Shifts in rhizosphere microbial communities and enzyme activity of Poa alpina across an alpine chronosequence

This study quantifies the influence of Poa alpina on the soil microbial community in primary succession of alpine ecosystems, and whether these effects are controlled by the successional stage. Four successional sites representative of four stages of grassland development (initial, 4 years (non-vegetated); pioneer, 20 years; transition, 75 years; mature, 9500 years old) on the Rotmoos glacier foreland, Austria, were sampled. The size, composition and activity of the microbial community in the rhizosphere and bulk soil were characterized using the chloroform-fumigation extraction procedure, phospholipid fatty acid (PLFA) analysis and measurements of the enzymes β-glucosidase, β-xylosidase, N-acetyl-β-glucosaminidase, leucine aminopeptidase, acid phosphatase and sulfatase. The interplay between the host plant and the successional stage was quantified using principal component (PCA) and multidimensional scaling analyses. Correlation analyses were applied to evaluate the relationship between soil factors (C_org, N_t, C/N ratio, pH, ammonium, phosphorus, potassium) and microbial properties in the bulk soil. In the pioneer stage microbial colonization of the rhizosphere of P. alpina was dependent on the reservoir of microbial species in the bulk soil. As a consequence, the rhizosphere and bulk soil were similar in microbial biomass (ninhydrin-reactive nitrogen (NHR-N)), community composition (PLFA), and enzyme activity. In the transition and mature grassland stage, more benign soil conditions stimulated microbial growth (NHR-N, total amount of PLFA, bacterial PLFA, Gram-positive bacteria, Gram-negative bacteria), and microbial diversity (Shannon index H) in the rhizosphere either directly or indirectly through enhanced carbon allocation. In the same period, the rhizosphere microflora shifted from a G⁻ to a more G⁺, and from a fungal to a more bacteria-dominated community. Rhizosphere β-xylosidase, N-acetyl-β-glucosaminidase, and sulfatase activity peaked in the mature grassland soil, whereas rhizosphere leucine aminopeptidase, β-glucosidase, and phosphatase activity were highest in the transition stage, probably because of enhanced carbon and nutrient allocation into the rhizosphere due to better growth conditions. Soil organic matter appeared to be the most important driver of microbial colonization in the bulk soil. The decrease in soil pH and soil C/N ratio mediated the shifts in the soil microbial community composition (bacPLFA, bacPLFA/fungPLFA, G⁻, G⁺/G⁻). The activities of β-glucosidase, β-xylosidase and phosphatase were related to soil ammonium and phosphorus, indicating that higher decomposition rates enhanced the nutrient availability in the bulk soil. We conclude that the major determinants of the microflora vary along the successional gradient: in the pioneer stage the rhizosphere microflora was primarily determined by the harsh soil environment; under more favourable environmental conditions, however, the host plant selected for a specific microbial community that was related to the dynamic interplay between soil properties and carbon supply.     

Potassium efficiency mechanisms of wheat,barley, and sugar beet grown on a K fixing soil under controlled conditions

Plant species differ in their potassium (K) efficiency, but the mechanisms are not clearly documented and understood. Therefore, K efficiency of spring wheat, spring barley, and sugar beet was studied under controlled conditions on a K fixing sandy clay loam. The effect of four K concentrations in soil solution ranging from low (5 and 20 μM K) to high (2.65 and 10 mM K) on plant growth and K uptake was investigated at 3 harvest dates (14, 21, and 31 days after sowing). The following parameters were determined: shoot dry matter (DM), K concentration in shoot dry matter, root length (RL), root length/shoot weight ratio (RSR), shoot growth rate/average root length ratio (GR_s/aRL), K influx, and soil solution K concentrations. Wheat proved to have a higher agronomic K efficiency than barley and sugar beet, indicated by a greater relative yield under K‐deficient conditions. As compared to both cereals, sugar beet was characterized by higher K concentrations in the shoot dry matter, only 30—50 % of the root length, 15—30 % of the RSR and a 3 to 6 times higher GR_s/aRL. This means that the shoot of sugar beet had a 3 to 6 times higher K demand per unit root length. Even at low K concentrations in the soil solution, sugar beet had a 7 to 10 times higher K influx than the cereals, indicating that sugar beet was more effective in removing low available soil K. Wheat and barley were characterized by slow shoot growth, low internal K requirement, i.e. high K utilization efficiency, and high RSR, resulting in a low K demand per unit root length. At low soil K concentrations, both cereals increased K influx with age, an indication of adaptation to K deficiency. The mechanism of this adaptation merits closer investigation. Model calculations were performed to estimate the K concentration difference between the bulk soil and the root surface (ΔC_L) needed to drive the measured K influx. For the two cereals, the calculated ΔC_L was smaller than the K concentration in the soil solution, but for sugar beet, ΔC_L was up to seven times higher. This indicates that sugar beet was able to mobilize K in the rhizosphere, but the mechanisms responsible for this mobilization remain to be studied.     

