Changes of microbial properties in (near-) rhizosphere soils after phytoextraction by Sedum alfredii H: A rhizobox approach with an artificial Cd-contaminated soil

The ultimate goal of soil remediation is to restore soil health. Soil microbial parameters are considered to be effective indicators of soil health. The aim of this study was to determine the effects of phytoextraction on microbial properties through the measurement of soil microbial biomass carbon, soil basal respiration and enzyme activities. For this purpose, a pre-stratified rhizobox experiment was conducted with the Cd hyperaccumulator Sedum alfredii H. for phytoextraction Cd from an artificial contaminated soil (15.81 mg kg⁻¹) under greenhouse conditions. The plant and soil samples were collected after growing the plant for three and six months with three replications. The results indicated that the ecotype of S. alfredii H. originating from an ancient silver mining site was a Cd-hyperaccumulator as it showed high tolerance to Cd stress, the shoot Cd concentration were as high as 922.6 mg kg⁻¹ and 581.9 mg kg⁻¹ at the two samplings, and it also showed high BF (58.4 and 36.8 after 3 and 6 months growth), and TF (5.8 and 5.1 after 3 and 6 months growth). The amounts of Cd accumulated in the shoots of S. alfredii reached to an average of 1206 μg plant⁻¹ after 6 months growth. Basal respiration, invertase and acid phosphatase activities of the rhizosphere soil separated by the shaking method were significantly higher (P < 0.01) than that of the near-rhizosphere soil and the unplanted soil after 3 months growth, so were microbial biomass carbon, urease, invertase and acid phosphatase activities of the rhizosphere soil after 6 months growth. Acid phosphatase activity of the 0–2 mm sub-layer rhizosphere soil collected by the pre-stratified method after 3 months growth was significantly higher (P < 0.05) than that of other sub-layer rhizosphere soils and bulk soil, and so were microbial biomass carbon, basal respiration, urease, invertase and acid phosphatase activities of the 0–2 mm sub-layer rhizosphere soil after 6 months growth. It was concluded that phytoextraction by S. alfredii could improve soil microbial properties, especially in rhizosphere, and this plant poses a great potential for the remediation of Cd contaminated soil.     

Plant growth promoting potential of Pontibacter niistensis in cowpea (Vigna unguiculata (L.) Walp.)

During the past couple of decades, understanding of rhizosphere biology has progressed with the discovery of a special group of microorganisms known as plant growth promoting rhizobacteria (PGPR) and its application for sustainable agriculture has increased tremendously in various parts of the world. The search for microorganisms that improve soil fertility and enhance plant nutrition has continued to attract attention due to the increasing cost of fertilizers and some of their negative environmental impacts. In this study we demonstrated, a novel bacterial species Pontibacter niistensis NII-0905 isolated from forest soil in Western ghat forest soil with potential plant growth promoting ability (PGP) such as phosphate solubilization, indole acetic acid (IAA), siderophore and hydrogen cyanide (HCN) production. The activity varies with different growth temperatures, strain solubilize 28.5 ± 0.9, 48.02 ± 1.9 and 65.07 ± 2.1 μg mL⁻¹ at 4, 15 and 30 °C respectively and produced 24.8 μg mL⁻¹ day⁻¹ of indole acetic acid (IAA) in tryptophan amended media. Qualitative detection of siderophore production and HCN were also detected at all temperature tested. At a lower temperature (4 °C) strain NII-0905 retained all the plant growth promotion attributes. A significant increase in the growth of cow pea was recorded with inoculations of strain NII-0905 in pot experiments. Scanning electron microscopic study revealed the root colonization on cow pea seedlings against the untreated one. These results demonstrate that, the isolate NII-0905 has the promising PGPR attributes for both in cold as well as in humid condition. It has potential as a biofertilizer to enhance soil fertility and promote the plant growth.     

