Influence of acetamiprid on soil enzymatic activities and respiration

The effect of a new pesticide, acetamiprid, applied at normal field concentration (0.5 mg kg⁻¹ dried soil) and at high concentration (5 and 50 mg kg⁻¹ dried soil), on soil enzyme activities and soil respiration in upland soil was studied. The results showed that acetamiprid had a strong negative influence on soil respiration and phosphatase activity, and the enzyme activities in soil treated with 5 and 50 mg kg⁻¹ dry soil were significantly (P < 0.05) lower than the CK over the course of incubation. The 7-, 14-, and 35-day EC10 for phosphatase were 11, 15, and 11 mg kg⁻¹ dry soil, respectively. The 21-day EC10 and EC50 for soil respiration was 0.005 and 83 mg kg⁻¹ dry soil. The activity of dehydrogenase was enhanced after acetamiprid application for 2 weeks and the enzyme activities in samples treated with 0.5, 5 and 50 mg kg⁻¹ dry soil was about 2.5-, 1.5- and 2-fold to that of the control on sample day 28. Variance of urease and catalase had no distinct relationship with the application concentration. The activity of proteinase was not significantly affected within the first 2 weeks but inhibited from the fourth week after acetamiprid application and was only 0.45-fold to that of the control on sample day 28. Overall, acetamiprid at normal field dose would not pose a toxicological threat to soil enzymes, but a certain potential threat to soil respiration.     

Assessing impacts of copper on soil enzyme activities in regard to their natural spatiotemporal variation under long-term different land uses

Enzymes are known to be sensitive indicators of soil quality. Nevertheless, the natural variability of their activities needs to be considered for a relevant interpretation of activity levels, especially in contaminated soils. We first monitored the variability of enzymatic activities (acid and alkaline phosphatases, β-glucosidase, N-acetyl-β-glucosaminidase, urease and dehydrogenase) in two agricultural sites in north-western France during a seasonal April–October cycle. For both sites, two types of long-term different agricultural managements, grassland and intensive cropping were considered. Our results revealed great variability of enzymatic activities in space and over time, yet more pronounced for grassland than for cropped soils. Then, we assessed the impact of copper on enzymes activity on Terrestrial Model Ecosystems (TMEs) filled with undisturbed soil, and incubated for 70 days in open-field conditions. Copper was added at two concentration levels corresponding to a regulated annual agronomic input (2 mg kg⁻¹) or to a high soil contamination (200 mg kg⁻¹). In comparison to effects of natural spatiotemporal variability of soil conditions, copper addition did not show significant impacts on enzymatic activities. Finally, our results confirmed that for assessing effective impacts of contaminants in soils under real field conditions, natural spatiotemporal soil variability must be considered.     

Toxicity of imidacloprid to the earthworm Eisenia andrei and collembolan Folsomia candida in three contrasting tropical soils

Purpose

Imidacloprid is a widely used seed dressing insecticide in Brazil. However, the effects of this pesticide on non-target organisms such as soil fauna still present some knowledge gaps in tropical soils. This study aimed to assess the toxicity and risk of imidacloprid to earthworms Eisenia andrei and collembolans Folsomia candida in three contrasting Brazilian tropical soils.

Materials and methods

Acute and chronic toxicity assays were performed in the laboratory with both species in a tropical artificial soil (TAS) and in two natural soils (Oxisol and Entisol), at room temperature of 25 °C. The ecological risk was calculated for each species and soil by using the toxicity exposure ratio (TER) and hazard quotient (HQ) approaches.

Results and discussion

Acute toxicity for collembolans and earthworms was higher in Entisol (LC₅₀?=?4.68 and 0.55 mg kg^?1, respectively) when compared with TAS (LC₅₀?=?10.8 and 9.18 mg kg^?1, respectively) and Oxisol (LC_{50collembolans}?=?25.1 mg kg^?1). Chronic toxicity for collembolans was similar in TAS and Oxisol (EC_{50 TAS}?=?0.80 mg kg^?1; EC_{50 OXISOL}?=?0.83 mg kg^?1), whereas higher toxicity was observed in Entisol (EC₅₀?=?0.09 mg kg^?1). In chronic assays with earthworms, imidacloprid was also more toxic in Entisol (EC₅₀?=?0.21 mg kg^?1) when compared to TAS (EC₅₀?=?1.89 mg kg^?1). TER and HQ values indicated a significant risk of exposure of the species to imidacloprid in all soils tested, and the risk in Entisol was at least six times higher than in Oxisol or TAS.

