Response of microbial activity and microbial community composition in soils to long-term arsenic and cadmium exposure

Arsenic (As) and cadmium (Cd) in soils can affect soil microbial function and community composition and, therefore, may have effects on soil ecosystem functioning. The aim of our study was to assess the effects of long-term As and Cd contamination on soil microbial community composition and soil enzyme activities. We analyzed soils that have been contaminated 25 years ago and at present still show enhanced levels of either As, 18 and 39 mg kg⁻¹, or Cd, 34 and 134 mg kg⁻¹. Soil without heavy metal addition served as control. Polymerase chain reaction (PCR) followed by denaturing gradient gel electrophoresis (DGGE) showed that bacterial community composition in As and Cd contaminated soils differed from that in the control soil. The same was true for the microbial community composition assessed by analysis of respiratory quinones. Soil fungi and Proteobacteria appeared to be tolerant towards As and Cd, while other groups of bacteria were reduced. The decline in alkaline phosphatase, arylsulphatase, protease and urease activities in the As- and Cd-contaminated soils was correlated with a decrease of respiratory quinones occuring in Actinobacteria and Firmicutes. Xylanase activity was unaffected or elevated in the contaminated soils which was correlated with a higher abundance of fungal quinones, and quinones found in Proteobacteria.     

Effects of sulfamethoxazole on soil microbial communities after adding substrate

The effect of the antibiotic sulfamethoxazole (SMX) on soil bacteria was studied using two methods (leucine incorporation and Biolog plates) of estimating pollution-induced community tolerance (PICT). SMX was added to an agricultural soil in a microcosm setup. The addition of different substrates (manure and alfalfa), and a non-amended soil, were also studied over 5 weeks. PICT measurements were validated by comparison with other measurements. Community structure was assessed using phospholipid fatty acid (PLFA) analysis and community-level physiological profiling (CLPP), and bacterial growth was estimated using leucine incorporation. Increased PICT was found at SMX concentrations of 20 and 500 mg SMX kg⁻¹ soil in samples containing manure and alfalfa, and at 500 mg SMX kg⁻¹ soil in non-amended soil (only concentration tested) using leucine incorporation. No effect was seen at 1 mg SMX kg⁻¹ soil. It was not necessary to add any substrate to increase the microbial activity in order to detect the effects of a bacteriostatic toxicant such as SMX when using measures based on bacterial growth. Direct inhibition of bacterial growth 2 days after SMX addition was correlated to PICT. No major changes in PICT due to SMX addition were found when using Biolog plates. However, there was a tendency towards increased PICT at the higher SMX concentrations in the manure-amended soil. Thus, different methods of detecting PICT have different sensitivities in detecting the toxic effects of SMX. The effects of substrate amendment were reflected by changes in the microbial community, estimated using both PLFA and CLPP. SMX was found to have a clear effect at the two highest levels of SMX in the manure- and alfalfa-amended soils, with an increase in fungal and a decrease in bacterial PLFAs. Little difference in the PLFA composition was found in the non-amended soil. CLPP was only affected at the highest SMX concentration. Although different variables showed different sensitivities to the effects of SMX, the results were consistent with an initial decrease in bacterial growth rates of sensitive species, which eventually transformed into more tolerant species, altering the community composition.     

A single application of Cu to field soil has long-term effects on bacterial community structure, diversity, and soil processes

Long-term diversity-disturbance responses of soil bacterial communities to copper were determined from field-soils (Spalding; South Australia) exposed to Cu in doses ranging from 0 through to 4012 mg Cu kg⁻¹ soil. Nearly 6 years after application of Cu, the structure of the total bacterial community showed change over the Cu gradient (PCR-DGGE profiling). 16S rRNA clone libraries, generated from unexposed and exposed (1003 mg Cu added kg⁻¹ soil) treatments, had significantly different taxa composition. In particular, Acidobacteria were abundant in unexposed soil but were nearly absent from the Cu-exposed sample (P<0.05), which was dominated by Firmicute bacteria (P<0.05). Analysis of community profiles of Acidobacteria, Bacillus, Pseudomonas and Sphingomonas showed significant changes in structural composition with increasing soil Cu. The diversity (Simpsons index) of the Acidobacteria community was more sensitive to increasing concentrations of CaCl-extractable soil Cu (Cu_Ext) than other groups, with decline in diversity occurring at 0.13 Cu_Ext mg kg⁻¹ soil. In contrast, diversity in the Bacillus community increased until 10.4 Cu_Ext mg kg⁻¹ soil, showing that this group was 2 orders of magnitude more resistant to Cu than Acidobacteria. Sphingomonas was the most resistant to Cu; however, this group along with Pseudomonas represented only a small percentage of total soil bacteria. Changes in bacterial community structure, but not diversity, were concomitant with a decrease in catabolic function (BioLog). Reduction in function followed a dose-response pattern with Cu_Ext levels (R²=0.86). The EC₅₀ for functional loss was 0.21 Cu_Ext mg kg⁻¹ soil, which coincided with loss of Acidobacteria diversity. The microbial responses were confirmed as being due to Cu and not shifts in soil pH (from use of CuSO₄) as parallel Zn-based field plots (ZnSO₄) were dissimilar. Changes in the diversity of most bacterial groups with soil Cu followed a unimodal response - i.e. diversity initially increased with Cu addition until a critical value was reached, whereupon it sharply decreased. These responses are indicative of the intermediate-disturbance-hypothesis, a macroecological theory that has not been widely tested in environmental microbial ecosystems.     