Soil available phosphorus status determines indigenous mycorrhizal colonization of field and glasshouse-grown spring wheat from Argentina

Wheat production (Triticum aestivum L.) has increased across the world during last century with the intensification of agriculture. Phosphorus (P) fertilization is a common practice to improve wheat growth in Argentina. We investigate whether indigenous arbuscular mycorrhizal colonization (AMC) of hard red spring wheat is controlled by shoot P content (SPc) or by available soil P in an agricultural soil from the southeastern Argentine Pampas. In the field, AMC was monitored four times during two growing seasons of a conventional wheat crop. Treatments were: without P supply, annual supply of 11 and 22 kg P ha⁻¹ during the last 5 years, and 164 kg P ha⁻¹ applied once 5 years before the experiment. In the glasshouse, AMC was assessed three times in wheat growing in pots filled with the soil from unfertilized plots; treatments were: P (0 and 20 mg P pot⁻¹), and nitrogen (N) fertilization (0 and 150 mg N pot⁻¹). A range of soil P between 6 and 60 mg P kg⁻¹ was obtained and the AMC ranged from 1% to 67% of root length colonized under both field and glasshouse conditions. P supplied annually increased growth and SPc but decreased AMC. N fertilization did not affect growth or AMC. Variations in SPc did not account for AMC. Variability in AMC was best accounted for local current soil available P content (r² = 0.59). A linear-plateau relationship between soil P and indigenous AMC was established in wheat plants growing under contrasting environmental and experimental (field and glasshouse) conditions. Indigenous AMC was depressed by available soil P in the range 0–27 mg P kg⁻¹ (a decrease of 2.8% mg P⁻¹ kg⁻¹). Above 27 mg P kg soil⁻¹, AMC was stabilized at about 10%. Grain yield increased with fertilization and the highest relative shoot dry matter in field was obtained at 15.5 mg P kg soil⁻¹. The soil P range that ensures high wheat production without deterring indigenous AMC is discussed.     

Allyl isothiocyanate: an alternative chemical expellant for sampling earthworms

Allyl isothiocyanate (AITC), a natural breakdown product of glucosinolates in many Cruciferae and a component imparting the sharp taste to mustard, was tested for its effectiveness as a chemical expellant for sampling earthworms. Testing was performed in an arable field with earthworm populations dominated by Lumbricus terrestris Linnaeus, 1758 and Aporrectodea tuberculata (Eisen, 1874). The optimal concentration was found by comparing concentrations ranging from 5 to 250 mg l⁻¹ in water. Total biomass and numbers of earthworms collected increased hyperbolically with increasing AITC concentration, with the highest biomass and numbers collected using 100 mg l⁻¹ AITC. Biomass and numbers of earthworms collected using 250 mg l⁻¹ AITC, but not 150 or 200 mg l⁻¹ AITC, were significantly less than with 100 mg l⁻¹ AITC. Less earthworm biomass was collected by hand sorting than with chemical expulsion using 100 mg l⁻¹ AITC, but the number of earthworms collected by the two methods were not different. A comparison of hand sorting and 100 mg l⁻¹ AITC expulsion using analysis of similarities (ANOSIM) showed that the two methods produced samples differing in the distribution of both numbers and biomass of species-by-size classes. Hand sorting collected more of the smallest size class of L. terrestris and the largest size classes of A. tuberculata than AITC expulsion, whereas AITC expulsion recovered more of the largest size classes of L. terrestris than hand sorting. When 100 mg l⁻¹ AITC expulsion was compared with chemical expulsion using 200 mg l⁻¹ formalin, no differences were found in the total number, total biomass or in the species-by-size class distribution of the earthworms collected. This suggests that the AITC method may be a favorable alternative to formalin expulsion for sampling earthworms. Further studies under other environmental conditions and with other species of earthworms are warranted to establish its general utility.     

Removal of benzo (a) pyrene from soil using an endogeic earthworm Pontoscolex corethrurus ()