Short-term effects of defoliation on the soil microbial community associated with two contrasting Lolium perenne cultivars

Intra-species variation in response to defoliation and soil amendment has been largely neglected in terms of the soil microbial community (SMC). The influence of defoliation and soil fertiliser amendment on the structure of the SMC was assessed with two Lolium perenne cultivars contrasting in ability to accumulate storage reserves. Plant response to defoliation was cultivar specific and depended on the nutrient amendment of the soil. Results suggested a greater ability to alter plant biomass allocation in the low carbohydrate accumulating cultivar (S23) compared to the high carbohydrate cultivar (AberDove) when grown in improved (IMP), but not in unimproved (UNI), soil. Although differences in plant growth parameters were evident, no treatment effects were detected in the size of the active microbial biomass (total phospholipid fatty acid (PLFA) 313.8 nmol g⁻¹ soil±33.9) or proportions of PLFA signature groups. A lower average well colour development (AWCD) of Biolog sole carbon source utilisation profiles (SCSUPs) in defoliated (D) compared to non-defoliated (ND) treatments may be indicative of lower root exudation 1 week following defoliation, as a consequence of lower root non-structural carbohydrate (NSC) concentrations. Within the bacterial community the lower cyclopropyl-to-precursor ratio of PLFAs, and the trans/cis ratio of 16:1w7, in UNI relative to IMP soil treatments indicates lower physiological stress in UNI soils regardless of L. perenne cultivar. Discrimination of broad scale SMC structure, measured by PLFA analysis, revealed that soil treatment interacted strongly with cultivar and defoliation. In IMP soils the SMCs discriminated between cultivars while defoliation had little effect. Conversely, in UNI soils defoliation caused a common shift in the SMC associated with both cultivars, causing convergence of overall community structure. Separation of SMC structure along the primary canonical axis correlated most strongly (P<0.001) with root:shoot ratio (47.6%), confirming that differences in cultivar C-partitioning between treatments were influential in defining the rhizosphere microbial community.     

Soil inoculation with the biocontrol agent Clonostachys rosea and the mycorrhizal fungus Glomus intraradices results in mutual inhibition, plant growth promotion and alteration of soil microbial communities

Interactions between the biocontrol fungus Clonostachys rosea IK 726 and a tomato/Glomus intraradices BEG87 symbiosis were examined with and without wheat bran, which served as a food base for C. rosea. In soil without wheat bran amendment, inoculation with C. rosea increased plant growth and altered shoot nutrient content resulting in an increase and decrease in P and N content, respectively. Inoculation with G. intraradices had no effect on plant growth, but increased the shoot P content. Dual inoculation with G. intraradices and C. rosea followed the pattern of C. rosea in terms of plant growth and nutrient content. Wheat bran amendment resulted in marked plant growth depressions, which were counteracted by both inoculants and dual inoculation increased plant growth synergistically. Amendment with wheat bran increased the population density of C. rosea and reduced mycorrhizal fungus colonisation of roots. The inoculants were mutually inhibitory, which was shown by a reduction in root colonisation with G. intraradices in treatments with C. rosea and a reduction in colony-forming units (cfu) of C. rosea in treatments with G. intraradices, irrespective of wheat bran amendment. Moreover, both inoculants markedly influenced soil microbial communities examined with biomarker fatty acids. Inoculation with G. intraradices increased most groups of microorganisms irrespective of wheat bran amendment, whereas the influence of C. rosea on other soil microorganisms was affected by wheat bran amendment. Overall, inoculation with C. rosea increased and decreased most groups of microorganisms without and with wheat bran amendment, respectively. In conclusion, despite mutual inhibition between the two inoculants this interaction did not impair their observed plant growth promotion. Both inoculants also markedly influenced other soil microorganisms, which should be further studied in relation to their plant growth-promoting features.     

