Effects of Zeolite-Containing Material and Ammonium Nitrate on Biodegradation of n-Tridecane in Heavy Clay-Loam Leached Chernozem with High Organic Matter Content

Biostimulation based on usage of soil amendments is growing due to their efficiency in removing different petroleum hydrocarbons (PHC) from contaminated sand or loam-sand soils. However, the research on clay-rich soils with higher organic carbon content, in which PHC biodegradation may proceed differently and which are more difficult to clean up, has been less extensive. In a pot experiment, we studied and compared the effects of two soil amendments, natural zeolite-containing material (ZCM, 50 g kg^?1) as a bulking agent and ammonium nitrate (0.3 g N kg^?1) as a nitrogen fertilizer, on biodegradation of n-tridecane (1 wt.%) in a weakly acidic heavy clay loam leached chernozem with fairly high organic carbon content (3.71%). After 48 days, the nitrogen-amended contaminated soil showed enhancement of both respiratory activity (basal and substrate-induced respiration rates) and the number of n-tridecane- degraders. As a consequence, the extent of n-tridecane biodegradation (86.5%) was essentially higher in the presence of added nitrogen than that in the non-amended soil (73.7%). In contrast, due to the partial retention of n-tridecane molecules in its pores, ZCM retarded biodegradation to 56.0%, showed no significant effect on the number of n-tridecane-degraders and, moreover, enhanced the decomposition of the soil intrinsic organic matter. The obtained data indicate that more precautions should be considered when using porous sorbents such as ZCM for remedial arrangements in PHC-contaminated soils.     

Biological control of nematodes: Soil amendments and microbial antagonists

Summary Organic matter amendments to soil can be used to manage phytoparasitic nematodes. The most effective amendments are those with narrow C:N ratios and high protein or amine-type N content. For soil with 1.0% (w/w) organic matter amendment there is a direct relation between extent of nematode control and the N content of amendments. A special group of amendments are those containing chitinous materials. Chitin addition to soil results in stimulation of a select microflora capable of degrading the polymer. Several microbial species are known to destroy the eggs of phytonematodes (Meloidogyne spp). Organic matter can be modified by addition of specific compounds or by inoculation with particular microbial species to produce an amendment that will induce suppressiveness.     

Effect of green manure on Pythium spp. population and microbial communities in intensive cropping systems

Saprophytic soil-borne pathogens can be either actively suppressed by organic amendments or enhanced, depending on soil health conditions. This can be deleterious in the event of selection of a soil-borne population by previous soil management and short crop rotation. Trials were performed in the open field and in pots, using naturally infected soil from intensive crop systems, i.e., soil from fields with 8 years of strawberry cultivation. The aim was to study short-term response of Pythium and soil microbial populations to green manure. The use of green manure in these naturally infested soils, 3–10 weeks after fresh tissue incorporation, caused Pythium populations to increase concurrent with an increase in soil microbial populations, and did not result in the suppression of the pathogen. A more elaborate trial was performed under controlled conditions, amending soil with fresh wheat plant material, air-dried wheat plant material and an organic fertilizer with a high level of humic substances. Although compared to the original soil, all amendments caused a similar increase in organic matter content and small differences in soil respiration, incorporation of fresh, not decomposed, plant material strongly increased Pythium, while the organic fertilizer did not affect the original level of the pathogen population. The increase in total number of fungi and bacteria did not have any suppressive effect on the Pythium population in naturally infested soil used for this study.     

