Influence of Nitrogen Addition and Plant Root Parameters on Phytoremediation of Pyrene-contaminated Soil

Phytoremediation is a method in which plants, soil microorganisms, amendments, and agronomic techniques interact to enhance contaminant degradation. We hypothesized that bermudagrass (Cynodon dactylon L) and an appropriate amount of N fertilizer would improve remediation of pyrene-contaminated Captina silt loam soil. The soil was contaminated with 0 or 1,000 mg pyrene/kg of soil and amended with urea at pyrene-C:urea-N (C:N) ratios of 4.5:1, 9:1, 18:1, or unamended (36:1). Either zero, one, two, or three bermudagrass sprigs were planted per pot and ?33 kPa moisture potential was maintained. Pyrene concentrations, inorganic-N levels, shoot and root parameters, and pyrene degrader microbial numbers were measured following a 100-day greenhouse study. At a C:N ratio of 4.5:1, the presence of plants increased pyrene biodegradation from 31% for the no plant treatment to a mean of 62% for the one, two, and three plant treatments. With no plants and C:N ratios of 4.5:1, 9:1, 18:1, and 36:1, the mean pyrene biodegradation was 31, 52, 77, and 88%, respectively, indicating that increased inorganic-N concentration in the soil reduced pyrene degradation in the treatments without plants. Additionally, none of the one, two, or three plant treatments at any of the C:N ratios were different with a mean pyrene degradation value of 69% after 100 days. Pyrene resulted in reduced shoot and root biomass, root length, and root surface area, but increased root diameter. The pyrene degrading microbial numbers were approximately 10,000-fold higher in the pyrene-contaminated soil compared to the control. At the highest N rate, bermudagrass increased pyrene degradation compared to the no plant treatment, however, in the unvegetated treatment pyrene degradation was reduced with added N.     

Enrichment of functional microbes and genes during pyrene degradation in two different soils

Purpose

Stimulating microbial degradation is a promising strategy for the remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs). To better understand the functional microbial populations and processes involved in pyrene biodegradation in situ, the dynamics of pyrene degradation and functional microbial abundance were monitored during pyrene incubation in soils. We hope our findings will provide new insights into in situ pyrene biodegradation in soils and help to identify functional microbes from soils.

Materials and methods

Pyrene (60 mg kg^?1) was incubated with two different soils, one is lower PAH-containing agricultural soil (LS), and the other is higher PAH-containing industrial soil (HS). During incubation, triplicate samples were collected on days 0, 3, 7, 14, and 35. Pyrene in soil samples was analyzed using an Agilent gas chromatograph (7890A) equipped with a mass-selective detector (model 5897). DNA in soils was extracted with a FastDNA Spin kit for soil (Bio101, USA). The abundance of functional microbes and genes was monitored by a Taqman or SYBR Green based real-time PCR quantification using an iCycler iQ5 themocycler (Bio-Rad, USA). The diversity of PAH-RHDα GP genes was evaluated by constructing clone libraries and sequencing.

Results and discussion

In both soils, more than 80 % of the added pyrene was degraded within 35 days. After 35-day incubation, there was a significant enrichment of Gram-positive bacteria harboring PAH-ring hydroxylation dioxygenase (PAH-RHD_α GP) genes, and the abundance of Mycobacterium increased significantly. In PAH-RHD_α GP clone libraries from two soils, Mycobacterium was detected, while most sequences were closely related to uncultured Gram-positive bacteria. In addition, two pyrene catabolic pathways might be involved in pyrene degradation, as pyrene dioxygenase genes, nidA and nidA3, were dramatically enriched during incubation. Moreover, the abundance and diversity of potential degraders in two soils showed significantly difference in responding to pyrene stress. This result indicates that soil condition can significantly affect functional microbial populations and biological process for pyrene biodegradation.

Conclusions

These results revealed that Mycobacterium as well as uncultured Gram-positive PAH-RHD_α genotypes may be the important group of pyrene degraders in soils, and two pyrene catabolic pathways, targeted by nidA and nidA3, might potentially contribute to in situ biodegradation of pyrene. This study characterized the response pattern of potential pyrene degraders to pyrene stress in two different soils, which would increase our understanding of the indigenous processes of pyrene biodegradation in soil environment.