2-Phenylethylisothiocyanate concentration and microbial community composition in the rhizosphere of canola

Canola crops have been shown to inhibit soil-borne pathogens in following crops. This effect is mainly attributed to the release of low molecular S-containing compounds, such as isothiocyanates, during microbial degradation of the crop residues. We have assessed the effect of low concentrations of phenylethylisothiocyanate (PEITC) on soil microbial communities as well as its rate of degradation in soil and determined the concentration of PEITC and the microbial community structure in the rhizosphere of canola. PEITC was degraded within 96 h by soil microorganisms. PEITC added to the soil daily for 5 d affected both bacterial and eukaryotic community structure, determined by PCR-DGGE. Community structures of bacteria and eukaryotes changed at PEITC concentrations between 1300 and 3790 pmol g⁻¹ soil fresh weight but was unaffected at lower concentrations. The PEITC concentration in the rhizosphere of living canola roots was greater in first order laterals than in second order laterals. The maximal PEITC concentration detected in the rhizosphere was 1827 pmol g⁻¹. Redundancy analysis of the DGGE banding patterns indicated a significant correlation between the PEITC concentration in the rhizosphere and the community structure of the active fraction of eukaryotes and bacteria in the rhizosphere. Other important factors influencing the microbial community structure were soil moisture and plant dry matter. It is concluded that canola may affect the soil microbial community structure not only after incorporation of canola residues but also during active growth of the plants.     

Structure and function of the soil microbial community in a long-term fertilizer experiment

The effect of organic and inorganic fertiliser amendments is often studied shortly after addition of a single dose to the soil but less is known about the long-term effects of amendments. We conducted a study to determine the effects of long-term addition of organic and inorganic fertiliser amendments at low rates on soil chemical and biological properties. Surface soil samples were taken from an experimental field site near Cologne, Germany in summer 2000. At this site, five different treatments were established in 1969: mineral fertiliser (NPK), crop residues removed (mineral only); mineral fertiliser with crop residues; manure 5.2 t ha⁻¹ yr⁻¹; sewage sludge 7.6 t ha⁻¹ yr⁻¹ or straw 4.0 t ha⁻¹ yr⁻¹ with 10 kg N as CaCN₂ t straw⁻¹. The organic amendments increased the C_org content of the soil but had no significant effect on the dissolved organic C (DOC) content. The C/N ratio was highest in the straw treatment and lowest in the mineral only treatment. Of the enzymes studied, only protease activity was affected by the different amendments. It was highest after sewage amendment and lowest in the mineral only treatment. The ratios of Gram+ to Gram− bacteria and of bacteria to fungi, as determined by signature phospholipid fatty acids, were higher in the organic treatments than in the inorganic treatments. The community structure of bacteria and eukaryotic microorganisms was assessed by denaturing gradient gel electrophoresis (DGGE) and redundancy discriminate analyses of the DGGE banding patterns. While the bacterial community structure was affected by the treatments this was not the case for the eukaryotes. Bacterial and eukaryotic community structures were significantly affected by C_org content and C/N ratio.     