The heterogeneous nature of microbial products as shown by solid-state13C CP/MAS NMR spectroscopy

Homoionic Na-, Ca-, and Al-clays were prepared from the <2 m fractions of Georgia kaolinite and Wyoming bentonite and mixed with sand to give artificial soils with 5, and 25% clay. The artificial soils were inoculated with microbes from a natural soil before incubation. Unlabelled and uniformly¹³C-labelled (99.9% atom) glucose were incorporated into the artificial soils to study the effects of clay types, exchangeable cations and clay contents on the mineralization of glucose-carbon and glucose-derived organic materials. Chemical transformation of glucose-carbon upon incorporation into microbial products and metabolites, was followed using solid-state¹³C CP/MAS NMR spectroscopy.There was a significant influence of exchangeable cations on the mineralization of glucose-carbon over a period of 33 days. At 25% clay content, mineralization of glucose-carbon was highest in Ca-soils and lowest in Al-soils. The influence of exchangeable cations on mineralization of glucose-carbon was more pronounced in soils with bentonite clay than those with kaolinite clay. Statistical analysis of data showed no overall effect of clay type on mineralization of glucose-carbon. However, the interactions of clay type with clay content and clay type with clay content and exchangeable cations were highly significant. At 25% clay content, the mineralization of glucose-carbon was significantly lower in Na- and Al-soils with Wyoming bentonite compared with Na- and Al-soils with Georgia kaolinite. For Ca-soils this difference was not significant. Due to the increased osmotic tension induced by the added glucose, mineralization of glucose-carbon was slower in soils with 5% clay than soils with 25% clay.Despite the differences in the chemical and physical characteristics of soils with Ca-, Na- and Al-clays, the chemical composition of organic materials synthesised in these soils were similar in nature. Assuming CP/MAS is quantitative, incorporation of uniformly¹³C-labelled glucose (99.9% atom) in these soils resulted in distribution of carbon in alkyl (24–25%), O-alkyl (56–63%), carbonyl (11–15%) and small amounts of aromatic and olefinic carbon (2–4%). However, as decomposition proceeded, the chemistry of synthesised material showed some changes with time. In the Ca- and Na-soils, the proportions of alkyl and carbonyl carbon decreased and that of O-alkyl carbon increased with time of incubation. However, the opposite trend was found for the Al-soil.Proton-spin relaxation editing (PSRE) subspectra clearly showed heterogeneity within the microbial products. Subspectra of the slowly-relaxing (long T₁(H)) domains were dominated by alkyl carbon in long- and short-chain structures. The signals due to N-alkyl (55 ppm) and carbonyl carbon were also strong in these subspectra. These subspectra were very similar to those obtained for microbial and fungal materials and were probably microbial tissues attached to clay surfaces by polysaccharide extracellular mucilage. Subspectra of fast-relaxing (short T₁(H)) domains comprised mostly O-alkyl and carbonyl carbon and were probably microbial metabolites released as neutral and acidic sugars into the extracellular environment, and strongly sorbed by clay surfaces.     

Organic Amendments to Enhance Atrazine and Metamitron Degradation in Two Contaminated Soils with Contrasting Textures

Among all types of xenobiotics, pesticides such as herbicides play a significant role in soil and water pollution due to their wide usage all over the world. This study addresses the ability of organic amendments to enhance atrazine and metamitron degradation in two herbicide-contaminated soils with contrasting textures under laboratory conditions. Soil samples were collected from surface soils with textures of sandy loam and silty clay, from northeastern Iran. Initial concentration of herbicides was 50 mg · kg^{? 1} soil. Contaminated soil samples were treated with manure, compost and vermicompost at rates of 0, 0.5, and 2% (w/w). Residual concentrations of atrazine and metamitron were determined by HPLC at the end of incubation periods of 20, 40, and 60 days. Residual concentrations of atrazine were 46.5, 38.9, and 36.2 mg · kg^{? 1} after 20, 40, and 60 days incubation, respectively. Residual metamitron concentrations were clearly lower than atrazine. After 20, 40, and 60 days, concentrations of metamitron were 2.9, 1.0, and 0.6 mg · kg^{? 1}, respectively. Organic amendments at the rates of 0.5 and 2% showed similar effects on the enhancement of herbicide degradation in soils. However, no statistically significant effect was observed among types of organic amendments (α = 0.05). Degradation was affected by soil textures. Residual concentrations of herbicides were higher in sandy loam than in silty clay soil.     