     

Ammonium oxidizer numbers,potential and actual oxidation rates in two swedish arable soils

Summary The number of ammonium-oxidizing bacteria was measured with the most probable number (MPN) method while potential ammonium oxidation rates were determined with a chlorate inhibition technique in two arable soils. A new method for measuring actual in situ ammonium oxidation in soil cores is presented.One soil was cropped for 4 years with one of four crop-fertilizer combinations: Unfertilized lucerne ley, unfertilized barley, nitrate-fertilized grass ley, or nitrate-fertilized barley. The highest ammonium oxidizer numbers and potential rates were found in the grass ley. The unfertilized barley had one-third the number and activity of the grass ley. Actual rates were in general 5–25 times lower than potential rates.The other soil was that undergoing a 27-year-old field trial with a fallow and four different cropping treatments: No addition, nitrate, nitrate + straw, or manure. Ammonium oxidizer numbers were highest in the manure and straw treatments. MPN numbers and potential rates were lowest in the fallow treatment. Typical specific potential rates were 30 ng N oxidized cell^–1 h^–1. Actual rates were in general 40 times lower than potential rates.Actual ammonium oxidation measurements seem to correspond to actual in situ activity at the moment of sampling, whereas the MPN technique and the potential measurements reflect events that occurred weeks to months before the sampling.     

Atrazine and phenanthrene degradation in grass rhizosphere soil

Organic contaminants often disappear more quickly from planted than from non-planted soils. Five grass species (Sudan grass, ryegrass, tall fescue, crested wheatgrass and switch grass) were grown in soils without (Phase I) or with (Phase II) prior atrazine (ATR) and phenanthrene (PHE) amendment to study the degradation of these compounds by rhizosphere microorganisms. In suspensions of soil without prior chemical exposure, no significant loss of ATR was observed after 16 days incubation. The most probable number (MPN) of ATR-degrading bacteria in the soils was below detection. Phenanthrene degradation was observed in suspensions inoculated with all soils, but the rates of degradation were not significantly different among them. The number of PHE-degrading bacteria was similar in planted and non-planted soils (10⁵ cells g⁻¹ soil) except the number in tall fescue soil was significantly higher than in non-planted soil. In the Phase II study, both compounds were mineralized whether or not soils had been conditioned with ATR or PHE. Prior amendment with either ATR or PHE significantly reduced the acclimation period preceding the onset of mineralization. However, enumeration procedures detected ATR-degrading bacteria only in ATR-amended soils. Prior exposure to PHE did not alter the number of PHE-degrading bacteria significantly.     

Effects of Bacterial-Feeding Nematode Grazing and Tea Saponin Addition on the Enhanced Bioremediation of PyreneContaminated Soil Using Polycyclic Aromatic Hydrocarbon-Degrading Bacterial Strain

As one of the most widely distributed bacterial predators in the soil, the role of bacterivorous nematodes on the enhanced bioremediation of polycyclic aromatic hydrocarbon-contaminated soils is crucial, but remains to be investigated.A microcosm-level study was conducted to examine the effects of bacterial-feeding nematode grazing and tea saponin(TS) addition on bioremediation of a pyrene-contaminated soil enhanced by the polycyclic aromatic hydrocarbon(PAH)-degrading bacterial strain Sphingobium sp.PHE9.After 180 d of incubation, the highest pyrene dissipation(71.3%) was achieved through a combination of Sphingobium sp.PHE9 inoculation with nematode and TS addition.Meanwhile, high counts of culturable PAH-degrading bacteria, soil enzyme activity, and biodiversity indices were observed under the combined treatment, implying that the microbiological function of the contaminated soil was significantly restored.Additionally, the results of Tenax~ extraction with the first-order three-compartment model indicated that rate-limiting factors varied among treatments.The lack of degrading microorganisms was the main rate-limiting factor for the treatments involving TS/nematode addition, and inadequate bioaccessible pyrene was the vital rate-limiting factor in the treatments involving Sphingobium sp.PHE9 inoculation.The proposed combined clean-up strategy proved to be a promising bioremediation technology for aged pyrene-contaminated soils.     

