Relationships Between Soil Microorganisms, Plant Communities, and Soil Characteristics in Chinese Subtropical Forests

We analyzed the influence of above- and belowground factors on the soil microbial community in a Chinese subtropical forest, one of the most diverse biomes in the northern hemisphere. Soil samples were taken at different depths from four replicate comparative study plots in each of three forest age classes (young 10–40?years, medium 40–80?years, old ≥80?years). Microbial biomass and community structure were then determined using phospholipid fatty acid (PLFA) analysis, and basal respiration and microbial biomass carbon (C_mic) were determined by substrate-induced respiration. These data were then related to plant community and soil variables using non-metric multidimensional scaling analysis and post-hoc permutational correlations. We found that microbial lipid composition and abundance were not related to forest age class. Instead, microbial lipid composition and abundance were related to factors reflecting primary production, i.e., percent litter cover, percent dead wood cover, and percent tree layer cover. Specifically, the relative abundance (mol fraction) of indicators for arbuscular mycorrhizal fungi, Gram-positive and Gram-negative bacteria were positively significantly correlated with percent litter cover. We also found that the biomass of all microbial groups and total PLFA were negatively significantly related to percent deadwood cover. In addition, $ {\text{pH}}_{{{\text{H}}_{ 2} {\text{O}}}} $ was the only soil parameter that was correlated significantly to microbial biomass. Our results indicate that overarching ecological factors such as plant productivity and soil pH are important factors influencing the soil microbial community, both in terms of biomass and of community composition in this subtropical ecosystem.     

The Relative Importance of Seeding Method, Soil Ripping, and Soil Variables on Seeding Success

We conducted a field experiment to examine how seeding method, soil ripping, and soil characteristics affected the initial establishment and growth of seeded species and if differences among treatments persisted into the second growing season. We planted seeds into a compacted field plot where most of the topsoil had been removed. The native seed mixture of Artemisia californica, Eschscholzia californica, Eriogonum fasciculatum, Lupinus succulentus, Nassella pulchra, and Vulpia microstachys represented different seed sizes and life histories. Three seeding methods (hydroseeding, imprinting, and drilling) and three ripping depths (0, 20, and 40 cm) were combined in a factorial experiment. Soil organic matter and NO₃ ^? were used as covariates. For two years, we measured density, percent cover, mean size, and flower production of the selected species, and weed emergence. Only seeding method and soil variation affected initial establishment of natives. Small‐seeded species had higher density in imprinted and hydroseeded than drilled treatments, whereas large‐seeded species had higher density in imprinted and drilled than hydroseeded treatments. These patterns persisted with only slight modification into the second year. Weed density in year 1 decreased with soil ripping. In year 1, Vulpia height and Lupinus height and flowering were greater with drilling or imprinting than hydroseeding but were not affected by ripping. Eschscholzia, Lupinus, and Vulpia produced seeds in the first year, but only Vulpia reestablished successfully in the second year. Vulpia had high cover in the second year that increased with increasing NO₃ ^? , but did not vary by treatment. In year 2 perennial Nassella, and to some extent Eriogonum, grew largest, produced more inflorescences, and had their highest percent cover in the 40‐cm rip treatment. Size and inflorescence production also increased with increasing NO₃ ^? ; sometimes this relationship was stronger than the effects of treatments. We found only positive associations between estimated biomass (density × height) of annuals and survival of shrubs. Potential for erosion control, as measured by total density in year 1 and total vegetative cover in year 2, was greatest in imprinted and hydroseeded treatments and increased with increasing NO₃ ^? . This relatively simple experiment yielded information critical to understanding optimum seeding methods and seedbed preparation and indicates that seeding method can be determined by seed size and germination biology. Although an experiment such as this enables some generalizations, it does not eliminate the need for site‐specific experiments prior to restoration.     