Nutrient Amendments of Inorganic Fertiliser and Oil Palm Empty Fruit Bunch and Their Influence on Bacterial Species Dominance and Degradation of the Associated Crude Oil Constituents

The effects of inorganic commercial fertiliser (N:P:K = 8:8:1) and oil palm empty fruit bunch (EFB) as nutrient amendments for crude oil degradation and microbial population shift by a microbial consortium [Pseudomonas sp. (UKMP-14T), Acinetobacter sp. (UKMP-12T), Trichoderma sp. (TriUKMP-1M and TriUKMP-2M)] were assessed. The bacterial populations present during crude oil degradation were analysed by spread plate method and 16S rRNA sequences, whereas the presence of fungi was assessed by growth on potato dextrose agar. Crude oil degradation analysed using gas chromatography-flame ionisation detection showed total petroleum hydrocarbon reduced between 70 and 100%, depending on the type of amendments compared to control (≈55%) after 30 days of incubation. Nutrient amendments using NPK fertiliser or EFB were found to influence the domination of different bacterial species, which in turn preferentially utilised different hydrocarbons. This study suggested different nutrient amendments could be used to preferentially select bacteria to degrade different components of crude oil, particularly pertaining to the recalcitrant phytane. This information is very useful for application of in situ bioremediation of soil hydrocarbon contamination.     

Solidification/Stabilization of Cresol-Contaminated Soil: Mechanical and Leaching Behavior

In this research, the effects of a solidification/stabilization (S/S) technique on the remediation of cresol-contaminated soil were investigated. The soil samples were collected from the Tehran Oil Refinery (TOR) in Ray District, Iran. Cresols are hazardous chemicals whose exposure at high dosage results in irreparable damage to animal and human health. S/S process progresses through physicochemical reactions, reducing the leachability of a waste as well as changing its physical properties in order to encapsulate the contaminants and form a solid material. The samples were spiked by certain concentrations of cresols including meta-, ortho-, and para-isomers. The treatment process was conducted using Portland cement as the binder and modified bentonite as a stabilizer. The efficiency of the S/S technique was assessed by the unconfined compressive strength (UCS) test and toxicity characteristic leaching procedure (TCLP) test. The results of the tests showed that adding 20% and 30% Portland cement in combination with 15% and 30% modified bentonite to contaminated samples increased the 28-day compressive strength of S/S blocks to a range of 2.44 to 3.08 MPa. In fact, increasing the proportion of cement in the mix design resulted in enhanced compressive strength, while modified bentonite inversely affected samples’ strength. Regarding leaching behavior, adding organophilic clay to polluted samples noticeably declined cresol's concentrations in leachate. Overall efficiency of S/S in terms of leaching was about 70%, which was obtained by adding 20% cement and 30% modified clay. A logarithmic relationship between leaching percent and modified clay ratio was also detected with high logarithmic and linear correlation coefficients of 0.96 and 0.9, respectively, hence presenting the efficiency of S/S in stabilizing the cresols in the samples.     

Decrease in Hydraulic Conductivity of a Paddy Field using Biocalcification in situ

The hydraulic conductivity of a paddy field (Anthraquic Dystrustept), a silty clay soil containing more than 29% (w/w) of gravel, in Nagoya University Farm was reduced by in situ treatment of subsurface soil using bentonite and biocalcification (microbial calcium carbonate precipitation) through the addition of CaCl₂, urea, and corn steep liquor (CSL). The treatment decreased the hydraulic conductivity of the field from an average of 10^?3 cm/s to a range of 10^?5 to 10^?7 cm/s during 69 days, with reducing the proportion of pores of subsurface soil larger than 75 µm in diameter. The biocalcification effect was observed at 10-cm thickness from the treated subsurface. Laboratory soil core experiments demonstrated that the decrease in the hydraulic conductivity was not attributed to the effect of bentonite but mainly to the effect of biocalcification. The addition of CSL enhanced the urease activity of soil required for biocalcification, even at 4°C, as indicated by a decrease in urease activation energy temperature sensitivity. These experimental results agreed with the gradual decrease in hydraulic conductivity observed in the field when the average daily temperature was 7°C (days 24–69). It was suggested that the biocalcification is a potential technique to reduce the hydraulic conductivity of paddy field.     