Characterization of rhizosphere microbial community structure in five similar grass species using FAME and BIOLOG analyses

Accelerated biodegradation of organic contaminants in planted soil is frequently reported yet our current understanding of plant–microbe interactions does not allow us to predict which plant species can encourage the development of rhizosphere communities with enhanced degradation capacity. In a companion study, five grass species (Sudan grass, ryegrass, tall fescue, crested wheatgrass, and switch grass) were grown in a Matapeake silt loam soil to study the degradation of atrazine and phenanthrene by rhizosphere microorganisms (see Fang et al., 2000, this vol., Fang, C., Radosevich, M., Fuhrmann, J. J., 2000. Atrizine and phenanthrene degradation in grass rhizosphere soil. Soil Biology & Biochemistry, in press). In the present paper substrate utilization patterns (BIOLOG^®), and fatty acid methyl ester (FAME) profiles of the same rhizosphere microbial communities were determined. Both FAME and BIOLOG^® analyses detected changes in soil microbial community structure among treatments. However, community structure did not directly correlate to either ATR or PHE degradation rates.     

Effects of thiamine and nitrogen fertilizer form on the number of N2−fixing and total bacteria in the rhizosphere of maize plants

The effects of thiamine (vitamin B1) application as seed dressing and of N form supplied (NH₄⁺ versus NO₃^?) on rhizosphere pH and on rhizosphere microorganisms were evaluated in two different soils. Imbibition of maize (Zea mays L.) seeds with thiamine (1 g kg^?1) increased seed thiamine content by a factor of 370. Maize plants from untreated and treated seeds were cultivated in a growth chamber under controlled conditions for 10 d in a sandy loam soil, pH 7.1 (Mascherode soil) or in a sandy soil, pH 4.8 (Niger soil) fertilized with two different N sources (NO₃?N or NH₄?N with dicyandiamide, 100 and 250 mg N kg^?1 soil). The rhizosphere pH was not affected by thiamine, only slightly affected by N source in the Mascherode soil, but markedly affected in the Niger soil. Thiamine application and N source affected the most probable number (MPN) of diazotrophs and total bacteria isolated from the rhizosphere soil of 10 d old maize plants. In the Mascherode soil, thiamine application increased MPN of diazotrophs 4-fold and total bacteria 2-fold when the soil was fertilized with 100 mg NO₃?N compared to untreated seedlings. Compared to Mascherode soil, in the Niger soil, MPN of diazotrophs was extremely low, especially after NH₄?N treatment which significantly decreased pH of the rhizosphere. Thiamine application had only marginal effects on the MPN of diazotrophs and total bacteria. Total bacteria isolated from Niger soil fertilized with NH₄?N were about 10-fold lower compared to the soil from Mascherode. However, in the other two treatments, total bacteria were higher in the Niger soil compared to the Mascherode soil. In the Niger soil, apparently some of the heterotrophs (the Actinomycetes dominated in this soil) might have suppressed the diazotrophs. The results of the present study demonstrate that in many cases seed treatment with thiamine enhances MPN of diazotrophs and total bacteria in the rhizosphere of maize seedlings.     

Nitrification of ammonium in different components of a flooded rice soil system

Nitrification associated with the various components [subsurface soil from unplanted and planted (rhizosphere) fields, standing water and surface soil from planted and unplanted fields and leaf sheath suspensions] of submerged rice paddies was examined in incubation experiments with solutions inoculated with soil or water samples. Substantial nitrification occurred in all samples, standing water and surface soil samples in particular, during their 40-day incubation with NH ₄ ⁺ –N. Almost all the NH ₄ ⁺ –N, disappeared during incubation with standing water, was recovered as NO _inf3 ^sup- –N. This, compared to 70–80% from all soil samples and only 29% from leaf sheath suspensions. Significant loss of nitrogen, especially from leaf sheath suspensions, is probably due to nitrification-denitrification as evidenced by its complete recovery in the presence of N-Serve. Nitrification potential of the soil and water samples varied with the crop growth stage and was more pronounced at tillering and panicle inititation stages than at other stages. Nitrification potential of samples from green-manure-amended plots was distinctly less than that of samples from control and urea-amended plots. Most probable number (MPN) estimates of ammonium-oxidizing bacteria were always higher in surface soil in both planted and unplanted plots at all stages of crop growth.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

A comparison of microbial-feeding nematodes and protozoa in the rhizosphere of different plants