Increased degradation of phenanthrene in soil by Pseudomonas sp. GF3 in the presence of wheat

A phenanthrene-degrading bacterial strain Pseudomonas sp. GF3 was examined for plant-growth promoting effects and phenanthrene removal in soil artificially contaminated with low and high levels of phenanthrene (0, 100 and 200 mg kg⁻¹) in pot experiments. Low and high phenanthrene treatments significantly decreased the growth of wheat. Inoculation with bacterial strain Pseudomonas sp. GF3 was found to increase root and shoot growth of wheat. Strain GF3 was able to degrade phenanthrene effectively in the unplanted and planted soils. Over a period of 80 days the concentration of phenanthrene in soil in which wheat was grown was significantly lower than in unplanted soil (p<0.05). At the end of the 80-d experiments, 62.2% and 42.3% of phenanthrene had disappeared from planted soils without Pseudomonas sp. GF3 when the phenanthrene was added at 100 and 200 mg kg⁻¹ soil, respectively, but 84.8% and 70.2% of phenanthrene had disappeared from planted soils with the bacterial inoculation. The presence of vegetation significantly enhances the dissipation of phenanthrene in the soil. There was no significant difference in soil polyphenol oxidase activities among the applications of 0, 100 and 200 mg kg⁻¹ of phenanthrene. However, the enzyme activities in planted and unplanted soils inoculated with the strain Pseudomonas sp. GF3 were significantly higher than those of non-inoculation controls. The bacterial isolate was also able to colonize and develop in the rhizosphere soil of wheat after inoculation.     

Gross nitrogen transformations in adjacent native and plantation forests of subtropical Australia

The impact of land-use change on soil nitrogen (N) transformations was investigated in adjacent native forest (NF), 53 y-old first rotation (1R) and 5 y-old second rotation (2R) hoop pine (Araucaia cunninghamii) plantations. The ¹⁵N isotope dilution method was used to quantify gross rates of N transformations in aerobic and anaerobic laboratory incubations. Results showed that the land-use change had a significant impact on the soil N transformations. Gross ammonification rates in the aerobic incubation ranged between 0.62 and 1.78 mg N kg⁻¹ d⁻¹, while gross nitrification rates ranged between 2.1 and 6.6 mg N kg⁻¹ d⁻¹. Gross ammonification rates were significantly lower in the NF and the 1R soils than in the 2R soils, however gross nitrification rates were significantly higher in the NF soils than in the plantation soils. The greater rates of gross nitrification found in the NF soil compared to the plantation soils, were related to lower soil C:N ratios (i.e. more labile soil N under NF). Nitrification was found to be the dominant soil N transformation process in the contrasting forest ecosystems. This might be attributed to certain site conditions which may favour the nitrifying community, such as the dry climate and tree species. There was some evidence to suggest that heterotrophic nitrifiers may undertake a significant portion of nitrification.     

Enzyme activities and microbial biomass in coastal soils of India

Soil salinity is a serious problem for agriculture in coastal regions, wherein salinity is temporal in nature. We studied the effect of salinity, in summer, monsoon and winter seasons, on microbial biomass carbon (MBC) and enzyme activities (EAs) of the salt-affected soils of the coastal region of the Bay of Bengal, Sundarbans, India. The average pH of soils collected from different sites, during different seasons varied from 4.8 to 7.8. The average organic C (OC) and total N (TN) content of the soils ranged between 5.2-14.1 and 0.6-1.4 g kg⁻¹, respectively. The electrical conductivity of the saturation extract (ECe) of soils, averaged over season, varied from 2.2 to 16.3 dSm⁻¹. The ECe of the soils increased five fold during the summer season (13.8 dSm⁻¹) than the monsoon season (2.7 dSm⁻¹). The major cation and anion detected were Na⁺ and Cl⁻, respectively. Seasonality exerted considerable effects on MBC and soil EAs, with the lowest values recorded during the summer season. The activities of β-glucosidase, urease, acid phosphatase and alkaline phosphatase were similar during the winter and monsoon season. The dehydrogenase activity of soils was higher in monsoon than in winter. Average MBC, dehydrogenase, β-glucosidase, urease, acid phosphatase and alkaline phosphatase activities of the saline soils ranged from 125 to 346 mg kg⁻¹ oven dry soil, 6-9.9 mg triphenyl formazan (TPF) kg⁻¹ oven dry soil h⁻¹, 18-53 mg p-nitro phenol (PNP) kg⁻¹ oven dry soil h⁻¹, 38-86 mg urea hydrolyzed kg⁻¹ oven dry soil h⁻¹, 213-584 mg PNP kg⁻¹ oven dry soil h⁻¹ and 176-362 mg PNP g⁻¹ oven dry soil h⁻¹, respectively. The same for the non-saline soils were 274-446 mg kg⁻¹ oven dry soil, 8.8-14.4 mg TPF kg⁻¹ oven dry soil h⁻¹, 41-80 mg PNP kg⁻¹ oven dry soil h⁻¹, 89-134 mg urea hydrolyzed kg⁻¹ oven dry soil h⁻¹, 219-287 mg PNP kg⁻¹ oven dry soil h⁻¹ and 407-417 mg PNP kg⁻¹ oven dry soil h⁻¹, respectively. About 48%, 82%, 48%, 63%, 40% and 48% variation in MBC, dehydrogenase activity, β-glucosidase activity, urease activity, acid phosphatase activity and alkaline phosphatase activity, respectively, could be explained by the variation in ECe of saline soils. Suppression of EAs of the coastal soils during summer due to salinity rise is of immense agronomic significance and needs suitable interventions for sustainable crop production.     