长期施肥下农田土壤-有机质-微生物的碳氮磷化学计量学特征

探讨外源养分的输入对土壤系统内碳、氮、磷化学计量特征的影响,对于深刻认识农田土壤有机碳(C)和养分循环及其相互作用过程具有重要意义。以26年的农田长期定位施肥试验为平台,分析长期不同施肥条件下土壤、有机态及微生物生物量碳、氮、磷含量及其化学计量学特征,并根据内稳性模型y=c x~(1/H)计算其化学计量内稳性指数H。结果表明:与长期撂荒处理(CK_0)相比,种植作物条件下26年化肥配施有机肥处理(MNPK和1.5MNPK)显著降低微生物生物量氮含量,但显著提高了微生物生物量磷的含量。相对于撂荒处理,即使长期配施化肥磷处理(NP、PK、NPK),其土壤有机磷降低显著。对于C∶N比而言,化肥配施有机物料处理(秸秆或有机肥)的土壤C∶N比、有机质C∶N及微生物生物量C∶N比均显著低于化肥处理(N、NP、PK和NPK)。对于C∶P比而言,相对于撂荒处理,26年施用磷肥(化肥磷或有机磷)显著降低了土壤C∶P比和微生物生物量C∶P比,而CK和偏施化肥处理(N、NP和PK)显著降低了土壤有机质C∶P比。对于土壤N∶P比而言,撂荒处理土壤N∶P比显著高于其他处理,而撂荒处理土壤有机质N∶P比显著高于CK和化肥处理,表明不施肥或化肥条件下作物种植加剧了土壤有机质中氮素的消耗。微生物生物量C∶N、C∶P、N∶P比的内稳性指数H分别为0.24、0.75、0.64,不具有内稳性特征。微生物生物量C∶N、C∶P、N∶P比分别与土壤C∶N、C∶P、N∶P比呈显著正相关关系,但与土壤有机质碳氮磷化学计量比之间无显著相关性。表明土壤碳、氮、磷元素的改变会直接导致微生物生物量碳、氮、磷化学计量比的改变,但微生物生物量碳氮磷化学计量比对土壤有机质碳氮磷化学计量比无显著影响,土壤有机质的碳氮磷计量比可能更多是受到作物和施肥等养分管理措施的影响。     

Microbial community structure changes during bioremediation of PAHs in an aged coal-tar contaminated soil by in-vessel composting

The microbial community structure changes of an aged-coal-tar soil contaminated with polycyclic aromatic hydrocarbons (PAHs) were investigated during simulated bioremediation at the laboratory-scale using an in-vessel composting approach. The composting reactors were operated using a logistic three-factor factorial design with three temperatures (T=38, 55 or 70 °C), four soil to green-waste amendment ratios (S:GW=0.6:1, 0.7:1, 0.8:1 or 0.9:1 on a dry weight basis) and three moisture contents (MC=40%, 60% or 80%). Relative changes in microbial populations were investigated by following the dynamics of phospholipid fatty acid (PLFA) signatures using a ¹³C-labeled palmitic acid internal standard and sensitive GC/MS analysis during in-vessel composting over 98 days. The results of this investigation indicated that fungal to bacterial PLFA ratios were significantly influenced by temperature (p<0.05), and Gram-positive to Gram-negative bacterial ratios were significantly influenced by temperature (p<0.001) and S:GW ratio (p<0.01) during in-vessel composting. Additionally, the Gram-positive to Gram-negative bacterial ratios were correlated to the extent of PAH losses (p<0.005) at 70 °C.     

