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.     

Survival and persistence of genetically modified Sinorhizobium meliloti in soil

Studies were conducted to evaluate the survival and persistence of Sinorhizobium meliloti 104A14 and two acid phosphatase-negative mutants in Kirkland (fine, mixed, thermic Udertic Paleustolls) silt loam soils with various fertility levels, and to assess the impact of inoculation on nodule occupancy and soil microbial community structure in the inoculated alfalfa (Medicago sativa L.) rhizosphere. Recovery of the inoculated strains was 100% (in the order of 10⁸ cells g⁻¹ soil) immediately following inoculation to soils, but decreased from 10⁸ cells g⁻¹ soil to undetectable levels in a nutrient-poor soil within 32 days. In a nutrient-rich soil, approximately 2–3% (4.7–7.43×10⁶ cells g⁻¹ soil) of the mutants and 23% (5.84×10⁷ cells g⁻¹ soil) of the wild-type inocula persisted for more than 64 days. Survivability and persistence of the wild-type S. meliloti were significantly greater than that of the genetically modified acid phosphatase negative mutants in all the soils tested. The persistence and nodule occupancy of the introduced S. meliloti in sterile and non-sterile soils were also tested for two repeated alfalfa growth periods in the same plant growth units, with a 1 month interval in between and no additional inoculation for the second period. Nodule occupancy of the introduced S. meliloti in non-sterile soils ranged from 30 to 60% for the first period and 85 to 100% for the second period. Our results suggest that survival and persistence of S. meliloti was enhanced by alfalfa cultivation and increased soil fertility, but impaired by mutation of acid phosphatase genes regardless of phosphorus nutritional levels. Moreover, inoculation with genetically modified S. meliloti strain 104A14 promoted indigenous bacterial growth in soil (increased bacterial population from 1.4×10⁶ to 4.3×10⁶ cells g⁻¹ soil), but not the growth of fungi and yeast. However, inoculation of the wild-type S. meliloti or genetically modified mutants did not result in significant changes in microbial community structure as indicated by EP indices and ratios of r/K strategists.     

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.     

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.     

Evaluation of an optimal extraction method for measuring d-ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) in agricultural soils and its association with soil microbial CO2 assimilation

Assimilating atmospheric carbon (C) into terrestrial ecosystems is recognized as a primary measure to mitigate global warming. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) is the dominant enzyme by which terrestrial autotrophic bacteria and plants fix CO₂. To investigate the possibility of using RubisCO activity as an indicator of microbial CO₂ fixation potential, a valid and efficient method for extracting soil proteins is needed. We examined three methods commonly used for total soil protein extraction. A simple sonication method for extracting soil protein was more efficient than bead beating or freeze–thaw methods. Total soil protein, RubisCO activity, and microbial fixation of CO₂ in different agricultural soils were quantified in an incubation experiment using ¹⁴C-CO₂ as a tracer. The soil samples showed significant differences in protein content and RubisCO activity, defined as nmol CO₂ fixed g⁻¹ soil min⁻¹. RubisCO activities ranged from 10.68 to 68.07 nmol CO₂ kg⁻¹ soil min⁻¹, which were closely related to the abundance of cbbL genes (r = 0.900, P = 0.0140) and the rates of microbial CO₂ assimilation (r = 0.949, P = 0.0038). This suggests that RubisCO activity can be used as an indicator of soil microbial assimilation of atmospheric CO₂.     

Rapid estimation of microbial biomass in grassland soils by ultra-violet absorbance

A reliable and simple technique for estimating soil microbial biomass (SMB) is essential if the role of microbes in many soil processes is to be quantified. Conventional techniques are notoriously time-consuming and unreproducible. A technique was investigated that uses the UV absorbance at 280 nm of 0.5 M K₂SO₄ extracts of fumigated and unfumigated soils to estimate the concentrations of carbon, nitrogen and phosphorus in the SMB. The procedure is based on the fact that compounds released after chloroform fumigation from lysed microbial cells absorb in the near UV region. Using 29 UK permanent grassland soils, with a wide range of organic matter (2.9–8.0%) and clay contents (22–68%), it was demonstrated that the increase in UV absorbance at 280 nm after soil fumigation was strongly correlated with the SMB C (r=0.92), SMB N (r=0.90) and SMB P (r=0.89), as determined by conventional methods. The soils contained a wide range of SMB C (412–3412 μg g⁻¹ dry soil), N (57–346 μg g⁻¹ dry soil) and P (31–239 μg g⁻¹ dry soil) concentrations. It was thus confirmed that the UV absorbance technique described was a rapid, simple, precise and relatively inexpensive method of estimating soil microbial biomass.     