Dynamics of the nitrous oxide reducing community during adaptation to Zn stress in soil

Laboratory studies show that the nitrous oxide (N₂O) reduction rate in soil is strongly inhibited by trace metal contamination; however, this effect appears transient. Here we assess if this recovery is due to microbial adaptation associated with shifts in community composition. Soils were spiked with zinc chloride (0-5000 mg Zn kg⁻¹) in a factorial design with 3 application rates of organic matter (OM), i.e. 0, 2 and 4 g milled hay kg⁻¹, to accelerate growth and, potentially, adaptation rate. The soil treatments were incubated outdoors with free drainage during 1 year and periodically sampled. The potential N₂O reduction rate, measured in an anaerobic laboratory assay, was inhibited by Zn during the first 2 months after spiking with 50% inhibition at 500-1000 mg Zn kg⁻¹. After 6 months exposure, the N₂O reduction rate recovered to at least 80% of the rate in the control treatment in the series receiving OM up to the largest Zn dose, but strong inhibition remained in the series which did not receive OM. In this series recovery was only observed after 12 months exposure. Soil pore water Zn concentrations did not explain the recovery of the N₂O reduction rate in the control series suggesting that recovery is due to adaptation and not to reduced Zn bioavailability. The faster recovery in the series receiving OM was partially, but not fully related to the effects of OM on Zn bioavailability. The recovery at all Zn and OM treatments co-varied with a recovery of nosZ gene abundance from about 1 × 10⁷ copies g⁻¹ soil in the soil treatments with decreased activity to 5 × 10⁸ copies g⁻¹ soil in the other soil treatments. The nosZ gene DGGE profile of the soil microbial communities revealed minor changes in the nosZ containing community. This study strongly suggests that the transient effects of trace metal inhibition of N₂O reduction is due to the development of a Zn tolerant denitrifying community.     

In-situ enrichment and analysis of atrazine-degrading microbial communities using atrazine-containing porous beads

We examined the community composition of microbes that colonized atrazine-containing beads buried in agricultural soils that differed in atrazine treatment history. Bacterial abundance was 5-40-fold greater in atrazine-fortified beads. In beads containing 20 mg atrazine kg⁻¹ buried in soil with a history of atrazine application (conditioned soil), the abundance of Actinobacteria increased approximately 80-fold whereas in control soil, Actinobacteria were enriched only 10-fold and the gamma-Proteobacteria and Planctomycetes increased by 60- and 25-fold, respectively. The gamma-Proteobacteria were enriched by 120- and 230-fold in beads containing 200 mg atrazine kg⁻¹ in conditioned and control soil, respectively. The results demonstrate that BioSep^® beads are a suitable matrix for recruiting a diverse subset of the bacterial community involved in atrazine degradation.     

Impact of the antibiotic sulfadiazine and pig manure on the microbial community structure in agricultural soils

Large amounts of veterinary antibiotics enter soil via manure of treated animals. The effects on soil microbial community structure are not well investigated. In particular, the impact of antibiotics in the presence of manure is poorly understood. In this study, two agricultural soils, a sandy Cambisol (KS) and a loamy Luvisol (ML), were spiked with manure and sulfadiazine (SDZ; 0, 10 and 100 μg g^?1) and incubated for 1, 4, 32 and 61 days. Untreated controls received only water. The microbial community structure was characterised by investigating phospholipid fatty acids (PLFA) and using PCR–denaturing gradient gel electrophoresis (DGGE) of 16S rDNA. The total concentration of PLFA increased with addition of manure and was reduced by both SDZ concentrations at incubation times >4 days. The SDZ addition decreased the bacteria:fungi ratio. The largest stress level, measured as ratio of PLFA (cyc17:0 + cyc19:0)/(16:1ω7c + 18:1ω7c), was found for the controls, followed by the manure treatments and the SDZ treatments. A discriminant analysis of the PLFA clearly separated treatments and incubation times. Both soils differed in total PLFA concentrations and Gram^?:Gram⁺ ratios, but showed similar changes in PLFA pattern upon soil treatment. Effects of manure and SDZ on the bacterial community structure were also revealed by DGGE analysis. Effects on pseudomonads and β-proteobacteria were less pronounced. While community structure remained altered even after two months, the extractable concentrations of SDZ decreased exponentially and the remaining solution concentrations after 32 days were ≤27% of the spiking concentration. Our results demonstrate that a single addition of SDZ has prolonged effects on the microbial community structure in soils.     