Conclusions

The toxicity and risk of imidacloprid varied significantly between tropical soils, being the species exposure to this pesticide particularly hazardous in very sandy natural soils such as Entisol.

     

Nitrogen mineralization and CO2 and N2O emissions in a sandy soil amended with original or acidified pig slurries or with the relative fractions

The following six pig slurries obtained after acidification and/or solid/liquid separation were used in the research: original (S) and acidified (AS) pig slurry, nonacidified (LF) and acidified (ALF) pig slurry liquid fraction, and nonacidified (SF) and acidified (ASF) pig slurry solid fraction. Laboratory incubations were performed to assess the effect of the application of these slurries on N mineralization and CO₂ and N₂O emissions from a sandy soil. Acidification maintained higher NH₄ ⁺-N contents in soil particularly in the ALF-treated soil where NH₄ ⁺-N contents were two times higher than in LF-treated soil during the 55–171-day interval. At the end of the incubation (171 days), 32.9 and 24.2 mg N kg⁻¹ dry soil were mineralized in the ASF- and SF-treated soils, respectively, but no mineralization occurred in LF- and S-treated soils, although acidification decreased N immobilization in ALF- (−25.3 mg N kg⁻¹ soil) and AS- (−12.7 mg N kg⁻¹ soil) compared to LF- (−34.4 mg N kg⁻¹ soil) and S-treated (−18.6 mg N kg⁻¹ soil) soils, respectively. Most of the dissolved CO₂ was lost during the acidification process. More than 90% of the applied C in the LF-treated soil was lost during the incubation, indicating a high availability of the added organic compounds. Nitrous oxide emissions occurred only after day 12 and at a lower rate in soils treated with acidified than nonacidified slurries. However, during the first 61 days of incubation, 1,157 μg N kg⁻¹ soil was lost as N₂O in the AS-treated soil and only 937 in the S-treated soil.     

Organic nitrogen stimulates the heterotrophic nitrification rate in an acidic forest soil

Many previous studies have demonstrated that heterotrophic nitrification processes play an important role in the production of NO₃⁻ in acidic soils. However, it is not clear whether a low concentration of nitrogenous organic compounds support heterotrophic nitrification processes in natural soils. In this study, we performed an ¹⁵N tracer experiment with a glycine concentration gradient (20, 40, 80, and 160 mg N kg⁻¹) to investigate the effect of the organic nitrogen concentration on the heterotrophic nitrification rate and its relative contribution to the total nitrification of the studied acidic forest soil. This experiment demonstrated that ¹⁵N–NO₃⁻ accumulated over time with all nitrogen treatments in the presence of acetylene, confirming that heterotrophic nitrification occurred even at a low organic nitrogen concentration (20 mg kg⁻¹) in the studied acidic forest soil. In the presence of acetylene, the ¹⁵N–NO₃⁻ concentration in the 20 and 40 mg kg⁻¹ glycine-N treatments was significantly lower than in the 80 and 160 mg kg⁻¹ glycine-N treatments (p < 0.05), indicating that a high organic nitrogen concentration stimulated the heterotrophic nitrification rate. There was no significant difference in the average contribution of heterotrophic nitrification to total nitrification among the different nitrogen treatments, suggesting that the organic nitrogen concentration did not affect the relative contribution of heterotrophic nitrification to total nitrification in the studied acidic soil. Our results confirmed that a low concentration of organic N (20 mg kg⁻¹) supported heterotrophic nitrification in the studied soil. The organic nitrogen concentration stimulates the heterotrophic nitrification rate, but does not affect the relative contribution of heterotrophic nitrification to total nitrification in the studied acidic soil.     

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.     

Impact of carbon nanomaterials on microbial activity in soil

In this study, effects of an increase in concentration of fullerene-C_60, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs) or fullerene soot (FS) on overall microbial activity was investigated over a 21 d incubation period. Microbial utilisation of ¹⁴C-glucose and uptake of ¹⁴C-glucose into the microbial biomass was investigated. For CNM-amended soils, greater extents of ¹⁴C-glucose mineralisation were found in the C₆₀-amended soils compared to MWCNT-, SWCNT- or FS-amended soils. In addition, the 100 and 1000 mg kg⁻¹ were consistently found to have higher extents of mineralisation in C₆₀, MWCNT, SWCNT or FS-amended soils, respectively. Further, the incorporation of ¹⁴C-glucose into the microbial biomass declined slightly with an increase in concentration in the amended soils, but no consistent pattern was observed. As a result, the biophysical quotient (BQ) increased significantly (P < 0.05), as concentrations increased from 1 mg kg⁻¹ to 1000 mg kg⁻¹ in all C₆₀-, MWCNT-, SWCNT- and FS-amended soils. The results obtained from this study showed that the addition to carbon nanomaterials had no profound impacts on the overall microbial activity, and the overall influence of CNMs on soil microbial activity does not reveal a specific pattern in the short term.     