Effects of cultivation on soil microbiological properties in a freshwater marsh soil in Northeast China

The Sanjiang Plain has become an intensive area of land use/cover change in China. However, little is known about the effect of cultivation on soil microbiological properties in this freshwater marsh ecosystem. Our objective was to evaluate the effect of cultivation on mineralizable, microbial biomass, and total C in the Sanjiang Plain of Northeast China. Soil microbial biomass C (MBC) was 4346 ± 309 mg kg⁻¹ in undisturbed marsh and 229 mg kg⁻¹ in soil cultivated for 15 years. Undisturbed marsh soil had the highest microbial quotient (3.64%), which declined with increasing cultivation time (R² = 0.97, p < 0.01). Metabolic quotient increased with increasing cultivation time. Soil C mineralization in undisturbed marsh was 3.5 times that in soil cultivated for 1 year, and was 12 times that in soil cultivated for 15 years. Cultivation strongly affected measured soil microbiological properties.     

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.     

Accounting for variability in soil microbial communities of temperate upland grassland ecosystems

This study aimed to determine the factors which regulate soil microbial community organisation and function in temperate upland grassland ecosystems. Soil microbial biomass (C_mic), activity (respiration and potential carbon utilisation) and community structure (phospholipid fatty acid (PLFA) analysis, culturing and community level physiological profiles (CLPP) (Biolog^®)) were measured across a gradient of three upland grassland types; Festuca–Agrostis–Galium grassland (unimproved grassland, National Vegetation Classification (NVC) — U4a); Festuca–Agrostis–Galium grassland, Holcus–Trifolium sub-community (semi-improved grassland, NVC — U4b); Lolium–Cynosurus grassland (improved grassland, NVC — MG6) at three sites in different biogeographic areas of the UK over a period of 1 year. Variation in C_mic was mainly due to grassland type and site (accounting for 55% variance, v, in the data). C_mic was significantly (P<0.001) high in the unimproved grassland at Torridon (237.4 g C m⁻² cf. 81.2 g C m⁻² in semi- and 63.8 g C m⁻² in improved grasslands) and Sourhope (114.6 g C m⁻² cf. in 44.8 g C m⁻² semi- and 68.3 g C m⁻² in improved grasslands) and semi-improved grassland at Abergwyngregyn (76.0 g C m⁻² cf. 41.7 g C m⁻² in un- and 58.3 g C m⁻² in improved grasslands). C_mic showed little temporal variation (v=3.7%). Soil microbial activity, measured as basal respiration was also mainly affected by grassland type and site (n=32%). In contrast to C_mic, respiration was significantly (P<0.001) high in the improved grassland at Sourhope (263.4 l h⁻¹m⁻² cf. 79.6 l h⁻¹m⁻² in semi- and 203.9 l h⁻¹m⁻² unimproved grasslands) and Abergwyngregyn (198.8 l h⁻¹m⁻² cf. 173.7 l h⁻¹m⁻² in semi- and 88.2 l h⁻¹m⁻² unimproved grasslands). Microbial activity, measured as potential carbon utilisation, agreed with the respiration measurements and was significantly (P<0.001) high in the improved grassland at all three sites (A₅₉₀ 0.14 cf. 0.09 in semi- and 0.07 in unimproved grassland). However, date of sampling also had a significant (P<0.001) impact on C utilisation potential (v=24.7%) with samples from April 1997 having highest activity at all three sites. Variation in microbial community structure was due, predominantly, to grassland type (average v=23.6% for bacterial and fungal numbers and PLFA) and date of sampling (average v=39.7% for bacterial and fungal numbers and PLFA). Numbers of culturable bacteria and bacterial PLFA were significantly (P<0.001) high in the improved grassland at all three sites. Fungal populations were significantly (P<0.01) high in the unimproved grassland at Sourhope and Abergwyngregyn. The results demonstrate a shift in soil microbial community structure from one favouring fungi to one favouring bacteria as grassland improvement increased. Numbers of bacteria and fungi were also significantly (P<0.001) higher in August than any other sampling date. Canonical variate analysis (CVA) of the carbon utilisation data significantly (P<0.05) differentiated microbial communities from the three grassland types, mainly due to greater utilisation of sugars and citric acid in the improved grasslands compared to greater utilisation of carboxylic acids, phenolics and neutral amino acids in the unimproved grasslands, possibly reflecting substrate availability in these grasslands. Differences in C_mic, activity and community structure between grassland types were robust over time. In addition, broad scale measures of microbial growth and activity (C_mic and respiration) showed little temporal variation compared to measures of soil microbial community structure, which varied quantitatively with respect to environmental variables (temperature, moisture) and plant productivity, hence substrate supply.     