Relationship between Soil Arthropods and Soil Properties in a Suburb of Qianjiang City,Hubei, China

Nine categories of soil arthropods and 24 soil phy sicochemical properties were surveyed in 31 plots of a suburb of Qianjiang City, Hubei, China, in 1993 to 1994. Using factor analysis, 24 soil properties were reduced to five key soil factors (salt content, available K and P, organic matter, soil particles, Cd content), which were composed of 12 variables. Both simple correlation and canonical correlation analysis of the key soil factors and soil arthropods revealed positive correlations between Isopoda and Formicidae and the salt content factor, and between Chilopoda and the available P and K factor. Canonical correlation analysis also revealed a positive relationship between Formicidae, Isopoda, Diplopoda, and Staphylinidae and available K and P, and between Araneae, Staphylinidae, Diplopoda, and Formicidae and organic matter; and indicated a close relationship between Araneae, Staphylinidae, Diplopoda, and Formicidae and sandy loam soil. Results also indicated that Cd concentrations of 0.15?mg/kg to 0.41?mg/kg in soil with a high content of water did not significantly influence the nine arthropod categories.     

Assessing Cross-disciplinary Efficiency of Soil Amendments for Agro-biologically, Economically, and Ecologically Integrated Soil Health Management

Preventive and/or manipulative practices will be needed to maintain soil's biological, physiochemical, nutritional, and structural health in natural, managed, and disturbed ecosystems as a foundation for food security and global ecosystem sustainability. While there is a substantial body of interdisciplinary science on understanding function and structure of soil ecosystems, key gaps must be bridged in assessing integrated agro-biological, ecological, economical, and environmental efficiency of soil manipulation practices in time and space across ecosystems. This presentation discusses the application of a fertilizer use efficiency (FUE) model for assessing agronomic, economic, ecological, environmental, and nematode (pest) management efficiency of soil amendments. FUE is defined as increase in host productivity and/or decrease in plant-parasitic nematode population density in response to a given fertilizer treatment. Using the effects of nutrient amendment on Heterodera glycines population density and normalized difference vegetative index (indicator of physiological activities) of a soybean cultivar 'CX 252', how the FUE model recognizes variable responses and separates nutrient deficiency and toxicity from nematode parasitism as well as suitability of treatments designed to achieve desired biological and physiochemical soil health conditions is demonstrated. As part of bridging gaps between agricultural and ecological approaches to integrated understanding and management of soil health, modifications of the FUE model for analyzing the relationships amongst nematode community structure, soil parameters (eg. pH, nutrients, %OM), and plant response to soil amendment is discussed.     