Summary The biomass of microbial-feeding nematodes and protozoa was measured in the rhizospheres of peas, barley, grass and turnips grown for 10 weeks in pots containing a clay-loam soil; in the rhizospheres of peas and barley grown for 3 weeks in a sandy soil; and in the rhizosphere of barley grown for 11 weeks in an unfertilised and a fertilised clay-loam soil. The nematode biomass was consistently larger in the rhizosphere of all plants in both soils than in the bulk soil, but the protozoa biomass showed a rhizosphere effect only under pea and fertilised barley. The biomass of nematodes in the rhizosphere (1.2–22.3 g dry weight g^-1 dry soil) was greater than the biomass of protozoa (0.1–3.2 g g^-1), and greater under pea>barley>grass>turnip. It is suggested that nematodes are more able to exploit low bacterial densities than protozoa and that they initially migrate into the rhizosphere from the bulk soil. In samples of potato rhizosphere from field-grown plants, the nematode biomass was also greater than the active and total protozoan biomass. It is argued that in the rhizosphere the biomass of microbially feeding nematodes exceeds that of protozoa and that nematodes are more important in terms of nutrient cycling.     

Time-course of nitrogen fixation in two bambara groundnut (Vigna subterranea L. Verdc.) cultivars

TheA-value method, involving the application of a higher¹⁵N rate to a reference non-N₂-fixing plant, was used to assess the magnitude of N₂ fixation in two bambara groundnut cultivars at four growth stages [vegetative, 0–47 days after planting (DAP); early pod-filling, 47–99 DAP; mid-pod-filling, 99–120 DAP; physiological maturity, 120–148 DAP). The cultivars were Ex-Ada, a bunchy type, and CS-88-11, a slightly spreading type. They were grown on a loamy sand. Uninoculated Ex-Ada and CS-88-11 were used as reference plants to measure the N₂ fixed in the inoculated bambara groundnuts. In this greenhouse study, soil was the major source of N in bambara groundnuts during vegetative growth, and during this period it accounted for over 80% of the N accumulaed in the plants. However, N₂ fixation became the major source of plant N during reproductive growth. There were significant differences between the two cultivars in the ability to fix N₂, and at physiological maturity, almost 75% of the N in CS-88-11 was derived from the atmosphere compared to 55% in Ex-Ada. Also, the total N fixed in CS-88-11 at physiological maturity was almost double that in Ex-Ada. Our data indicate that the higher N₂ fixation in CS-88-11 was due to two factors, a higher intensity of N₂ fixation and a longer active period of N₂ fixation. The results also suggest that bambara groundnut genotypes could be selected for higher N₂ fixation in farining systems.     

Vertical distribution of nematodes in arable soil under grass (Festuca pratensis) and barley (Hordeum distichum)

Summary The connection between faunal composition and soil factors is discussed in this study on vertical distribution of soil nematodes under grass and barley. The investigation was undertaken on the field site of a Swedish integrated research project Ecology of Arable Land. The Role of Organisms in Nitrogen Cycling. Higher nematode number (7.6 × 10⁶ m^–2) and biomass (340 mg dry wt. m^–2) were found under a 4-year-old grass ley than under barley (5.0 × 10⁶ m^–2; biomass, 136 mg dry wt. m^–2). Plant feeders dominated under the grass ley (3.2 × 10⁶ m^–2 whereas under barley the bacterial feeders (2.4 × 10⁶ m^–2) were the most abundant feeding group. Number, biomass, mean individual size and various community parameters indicated a much better nutritive situation for the nematodes under grass than under barley. The vertical changes in the various parameters, including proportion of egg-carrying females, indicated an increasing food shortage for the nematode populations towards greater depths. In the top soil, predation could be an important factor in regulating nematode number.Dedicated to the late Prof. Dr. M.S. Ghilarov     

Distribution of microbial biomass across the rhizosphere of barley (Hordeum vulgare L.) in soils

Summary The microbial activity at the soil-root interface (rhizosphere) of barley was examined using a rhizobox system. In this system, the soil was placed in several compartments separated from each other by a 500-mesh nylon cloth. Plants were grown in the central compartment and after a 2-month growing period the roots were still confined to this compartment. The soil from each compartment was then analyzed for ATP, NO₃ ^/–, total N, total C and CO₂ production. The increase in ATP concentration was found in a range of 4 mm around the roots. The ATP content and CO₂ production across the rhizosphere were correlated in all the soils used, but changes in NO₃ ^– were not correlated with ATP changes. The range of NO₃ ^– change was wider than that of the ATP change, indicating that NO₃ ^– production is not influenced by the biological activity in the rhizosphere.     