Microbial properties and soil respiration in submontane forests of Venezuelian Guyana: characteristics and response to fertilizer treatments

The distribution of vegetation types in Venezuelan Guyana (in the ‘Canaima’ National Park) represents a transitional stage in a long term process of savannization, a process considered to be conditioned by a combined chemical and intermittent drought stress. All types of woody vegetation in this environment accumulate large amounts of litter and soil organic carbon (SOC). We hypothesized that this accumulation is caused by low microbial activity. During 1 year we measured microbial biomass carbon (Cmic), microbial respiration and soil respiration of stony Oxisols (Acrohumox) at a tall, a medium and a low forest and with three chemical modifications of site conditions by the addition of NO₃⁻, Ca²⁺ and PO₄³⁻ as possible limiting elements. Due to high SOC contents, mean Cmic was 1 mg g soil⁻¹ in the mineral topsoil and 3 mg g soil⁻¹ in the forest floor. Mean microbial respiration in the mineral topsoil and the forest floor were 165 and 192 μg CO₂-C g soil⁻¹ d⁻¹, respectively. We calculated high mean metabolic quotients (qCO₂) of 200 mg CO₂-C g Cmic⁻¹ d⁻¹ in the litter layer and 166 mg CO₂-C g Cmic⁻¹ d⁻¹ in the mineral topsoil, while the Cmic-to-SOC ratios were as low as 1.0% in the litter layer and 0.8% in the mineral topsoil. Annual soil respiration was 9, 12 and 10 Mg CO₂-C ha⁻¹ yr⁻¹ in the tall, medium and low forest, respectively. CO₂ production was significantly increased by CaHPO₄ fertilization, but no consistent effects were caused by Ca²⁺ and NO₃⁻, fertilization. Our findings indicate that Cmic and microbial respiration are reduced by low nutrient concentrations and low litter and SOC quality. Reduced microbial decomposition may have contributed to SOC accumulation in these forests.     

Characteristics of earthworms (Eisenia fetida) in PAHs contaminated soil amended with sewage sludge or vermicompost