The endogeic earthworm Pontoscolex corethrurus (Müller, 1857) was the most abundant species (75%) in soil contaminated with hydrocarbons, mostly benzo(a)pyrene (BaP), in the state of Tabasco (Mexico). The earthworm P. corethrurus was tested for its capacity to remove 100 mg BaP kg⁻¹ from an Anthrosol soil (sterilized or not) and amended with legume Mucuna pruriens (L.) DC. var. utilis (Wall. ex Wight) Baker ex Burck (3%) or the grass Brachiaria humidicola (L.) DC (3%) (recently renamed as Urochloa humidicola (Rendle) Morrone & Zuloaga) in an aerobic incubation experiment. P. corethrurus removed 26.6 mg BaP kg⁻¹ from the sterilized soil and application of B. humidicola as feed increased this to 35.7 mg BaP kg⁻¹ and M. pruriens to 34.2 mg BaP kg⁻¹ after 112 days. The autochthonous microorganisms removed 9.1 mg BaP kg⁻¹ from the unsterilized soil and application of B. humidicola increased this to 18.0 mg BaP kg⁻¹ and M. pruriens to 11.2 mg BaP kg⁻¹. Adding P. corethrurus to the unsterilized soil accelerated the removal of BaP and 36.1 mg kg⁻¹ was dissipated from soil. It was found that the autochthonous microorganisms removed BaP from soil, but addition of P. corethrurus increased the dissipation 4-fold. The endogeic earthworm P. corethrurus can thus be used to remediate hydrocarbon-contaminated soils in tropical regions.     

Climatic influences on active fractions of soil organic matter

Biologically active fractions of soil organic matter are important in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. We evaluated the quantitative relationships among potential C and net N mineralization, soil microbial biomass C (SMBC), and soil organic C (SOC) under four contrasting climatic conditions. Mean SOC values were 28±11 mg g⁻¹ (n=24) in a frigid–dry region (Alberta/British Columbia), 25±5 mg g⁻¹ (n=12) in a frigid–wet region (Maine), 11±4 mg g⁻¹ (n=117) in a thermic–dry region (Texas), and 12±5 mg g⁻¹ (n=131) in a thermic–wet region (Georgia). Higher mean annual temperature resulted in consistently greater basal soil respiration (1.7 vs 0.8 mg CO₂–C g⁻¹ SOC d⁻¹ in the thermic compared with the frigid regions, P<0.001), greater net N mineralization (2.8 vs 1.3 mg inorganic N g⁻¹ SOC 24 d⁻¹, P<0.001), and greater SMBC (53 vs 21 mg SMBC g⁻¹ SOC, P<0.001). Specific respiratory activity of SMBC was, however, consistently lower in the thermic than in the frigid regions (29 vs 34 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Higher mean annual precipitation resulted in consistently lower basal soil respiration (1.1 vs 1.3 mg CO₂–C g⁻¹ SOC d⁻¹ in the wet compared with the dry regions, P<0.01) and lower SMBC (31 vs 43 mg SMBC g⁻¹ SOC, P<0.001), but had inconsistent effects on net N mineralization that depended upon temperature regime. Specific respiratory activity of SMBC was consistently greater in the wet than the dry regions (≈33 vs 29 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Although the thermic regions were not able to retain as high a level of SOC as the frigid regions, due likely to high annual decomposition rates, biologically active soil fractions were as high per mass of soil and even 2–3-times greater per unit of SOC in the thermic compared with the frigid regions. These results suggest that macroclimate has a large impact on the portion of soil organic matter that is potentially active, but a relatively small impact on the specific respiratory activity of SMBC.     

Tolerance,growth and degradation of phenanthrene and benzo[a]pyrene by Rhizobium tropici CIAT 899 in liquid culture medium

Polycyclic aromatic hydrocarbons (PAH) are ubiquitous pollutants that are toxic and recalcitrant to degradation by bacteria. This research evaluated the toxicity of different concentrations [10, 20, 40, 60, 80 and 100 μg mL⁻¹] of phenanthrene (PHE) or benzo[a]pyrene (BaP) on the growth of Rhizobium tropici CIAT899 under in vitro conditions as well as the potential degradation of PHE and BaP by this bacterium. At 24 h, a 40% decrease in Rhizobium growth was observed when exposed to 40 μg mL⁻¹ of either PHE or BaP. Furthermore, bacterial growth was completely inhibited by PHE or BaP applied in 80 and 100 μg mL⁻¹. After 96 h, the growth of R. tropici at 40 μg PHE mL⁻¹ or 60 μg BaP mL⁻¹ was similar to those treatments without PAH. To evaluate R. tropici degrading capabilities, supernatants of cultures with 40 μg PHE mL⁻¹ or 60 μg BaP mL⁻¹ were analyzed by gas chromatography coupled to mass spectrophotometer (GC–MS). R. tropici was able to degrade either PHE or BaP diminishing its concentration in 20% and 25% during the first 24 h, degradation obtained at 120 h was 50% and 45% for PHE or BaP, respectively. This research shows for the first time that R. tropici CIAT 899 grows in liquid culture medium contaminated with PAH, and moreover is able to growth and to degrade either PHE or BaP.     