Interplay between Wheat Cultivars,Biocontrol Pseudomonads,and Soil

There is a significant potential to improve the plant-beneficial effects of root-colonizing pseudomonads by breeding wheat genotypes with a greater capacity to sustain interactions with these bacteria. However, the interaction between pseudomonads and crop plants at the cultivar level, as well as the conditions which favor the accumulation of beneficial microorganisms in the wheat rhizosphere, is largely unknown. Therefore, we characterized the three Swiss winter wheat (Triticum aestivum) cultivars Arina, Zinal, and Cimetta for their ability to accumulate naturally occurring plant-beneficial pseudomonads in the rhizosphere. Cultivar performance was measured also by the ability to select for specific genotypes of 2,4-diacetylphloroglucinol (DAPG) producers in two different soils. Cultivar-specific differences were found; however, these were strongly influenced by the soil type. Denaturing gradient gel electrophoresis (DGGE) analysis of fragments of the DAPG biosynthetic gene phlD amplified from natural Pseudomonas rhizosphere populations revealed that phlD diversity substantially varied between the two soils and that there was a cultivar-specific accumulation of certain phlD genotypes in one soil but not in the other. Furthermore, the three cultivars were tested for their ability to benefit from Pseudomonas inoculants. Interestingly, Arina, which was best protected against Pythium ultimum infection by inoculation with Pseudomonas fluorescens biocontrol strain CHA0, was the cultivar which profited the least from the bacterial inoculant in terms of plant growth promotion in the absence of the pathogen. Knowledge gained of the interactions between wheat cultivars, beneficial pseudomonads, and soil types allows us to optimize cultivar-soil combinations for the promotion of growth through beneficial pseudomonads. Additionally, this information can be implemented by breeders into a new and unique breeding strategy for low-input and organic conditions.Improvement of plant fitness and yield by root-colonizing microorganisms is of special value in low-input or organic wheat production. Beneficial soil bacteria, such as certain Pseudomonas strains, are known to promote plant growth, which might help to circumvent potential negative consequences of low-input cropping systems, such as the limited supply of nutrients and higher disease pressure. A wide range of traits in Pseudomonas spp. are responsible for plant-beneficial effects. Many pseudomonads are capable of solubilizing poorly soluble or insoluble mineral phosphates, thereby rendering this element available for the plant and promoting plant growth (25, 43). Root-colonizing pseudomonads are also able to indirectly promote plant growth by providing protection against plant diseases. The most important mechanisms for plant protection against attacking pathogens are the induction of systemic resistance in plants (3) and the direct suppression of soilborne pathogens through the production of antimicrobial metabolites (16). The protection of wheat plants against Gaeumannomyces graminis var. tritici by naturally occurring pseudomonads in take-all decline soils is a well-described phenomenon and highlights the importance of these bacteria in a successful and environmentally friendly wheat production (53). Interestingly, in many naturally disease-suppressive soils a specific group of fluorescent pseudomonads is enriched, which is able to produce the antimicrobial compound 2,4-diacetylphloroglucinol (DAPG) (6, 38, 53). The production of the polyketide DAPG, which has broad-spectrum activity against bacteria, plants, fungi, and nematodes (8, 9, 21, 28, 33, 45), has been shown to be a key factor in the suppression of soilborne plant diseases by various Pseudomonas biocontrol strains (16).The degree of plant protection and plant growth promotion provided by root-colonizing pseudomonads is highly dependent on different environmental factors. For example, the expression of important biocontrol genes such as DAPG or HCN biosynthetic genes in the rhizosphere is modulated by biotic factors such as fungi and other bacteria present in the rhizosphere and the secondary metabolites they release (7, 19, 27, 29, 32). Moreover, it has been observed that the plant species and cultivar as well as the physiological stage of the plant can influence the expression of biocontrol genes and the production of antimicrobial metabolites (4, 7, 19, 32, 35). In addition to the production of DAPG and other antimicrobial metabolites, efficient colonization of roots is a prerequisite for beneficial plant-Pseudomonas interactions. Root colonization is dependent not only upon specific characteristics of the bacterium itself but also on root morphology and root exudates that vary between host plant species and even between cultivars of the same species (5, 34). The host species/cultivar also influences the abundance and diversity of naturally occurring pseudomonads (13). This has been shown in particular for DAPG-producing populations (4, 5, 26, 30, 36).Wheat is a crop known to benefit strongly from naturally occurring DAPG-producing pseudomonad populations (52). It has been shown that the size and composition of DAPG-producing populations in the wheat rhizosphere and also the amount of DAPG produced by these populations may vary substantially between different cultivars (4, 35). However, holistic studies which evaluate specific wheat cultivars for both their ability to benefit from plant growth-promoting pseudomonads and their influence on bacterial populations and production of biocontrol compounds are missing. A comprehensive characterization of different cultivars is needed in order to better understand which cultivars promote beneficial interactions with the pseudomonads. This knowledge has potential in future breeding strategies to be used for selection of new cultivars that optimally attract and respond to these bacteria.In order to address this gap in knowledge, this study evaluated three Swiss winter wheat (Triticum aestivum) cultivars for several characteristics considered important in a successful wheat-pseudomonas interplay: (i) the ability to accumulate pseudomonads and phlD⁺ pseudomonads in two different Swiss soils, (ii) the ability to select for individual phlD⁺ genotypes in two different soils, (iii) the ability to benefit from the two model biocontrol strains, Pseudomonas fluorescens strain CHA0 (a DAPG producer) and P. putida KD (a DAPG nonproducer), in terms of direct plant growth promotion and disease suppression, and finally (iv) the level of biocontrol gene expression (DAPG-biosynthetic gene phlA) in the rhizosphere.     