The Effectiveness and Feasibility of Using Ochre as a Soil Amendment to Sequester Dissolved Reactive Phosphorus in Runoff

Incidental losses of dissolved reactive phosphorus (DRP) to a surface waterbody originate from direct losses during land application of fertilizer, or where a rainfall event occurs immediately thereafter. Another source is the soil. One way of immobilising DRP in runoff before discharge to a surface waterbody, is to amend soil within the edge of field area with a high phosphorus (P) sequestration material. One such amendment is iron ochre, a by-product of acid mine drainage. Batch experiments utilising two grassland soils at two depths (topsoil and sub-soil), six ochre amendment rates (0, 0.15, 1.5, 7.5, 15 and 30 g kg⁻¹ mass per dry weight of soil) and five P concentrations (0, 5, 10, 20 and 40 mg L⁻¹) were carried out. A proportional equation, which incorporated P sources and losses, was developed and used to form a statistical model. Back calculation identified optimal rates of ochre amendment to soil to ameliorate a specific DRP concentration in runoff. Ochre amendment of soils (with no further P inputs) was effective at decreasing DRP concentrations to acceptable levels. A rate of 30 g ochre kg⁻¹ soil was needed to decrease DRP concentrations to acceptable levels for P inputs of ≤10 mg L⁻¹, which represents the vast majority of cases in grassland runoff experiments. However, although very quick and sustained metal release above environmental limits occurred, which makes it unfeasible for use as a soil amendment to control P release to a waterbody, the methodology developed within this paper may be used to test the effectiveness and feasibility of other amendments.     

铜和强力霉素复合污染对土壤微生物与酶活性的影响

当前土壤环境中重金属和抗生素的广泛共存及二者复合存在所诱导出的细菌抗性,与单一物质的污染相比,均能够加剧对土壤质量和作物安全的破坏。在在实验室模拟培养条件下,向土壤中加入不同浓度的重金属(铜)和抗生素(强力霉素),探讨抗生素和重金属复合污染对土壤微生物呼吸、脲酶、蔗糖酶和过氧化氢酶活性和四环素抗性基因的丰度等土壤微生物指标的影响。结果显示,在整个培养期(30 d)内,铜和强力霉素单一及复合污染均会显著抑制土壤微生物呼吸强度,对脲酶活性主要表现为促进作用,对蔗糖酶、过氧化氢酶活性主要为抑制作用,对过氧化氢酶活性的抑制强度明显大于蔗糖酶。综合而言,铜和强力霉素的复合污染相对于单一污染对上述微生物指标的影响较大,强力霉素的加入可以促进铜对微生物呼吸或酶活性的初始影响。此外,该研究还表明添加为400mg·kg~(–1)铜可以提高强力霉素在土壤培养中后期诱导的抗性基因相对丰度的能力水平。本研究从微生物角度定量探讨铜与强力霉素单一及复合污染对土壤微生物指标的影响程度,以期为重金属与抗生素协同污染的土壤构建微生物预警体系,并为土壤修复和风险评估工作提供理论依据。     

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.     

Toxicity of chemical-warfare agent HD to Folsomia candida in different soil types

We investigated the ecotoxicity of the chemical-warfare agent (CWA) HD (Mustard) using Collembola reproduction test with numbers of adults and juveniles as measurement endpoints. Toxicity tests were conducted using soils with contrasting parameters to investigate the effects of soil properties on chemical toxicity. These included standard artificial soil (SAS; 10% OM; 6 pH), O’Neill-Hall sandy loam (OHSL; natural soil with 4.3% OM; 5.1 pH), and Sassafras sandy loam (SSL; natural soil with 2% OM; 4.9 pH). Soils were individually spiked with HD concentrations ranging from 1 to 125 mg kg^–1. Lowest observed effect concentrations (LOECs) for adult mortality were 25, 50, and 6.97 mg kg^–1 for SAS, OHSL, and SSL, respectively. The LOECs for reproduction were 6.97, 6.25, and 1.9 mg kg^–1 for SAS, OHSL, and SSL, respectively. HD toxicity to both adults and juveniles was greater in SSL. These results show that soil toxicity testing should not rely solely on the adult acute endpoints using artificial soils, but should include assays with reproductive endpoints using natural soils with varying physical and chemical parameters to adequately assess toxicity to test species.     