Influence of selenium on oxidoreductive enzymes activity in soil and in plants

The aim of this greenhouse experiment was the assessment of the influence of H₂SeO₃ at soil concentrations of 0.05, 0.15 and 0.45 mmol kg⁻¹, on the activity of selected oxidoreductive enzymes in wheat (Triticum aestivum). The wheat plants were grown in 2 dm³ pots filled with dust-silt black soil of pH 7.7. Applied H₂SeO₃ caused activation of plant nitrate reductase at all concentrations, but activation of plant polyphenol oxidase at only two lower concentrations. The highest concentration caused inhibition of polyphenol oxidase and peroxidase. Plant catalase activity decreased under the influence of 0.15 and 0.45 mmol kg⁻¹ concentration. After the final analysis Se was quantified in plants and soil. The amounts in plants were: control (unamended soil) 1.95 mg kg⁻¹; I dose (0.05 mmol kg⁻¹) 18.27 mg kg⁻¹; II dose (0.15 mmol kg⁻¹) 33.20 mg kg⁻¹ and III dose (0.45 mmol kg⁻¹) 38.37 mg kg⁻¹, in soil: 0.265 mg kg⁻¹; 3.61 mg kg⁻¹; 10.53 mg kg⁻¹; 30.53 mg kg⁻¹; respectively. Simultaneously, a laboratory experiment was performed, where the activity of soil catalase and peroxidase were tested after 1, 3, 7, 14, 28, 56, and 112 days after Se treatment. Peroxidase activity in soil decreased with increasing Se content, over the whole experiment. The lowest dose of Se caused activation a significant 10% increase in catalase activity, but the influence of others doses was unclear.     

Hydrolase activity, microbial biomass and community structure in long-term Cd-contaminated soils

Long-term effects of high Cd concentrations on enzyme activities, microbial biomass and respiration and bacterial community structure of soils were assessed in sandy soils where Cd was added between 1988 and 1990 as Cd(NO₃)₂ to reach concentrations ranging from 0 to 0.36 mmol Cd kg⁻¹ dry weight soil. Soils were mantained under maize and grass cultivation, or ‘set-aside’ regimes, for 1 year. Solubility of Cd and its bioavailability were measured by chemical extractions or by the BIOMET bacterial biosensor system. Cadmium solubility was very low, and Cd bioavailability was barely detectable even in soils polluted with 0.36 mmol Cd kg⁻¹. Soil microbial biomass carbon (B_C) was slightly decreased and respiration was increased significantly even at the lower Cd concentration and as a consequence the metabolic quotient (qCO₂) was increased, indicating a stressful condition for soil microflora. However, Cd-contaminated soils also had a lower total organic C (TOC) content and thus the microbial biomass C-to-TOC ratio was unaffected by Cd. Alkaline phosphomonoesterase, arylsulphatase and protease activities were significantly reduced in all Cd-contaminated soils whereas acid phosphomonoesterase, β-glucosidase and urease activites were unaffected by Cd. Neither changes in physiological groups of bacteria, nor of Cd resistant bacteria could be detected in numbers of the culturable bacterial community. Denaturing gradient gel electrophoresis analysis of the bacterial community showed slight changes in maize cropped soils containing 0.18 and 0.36 mmol Cd kg⁻¹ soil as compared to the control. It was concluded that high Cd concentrations induced mainly physiological adaptations rather than selection for metal-resistant culturable soil microflora, regardless of Cd concentration, and that some biochemical parameters were more sensitive to stress than others.     

Sulphur immobilization and arylsulphatase activity in two calcareous arable and fallow soils as affected by glucose additions