Effects of Pseudomonas aeruginosa on the diversity of culturable microfungi and nematodes associated with tomato: impact on root-knot disease and plant growth

The available information on Pseudomonas biocontrol inoculants on the non-target fungal and nematode community is scant. The current paper addresses this issue and investigates the effects of biocontrol agents Pseudomonas aeruginosa IE-6 and IE-6S⁺ (previously shown to suppress several soil-borne plant pathogens) on soil microfungi and plant-parasitic nematodes as well as on the root-knot development and growth of tomato (Lycopersicon esculentum). Furadan, a granular nematicide was included as a treatment for comparative purposes. Treatments were applied to soil at the start of each 52-day-long tomato growth cycle, and their effects on the composition and diversity of rhizosphere and endophytic microfungi and plant-parasitic nematodes were examined at the end of first and fourth growth cycle. Several diversity indices were employed to assess community diversity. A total of 16 genera comprising 23 microfungal species were isolated from the tomato rhizosphere. The most abundant fungal species belonged to the genera Aspergillus, Fusarium, and Penicillium. With a few exceptions, fungi were neither exclusively inhibited nor specifically promoted by the application of treatments at any of the growth cycles studied. However, Paecilomyces lilacinus, an egg and female parasite of root-knot nematode, though exclusively absent in the controls was isolated from the treatments. Both general diversity and equitability of rhizosphere microfungi were greater at first compared to the fourth growth cycle while species richness remained uninfluenced across the growth cycles and treatments. However, Furadan and IE-6S⁺ treatments considerably abated general diversity and equitability. Of the microfungal species isolated from the rhizosphere seven were also recovered from surface-sterilized root tissue of tomato suggesting that all the endophytes are primarily rhizosphere organisms. Diversity of endophytic fungi was consistently lower compared with that of the rhizosphere. Both general diversity and equitability declined in all three treatments relative to controls in the root tissue but species richness remained unaltered. Diversity and equitability of plant-parasitic nematodes in soil were reduced by all three treatments over the controls at fourth growth cycle whilst species richness did not change at either growth cycle. The biocontrol agents significantly reduced root-knot development and enhanced shoot growth of tomato over the controls. The possible implications of fungal composition and abundance because of biocontrol by Pseudomonas application are discussed.     

Long-term plant growth legacies overwhelm short-term plant growth effects on soil microbial community structure

Plant-soil feedbacks are gaining attention for their ability to determine plant community development. Plant-soil feedback models and research assume that plant-soil interactions occur within days to weeks, yet, little is known about how quickly and to what extent plants change soil community composition. We grew a dominant native plant (Pseudoroegneria spicata) and a dominant non-native plant (Centaurea diffusa) separately in both native- and non-native-cultivated field soils to test if these species could overcome soil legacies and create new soil communities in the short-term. Soil community composition before and after plant growth was assessed in bulk and rhizosphere soils using phospholipid fatty acid analyses. Nematode abundance and mycorrhizal colonization were also measured following plant growth. Field-collected, native-cultivated soils showed greater bacterial, Gram (−), fungal, and arbuscular mycorrhizal PLFA abundance and greater PLFA diversity than field-collected, non-native-cultivated soils. Both plant species grew larger in native- than non-native-cultivated soils, but neither plant affected microbial composition in the bulk or rhizosphere soils after two months. Plants also failed to change nematode abundance or mycorrhizal colonization. Plants, therefore, appear able to create microbial legacies that affect subsequent plant growth, but contrary to common assumptions, the species in this study are likely to require years to create these legacies. Our results are consistent with other studies that demonstrate long-term legacies in soil microbial communities and suggest that the development of plant-soil feedbacks should be viewed in this longer-term context.     