Plant Availability and Uptake of Lead, Zinc, and Cadmium in Soils Contaminated with Anti-corrosion Paint from Pylons in Comparison to Heavy Metal Contaminated Urban Soils

Red lead (Pb₃O₄) has been used extensively in the past as an anti-corrosion paint for the protection of steel constructions. Prominent examples being some of the 200,000 high-voltage pylons in Germany which have been treated with red lead anti-corrosion paints until about 1970. Through weathering and maintenance work, paint compounds and particles are deposited on the soils beneath these constructions. In the present study, six such “pylon soils” were investigated in order to characterize the plant availability and plant uptake of Pb, Cd, and Zn. For comparison, three urban soils with similar levels of heavy metal contamination were included. One phase extractions with 1 M NH₄NO₃, sequential extractions (seven steps), and extractions at different soil pH were used to evaluate the heavy metal binding forms in the soil and availability to plants. Greenhouse experiments were conducted to determine heavy metal uptake by Lolium multiflorum and Lactuca sativa var. crispa in untreated and limed red lead paint contaminated soils. Concentrations of Pb and Zn in the pylon soils were elevated with maximum values of 783 mg Pb kg⁻¹ and 635 Zn mg kg⁻¹ while the soil Cd content was similar to nearby reference soils. The pylon soils were characterized by exceptionally high proportions of NH₄NO₃-extractable Pb reaching up to 17% of total Pb. Even if the relatively low pH of the soils is considered (pH 4.3–4.9), this appears to be a specific feature of the red lead contamination since similarly contaminated urban soils have to be acidified to pH 2.5 to achieve a similarly high Pb extractability. The Pb content in L. multiflorum shoots reached maximum values of 73 mg kg⁻¹ after a cultivation time of 4 weeks in pylon soil. Lime amendment reduced the plant uptake of Pb and Zn significantly by up to 91%. But L. sativa var. crispa cultivated on soils limed to neutral pH still contained critical Pb concentrations (up to 0.6 mg kg⁻¹ fresh weight). Possible mechanisms for the exceptionally high plant availability of soil Pb derived from red lead paint are discussed.     

Long-term effects of land use and fertiliser treatments on sulphur transformations in soils from the Broadbalk experiment

Sulphur transformations were monitored in a unique set of arable, grassland and woodland soils from the Broadbalk Classical Experiment, which started in 1843. In an open incubation experiment with periodic leaching, 14–35 mg SO₄²⁻-S kg⁻¹ was mineralised in 28 weeks at 25°C, equivalent to 4.4–8.3% soil organic S. Cumulative amounts of S mineralised increased linearly during the 28 weeks, indicating constant rates of mineralisation. The rate of mineralisation was the greatest in the woodland soil (170 μg SO₄-S kg⁻¹ day⁻¹), followed by the grassland (120 μg SO₄-S kg⁻¹ day⁻¹) and the arable soil from the farmyard manure (FYM) plot (110 μg SO₄-S kg⁻¹ day⁻¹). Three soils from arable plots receiving different inorganic fertiliser treatments but no FYM had similar rates of S mineralisation (~70 μg SO₄-S kg⁻¹ day⁻¹). In an incubation experiment with ³⁵SO₄²⁻, addition of glucose greatly enhanced S immobilisation. In 132 days, the woodland and grassland soils immobilised more S than the arable soils, with or without glucose amendment. Immobilisation and mineralisation of S occurred concurrently, and both were stimulated by glucose addition. The results show that S mineralisation and immobilisation were influenced strongly by the type of land-use and long-term organic manuring, whereas annual application of sulphate-containing fertilisers for over 150 years had few effects on short-term S transformations.     