土壤保持服务:概念、评估与展望

土壤保持服务作为一项重要的生态系统调节服务,是防止区域土地退化、降低洪涝灾害风险的重要保障。针对在全球范围内影响最大的土壤水蚀,基于土壤侵蚀、运移和输出过程,对土壤保持服务的概念、评估方法进行了梳理总结。土壤保持服务是指生态系统防止土壤流失的侵蚀调控能力及对泥沙的储积保持能力。土壤保持服务的评估往往是基于通用土壤流失方程RUSLE(Revised Universal Soil Loss Equation),以潜在土壤侵蚀量(裸地时土壤侵蚀量)与实际土壤侵蚀量之差,作为指标衡量。由于生态过程具有尺度依赖性,对土壤保持服务的有效评估,需要采用多尺度方法。土壤保持服务与人类需求紧密相关,在未来土壤保持服务研究中应强调连接土壤保持服务与人类福祉,对土壤保持服务产生、流动、使用的全过程及土壤保持服务时空动态与人类福祉变化的关系进行探究。     

Soil Properties,Nutrient Dynamics,and Soil Enzyme Activities Associated with Garlic Stalk Decomposition under Various Conditions

The garlic stalk is a byproduct of garlic production and normally abandoned or burned, both of which cause environmental pollution. It is therefore appropriate to determine the conditions of efficient decomposition, and equally appropriate to determine the impact of this decomposition on soil properties. In this study, the soil properties, enzyme activities and nutrient dynamics associated with the decomposition of garlic stalk at different temperatures, concentrations and durations were investigated. Stalk decomposition significantly increased the values of soil pH and electrical conductivity. In addition, total nitrogen and organic carbon concentration were significantly increased by decomposing stalks at 40°C, with a 5∶100 ratio and for 10 or 60 days. The highest activities of sucrase, urease and alkaline phosphatase in soil were detected when stalk decomposition was performed at the lowest temperature (10°C), highest concentration (5∶100), and shortest duration (10 or 20 days). The evidence presented here suggests that garlic stalk decomposition improves the quality of soil by altering the value of soil pH and electrical conductivity and by changing nutrient dynamics and soil enzyme activity, compared to the soil decomposition without garlic stalks.     

Interaction of Rotylenchulus reniformis,Soil Salinity,and Cotton

Rotylenchulus reni]ormis occurred equally in relatively non-saline (4.0 mmhos/cm) and highly-saline (16.5 mmhos/cm) soils in sampling transects across zones of depressed plant growth in six Texas cotton fields.Results from greenhouse pot experiments indicated progressive positive interaction of salinity and R. reni[ormis pathogenicity in the range 6-18 mmhos/cm.     

Regulation of Microbial Community Composition and Activity by Soil Nutrient Availability, Soil pH, and Herbivory in the Tundra

Soil nitrogen (N) availability and pH constitute major abiotic controls over microbial community composition and activity in tundra ecosystems. On the other hand, mammalian grazers form an important biotic factor influencing resource coupling between plants and soil microorganisms. To investigate individual effects and interactions among soil nutrients, pH, and grazing on tundra soils, we performed factorial treatments of fertilization, liming, and grazer exclusion in the field for 3 years at 2 contrasting tundra habitats, acidic (N-poor) and non-acidic (N-rich) tundra heaths. The effects of all treatments were small in the non-acidic tundra heaths. In the acidic tundra heaths, fertilization decreased the fungal:bacterial ratio as analyzed by soil PLFAs, but there were no effects of liming. Fertilization increased soil N concentrations more drastically in ungrazed than grazed plots, and in parallel, fertilization decreased the fungal:bacterial ratio to a greater extent in the ungrazed plots. Liming, on the other hand, partly negated the effects of fertilization on both soil N concentrations and PLFAs. Fertilization drastically increased the activity of phenol oxidase, a microbial enzyme synthesized for degradation of soil phenols, in grazed plots, but had no effect in ungrazed plots. Taken together, our results demonstrate that grazers have the potential to regulate the fungal:bacterial ratio in soils through influencing N availability for the soil microorganisms.     