Phytoremediation of Total Petroleum Hydrocarbons From Highly Saline and Clay Soil Using Sorghum halepense (L.) Pers. and Aeluropus littoralis (Guna) Parl

This study evaluated the effects of native plants (Sorghum halepense and Aeluropus littoralis), total petroleum hydrocarbons (TPH) concentrations, and nutrients on the removal of TPHs from a highly saline clay soil. For a period of 180 days, rhizosphere microbial number, plant biomass, and residual TPHs were determined monthly. Results showed that TPH removal from soil in the rhizosphere was 13% higher than that in the control (unplanted soil). In addition, the number of heterotrophic bacteria in the rhizosphere and non-rhizosphere soil was 7.407 and 6.629 log₁₀CFU/g, respectively. The maximum TPH removal, microbial numbers, and plant biomass were measured in the treated soil, polluted with 0.86% (w/w) of TPH. The high clay and salinity of the experimental soil had a negative effect on the phytoremediation efficiency. Hence, it was necessary to improve the physicochemical properties of the soil to provide a good condition for plants and microbes, thereby increasing the phytoremediation efficiency.     

Saprophytic and pathogenic behaviour of R. solani AG2-1 (ZG-5) in a soil amended with Diplotaxis tenuifolia or Brassica nigra manures and incubated at different temperatures and soil water content

The Brassicaceae species Diplotaxis tenuifolia and Brassica nigra contain high concentrations of glucosinolates, the precursors of isothiocyanates (ITCs) that can have biofumigation effects in amended soils. In a laboratory experiment, incorporation of these plants as green manures into soil was expected to suppress Rhizoctonia solani AG2-1 (ZG5), the causal agent of damping-off in canola (Brassica napus). The manures were incorporated at 1 (1% w/w) or 5 (5% w/w) g fresh material per 100 g dry soil and incubated for 6 months at 10, 20, or 30°C and at soil water contents of 10%, 40%, or 70% of water holding capacity. R. solani survived for up to 6 months as a saprophyte in un-amended soil at all soil water contents and at 10 and 20°C. A temperature of 30°C suppressed R. solani below the level of detection in all treatments after one week. At 1% concentration, the green manures increased the colonisation of the soil by R. solani, which caused severe damping-off of canola subsequently sown in this soil treatment. Soil amendments at 1% temporarily increased soil microbial activity. The addition of B. nigra or D. tenuifolia green manure at 5% concentration suppressed the saprophytic growth of R. solani incubated at 10 or 20°C over all soil water contents and significantly increased the microbial activity at all soil temperatures and water contents. Canola sown into these pots did not succumb to damping-off. The efficiency of hydrolysis of glucosinolates in the 5% treatment in the first week of incubation ranged from 1.6% for 2-propenyl ITC, extracted from soil containing tissues of B. nigra, to 3.4% for 3-butenyl ITC extracted from soil containing tissues of D. tenuifolia. 2-propenyl ITC could not be detected after 7 d of incubation. In the longer term (weeks to months), the increase of microbial activity, caused by adding green manures at 5%, or volatiles from the green manures, most likely played a dominant role in suppressing R. solani. The impact of ITCs, if any, appears to be short-term (days). Responsible Editor: Peter A. H. Bakker     

In vitro evaluation of the capacity of zeolite and bentonite to adsorb aflatoxin B1 in simulated gastrointestinal fluids

Anin vitro study using single concentration and isotherm adsorption was carried out to evaluate the capacity of Vietnamese produced zeolite and bentonite to adsorb aflatoxin B₁ (AFB₁) in simulated gastrointestinal fluids (SGFs), and a commercial sorbent hydrated sodium calcium aluminosilicate (HSCAS) was used as reference. In this study, AFB₁ solution was mixed with sorbents (0.3, 0.4 and 0.5% w/v) in SGFs at pH 3 and pH 7 and shaken for 8 h, centrifuged and the supernatant measured by Vicam fluorometer. Adsorption of AFB₁ onto zeolite and bentonite varied according to the pH of SGFs and was lower than HSCAS. Linearity between the increased amount of AFB₁ adsorbed on sorbents and the decrease of sorbent concentration was observed for bentonite and HSCAS, except for zeolite in SGFs at pH 7. The observed maximum amounts of AFB₁ adsorbed on bentonite and HSCAS were 1.54 and 1.56 mg/g, respectively. The adsorption capacities of bentonite and HSCAS for AFB₁ were 12.7 and 13.1 mg/g, respectively, from fitting the data to the Freundlich isotherm equation. Improvement in processing and purification for bentonite is needed to enhance the surface area, which would probably result in better adsorptive capacity for this sorbent.     