Earthworms can be used to remove polycyclic aromatic hydrocarbons (PAHs) from soil, but this might affect their survival and they might accumulate the contaminants. Sterilized and unsterilized soil was contaminated with phenanthrene (Phen), anthracene (Anth) and benzo(a)pyrene (BaP), added with or without Eisenia fetida, sewage sludge or vermicompost. Survival, growth, cocoon formation and concentrations of PAHs in the earthworms were monitored for 70 days. Addition of sewage sludge to sterilized or unsterilized soil maintained the number of earthworms and their survival was 94%. The addition of sludge significantly increased the weight of earthworms 1.3 times compared to those kept in the unamended soil or in soil amended with vermicompost. The weight of earthworms was significantly lower in sterilized than in unsterilized soil. Cocoons were only detected when sewage sludge was added to unsterilized soil. A maximum concentration of 62.3 μg Phen kg⁻¹ was found in the earthworms kept in sterilized soil amended with vermicompost after 7 days and 22.3 μg Phen kg⁻¹ when kept in the unamended unsterilized soil after 14 days. Concentrations of Phen in the earthworms decreased thereafter and ≤2 μg kg⁻¹ after 28 days. A maximum Anth concentration of 82.5 μg kg⁻¹ was found in the earthworms kept in sterilized soil amended with vermicompost and 45.8 μg Anth kg⁻¹ when kept in the unamended unsterilized soil after 14 days. A maximum concentration of 316 μg BaP kg⁻¹ was found in the earthworms kept in sterilized soil amended with vermicompost after 56 days and 311 μg BaP kg⁻¹ when kept in the unsterilized soil amended with vermicompost after 28 days. The amount of BaP in the earthworm was generally largest after 28 days, but after 70 days still 60 μg kg⁻¹ was found in E. fetida when kept in the sterilized soil amended with sewage sludge. It was found that E. fetida survived in PAHs contaminated soil and accumulated only small amounts of the contaminants, but sewage sludge was required as food for its survival and cocoon production.     

Effects of nonylphenols on soil microbial activity and water retention

The main aim of this study is to analyze the influence of 4-nonylphenol (NP) on soil water retention and biological activity. Two doses of 4-nonylphenol (25 and 50 mg kg⁻¹) were tested in a loam soil with and without peat amendment. In general, one week after the start of the experiment, the soil water content retained at −0.75 MPa of soil suction was 18% higher in the soil amended and its basal respiration (BR) was 15% higher than soil without peat. In contrast, the microbial activity indices (CM: coefficient of mineralization or BR:total organic carbon (TOC) ratio; Cmic:Corg: microbial biomass carbon (MBC):TOC ratio; qCO₂: metabolic quotient or BR:MBC ratio) were higher in the soil without peat, compared to the soil amended with peat. On the other hand, the addition of NP to soil was able to modify soil biological but not physical (water retention, desorption) properties. When soil was amended with peat, MBC was reduced one week after applying NP. In contrast, no effects of NP on MBC were observed in the soil without peat. BR was reduced by 16% one week after applying 50 mg kg⁻¹ of NP to soil with peat, and was increased by 46% one week after applying 25 mg kg⁻¹ of NP to soil without peat. The effects of NP on MBC and BR could be associated more with the adsorption of NP by soil organic matter, while changes in CM or Cmic:Corg ratio were more closely related to changes in soil water retention. The potential toxic effects of NP (high qCO₂ values) were only observed in the absence of peat amendments. Peat addition reduced NP toxic effects on microorganisms.     

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.     

Effects of vegetation on soil microbial C,N, and P dynamics in a tropical forest and savanna of Central Africa

The forest–savanna transition zone is widely distributed on nutrient-poor oxisols in Central Africa. To reveal and compare the nutrient cycle in relation to soil microbes for forest and savanna vegetation in this area, we evaluated seasonal fluctuations in microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP) for 13 months as well as soil moisture, temperature, soil pH levels, and nutrients for both vegetation types in eastern Cameroon. Soil pH was significantly lower in forest (4.3) than in savanna (5.6), and soil N availability was greater in forest (87.1 mg N kg⁻¹ soil) than in savanna (32.9 mg N kg⁻¹ soil). We found a significant positive correlation between soil moisture and MBP in forest, indicating the importance of organic P mineralization for MBP, whereas in savanna, we found a significant positive correlation between soil N availability and MBP, indicating N limitation for MBP. These results suggest that for soil microbes, forest is an N-saturated and P-limited ecosystem, whereas savanna is an N-limited ecosystem. Additionally, we observed a significantly lower MBN and larger MB C:N ratio in forest (50.7 mg N kg⁻¹ soil and 8.6, respectively) than in savanna (60.0 mg N kg⁻¹ soil and 6.5, respectively) during the experimental period, despite the rich soil N condition in forest. This may be due to the significantly lower soil pH in forest, which influences the different soil microbial communities (fungi-to-bacteria ratio) in forest versus savanna, and therefore, our results indicate that, in terms of microbial N dynamics, soil pH rather than soil substrate conditions controls the soil microbial communities in this area. Further studies should be focused on soil microbial community, such as PLFA, which was not evaluated in the present study.     