Temperature Increases Soil Respiration Across Ecosystem Types and Soil Development,But Soil Properties Determine the Magnitude of This Effect

Ecosystems - Soil carbon losses to the atmosphere, via soil heterotrophic respiration, are expected to increase in response to global warming, resulting in a positive carbon-climate feedback....     

Responses of Soil,Heterotrophic, and Autotrophic Respiration to Experimental Open-Field Soil Warming in a Cool-Temperate Deciduous Forest

How global warming will affect soil respiration (R _S) and its source components is poorly understood despite its importance for accurate prediction of global carbon (C) cycles. We examined the responses of R _S, heterotrophic respiration (R _H), autotrophic respiration (R _A), nitrogen (N) availability, and fine-root biomass to increased temperature in an open-field soil warming experiment. The experiment was conducted in a cool-temperate deciduous forest ecosystem in northern Japan. As this forest is subjected to strong temporal variation in temperature, on scales ranging from daily to seasonal, we also investigated the temporal variation in the effects of soil warming on R _S, R _H, and R _A. Soil temperature was continuously elevated by about 4.0°C from 2007 to 2014 using heating wires buried in the soil, and we measured soil respiratory processes in all four seasons from 2012 to 2014. Soil warming increased annual R _S by 32–45%, but the magnitude of the increase was different between the components: R _H and R _A were also stimulated, and increased by 39–41 and 17–18%, respectively. Soil N availability during the growing season and fine-root biomass were not remarkably affected by the warming treatment. We found that the warming effects varied seasonally. R _H increased significantly throughout the year, but the warming effect showed remarkable seasonal differences, with the maximum stimulation in the spring. This suggests that warmer spring temperature will produce a greater increase in CO₂ release than warmer summer temperatures. In addition, we found that soil warming reduced the temperature sensitivity (Q ₁₀) of R _S. Although the Q ₁₀ of both R _H and R _A tended to be reduced, the decrease in the Q ₁₀ of R _S was caused mainly by a decrease in the response of R _A to warming. These long-term results indicate that a balance between the rapid and large response of soil microbes and the acclimation of plant roots both play important roles in determining the response of R _S to soil warming, and must be carefully considered to predict the responses of soil C dynamics under future temperature conditions.     

Soil Loss and Runoff in Semiarid Ecosystems: A Complex Interaction Between Biological Soil Crusts, Micro-topography, and Hydrological Drivers

Biological soil crusts (BSCs) cover non-vegetated areas in most arid and semiarid ecosystems. BSCs play a crucial role in the redistribution of water and sediments and, ultimately, in the maintenance of ecosystem function. The effects of BSCs on water infiltration are complex. BSCs increase porosity and micro-topography, thus enhancing infiltration, but, at the same time, they can increase runoff by the secretion of hydrophobic compounds and clogging of soil pores upon wetting. BSCs confer stability on soil surfaces, reducing soil detachment locally; however, they can also increase runoff, which may increase sediment yield. Although the key role of BSCs in controlling infiltration–runoff and erosion is commonly accepted, conflicting evidence has been reported concerning the influence of BSCs on runoff generation. Very little is known about the relative importance of different BSC features such as cover, composition, roughness, or water repellency, and the interactions of these attributes in runoff and erosion. Because BSC characteristics can affect water flows and erosion both directly and indirectly, we examined the direct and indirect effects of different BSC features on runoff and erosion in a semiarid ecosystem under conditions of natural rainfall. We built structural equation models to determine the relative importance of BSC cover and type and their derived surface attributes controlling runoff and soil erosion. Our results show that the hydrological response of BSCs varies depending on rainfall properties, which, in turn, determine the process governing overland flow generation. During intense rainfalls, runoff is controlled not only by rainfall intensity but also by BSC cover, which exerts a strong direct and indirect influence on infiltration and surface hydrophobicity. Surface hydrophobicity was especially high for lichen BSCs, thus masking the positive effect of lichen crust on infiltration, and explaining the lower infiltration rates recorded on lichen than on cyanobacterial BSCs. Under low intensity, rainfall volume exerts a stronger effect than rainfall intensity, and BSC features play a secondary role in runoff generation, reducing runoff through their effect on surface micro-topography. Under these conditions, lichen BSCs presented higher infiltration rates than cyanobacterial BSCs. Our results highlight the significant protective effect against erosion exerted by BSCs at the plot scale, enhancing surface stability and reducing sediment yield in both high- and low-magnitude rainfall events.     