Assessing the toxic effect of nitrapyrin on nitrogen transformation in soil

A 28 d N transformation test was developed according to the OECD guideline 216. In the laboratory-based test, a suitable soil was amended with powdered plant meal as an organic N source. Soil samples of 1 kg treated with five concentrations of nitrapyrin (2-chloro-6-(trichloromethyl)-pyridine), in the range 1.0-100 mg kg⁻¹ dry weight were incubated for 28 d at 20±2 °C. A dose response was produced and the N mineralisation EC₅₀ (95% C.I.) for nitrapyrin was 3.1 (1.9-4.3) mg kg⁻¹ dry soil. The determined EC₅₀ was compared with literature figures for similar end points but using different methodology.     

Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N₂O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization–immobilization and N₂O emission. Residues were added at the rate of 3 t C ha⁻¹ to soil with, and without, 150 kg urea N ha⁻¹. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P < 0.05) lower NO₃⁻–N, which reached minimum values of 2.8 mg N kg⁻¹ for sugarcane (at day 28), 10.3 mg N kg⁻¹ for maize (day 7), and 5.9 mg N kg⁻¹ for sorghum (day 7), compared to 22.7 mg N kg⁻¹ for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg⁻¹ in sugarcane, 34 mg kg⁻¹ in maize, 29 mg kg⁻¹ in sorghum, and 16 mg kg⁻¹ in cotton amended soil compared to soil + N fertilizer, although soil NO₃⁻–N increased by 7 mg kg⁻¹ in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N₂O from crop residue amended soils occurred in the first 4–5 days of incubation. Overall, after 84 days of incubation, the cumulative N₂O emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative N₂O emission was significantly and positively correlated with NO₃⁻–N (r = 0.92, P < 0.01) and total mineral N (r = 0.93, P < 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO₂ in the first 3 and 5 days of incubation (r = 0.59, P < 0.05).     

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.     

Soil organic carbon buffers heavy metal contamination on semiarid soils: Effects of different metal threshold levels on soil microbial activity

Traditionally, three threshold levels have been accepted for heavy metal concentrations in agricultural soils, depending on soil pH. The aim of this work was to ascertain how the three threshold values proposed for Cd (3, 6.5, and 12.5 mg kg^?1) and Zn (300, 650, and 1300 mg kg^?1) really affect soil microbial activity. Two soils, a scrubland soil and a forest soil, differing widely in their organic C content, were used in this study. Despite the different soil characteristics, the fractions of Cd and Zn extracted with a solution of diethylenetriaminepentaacetic acid (DTPA) showed little difference between soils. Parameters, such as microbial biomass C (C_mic), soil basal respiration (BR), adenosine triphosphate (ATP) content, dehydrogenase activity (DHA), urease activity (UA), alkaline phosphatase activity (APA), and β-glucosidase (β-GA), were less affected by heavy metals in the forest soil than in the scrubland soil. In general, the simultaneous addition of both metals had a synergistic effect on microbial activity, and this treatment produced a significant decrease of microbial activity of both soils with respect to control. The highest level (L3) of Cd, Zn and Cd + Zn treatments produced significant decrease of microbial and biochemical parameters in both soils.     

Soil organic C stabilization and thresholds in C saturation

When building soil organic matter (SOM) contents in agricultural production systems, stabilization of both pre-existing as well as added C is important. A laboratory mineralization experiment was conducted over 374 days to evaluate the effect of pre-existing SOM on soil C mineralization after addition of organic matter (OM) using sugar cane. The SOM gradient used here stretched from 21 to 106 g C kg⁻¹ soil and was a result of different periods of continuous cultivation of 5, 20, 35 and 105 years in comparison to a forest soil. The rate of organic C mineralization was found to be dependent on the status of pre-existing soil organic C (SOC). Highly degraded soil which had been under continuous cultivation for 35 years and more showed the highest rate of C mineralization per unit SOC (117.9 mg C g⁻¹ C) while forest soil had the lowest amount of C mineralized per unit SOC (73.5 mg C g⁻¹ C). Forest soil had the highest amount of increased C mineralization as a result of organic matter (OM) additions (8.0 mg C g⁻¹ soil) followed by the highly degraded soil that had been under cultivation for 105 years (5.5 mg C g⁻¹ soil). Additional mineralized C as a function of time after forest conversion declined progressively within the first 20 years of continuous soil use. Soil which had been under continuous cultivation for 20 years had the lowest amount of additional mineralized C (4.0 mg C g⁻¹ soil). SOM stabilization efficiency in the studied soils appears to be highest with intermediate cultivation history of about 20 years. These soils that have been recently converted to cultivation also appear to have a greater ability to stabilize added OM than the most degraded soils investigated in this study. It is thus advisable to provide intervention strategies to reverse SOM decline for farming communities at an intermediate stage before the soils are highly depleted of SOC.     