Changes in the speciation,partitioning and phytoavailability of chromium induced by organic soil amendments

Abstract

This study investigated the effect of two organic amendments (compost of cattle ruminai content and Sphagnum-moss peat) on the reduction of hexavalent chromium and the distribution of this metal among the main solid phases of a soil with low organic matter content treated with different levels of Cr(VI) (0–2000 mg Cr kg^?1 soil). At the same level of added organic carbon, the peat reduced Cr(VI) added to the soil from 250 to 2000 mg kg^?1, with 100% efficiency. The reduction efficiency of the compost, however, decreased with the increasing dose of Cr(VI) soil. The distribution of Cr between the different soil components was evaluated by a sequential chemical extraction procedure. The concentration of water-soluble and exchangeable Cr decreased with the addition of organic amendments to the soil, whereas Cr increased in the organic fraction. The effect of added organic material on the Cr absorption was examined with two ornamental plants (Melissa officinalis and Begonia semperflorens). The increased Cr(VI) in the soil increased the Cr concentration in plant tissues. The addition of organic matter produced a greater aerial biomass for each level of added Cr in comparison with unamended soil. Sphagnum moss peat was more effective than the compost to decrease the total Cr and the Cr(VI) concentration in the water-soluble and exchangeable fraction of soil, thereby reducing the Cr accumulation in plants tissues and phytotoxic symptoms.     

Soil particle accumulation in termite (Macrotermes bellicosus) mounds and the implications for soil particle dynamics in a tropical savanna Ultisol

This study investigated the influence of mound-building termites on soil particle dynamics on the land surface and in soil-forming processes by examining the amount of soil particles in mound structures of Macrotermes bellicosus in a highly weathered Ultisol of tropical savanna. Soil particle turnover via the mounds was estimated using particle stock data and soil turnover data from previous studies. A 4-ha study plot with six mounds of relatively uniform shape and size was investigated. Soil mass constituting the mounds was 6,166 ± 1,581 kg mound⁻¹ within which the mound wall and nest body accounted for 5,002 ± 1,289 and 1,164 ± 293 kg, respectively. The mound wall contained a significantly larger amount of clay (252 ± 9.97 g kg⁻¹) balanced with a lower sand content (676 ± 26.5 g kg⁻¹) than in the adjacent surface (Ap1) horizon, (46.4 ± 12.8 g clay kg⁻¹; 866 ± 83.2 g sand kg⁻¹); the nest body had much higher clay content (559 ± 51.0 g kg⁻¹) but less sand (285 ± 79.2 g kg⁻¹) than the mound wall. As a result, the mounds of M. bellicosus accumulated clay of 2,874 ± 781 kg ha⁻¹ (corresponding to 2.52% of clay stock in the Ap1 horizon) along with an estimated clay turnover rate of 169 kg ha⁻¹ year⁻¹. These findings suggest a positive feedback effect from termite mound-building activity on soil particle dynamics in tropical savanna ecosystems: M. bellicosus preferentially use subsoil material for mound construction, resulting in relocation of illuvial clay in the subsoil to the land surface where clay eluviation from the surface soil and its illuviation in the subsoil are major soil-forming processes.     