In this study we examined the effects of glucose-C on the activities of fungi and bacteria determined by the method of substrate-induced respiration (SIR) in combination with the selective inhibition technique, the immobilized-S and the arylsulphatase (ARS) activity in two calcareous arable and fallow soils. The amounts of glucose-C were added at six doses: 0, 125, 250, 500, 750 and 1000 mg kg^− 1 soil to the soils and then incubated for one week with a Na₂³⁵SO₄ solution (518.9 kBq kg^− 1 dry soil and 20 mg S kg^− 1 dry soil) prior to analysis. At the highest dose of 1000 mg kg^− 1 soil, fungal activity increased by 59.1% (of the dose 0) versus 45.5% for bacterial activity in the arable soil, while in the fallow soil the increases were more marked and corresponded to 69.9% and 71.1%, respectively. Largest increase in immobilized-S was observed in the arable soil (300.7%) compared with the fallow soil (153.1%). In contrast, the ARS activity increased by 16.4% in the arable soil versus 32.1% in the fallow soil. These results indicate that glucose proportionately affected more the intensities of immobilized-S than those of ARS. Strong positive correlation coefficients were found between fungal activities and immobilized-S in the arable soil (r = 0.96, P < 0.01) and in the fallow soil (r = 0.98, P < 0.001). However, non-significant correlations were observed between fungal activities and ARS in both studied soils. As to bacterial activities, positive significant correlation coefficients were found with immobilized-S in the arable soil (r = 0.95, P < 0.01) and in the fallow soil (r = 0.90, P < 0.05) as well as with ARS activities in the arable soil (r = 0.83, P < 0.05) and in the fallow soil (r = 0.97, P < 0.01). Overall, we also found positive and significant correlation coefficients of immobilized-S with ARS activities in the arable soil (r = 0.86, P < 0.05) and in the fallow soil (r = 0.83, P < 0.05). Accordingly, the results showed a presence of extracellular arylsulphatase activity of 38.7 mg p-nitrophenol kg^− 1 soil h^− 1 in the arable soil and of 63.5 mg p-nitrophenol kg^− 1 soil h^− 1 in the fallow soil. It was concluded that fallowing maintained larger activities of fungi, bacteria and arylsulphatase compared with the arable soil.     

Combined effects of the antibiotic sulfadiazine and liquid manure on the soil microbial‐community structure and functions

Sulfonamides are the second most used antibiotic class in veterinary medicine and applied to livestock to treat bacterial infections. Subsequently, they are spread onto agricultural soils together with the contaminated manure used as fertilizer. Both manure and antibiotics affect the soil microbial community. However, the influence of different liquid manure loads on effects of antibiotics to soil microorganisms is not well understood. Therefore, we performed a microcosm experiment for up to 32 d to clarify whether the function and structure of the soil microbial community is differently affected by interactions of manure and the antibiotic sulfadiazine (SDZ). To this end selected concentrations of pig liquid manure (0, 20, 40, 80 g kg^–1) and SDZ (0, 10, 100 mg kg^–1) were combined. We hypothesized that incremental manure amendment might reduce the effect of SDZ in soils, due to an increasing sorption capacity of SDZ to organic compounds. Clear dose‐dependent effects of SDZ on microbial biomass and PLFA pattern were determined, and SDZ effects interacted with the liquid manure application rate. Soil microbial biomass increased with incremental liquid manure addition, whereas this effect was absent in the presence of additional SDZ. However, activities of enzymes such as urease and protease were only slightly affected and basal respiration was not affected by SDZ application, while differences mostly depended on the concentration of liquid manure. These results illustrated that the microbial biomass and structural composition react more sensitive to SDZ contamination than functional processes. Furthermore, effects disproportionally increased with incremental liquid manure addition, although extractable amounts of SDZ declined with increasing liquid manure application.     

Effects of municipal solid waste compost amendments on soil enzyme activities and bacterial genetic diversity

Municipal solid waste (MSW) composts have been used to maintain the long-term productivity of agroecosystems and to protect the soil environment from overcropping, changes in climatic conditions and inadequate management; they also have the additional benefit of reducing waste disposal costs. Since MSW may contain heavy metals and other toxic compounds, amendments cannot only influence soil fertility, but may also affect the composition and activity of soil microorganisms. The effects of MSW compost and mineral N amendments in a 6-year field trial on some physical-chemical properties, enzyme activities and bacterial genetic diversity of cropped plots (Beta vulgaris-Triticum turgidum rotation) and uncropped plots were investigated. The compost was added at the recommended and twice the recommended dosage (12, 24 t ha⁻¹). Amendments of cropped plots with MSW compost increased the contents of organic C from 13.3 to 15.0 g kg⁻¹ soil and total N from 1.55 to 1.65 g kg⁻¹ soil. There were significant increases in dehydrogenase (9.6%), β-glucosidase (13.5%), urease (15.4%), nitrate reductase (21.4%) and phosphatase (9.7%) activities. A significant reduction in protease activity (from 3.6 to 2.8 U g⁻¹ soil) was measured when a double dose of compost was added to the cropped plots. No dosage effect was detected for the other enzymes. Changes in the microbial community, as a consequence of MSW amendment, were minimal as determined using denaturing gradient gel electrophoresis, rDNA internal spacer analysis and amplified ribosomal DNA restriction analysis of bacteria, archaea, actinomycetes, and ammonia oxidizers. This indicates that there was no significant variation in the overall bacterial communities nor in selected taxonomic groups deemed to be essential for soil fertility.     