Structural and functional diversity of bacterial community in soil treated with the herbicide napropamide estimated by the DGGE,CLPP and r/K-strategy approaches

Napropamide is one of the most commonly used herbicide in agricultural practice and can exhibit toxic effect to soil microorganisms. Therefore, the main objective of this study was to examine the genetic and functional diversity of microbial communities in soil treated with napropamide at field rate (FR, 2.25 mg kg⁻¹ of soil) and 10 times the FR (10 × FR, 22.5 mg kg⁻¹ of soil) by the denaturing gradient gel electrophoresis (DGGE) and the community level physiological profile (CLPP) methods. In addition, the r/K-strategy approach was used to evaluate the effect of this herbicide on the community structure of the culturable soil bacteria. DGGE patterns revealed that napropamide affected the structure of microbial community; however, the richness (S) and genetic diversity (H) values indicated that the FR dosage of napropamide experienced non-significant changes. In turn, the 10 × FR dosage of herbicide caused significant changes in the S and H values of dominant soil bacteria. DGGE profiles suggest an evolution of bacteria capable of degrading napropamide among indigenous microflora. Analysis of the CLPPs indicated that the catabolic activity of microbial community expressed as AWCD (average well-color development) was temporary positively affected after napropamide application and resulted in an increase of the substrate richness (S_R) as well as functional biodiversity (H) values. Analysis of the bacterial growth strategy revealed that napropamide affected the r- or K-type bacterial classes (ecotypes). In treated-soil samples K-strategists dominated the population, as indicated by the decreased ecophysiological (EP) index. Napropamide significantly affected the physiological state of culturable bacteria and caused a reduction in the rate of colony formation as well as a prolonged time of growth rate. Obtained results indicate that application of napropamide may poses a potential risk for soil functioning.     

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.     

Botanical survey and screening of plant species which accumulate 226Ra from contaminated soil of uranium waste depot

A geobotanical study was performed of the wild plants growing in the area of the old uranium mill tailings waste depot of a former uranium ore reprocessing factory in South Bohemia and the distribution of ²²⁶Ra in selected plants was determined. The distribution of ²²⁶Ra in contaminated soil was found to be extremely variable (from 7 to 32 Bq ²²⁶Ra g⁻¹ of DW). The differences in plant distribution were caused by factors of disturbance, soil properties (nutrients and salt content, water supply), and successional stage. No direct relation was proved between plant species characteristics and their radioactivity content. The results showed a great range of variation in the accumulation of ²²⁶Ra by the plant species found. The highest activity of ²²⁶Ra was found in Potentilla reptans (4.09 Bq ²²⁶Ra g⁻¹ of DW), Mentha arvensis (4.00 Bq ²²⁶Ra g⁻¹ of DW), and Daucus carota (3.70 Bq ²²⁶Ra g⁻¹ of DW). About half of the plant species are used as medicinal plants and some of them are accumulators of ²²⁶Ra. However, no plants suitable for phytoextraction of ²²⁶Ra contaminated substrates were discovered.     

Soil losses due to cassava and sweet potato harvesting: A case study from low input traditional agriculture