土壤微生物产电信号评价芘污染毒性的研究

通过向土壤中加入葡萄糖促进微生物产电过程,研究了芘污染条件下土壤产电变化规律。利用双室微生物燃料电池(Microbial fuel cells,MFCs),实时、连续记录芘污染土壤产电电压。产电110 h后结束MFCs运行,采用循环伏安法检测芘对土壤微生物电化学活性的影响;结合PCR-DGGE及测序技术,分析芘对MFCs阳极表面细菌群落结构的影响。结果显示,MFCs产电电量随芘浓度增加显著降低。循环伏安检测显示芘降低了土壤微生物的电化学活性。DNA序列分析表明,阳极细菌与已报道的产电细菌高度相似,包括Sporolactobacillus、Clostridium、Enterobacter、Bacillus及Ethanoligenens。芘降低了Bacillus丰度。     

Consequences of sodium exclusion for the osmotic potential in the rhizosphere – Comparison of two maize cultivars differing in Na+ uptake

According to the biphasic model of growth response to salinity, growth is first reduced by a decrease in the soil osmotic potential (Ψ_o), i.e., growth reduction is an effect of salt outside rather than inside the plant, and genotypes differing in salt resistance respond identically in this first phase. However, if genotypes differ in Na⁺ uptake as it has been described for the two maize cultivars Pioneer 3906 and Across 8023, this should result in differences in Na⁺ concentrations in the rhizosphere soil solution and thus in the concentration of salt outside the plant. It was the aim of the present investigation to test this hypothesis and to investigate the effect of such potential differences in soil Ψ_o caused by Na⁺ exclusion on plant water relations. Sodium exclusion at the root surface of intact plants growing in soil was investigated by sampling soil solution from the rhizosphere of two maize cultivars (Across 8023, Pioneer 3906). Plants were grown in a model system, consisting of a root compartment separated from the bulk soil compartment by a nylon net (30 μm mesh size), which enabled independent measurements of the change of soil solution composition and soil water content with increasing distance from the root surface (nylon net). Across 8023 accumulated higher amounts of sodium in the shoot compared to the excluder (Pioneer 3906). The lower Na⁺ uptake in the excluder was partly compensated by higher K⁺ uptake. Pioneer 3906 not only excluded sodium from the shoot but also restricted sodium uptake more efficiently from roots relative to Across 8023. This was reflected by higher Na⁺ concentrations in the rhizosphere soil solution of the excluder 34 days after planting (DAP). The difference in Na⁺ concentration in rhizosphere soil solution between cultivars was neither due to differences in transpiration and thus in mass flow, nor due to differences in actual soil water content. As the lower Na⁺ uptake of the excluder (Pioneer 3906) was only partly compensated by increased uptake of K⁺, soil Ψ_o in the rhizosphere of the excluder was more negative compared to Across 8023. However, no significant negative effect of decreased soil Ψ_o on plant water relations (transpiration rate, leaf Ψ_o, leaf water potential, leaf area) could be detected. This may be explained by the fact that significant differences in soil Ψ_o between the two cultivars occurred only towards the end of the experiment (27 DAP, 34 DAP).     

Effect of manganese-reducing rhizosphere bacteria on the growth of Gaeumannomyces graminis var. tritici and on manganese uptake by wheat (Triticum aestivum L.)

Summary Five bacterial strains capable of Mn reduction were isolated from the rhizosphere of plants growing in different South Australian soils. They differed in their Mn-reducing capacity. The antagonism of these strains compared to the imported strain 2–79 (from the United States) against Gaeumannomyces graminis var. tritici was tested in agar and in a soil sandwich experiment at different Mn²⁺ concentrations in the soil. In addition, wheat seeds were coated with the different strains and with MnSO₄ or with MnSO₄ only in order to investigate their effect on plant growth and Mn uptake. With one exception, all strains inhibited the growth of G. graminis in agar, but to different degrees. In contrast, only two strains significantly inhibited the growth of the fungus in the soil. The hyphal density was decreased more than the hyphal length. The Mn²⁺ concentration in the soil also had a marked effect on fungal growth; low Mn concentrations slightly increased while high Mn concentrations strongly decreased the fungal growth. Seed treatment with MnSO₄ only (+Mn) increased Mn uptake above that of the control (no seed treatment). Only the weakest Mn reducer on agar significantly increased plant growth and Mn uptake from soil in comparison with the Mn treatment. One strain was tested as seed coating without adding MnSO₄; it increased the plant growth to an extent similar to the Mn treatment. Increasing the Mn uptake by plants may be one of the growth-promoting effects exerted by rhizosphere bacteria.     