Enhancement of arbuscular mycorrhizal fungus (Glomus versiforme) on the growth and Cd uptake by Cd-hyperaccumulator Solanum nigrum

Arbuscular mycorrhizal fungus (AMF) can enhance plant growth and resistance to toxicity produced by heavy metals (HMs), affect the bioavailability of HMs in soil and the uptake of HMs by plants, and thus has been emerged as the most prominent symbiotic fungus for contribution to phytoremediation. A greenhouse pot experiment was conducted to assess the effect of Glomus versiforme BGC GD01C (Gv) on the growth and Cd accumulation of Cd-hyperaccumulator Solanum nigrum in different Cd-added soils (0, 25, 50, 100 mg Cd kg⁻¹ soil). Mycorrhizal colonization rates were generally high (from 71% to 82%) in Gv-inoculated treatments at all Cd levels. Gv colonization enhanced soil acid phosphatase activity, and hence elevated P acquisition and growth of S. nigrum at all Cd levels. Moreover, the presence of Gv significantly increased DTPA-extractable (phytoavailable) Cd concentrations in 25 and 50 mg Cd kg⁻¹ soils, but did not affect phytoavailable Cd in 100 mg Cd kg⁻¹ soil. Similarly, inoculation with Gv significantly increased Cd concentrations of S. nigrum in 25 and 50 mg Cd kg⁻¹ soils, but decreased Cd concentrations of the plants in 100 mg Cd kg⁻¹ soil. Overall, inoculation with Gv greatly improved the total Cd uptakes in all plant tissues at all Cd levels. The present results indicated that S. nigrum associated with Gv effectively improved the Cd uptake by plant and would be a new strategy in microbe-assisted phytoremediation for Cd-contaminated soils.     

Accumulation of As,Pb, Zn,Cd and Cu and arbuscular mycorrhizal status in populations of Cynodon dactylon grown on metal-contaminated soils

Metal(loid) accumulation and arbuscular mycorrhizal (AM) status of the dominant plant species, Cynodon dactylon, growing at four multi-metal(loid)s-contaminated sites and an uncontaminated site of China were investigated. Up to 94.7 As mg kg^?1, 417 Pb mg kg^?1, 498 Zn mg kg^?1, 5.8 Cd mg kg^?1 and 27.7 Cu mg kg^?1 in shoots of C. dactylon were recorded. The plant was colonized consistently by AM fungi (33.0–65.5%) at both uncontaminated site and metal-contaminated sites. Based on morphological characteristics, fourteen species of AM fungi were identified in the rhizosphere of C. dactylon, with one belonging to the genus of Acaulospora and the other thirteen belonging to the genus of Glomus. Glomus etunicatum was the most common species associated with C. dactylon growing at metal-contaminated sites. Spore abundance in the rhizosphere of C. dactylon growing at the metal-contaminated soils (22–82 spores per 25 g soil) was significantly lower than that of the uncontaminated soils (371 spores per 25 g soil). However, AM fungal species diversity in the metal-contaminated soils was significantly higher than that in the uncontaminated soils. This is the first report of AM status in the rhizosphere of C. dactylon, the dominant plant survival in metal-contaminated soils. The investigation also suggests that phytorestoration of metal-contaminated sites might be facilitated using the appropriate plant with the aid of tolerant AM fungi.     