Microbial Activity and 13C/12C Ratio as Evidence of N-Hexadecane and N-Hexadecanoic Acid Biodegradation in Agricultural and Forest Soils

The dynamics of microbial degradation of exogenous contaminants, n-hexadecane and its primary microbial oxidized metabolite, n-hexadecanoic (palmitic) acid, was studied for topsoils, under agricultural management and beech forest on the basis the changes in O₂ uptake, CO₂ evolution and its associated carbon isotopic signature, the respiratory quotient (RQ) and the priming effect (PE) of substrates. Soil microbial communities in agricultural soil responded to the n-hexadecane addition more rapidly compared to those of forest soil, with lag-periods of about 23 ± 10 and 68 ± 13 hours, respectively. Insignificant difference in the lag-period duration was detected for agricultural (t_lag = 30 ± 13 h) and forest (t_lag = 30 ± 14 h) soils treated with n-hexadecanoic (palmitic) acid. These results demonstrate that the soil microbiota has different metabolic activities for using n-hexadecane as a reductive hydrocarbon and n-hexadecanoic acid as a partly oxidized hydrocarbon. The corresponding δ¹³C of respired CO₂ after the addition of the hydrocarbon contaminants to soils indicates a shift in microbial activity towards the consumption of exogenous substrates with a more complete degradation of n-hexadecane in the agricultural soil, for which some initial contents of hydrocarbons are inherent. It is supposed that the observed deviation of RQ from theoretically calculated value under microbial substrate mineralization is determined by difference in the time (Δt_i) of registration of CO₂ production and O₂ consumption. Positive priming effect (PE) of n-hexadecane and negative PE of n-hexadecanoic (palmitic) acid were detected in agricultural and forest soils. It is suggested that positive PE of n-hexadecane is conditioned by the induction of microbial enzymes that perform hydroxylation/oxygenation of stable SOM compounds mineralized by soil microbiota to CO₂. The microbial metabolism coupled with oxidative decarboxylation of n-hexadecanoic acid is considered as one of the most probable causes of the revealed negative PE value.     

Impact of microorganisms,humidity, and temperature on the enantioselective degradation of imazethapyr in two soils

Imazethapyr (IM) is a chiral herbicide composed of an (−)‐R‐enantiomer and an (+)‐S‐enantiomer with differential herbicidal activity. In this study, the effects of microbial organisms, humidity, and temperature on the selective degradation of the (−)‐R‐ and (+)‐S‐enantiomers of IM were determined in silty loam (SL) and clay loam (CL) soil with different pH values. The (−)‐R‐enantiomer of IM was preferentially degraded in two soils under different microorganism, humidity, and temperature conditions. The average half‐lives of R‐IM ranged from 43 to 66.1 days and were significantly shorter (P <  0.05) than those of S‐IM, which ranged from 51.4 to 79.8 days. The enantiomer fraction (EF = (+)‐S‐enantiomer/((−)‐R‐enantiomer + (+)‐S‐enantiomer)) values were used to describe the enantioselectivity of degradation of IM were >0.5 (P <  0.05) in two unsterilized soils under different humidity and temperature conditions. The highest EF values were observed at unsterilized CL soil samples under 50% maximum water‐holding capacity (MWHC) and 25 °C environmental conditions. The EF values of the IM enantiomers were significantly higher (P <  0.05) in CL soils (higher pH = 5.81) and were 0.581 (unsterilized) and 0.575 (50% MWHC; 25 °C) compared with those recorded in SL soil (lower pH = 4.85). In addition, this study revealed that microbial organisms preferentially utilized the more herbicidal active IM enantiomer.     

