. Moisture content, pH, and electrical conductivity

Purpose

Applying standard soil analytical methods to novel soil materials, such as tailings or soils with unusual properties, should be done with caution and with special consideration of potential interferents and possible pretreatments. The aim of this study was to investigate the effects of common variations in methods on calculated total moisture content, pH, and electrical conductivity (EC) of saline alkaline soil materials.

Materials and methods

Bauxite residue (an alkaline, saline–sodic Technosolic material) as well as two saline alkaline soils from coastal and lacustrine environments were dried under various temperatures and atmospheres, and then analyzed for pH and EC at various soil–solution ratios over time.

Results and discussion

Calculated moisture content of all samples increased with drying temperature. Dehydration of gypsum elevated calculated moisture content. Decreases in soil–solution ratio decreased suspension EC and pH in highly alkaline samples. The pH and EC of soil/water suspensions generally rose with equilibration time for bauxite residue; stable values were attained within 24–120 h. Atmospheric carbonation substantially decreased the pH of samples dried at lower temperatures.

Conclusions

Variations in temperature, time, and atmosphere during drying of highly alkaline and saline soil materials influenced calculated moisture content as well as chemical properties such as pH and EC. A drying temperature of 40 °C and drying to constant weight is recommended to minimize these effects. Soil–solution ratio, equilibration time, and sample preparation conditions influenced observed pH and EC, and should be standardized if attempting to compare results between studies.     

Multi-residue analysis of PAH, PCB, and OCP optimized for organic matter of forest soil

Purpose

Analyzing organic pollutants in forest soil is challenging because they are strongly physical and chemical bound to soil organic matter (SOM). Within the framework of a forest soil inventory, an analytical protocol for the determination of polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), and organochlorine pesticides (OCP) should be established and validated using one and the same extraction and cleanup procedure. The protocol should be applicable for reliable analysis of a high number of samples in a short timeframe.

Materials and methods

Two different soil samples representative for the humic layer from a typical mixed and coniferous forest soil had been used for the analysis. Three solvents of different polarity, namely cyclohexane (CH), ethylacetate (EA)/CH (1/1, v/v), and acetone (AC)/CH (2/1, v/v), and the six standard extraction techniques (pressurized liquid extraction (PLE), soxhlet extraction, fluidized bed extraction, sonication, shaking, and one-step extraction recommended for analyzing agricultural soil in Germany (VDLUFA 2008)) were compared concerning their extraction efficiency. For additional matrix separation, two different cleanup procedures (gel permeation chromatography (GPC) and solid-phase extraction (SPE) with different sorbents) were tested. Quantification was carried out using gas chromatography combined with mass spectrometry (GC–MS) and two different injection systems (split/splitless injection and programmable temperature vaporizer (PTV) injection). Labeled internal standards, added prior to extraction, were used for method evaluation.

Results and discussion

For the simultaneous extraction of PAH, PCB, and OCP from organic forest soil PLE with acetone/cyclohexane (2/1, v/v) provided the highest extraction efficiency. A two-step cleanup procedure consisting of GPC followed by SPE with silica gel was entirely sufficient for the separation of humic substances without discrimination of analytes. Recovery rates for the different extraction and cleanup steps ranged between 89% and 106%. For quantification, a GC–MS method was developed using two different injection systems and two capillary columns of different selectivity.

Conclusions

By comparing six standard extraction techniques for PAH, PCB, and OCP from forest soil, we obtained the highest extraction efficiency when using PLE with AC/CH (2/1). For sample injection, we achieved best results using an optimized PTV injection system as it highly reduced the breakdown of thermolabile pesticides. Using this combination of technical equipment, it is possible to determine a concentration of the analytes in the trace level range of 1–2 μg kg^?1 in humic soil.     

Contrasting response of two grassland soils to N addition and moisture levels: N<Subscript>2</Subscript>O emission and functional gene abundance

Purpose

Nitrification and denitrification processes dominate nitrous oxide (N₂O) emission in grassland ecosystems, but their relative contribution as well as the abiotic factors are still not well understood.

Materials and methods

Two grassland soils from Duolun in Inner Mongolia, China, and Canterbury in New Zealand were used to quantitatively compare N₂O production and the abundance of bacterial and archaeal amoA, denitrifying nirK and nirS genes in response to N additions (0 and 100 μg NH₄ ⁺–N g^?1 dry soil) and two soil moisture levels (40 and 80 % water holding capacity) using microcosms.

Results and discussion

Soil moisture rather than N availability significantly increased the nitrification rate in the Duolun soil but not in the Canterbury soil. Moreover, N addition promoted denitrification enzyme activities in the Canterbury soil but not in the Duolun soil. The abundance of bacterial and archaeal amoA genes significantly increased as soil moisture increased in the Duolun soil, whereas in the Canterbury soil, only the abundance of bacterial amoA gene increased. The increase in N₂O flux induced by N addition was significantly greater in the Duolun soil than in the Canterbury soil, suggesting that nitrification may have a dominant role in N₂O emission for the Duolun soil, while denitrification for the Canterbury soil.

Conclusions

Microbial processes controlling N₂O emission differed in grassland soils, thus providing important baseline data in terms of global change.