Molecular and physiological bacterial diversity of a semi-arid soil contaminated with different levels of formulated atrazine

A semi-arid soil treated with different concentrations of formulated atrazine in a laboratory experiment was studied over 45 days, by different biological and molecular parameters (bacterial enumeration (cfu), community level physiological profiles (CLPPs) measured by Biolog^® and denaturing gradient gel electrophoresis (DGGE)), to study the bacterial community diversity.Formulated atrazine was almost totally degraded at different concentrations after this incubation time. The number of colony forming units (cfu) for soils with 100 and 1000 mg kg⁻¹ atrazine was significantly (p ≤ 0.05) higher than for the control, 1 and 10 mg kg⁻¹ treatments. DGGE banding patterns showed that regardless of time elapsed, concentrations of 10, 100 and 1000 mg kg⁻¹ atrazine in soil, affected the bacterial community compared to control and 1 mg kg⁻¹.The Shannon diversity index (H′) based on CLPP data showed a significant (p ≤ 0.05) decrease at atrazine concentrations of 100 and 1000 mg kg⁻¹. The Shannon diversity indices for different guilds of source carbon and the parameters K and r (based on the kinetics of colour formation rather than on the degree of colour development) were related to guilds of carbon substrates and atrazine concentration at a sampling time. The parameter K was very sensitive to atrazine effects on microbial communities.These biological and molecular parameters can be used to monitor changes in soils treated with atrazine at different concentrations, even when the pesticide is degraded.     

Impact of high cadmium and nickel soil concentration on selected physiological parameters of Arundo donax L.

The purpose of this study was to investigate the effects of high cadmium and nickel soil concentrations on selected physiological parameters of Arundo donax L. A 2-year pot experiment was held in the field and the pots were irrigated with aqueous solutions of Cd and Ni in concentrations of 5, 50 and 100 ppm, against the control (tap water). At the end of the cultivation periods the pots soil was divided into three equal zones and total and NH₄OAc extractable Cd and Ni concentrations were determined. The top zone exhibited the highest metal content. Cadmium and nickel total concentrations at the end of the experiment were up to 973.8 mg kg⁻¹ and 2543.3 mg kg⁻¹ respectively, while NH₄OAc extractable Cd was up to 291.7 mg kg⁻¹ and Ni up to 510.3 mg kg⁻¹. Stomatal conductance ranged between 0.3 and 0.8 mol CO₂ m⁻² s⁻¹, intercellular CO₂ concentration ranged between 212.9 and 243.0 ppm CO₂, stomatal resistance between 0.6 and 1.3 s cm⁻¹, chlorophyll content (SPAD values) between 46.3 and 57.0 and chlorophyll fluorescence (F_v/F_m) ranged between 0.8 and 0.9. All studied physiological parameters did not show statistically significant differences among control and heavy metal treated plants for both years; therefore, high soil cadmium and nickel concentration did not inhibit stomatal opening and did not affect the function of the photosynthetic machine of A. donax plants.     

Mercury affects the distribution of culturable species of Pseudomonas in soil

Pseudomonas bacteria isolated during 52 days on Gould's S1 agar from soil spiked with 0, 3.5 and 15 mg Hg(II) kg soil⁻¹ were characterised to reveal whether mercury affected them differently. Isolates from the treatments with 0 and 15 mg Hg kg⁻¹ were characterised using FT-IR characterisation and subsequent 16S rDNA partial sequencing of representative isolates. To verify the selectivity of Gould's S1 agar and the FT-IR characterisation, all 450 isolates were subjected to the following tests: Gram-determination, catalase and oxidase activity, pigment production on PDA and growth at different temperatures. Furthermore, the isolates were tested for their ability to grow on agar amended with 10 mg Hg kg⁻¹ as an indication of mercury resistance. We found that up to 80% of the isolates in soil amended with 15 mg Hg kg⁻¹ were mercury-resistant, whereas only up to 20% were resistant in the treatments with 0 and 3.5 mg Hg kg⁻¹. We found two groups of Pseudomonas, which probably represent non-described species since they did not group closely with any known species of Pseudomonas in the dendrogram. Hg-enhanced isolates were closely related to P. frederiksbergensis. Furthermore, Hg resistance was almost exclusively restricted to P. frederiksbergensis and P. migulae groups. We conclude that Hg caused a shift in the dominating species of culturable Pseudomonas.     