Exploring the mechanisms behind elevated microbial activity after wood ash application

Wood ash fertilization increases the pH and concentration of dissolved organic carbon (DOC) in the soil solution and enhances the activity of soil microorganisms. However, it is unknown whether DOC or pH is primarily responsible for the increase in microbial activity. We designed an experiment to separate the effects of DOC and/or pH on soil microbial activity using suspensions of humus extracts and bacteria that had not previously been exposed to wood ash fertilization. After a 3-week incubation, DOC extracts were obtained from control (DOC_C) and ash (DOC_A) treatments with carbon concentrations of 9.1 and 32.5 mg C l⁻¹, respectively. These extracts were supplied to bacterial suspensions at concentrations of 0 and 5 mg C l⁻¹. We controlled for pH by matching adjustments, i.e. the original pH of the DOC_C extract was 4.5 and its adjusted pH was 6.9, whereas the DOC_A extract was pH 6.9 originally and pH 4.5 adjusted. The relative bacterial growth rate (RBGR), as measured by ³H-thymidine incorporation, increased in suspensions of 5 mg C l⁻¹ DOC as compared to control suspensions of 0 mg C l⁻¹. At pH 6.9, RBGR was higher for both DOC extracts than at pH 4.5. These results suggest that both DOC and pH influence microbial activity. As the growth rate at pH 6.9 with DOC_A was higher than with DOC_C, the quality of the DOC extract must also play a role since the carbon concentration was controlled for. The decrease in relative abundance of hydrophobic and hydrophilic acids in DOC_A compared to DOC_C indicates a quality shift. As measured by DGGE banding patterns, the bacterial community structure changed over the course of the 24-h experiment in the following three trials, all of which received 5 mg C l⁻¹: DOC_C at pH 6.9 and DOC_A at pH 4.5 and 6.9. These results demonstrate that both the DOC origin (control vs. ash) and the pH influence a subset of the bacterial community.     

Cyanobacterial inoculation of heated soils: effect on microorganisms of C and N cycles and on chemical composition in soil surface

Physiological groups of soil microorganisms, total C and N and available nutrients were investigated in four heated (350 °C, 1 h) soils (one Ortic Podsol over sandstone and three Humic Cambisol over granite, schist or limestone) inoculated (1.5 μg chlorophyll a g⁻¹ soil or 3.0 μg chlorophyll a g⁻¹ soil) with four cyanobacterial strains of the genus Oscillatoria, Nostoc or Scytonema and a mixture of them.Cyanobacterial inoculation promoted the formation of microbiotic crusts which contained a relatively high number of NH₄⁺-producers (7.4×10⁹ g⁻¹ crust), starch-mineralizing microbes (1.7×10⁸ g⁻¹ crust), cellulose-mineralizing microbes (1.4×10⁶ g⁻¹ crust) and NO₂⁻ and NO₃⁻ producers (6.9×10⁴ and 7.3×10³ g⁻¹ crust, respectively). These crusts showed a wide range of C and N contents with an average of 293 g C kg⁻¹ crust and 50 g N kg⁻¹ crust, respectively. In general, Ca was the most abundant available nutrient (804 mg kg⁻¹ crust), followed by Mg (269 mg kg⁻¹ crust), K (173 mg kg⁻¹ crust), Na (164 mg kg⁻¹ crust) and P (129 mg kg⁻¹ crust). There were close positive correlations among all the biotic and abiotic components of the crusts.Biofertilization with cyanobacteria induced great microbial proliferation as well as high increases in organic matter and nutrients in the surface of the heated soils. In general, cellulolytics were increased by four logarithmic units, amylolytics and ammonifiers by three logarithmic units and nitrifiers by more than two logarithmic units. C and N contents rose an average of 275 g C kg⁻¹ soil and 50 g N kg⁻¹ soil while the C:N ratio decreased up to 7 units. Among the available nutrients the highest increase was for Ca (315 mg kg⁻¹ soil) followed by Mg (189 mg kg⁻¹ soil), K (111 mg kg⁻¹ soil), Na (109 mg kg⁻¹ soil) and P (89 mg kg⁻¹ soil). Fluctuations of the microbial groups as well as those of organic matter and nutrients were positively correlated.The efficacy of inoculation depended on both the type of soil and the class of inoculum. The best treatment was the mixture of the four strains and, whatever the inoculum used, the soil over lime showed the most developed crust followed by the soils over schist, granite and sandstone. In the medium term there were not significant differences between the two inocula amounts tested.These results showed that inoculation of burned soils with alien N₂-fixing cyanobacteria may be a biotechnological means of promoting microbiotic crust formation, enhancing C and N cycling microorganisms and increasing organic matter and nutrient contents in heated soils.     