Soil loss due to crop harvesting (SLCH) has been established as an important soil erosion process that has significantly contributed to soil degradation in highly mechanised agriculture. This has stimulated the need to investigate the importance of this process of erosion under low input agriculture where, until now, only water and tillage erosion are known as important phenomena causing soil degradation. This study was conducted in Eastern Uganda with the following objectives: (1) to assess the amount of soil lost due to the harvesting of cassava roots and sweet potato tubers under low input agriculture, (2) to look into the factors that influence variations in these soil losses, and (3) to estimate the amount of plant nutrients lost due to SLCH for cassava and sweet potato. Soil sticking to roots and tubers was washed and the soil suspension oven dried to estimate the amount of soil lost after harvesting. Mean annual soil loss for cassava was 3.4 tonnes ha⁻¹ and for sweet potato was 0.2 tonnes ha⁻¹. Ammonium acetate lactate extractable soil nutrient losses for cassava were N = 1.71 kg ha⁻¹ harvest⁻¹, P = 0.16 kg ha⁻¹ harvest⁻¹, K = 1.08 kg ha⁻¹ harvest⁻¹ and for sweet potato were N = 0.14, P = 0.01 kg ha⁻¹ harvest⁻¹, K = 0.15 kg ha⁻¹ harvest⁻¹. Difference in soil loss due to crop harvesting for cassava and sweet potato could be due to: (1) smaller yields of sweet potato leading to smaller soil losses on an area basis, (2) smoother skin and less kinked morphology of sweet potato that allowed less soil to adhere, and (3) the fact that sweet potato is planted in mounds which dry out faster compared to the soil under cassava. Soil moisture content at harvesting time and crop age were significant factors that explained the variations in the soil lost at cassava harvesting. Soil loss under cassava justifies the need to conduct further investigations on this process of soil erosion under low input agriculture.     

Temporal variations of rhizosphere and bulk soil chemistry in a Douglas fir stand

Rhizosphere soil can play a central role in the maintenance of the soil–plant system and influencing the biogeochemistry of forest ecosystems. However, rhizospheric studies in situ to understand the ecosystem functions are still lacking. Therefore, the objectives of the present study were to examine the differences in the chemical properties such as the pH, organic C, exchangeable base cations (EBC = Mg²⁺ + Ca²⁺ + K⁺), exchangeable acidity and cation exchange capacity (CEC) of three soil fractions (Bulk, B; Rhizosphere, R and Rhizosphere Interface, RI) and the evolution of chemical properties of soil samples collected in March and June from a Douglas fir ecosystem located in the Beaujolais Mounts in France.Most of the variables measured (organic matter, CEC and EBC) increased significantly in the same order (B < R < RI), indicating that the rhizosphere was a favorable interface for tree nutrition. These processes were more pronounced in June than in March, as the temperature and biological activities are normally higher in June than in March. The temporal variations of Al charge in R and B seem to depend on OH⁻, organic complexation and H⁺ production by roots and/or the organic matter degradation in the bulk soil. In the rhizosphere, the H⁺ or OH⁻ production depends on the N cycle. The results of this study and the support of independent mineralogical study using the same soil fractions, and of other field studies on the same site have all pointed at the importance of rhizosphere as an excellent indicator for the understanding of the ecosystem dynamics in both short- and long-term.     

The effects of fresh and stabilized pruning wastes on the biomass,structure and activity of the soil microbial community in a semiarid climate

The incorporation of organic amendments from pruning waste into soil may help to mitigate soil degradation and to improve soil fertility in semiarid ecosystems. However, the effects of pruning wastes on the biomass, structure and activity of the soil microbial community are not fully known. In this study, we evaluate the response of the microbial community of a semiarid soil to fresh and composted vegetal wastes that were added as organic amendments at different doses (150 and 300 t ha⁻¹) five years ago. The effects on the soil microbial community were evaluated through a suite of different chemical, microbiological and biochemical indicators, including enzyme activities, community-level physiological profiles (CLPPs) and phospholipid fatty acid analysis (PLFA). Our results evidenced a long-term legacy of the added materials in terms of soil microbial biomass and enzyme activity. For instance, cellulase activity reached 633 μg and 283 μg glucose g⁻¹ h⁻¹ in the soils amended with fresh and composted waste, respectively. Similarly, bacterial biomass reached 116 nmol g⁻¹ in the soil treated with a high dose of fresh waste, while it reached just 66 nmol g⁻¹ in the soil amended with a high dose of composted waste. Organic amendments produced a long-term increase in microbiological activity and a change in the structure of the microbial community, which was largely dependent on the stabilization level of the pruning waste but not on the applied dose. Ultimately, the addition of fresh pruning waste was more effective than the application of composted waste for improving the microbiological soil quality in semiarid soils.