Effects of azoxystrobin,chlorothalonil, and ethoprophos on the reproduction of three terrestrial invertebrates using a natural Mediterranean soil

The potential terrestrial toxicity of three pesticides, azoxystrobin, chlorothalonil, and ethoprophos was evaluated using reproduction ecotoxicological tests with different non-target species: the collembolan Folsomia candida, the earthworm Eisenia andrei, and the enchytraeid Enchytraeus crypticus. All reproduction tests were performed with natural soil from a Mediterranean agricultural area (with no pesticide residues) in order to improve the relevance of laboratory data to field conditions. Controls were performed with natural and standard artificial soil (OECD 10% OM). The fungicide azoxystrobin showed the highest toxicity to earthworms (EC₅₀ = 42.0 mg a.i. kg⁻¹ dw soil). Collembolans were the most sensitive taxa in terms of sublethal effects of chlorothalonil with an EC₅₀ of 31.1 mg a.i. kg⁻¹ dw soil followed by the earthworms with an EC₅₀ of 40.9 mg a.i. kg⁻¹ dw soil. The insecticide ethoprophos was the most toxic to collembolans affecting their reproduction with an EC₅₀ of 0.027 mg a.i. kg⁻¹ dw soil. Enchytraeids were generally the least sensitive of the three species tested for long-term effects. Earthworms were not always the most sensitive species, emphasizing the need to increase the number of mandatory assays with key non-target organisms in the environmental risk assessment of pesticides.     

Miscanthus sinensis and wild flowers as food resources of Lumbricus terrestris L.

Senescent leaves of Miscanthus sinensis contained 36% soluble polysaccharides, 26% cellulose and had a C/N ratio of 45. In 11 wild flower species contents of soluble polysaccharides (21–30%), cellulose (3–16%) and C/N ratio (13–31) were lower. Decomposing leaves of M. sinensis lost weight at a rate of 0.002 day⁻¹, increased the C/N ratio from 45 to about 100, the bacterial biomass from 0.4 to 1 μg C mg⁻¹ dry weight, and decreased the tensile strength from 35 to 10 N. The withdrawal rate of Lumbricus terrestris with senescent leaves of M. sinensis was 30 mg g⁻¹ week⁻¹; the feeding rate was lower. With most senescent wild flowers withdrawal and feeding rates were higher. During decomposition of M. sinensis withdrawal rates increased to about 90, and feeding rates to about 30 mg g⁻¹ week⁻¹. The rates were not related to soluble polysaccharides, cellulose, acid-insoluble residue, C/N ratio and the presence of trichomes on the leaves. The abundance of L. terrestris decreased in a meadow turned into a field of M. sinensis from 55 to 26 earthworms m⁻² and increased in a rotational maize field turned into wild flower strips from 28 to 46 earthworms m⁻². The species richness of earthworms decreased with M. sinensis from 7.2 to 4.7 and increased with wild flowers from 4.7 to 6.7 species per sampling unit.     

Toxicity of three pesticides commonly used in Brazil to Pontoscolex corethrurus (Müller, 1857) and Eisenia andrei (Bouché, 1972)

The indiscriminate and excessive use of pesticides poses serious risks to humans and the environment, including soil biota. Ecotoxicological tests are useful to indicate the extent to which these chemicals are harmful and how and where their effects occur. Some of these tests were standardized by ISO (International Organization for Standartization) using the earthworm species Eisenia fetida and Eisenia andrei, both native to temperate climates. However, these species may be of lower relevance for soil ecotoxicological studies since they live in the litter and feed on fresh organic matter. The species Pontoscolex corethrurus, native to tropical regions, may be an alternative for more relevant ecotoxicological tests as it is an endogeic geophagous species. However, little is known of its sensitivity to pesticides. Therefore, avoidance and mortality tests were performed using E. andrei and P. corethrurus and three pesticides commonly used in Brazilian agriculture: carbendazim, carbofuran and glyphosate. The tests were conducted in tropical artificial soil (TAS). For carbendazim, the median avoidance concentration (AC₅₀) was 76.1 and 65.8 mg a.i. kg⁻¹ and the median lethal concentration (LC₅₀) 19.7 and 15.3 mg a.i. kg⁻¹ for E. andrei and P. corethrurus, respectively. For carbofuran, the AC₅₀ was 9.7 and 7.3 mg a.i. kg⁻¹ and LC₅₀ 13.5 and 9.3 mg a.i. kg⁻¹ for E. andrei and P. corethrurus, respectively. Concentrations applied in the field of these two pesticides have toxic effects on both species. Glyphosate showed no toxic effects for either species even at the highest concentration tested (47 mg a.i. kg⁻¹), although they displayed avoidance behavior at this concentration. The sensitivity of P. corethrurus appears to be similar to the standard species for the pesticides evaluated reinforcing the notion that E. andrei is a good test species. Nevertheless, further studies should be undertaken using other contaminants to confirm the similar sensitivity of both species and the relevance of E. andrei in ecotoxicological tests.     