Changes in Mycobacterium spp. population structure and pyrene mineralization in polycyclic aromatic hydrocarbon-amended soils

The effect of pyrene and phenanthrene contamination on soil Mycobacterium spp. community structure was examined using PCR-amplification of 16S rRNA genes with primers specific for the fast-growing group of Mycobacterium spp. and separation of phylotypes by temperature gradient gel electrophoresis (TGGE). The degradative potential of the soil microbial community was measured over time by mineralization of ¹⁴C-pyrene added to the contaminated soils. PCR-TGGE profiles, in combination with band sequencing and phylogenetic analysis of the prominent phylotypes, indicated shifts in the Mycobacterium spp. community during incubation. Reductions in species diversity and enrichments of specific populations were observed in all pyrene- and phenanthrene-treated soils, in contrast to the relatively stable control soil profiles. Mineralization studies indicated the shortest acclimation periods and the highest initial rates of pyrene degradation occurred in soils pre-exposed to phenanthrene, or a mixture of phenanthrene and pyrene, for 14 weeks. Pre-exposure of soil microorganisms to a single dose of pyrene for the same length of time also decreased the acclimation period for the degradation of pyrene. Monthly application of either pyrene or phenanthrene to soils, however, resulted in an increase in pyrene degradative potential 6 weeks after the first pre-exposure, but a decrease in degradative potential 14 weeks after the first pre-exposure. Similar PCR-TGGE profiles were obtained from soils with comparable pyrene mineralization curves or degradative potentials.     

Production of root-derived material and associated microbial growth in soil at different nutrient levels

Summary Maize plants were grown for 42 days in a sandy soil at two different mineral nutrient levels, in an atmosphere containing ¹⁴CO₂. The ¹⁴C and total carbon contents of shoots, roots, soil and soil microbial biomass were measured 28, 35 and 42 days after germination. Relative growth rates of shoots and roots decreased after 35 days at the lower nutrient level, but were relatively constant at the higher nutrient level. In the former treatment, 2% of the total ¹⁴C fixed was retained as a residue in soil at all harvests while at the higher nutrient level up to 4% was retained after 42 days. Incorporation of ¹⁴C into the soil microbial biomass was close to its maximum after 35 days at the lower nutrient level, but continued to increase at the higher level. Generally a good agreement existed between microbial biomass, ¹⁴C contents and numbers of fluorescent pseudomonads in the rhizosphere. Numbers of fluorescent pseudomonads in the rhizosphere were maximal after 35 days at the lower nutrient level and continued to increase at the higher nutrient level. The proportions of the residual ¹⁴C in soil, incorporated in the soil microbial biomass, were 28% to 41% at the lower nutrient level and 20%6 – 30% at the higher nutrient level. From the lower nutrient soil 18%6 – 52%6 of the residual soil ¹⁴C could be extracted with 0.5 N K₂SO₄, versus 14%6 – 16% from the higher nutrient soil.Microbial growth in the rhizosphere seemed directly affected by the depletion of mineral nutrients while plant growth and the related production of root-derived materials continued.     