Concept,Components, and Strategies of Soil Health in Agroecosystems

The terms ''''soil health'''' or ''''soil quality'''' as applied to agroecosystems refer to the ability of soil to support and sustain crop growth while maintaining environmental quality. High-quality soils have the following characteristics: (i) a sufficient, but not excess, supply of nutrients; (ii) good structure (tilth); (iii) sufficient depth for rooting and drainage; (iv) good internal drainage; (v) low populations of plant disease and parasitic organisms; (vi) high populations of organisms that promote plant growth; (vii) low weed pressure; (viii) no chemicals that might harm the plant; (ix) resistance to being degraded; and (x) resilience following an episode of degradation. Management intended to improve soil health involves creatively combining a number of practices that enhance the soil''s biological, chemical, and physical suitability for crop production. The most important general strategy is to add plentiful quantities of organic matter—including crop and cover crop residues, manures, and composts. Other important strategies include better crop rotations, reducing tillage and keeping the soil surface covered with living and dead residue, reducing compaction by decreasing heavy equipment traffic, and using best nutrient management practices. Practices that enhance soil quality frequently reduce plant pest pressures.     

Natural Attenuation, Biostimulation, and Bioaugmentation in 4-Chloroaniline-Contaminated Soil

Bioremediation treatments including natural attenuation (NA), biostimulation (BS), and bioaugmentation (BA) were performed and compared regarding the degradation of 4-chloroaniline (4CA) contaminating two types of agricultural soil collected from Nakornnayok (NN) and Chiangmai (CM) provinces, Thailand. Despite the different soil properties, both soil types exhibited intrinsic potential for biodegradation. 4CA degradation by NA in loam soil-NN was fairly effective (ca. 40%), while in sandy-clay loam soil-CM it occurred poorly (<10%). Compared to NA, BS with aniline and BA with 4CA-degrading Klebseilla sp. CA17 were comparatively more effective techniques, although the degradation occurred differently in each soil type. In soil-NN, the biodegradation of 4CA took place at a higher rate, achieving biodegradation of 70–75% within 4 weeks, than in soil-CM, i.e., up to 40–46% within 8 weeks. During each treatment, changes in soil microbial activity, numbers of 4CA-degrading micro-organisms, and dynamic modification of soil microbial community structure were also monitored. The results suggest that both BS and BA are feasible techniques for bioremediation of 4CA accumulated in soil, although the biodegrading efficiency in soil environment depends not only on site characteristics but also on the characteristics of either indigenous microbial population or the survival and stability of bioaugmented cultures.     

Adsorption,Bioavailability, and Toxicity of Cadmium to Soil Microorganisms

The influence of adsorption on cadmium toxicity to soil microorganisms in soils was quantified as a function of solution and sorbent characteristics. The influence of adsorption on cadmium toxicity to soil microorganisms was assessed indirectly through the relative change in microbial hydrolysis of fluorescein diacetate (FDA) as a function of total Cd concentration and sorbent characteristics. The sequence of relative percentage of FDA hydrolysis was reference smectite (RS) > untreated Vertisol (UV) > dithionate-citrate-bicarbonate (DCB)-treated Vertisol (DV) > H₂O₂-treated Vertisol (HV) in suspensions containing the same total Cd concentrations. The correlation between the percentage of FDA hydrolysis and activity of Cd²⁺ _(aq) illustrates that RS has a higher capacity of Cd adsorption. The microbial activity of RS was higher and the toxicity was lower than that of other soil samples. The HV had lower capacity of Cd adsorption so that its FDA hydrolysis was low and the Cd toxicity was high.     