Biochar as Electron Acceptor for Microbial Extracellular Respiration

Biochar is a charred carbonaceous material that has recently been identified to provide many potential environmental and agricultural applications. Biochar amendments are shown to effectively improve the quality of soil and increase soil microbial biomass. However, the interactions between biochar and microorganisms and the mechanisms through which biochar influences soil microbial growth and activities remain unclear. In this study, we investigated the potential for biochar to function as an electron acceptor for microbial extracellular respiration and growth. Anaerobic incubation of Geobacter sulfurreducens revealed that biochar was used as a sole terminal electron acceptor, as evidenced by a 31-fold increase of biomass and gradual increase in reducing equivalents of biochar and the consumption of acetate after 15 d. An electron stoichiometry analysis showed that 58.7% of the electrons released from acetate oxidation could be recovered in biochar, which was comparable to that of humic substances (44.8%). The finding that biochar participates in microbial extracellular respiration may have important environmental implications considering the widespread existence of both extracellular-respiring microorganisms and black carbon in the environment.     

Effects of antibiotics in soil on the population dynamics of transposon Tn5 carrying Pseudomonas fluorescens

The effects of kanamycin and streptomycin added to soil on the survival of transposon Tn5 modified Pseudomonas fluorescens strain R2f were investigated. Kanamycin in high (180 g g^-1 dry soil) or low (18 g g^-1) concentration or streptomycin in low concentration in Ede loamy sand soil had no noticeable effect on inoculant population dynamics in soil and wheat rhizosphere, whereas streptomycin in high concentration had a consistent significant stimulatory effect, in particular in the wheat rhizosphere. Streptomycin exerted its effect by selecting P. fluorescens with Tn5 insertion whilst suppressing the unmodified sensitive parent strain, as evidenced by comparing the behaviour of these two strains in separate and mixed inoculation studies.Soil textural type influenced the effect of streptomycin on the Tn5 carrying inoculant; the effect was consistently detected in rhizosphere and rhizoplane samples of wheat grown in Ede loamy sand after 7 and 14 days incubation, whereas it was only apparent after 7 days in rhizoplane or rhizosphere (and bulk soil) samples of wheat grown in two silt loam soils. Modification of soil pH by the addition of CaCO₃ or bentonite clay resulted in an enhancement of the selective effect of streptomycin by CaCO₃ and its abolishment by bentonite clay.The addition to soil of malic acid or wheat root exudate, but not of glucose, enhanced the streptomycin selective effect on the Tn5-modified P. fluorescens strain. Neither the streptomycin producer Streptomyces griseus nor two non-inhibiting mutants obtained following UV irradiation affected the dynamics of P. fluorescens (chr::Tn5) in soil and wheat rhizosphere.The effect of streptomycin in soil on inoculant Tn5 carrying bacteria depends on conditions such as soil type, the presence of (wheat) root exudates and the type of available substrate.     

Microbial Inoculants with Multifaceted Traits Suppress Rhizoctonia Populations and Promote Plant Growth in Cotton

The suppressive effects of microbial inoculants on cotton seedling mortality were assessed in Rhizoctonia solani‐infested soil. Per cent mortality ranged from 16 to 32 (60–120 days after sowing, DAS) and significant differences were recorded at 120 DAS, especially after drenching with compost tea of Azotobacter sp. and Anabaena torulosa—Trichoderma viride‐biofilmed formulations. The activity of hydrolytic enzymes was reduced in diseased root tissues due to a majority of the microbially inoculated treatments, compared with healthy root tissues. Per cent changes in the amounts of glomalin‐related soil proteins (GRSPs) were 2 to 85% greater than those of the uninoculated experimental controls. These microbial inoculants altered the rhizosphere bacterial communities as evident from the Denaturing gradient gel electrophoresis (DGGE) banding patterns and, also reduced the population of R. solani. While the copy numbers of the internal transcribed spacer (ITS) gene of R. solani in the uninoculated (infested soil) were approximately 1.47 × 10¹¹ per g soil, they were 1.34–1.42 × 10⁵ per g soil after the application of A. torulosa, Anabaena laxa and A. torulosa–Bacillus sp. Increases in yield (ranging from 3 to 23%) due to various microbial inoculants relative to uninoculated controls illustrated their promise as plant growth‐promoting and disease‐suppressing agents. This study illustrates the modulation of rhizosphere ecology through microbial inoculants as a mechanism of disease suppression and sustaining plant growth.