Severe soil frost reduces losses of carbon and nitrogen from the forest floor during simulated snowmelt: A laboratory experiment

Considerable progress has been made in understanding the impacts of soil frost on carbon (C) and nitrogen (N) cycling, but the effects of soil frost on C and N fluxes during snowmelt remain poorly understood. We conducted a laboratory experiment to determine the effects of soil frost on C and N fluxes from forest floor soils during snowmelt. Soil cores were collected from a sugar maple (Acer saccharum)–American beech (Fagus grandifolia) and a red spruce (Picea rubens)–balsam fir (Abies balsamea) forest at the Hubbard Brook Experimental Forest in New Hampshire, U.S.A. Soils were exposed to one of three temperature treatments, including severe (?15 °C), mild (?0.5 °C), and no soil frost (+5 °C) conditions. After one week the soils were incubated at +5 °C and snow was placed on top of the soils to simulate spring snowmelt. NO₃^? losses were up to 5.5 mg N kg^?1 soil greater in the mild soil frost treatment than the severe soil frost treatment. Net losses of NH₄⁺ and DON in leachate were up to 19 and 18 mg N kg^?1 soil greater in the no soil frost and mild soil frost treatments, respectively, than the severe soil frost treatment. In contrast, soil frost did not have a significant impact on dissolved organic C or cumulative gaseous fluxes of C and N throughout the snowmelt period. However, the total cumulative flux of C (i.e. dissolved organic C + CO₂ + CH₄) and N (i.e. dissolved organic N + NH₄ + NO₃ + N₂O) in the severe soil frost treatment were between one quarter and one half that observed in the no soil frost treatment for both forest types. Together, the results of this study show that total fluxes of N in leachate, as well as total cumulative C and N fluxes (gases + leachate), were significantly reduced following severe soil frost. We conclude that the extent to which C and N cycling during snowmelt is altered in response to changes in winter climate depend on both the presence and severity of soil frost.     

Soil particle surface electrochemical property effects on abundance of ammonia-oxidizing bacteria and ammonia-oxidizing archaea, NH4 activity, and net nitrification in an acid soil

Soil surface electrochemical properties may have a strong influence on nitrifying microorganisms, H⁺ and NH₄⁺ activities, and therefore on the nitrification process. A gradient of surface electrochemical parameters was obtained by amendment of a subtropical acid pine soil (Oxisol) with 0% (control), 3%, 5%, 8%, 10% and 12% pure Ca-Montmorillonite by weight. The H⁺ and NH₄⁺ activities, the abundance of the ammonia-oxidizing bacterial (AOB) and archaeal (AOA) amoA gene copies, and time-dependent kinetics of net nitrification were investigated. Soil particle surface specific area ranged from 53 to 103 m² g⁻¹ and increased with increasing montmorillonite application rate. Similar to specific area, surface charge quantity, surface charge density, electric field strength and surface potential increased after montmorillonite amendment. The H⁺ and NH₄⁺ activities decreased linearly after montmorillonite addition. AOB amoA gene copy number was 1.82 × 10⁵ copies g⁻¹ for unamended soil, and the highest AOB amoA gene copy numbers were found for the 10% montmorillonite amendment (3.11 × 10⁷ g⁻¹ soil), which was more than 150 times higher than unamended soil. AOA amoA gene copy numbers were 9.19 × 10³ copies g⁻¹ dry unamended soil, and the highest AOA amoA gene copy numbers were found in the 8% montmorillonite amendment (1.22 × 10⁵ g⁻¹ soil). Although pH significantly decreased during the first three weeks of incubation, no significant difference was observed between the unamended control and different rates of montmorillonite addition treatments during the whole incubation. The largest net nitrification (103 mg N kg⁻¹) was observed in the 10% montmorillonite amendment and the lowest in unamended soil (62 mg N kg⁻¹). While montmorillonite did not change the kinetic patterns of net nitrification, the highest nitrification potential (275 mg N kg⁻¹) for the 10% montmorillonite treatment was more than 3 times higher than unamended soil from simulation of time-dependent kinetics. Nitrification was significantly stimulated after montmorillonite amendment in acid soil mainly due to an increase in the quantity and activity of AOB and AOA. We concluded that soil particle surface parameters can significantly influence nitrification, especially in acid soils.