Occurrence of Enterococci: Bud, Blossom, and Soil Studies

The occurrence of enterococci (group D streptococci) on buds and flowers of plants and in soils has been studied. They were recovered from 27.5% of the flowers of seven species of plants, and from 6.8% of the buds of the same plants. They were recovered from 34% of the flowers of nonagricultural plants, from 32.2% of the flowers of ten species of agricultural dicotyledonous plants, and from 10.4% of the flowers of five species of grasses and cereals. The enterococci were invariably present or invariably absent from all samples taken from very few species. They occurred in small numbers on enclosed tassels and silks of corn of 22 of 60 samples, and in greater numbers on 90% or more of these after their floral parts had emerged. Interposition of a mechanical barrier reduced the incidence of recovery from flowers. The occurrence in soil, generally at a low level of population, may be correlated with occurrence on the plant growing on the soil or with nearby enterococcal-bearing plants.It is concluded that enterococci may be regarded as temporary residents on plants, capable of limited reproduction, and that they are disseminated among plants by the action of insects and wind, and spread to the ground by these agencies, gravity, and rain.     

Copper,Lead, Cadmium,and Zinc Sorption By Waterlogged and Air-Dry Soil

Competitive sorption of copper (Cu), lead (Pb), cadmium (Cd), and zinc (Zn) was studied in three soils of contrasting chemical and physical properties under air-dry and waterlogged conditions. Competitive sorption was determined using the standard batch technique using six solutions, each with Cu, Pb, Cd, and Zn concentrations of approximately 0, 2.5, 5, 10, 20, and 50?mg L^?1Waterlogged soils tended to sorb higher amounts of added Cu, Pb, Zn and Cd relative to soils in the air-dry condition; however, this increase in sorption was generally not statistically (p<0.05) significant. The magnitude of sorption under both waterlogged and air-dry conditions was affected by the type and amount of soil materials involved in metal sorption processes, and competition between other metals for the sorption sites. Metal sorption was closely correlated with soil properties such as cation exchange capacity, organic carbon, and Fe and Mn hydrous oxides. Exchangeable Al may have markedly reduced metal sorption due to its strong affinity for the sorption sites, while increases in exchangeable Mn may have enhanced Zn and Cd sorption. Heavy metal sorption was best described as a combination of both specific and nonspecific interactions. The extractability of Cu, Pb, Cd, and Zn under waterlogged and air-dry conditions was also studied. Three solutions containing these metals were mixed with each soil to achieve a final concentration of 0, 50, and 500?mg kg^?1. Each soil was extracted every 7 days using 1?M MgCl₂ (pH 7) to determine metal extractability. Metal extractability initially decreased then increased due to waterlogging. The increased extractability of added metals was closely related to increased solubility of Fe and Mn suggesting that dissolution of Fe and Mn, oxides under reducing conditions caused a release of previously sorbed Cu, Pb, Cd, and Zn.     

Metabolism of Benzene, Toluene, and Xylene Hydrocarbons in Soil

Enrichment cultures obtained from soil exposed to benzene, toluene, and xylene (BTX) mineralized benzene and toluene but cometabolized only xylene isomers, forming polymeric residues. This observation prompted us to investigate the metabolism of ¹⁴C-labeled BTX hydrocarbons in soil, either individually or as mixtures. BTX-supplemented soil was incubated aerobically for up to 4 weeks in a sealed system that automatically replenished any O₂ consumed. The decrease in solvent vapors and the production of ¹⁴CO₂ were monitored. At the conclusion of each experiment, ¹⁴C distribution in solvent-extractable polymers, biomass, and humic material was determined, obtaining ¹⁴C mass balances of 85 to 98%. BTX compounds were extensively mineralized in soil, regardless of whether they were presented singly or in combinations. No evidence was obtained for the formation of solvent-extractable polymers from xylenes in soil, but ¹⁴C distribution in biomass (5 to 10%) and humus (12 to 32%) was unusual for all BTX compounds and especially for toluene and the xylenes. The results suggest that catechol intermediates of BTX degradation are preferentially polymerized into the soil humus and that the methyl substituents of the catechols derived from toluene and especially from xylenes enhance this incorporation. In contrast to inhibitory residues formed from xylene cometabolism in culture, the humus-incorporated xylene residues showed no significant toxicity in the Microtox assay.