Influence of Glomus etunicatum/Zea mays mycorrhiza on atrazine degradation, soil phosphatase and dehydrogenase activities, and soil microbial community structure

The effects of an arbuscular mycorrhizal (AM) fungus (Glomus etunicatum) on atrazine dissipation, soil phosphatase and dehydrogenase activities and soil microbial community structure were investigated. A compartmented side-arm (‘cross-pot’) system was used for plant cultivation. Maize was cultivated in the main root compartment and atrazine-contaminated soil was added to the side-arms and between them 650 or 37 μm nylon mesh was inserted which allowed mycorrhizal roots or extraradical mycelium to access atrazine in soil in the side-arms. Mycorrhizal roots and extraradical mycelium increased the degradation of atrazine in soil and modified the soil enzyme activities and total soil phospholipid fatty acids (PLFAs). Atrazine declined more and there was greater stimulation of phosphatase and dehydrogenase activities and total PLFAs in soil in the extraradical mycelium compartment than in the mycorrhizal root compartment when the atrazine addition rate to soil was 5.0 mg kg⁻¹. Mycelium had a more important influence than mycorrhizal roots on atrazine degradation. However, when the atrazine addition rate was 50.0 mg kg⁻¹, atrazine declined more in the mycorrhizal root compartment than in the extraradical mycelium compartment, perhaps due to inhibition of bacterial activity and higher toxicity to AM mycelium by atrazine at higher concentration. Soil PLFA profiles indicated that the AM fungus exerted a pronounced effect on soil microbial community structure.     

Compositional shifts of bacterial groups in a solarized and amended soil as determined by denaturing gradient gel electrophoresis

Soil solarization is a widespread, nonchemical agricultural practice for disinfesting soils, which is often used in combination with organic amendment, and whose action represents an important factor impacting on soil bacterial communities structure and population dynamics. The present study was conducted to investigate whether and to which extent a 72-day plot-scale soil solarization treatment, either combined or not with organic amendment, could stimulate compositional changes in the genetic structure of indigenous soil bacterial communities. Soil solarization with transparent polyethylene film, in combination or not with farmyard manure addition, was carried out during a summer period on a clay loam agricultural soil located in Southern Italy. Soils from a four-treatment (NS, nonsolarized control soil; S, solarized soil; MA, manure-amended nonsolarized soil; MS, manure-amended and solarized soil) plot block were sampled after 0, 8, 16, 36 and 72 days. Compositional shifts in the genetic structure of indigenous soil bacterial communities were monitored by denaturing gradient gel electrophoresis (DGGE) fingerprinting of 16S rRNA gene fragments amplified from soil-extracted community DNA using primers specific for Bacteria, Actinomycetales, α- and β-Proteobacteria. Changes in soil temperature, pH, and electrical conductivity (EC_1:1) were also monitored from 0 to 72 days. Beneath the polyethylene film the average soil temperature at 8-cm depth reached 55 °C compared to 35 °C in nonsolarized soil. In general, without amendment both soil pH and EC_1:1 were not significantly affected by solarization, whereas in manured plots either variables were greatly increased (from 7.0 to 8.0 pH and from 271 to 3021 μS cm⁻¹ EC_1:1), and both showed long-lasting effects due to soil solar heating. The eubacterial DGGE profiles revealed that soil solarization was the main factor inducing strong time-dependent population shifts in the community structure either in unamended or amended soils. Conversely, the addition of organic amendment resulted in an altered bacterial community, which remained rather stable over time. A similar behaviour was also observed in the DGGE patterns of β-proteobacterial and actinomycete populations, and also, albeit to a lesser extent, in the DGGE profiles of α-Proteobacteria. An increased bacterial richness was evidenced by DGGE fingerprints in 16- and 36-day samplings, followed by a decrease appearing in 72-day samplings. This could be explained, other than by a direct thermal effect on soil microflora, by solarization-induced changes in the physico-chemical properties of soil microbial habitats or by other ecological factors (e.g. decreased competitiveness of dominating bacterial species, reduced grazing pressure of microfaunal predators, increased nutrient availability).