Microbial biomass in a semi arid soil of the central highlands of Mexico cultivated with maize or under natural vegetation

Microbial biomass (MB) is the key factor in nutrient dynamics in soil, but no information exists how clearing of vegetation to cultivate maize in the central highlands of Mexico might affect it. Soil MB was measured with the chloroform fumigation incubation (CFI) and fumigation extraction (CFE) techniques and the substrate-induced respiration (SIR) method in soil sampled under or outside the canopy of mesquite (Prosopis laevigata) and huisache (Acacia tortuoso), N₂ fixing shrubs, and from fields cultivated with maize. Microbial biomass C as measured with the CFI technique ranged from 122 mg C kg⁻¹ in agricultural soil to 373 mg C kg⁻¹ in soil sampled under mesquite shrubs. Microbial biomass N as measured with the CFI technique ranged from 11 mg N kg⁻¹ in agricultural soil to 116 mg N kg⁻¹ in soil sampled under mesquite shrub. The ratio of microbial biomass C as measured with CFI related to the ninhydrin-positive compounds (NPC) was 12.23 after 1 day and 8.43 after 10 days while the relationship with extractable C was 3.15 and 2.96, respectively. The metabolic quotient (qCO₂) decreased in the order OUTSIDE > MESQUITE > HUIZACHE > AGRICULTURE, and the microbial biomass:soil organic C ratio decreased in the order MESQUITE > HUIZACHE > OUTSIDE > AGRICULTURE using SIR to determine the microbial biomass. It was found that converting soil under natural vegetation to arable soil was not only detrimental for soil quality, but might be unsustainable as organic matter input is limited.     

A genotype dependent-response to cadmium contamination in soil is displayed by Pinus pinaster in symbiosis with different mycorrhizal fungi

Soil contamination with Cd is of primary concern and beneficial soil restoration strategies urge. The aim of this work is to evaluate the response of two different genotypes of Pinus pinaster (wild and selected) to Cd contamination and to assess how inoculation with ectomycorrhizal fungi, Suillus bovinus and Rhizopogon roseolus, influenced each genotype. Seedlings were exposed to soil contaminated at 15 and 30 mg Cd kg⁻¹. Plant growth, mycorrhizal traits and Cd accumulation in different tissues of the plant were determined at harvest. The fungal community was assessed by denaturing gradient gel electrophoresis. At 15 mg Cd kg⁻¹ S. bovinus increased aboveground development in both genotypes. At 30 mg Cd kg⁻¹ non-inoculated wild genotype accumulated more Cd in the shoots (1.7-fold) than the selected genotype; inoculation with R. roseolus decreased Cd concentration in the roots of the selected genotype whereas the opposite occurred in the wild genotype. Cd concentration in the root system was the parameter most influenced by the interaction between the three studied variables. The fungal community established was affected by the Cd concentration in the soil. Results show that different genotypes of P. pinaster react differently to Cd exposure depending on the mycorrhizal association. The importance of considering the combination between plant genotype and its symbiotic partners when aiming at the forestation of degraded land is highlighted.     