Compost Source and Rate Effects on Soil Macronutrient Availability Under Saint Augustine Grass and Bermuda Grass Turf

Compost application to turf grasses can increase availability of nutrients in soil and improve growth, but can potentially lead to accumulation of macronutrients in soil and contribute to leaching and runoff losses. The objectives of this study were to investigate the influence of compost source and application rate on concentrations of plant-available macronutrients in soil over 29 months after a one-time application to saint augustine grass [Stenotaphrum secundatum (Walt.) Kuntze] and Bermuda grass [Cynodon dactylon (L.) Pers.] turf. Compost application increased soil organic C, P, Ca, and S concentrations by 3 months after addition, but further increases from 3 to 29 months were seldom observed. In contrast, NO₃-N and K levels declined while Mg levels increased slightly from 3 to 29 months. Seasonal or cyclical patterns of soil macronutrient levels were apparent, as lower concentrations were observed during dormant stages of Bermuda grass growth in winter. Initial macronutrient concentrations of compost sources strongly influenced macronutrient dynamics in surface soil, while higher application rates resulted in higher levels of P, K, Ca, Mg, but not NO₃-N and S. Higher levels of macronutrients in Bermuda grass than saint augustine grass turf suggested plant-mediated uptake and assimilation differed between turf grass species. Utilization of turf grass systems for compost application should take into account plant species composition and the related impacts of plant uptake. Macronutrient concentrations were significantly correlated with both total organic C and dissolved organic C (DOC). Formation of organic matter-cation complexes appeared to influence macronutrient dynamics in soil, and may contribute to leaching and runoff losses.     

Microbial response to exudates in the rhizosphere of young beech trees (Fagus sylvatica L.) after dormancy

Plants act as an important link between atmosphere and soil: CO2 is transformed into carbohydrates by photosynthesis. These assimilates are distributed within the plant and translocated via roots into the rhizosphere and soil microorganisms. In this study, 3 year old European beech trees (Fagus sylvatica L.) were exposed after the chilling period to an enriched ¹³C–CO₂ atmosphere (δ¹³C = 60‰ – 80‰) at the time point when leaves development started. Temporal dynamics of assimilated carbon distribution in different plant parts, as well as into dissolved organic carbon and microbial communities in the rhizosphere and bulk soil have been investigated for a 20 days period. Photosynthetically fixed carbon could be traced into plant tissue, dissolved organic carbon and total microbial biomass, where it was utilized by different microbial communities. Due to carbon allocation into the rhizosphere, nutrient stress decreased; exudates were preferentially used by Gram-negative bacteria and (mycorrhizal) fungi, resulting in an enhanced growth. Other microorganisms, like Gram-positive bacteria and mainly micro eucaryotes benefited from the exudates via food web development. Overall our results indicate a fast turnover of exudates and the development of initial food web structures. Additionally a transport of assimilated carbon into bulk soil by (mycrorhizal) fungi was observed.     

Reduction of bacterial growth by a vesicular-arbuscular mycorrhizal fungus in the rhizosphere of cucumber (Cucumis sativus L.)

Summary Cucumber was grown in a partially sterilized sand-soil mixture with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum or left uninoculated. Fresh soil extract was places in polyvinyl chloride tubes without propagules of mycorrhizal fungi. Root tips and root segments with adhering soil, bulk soil, and soil from unplanted tubes were sampled after 4 weeks. Samples were labelled with [³H]-thymidine and bacteria in different size classes were measured after staining by acridine orange. The presence of VAM decreased the rate of bacterial DNA synthesis, decreased the bacterial biomass, and changed the spatial pattern of bacterial growth compared to non-mycorrhizal cucumbers. The [³H]-thymidine incorporation was significantly higher on root tips in the top of tubes, and on root segments and bulk soil in the center of tubes on non-mycorrhizal plants compared to mycorrhizal plants. At the bottom of the tubes, the [³H]-thymidine incorporation was significantly higher on root tips of mycorrhizal plants. Correspondingly, the bacterial biovolumes of rods with dimension 0.28–0.40×1.1–1.6 m, from the bulk soil in the center of tubes and from root segments in the center and top of tubes, and of cocci with a diameter of 0.55–0.78 m in the bulk soil in the center of tubes, were significantly reduced by VAM fungi. The extremely high bacterial biomass (1–7 mg C g^-1 dry weight soil) was significant reduced by mycorrhizal colonization on root segments and in bulk soil. The incorporation of [³H]-thymidine was around one order of magnitude lower compared to other rhizosphere measurements, probably because pseudomonads that did not incorporate [³H]-thymidine dominated the bacterial population. The VAM probably decreased the amount of plant root-derived organic matter available for bacterial growth, and increased bacterial spatial variability by competition. Thus VAM plants seem to be better adapted to compete with the saprophytic soil microflora for common nutrients, e.g., N and P, compared to non-mycorrhizal plants.