Arbuscular mycorrhizal abundance in contaminated soils around a zinc and lead deposit

In order to study the variations in spore abundance and root colonization parameters of arbuscular mycorrhizal (AM) fungi in a naturally heavy metals polluted site and their relationships with soil properties, 35 plots in the Anguran Zn and Pb mining region were selected along a transect from the mine to 4500 m away. Within each plot, a composite sample of root and rhizospheric soil from a dominant indigenous plant was collected. The soil samples were analyzed for their physico-chemical characteristics. Spores were extracted, counted and identified at genus level. The roots were examined for colonization, arbuscular abundance, mycorrhizal frequency and intensity. Along the transect, the total and available (DTPA-extractable) concentration of Zn decreased from 6472 to 45 mg kg⁻¹ and 75 to 5 mg kg⁻¹, respectively. For Pb the values varied from 5203 to 0 mg kg⁻¹ and 32 to 0 mg kg⁻¹, respectively. In parallel, root colonization rate in the dominant native plants (except Alyssum sp.) varied from 35% to 85% and the spore numbers from 80 to 1306 per 200 g dry soil along the transect. Spores of Glomus were abundantly found in all plots as dominant, while Acaulospora spores were observed only in some moderately polluted and in control plots. AM fungal propagules never disappeared completely even in soils with the highest rates of both heavy metals. Spore numbers were more affected by Zn and Pb concentrations than root colonization. The variations of AM fungi propagules were better related to available than to total concentration of both metals. Spore numbers were positively correlated with mycorrhizal colonization parameters, particularly with arbuscular abundance.     

Organic matter and polyacrylamide amendment of Norfolk loamy sand

Loamy sand soils of the southeastern USA Coastal Plains often have poor physical properties because they contain cemented subsurface hard layers that restrict root development and yield. Their physical properties can be improved by adding amendments. Polyacrylamide (PAM) amendments and/or organic matter (OM) in the form of ground wheat (Triticum aestivum L.) stubble or pecan (Carya illinoensis) branches were mixed into a blend of 90% E horizon and 10% Ap horizon (to assure microbial presence) obtained from a Norfolk soil (Acrisol or fine-loamy, siliceous, thermic Typic Kandiudult). We hypothesized that incorporation of these amendments would improve soil physical properties by reducing strength and improving aggregation. Amended treatments contained 450 g of soil, OM, and 30 or 120 mg kg⁻¹ of PAM (12 mg mol⁻¹, anionic, and 35% charge density); treatments were incubated for 96 days at 10% (w/w) water content. Twice during the incubation period, treatments were leached with 1.3 pore volumes of deionized water. After leaching and equilibrating to stable water contents, treatments were analyzed for bulk densities and probed with a 5-mm diameter flat-tipped bench-top penetrometer to measure penetration resistances. Though penetration resistances increased for the highest level of PAM amendment, they showed no significance when both PAM and OM were added to the soil. When compared to controls, treatments with PAM at 120 mg kg⁻¹ had decreased bulk densities. Treatments with both rates of PAM had decreased requirements for water needed to maintain treatments at 10% water contents. Aggregation increased with increasing amounts of PAM but showed no consistent trend when both PAM and OM were added to the soil. Because PAM increased aggregation and water holding capacities in these coastal soils, it could reduce the need for deep tillage. However, more work needs to be done to determine an effective mix of PAM and OM.     

Crop growth,culturable bacteria,and degradation of petrol hydrocarbons (PHCs) in a long-term contaminated field soil

A greenhouse pot experiment was conducted for investigating the capability of a grass (annual ryegrass), a legume (summer vetch), and a crucifer (white mustard) to grow in a soil with portions from a former coal gasification site, influence the soil bacterial community, and promote the biodegradation of petrol hydrocarbons (PHCs). Soil concentrations of 1517 mg kg⁻¹ of total petrol hydrocarbons (TPHs), including 71.4 mg kg⁻¹ of total US EPA priority polycyclic aromatic hydrocarbons (TPAHs) have caused a significant (P < 0.05) reduction in shoot and root dry matter yields by more than 50%. Culturable bacteria and actinomycetes in soil were as much as 18-fold more abundant and the species composition was largely altered because of PHC contaminants and depending on crop species and age. After 95 days, 68.7% of initial TPH amounts and 59% of the TPAHs had disappeared from unplanted soil. Mustard and vetch fostered the removal of PHCs from soil reaching final TPH concentrations that were 15.6% and 12% lower than in unplanted soil. Both crops elicited the greatest degradative root activities and sustained particularly great populations of rhizosphere bacteria that are known hydrocarbon degraders. None of the crops aided the reduction of TPAHs in soil.