Interactions between the arbuscular mycorrhizal fungus Glomus intraradices and the plant growth promoting rhizobacteria Paenibacillus polymyxa and P. macerans in the mycorrhizosphere of Cucumis sativus

Interactions between the arbuscular mycorrhizal (AM) fungus Glomus intraradices and bacteria from the genus Paenibacillus (P. macerans and P. polymyxa) were examined in a greenhouse pot experiment with Cucumis sativus with and without organic matter amendment (wheat bran). P. polymyxa markedly suppressed AM fungus root colonization irrespective of wheat bran amendment, whereas P. macerans only suppressed AM fungus root colonization in combination with wheat bran amendment. Dual inoculation with P. macerans and G. intraradices in combination with wheat bran amendment also caused severe plant growth suppression. Inoculation with G. intraradices was associated with increased levels of dehydrogenase activity and available P in the growth substrate suggesting that mycorrhiza formation accelerated the decomposition of organic matter resulting in mobilization of phosphorus. Inoculation with both Paenibacillus species increased all measured microbial fatty acid biomarkers in the cucumber rhizosphere, except for the AM fungus biomarker 16:1ω5, which was reduced, though not significantly. Similarly, inoculation with G. intraradices increased all measured microbial fatty acid biomarkers in the cucumber rhizosphere, except for the Gram-positive bacteria biomarker 15:0 anteiso, which was overall decreased by G. intraradices inoculation. In combination with wheat bran amendment G. intraradices inoculation caused a 39% reduction in the amount of 15:0 anteiso in the treatment with P. polymyxa, suggesting that G. intraradices suppressed P. polymyxa in this treatment. In conclusion, plant growth promoting species of Paenibacillus may have suppressive effects of AM fungi and plant growth, especially in combination with organic matter amendment. The use of an inert plant growth media in the present study allowed us to study rhizosphere microbial interactions in a relative simple substrate with limited interference from other soil biota. However, the results obtained in the present work mainly show potential interactions and should not be directly extrapolated to a soil situation.     

Non-target effects of bacterial biological control agents on soil Protozoa

Biological control agents (BCAs) have gained increasing interest as an alternative to chemical pesticides in agriculture. Before widespread environmental use, risk assessment of effects on target and non-target organisms are needed. However, the knowledge about the effect of BCAs on non-target soil Protozoa is insufficient to support thorough risk assessment. In this study we report on the effects of Pseudomonas fluorescens DR54 that is a potential BCA active against root pathogenic fungi. We present evidence of negative effects of P. fluorescens DR54 on growth of the amoebae Hartmanella vermiformis and Acanthamoeba sp. cultures and natural assemblages of soil protozoans. The observed effects were larger than those of the P. fluorescens type strain DSM50090 and Enterobacter aerogenes SC and were tentatively attributed to viscosinamide, which is an antimicrobial compound with surfactant properties produced by P. fluorescens DR54.     

雷公藤根际微生物特征研究

根际微生物的代谢作用, 直接促进或抑制根的营养吸收和生长, 也影响根际土壤中的物质转化, 雷公藤根系发达且多与其他树种混交栽培, 其根际微生物活性对雷公藤的生长和土壤肥力均有不可忽视的影响。以福建省泰宁县3 种不同栽培模式雷公藤林(野生雷公藤林、杉木雷公藤混交林、厚朴雷公藤混交林)为研究对象, 通过稀释平板法测定3 种不同雷公藤林分根际土壤和非根际土壤中细菌、真菌、放线菌的数量。结果表明: 根际土的微生物数量大于非根际土的微生物数量; 3 种林分, 无论是在根际土壤中, 还是在非根际土壤中, 均表现为细菌数量＞放线菌数量＞真菌数量; 根际微生物(R)比非根际微生物(S)更活跃, 3 种林分的3 大类微生物的R/S 数量比值均大于1; 3 种林分的微生物活性表现为厚朴雷公藤混交林＞杉木雷公藤混交林＞野生雷公藤林, 表明混交方式可促进雷公藤根际微生物活性。     

The mycorrhizal fungus (Glomus intraradices) affects microbial activity in the rhizosphere of pea plants (Pisum sativum)

Pea plants were grown in γ-irradiated soil in pots with and without addition of the AM fungus Glomus intraradices at sufficient N and limiting P. Depending on the growth phase of the plant presence of AM had negative or positive effect on rhizosphere activity. Before flowering during nutrient acquisition AM decreased rhizosphere respiration and number of protozoa but did not affect bacterial number suggesting top-down regulation of bacterial number by protozoan grazing. In contrast, during flowering and pod formation AM stimulated rhizosphere respiration and the negative effect on protozoa decreased. AM also affected the composition of the rhizosphere bacterial community as revealed from DNA analysis (DGGE). With or without mycorrhiza, rhizosphere respiration was P-limited on very young roots, not nutrient limited at more mature roots and C-limited at withering. This suggests changes in the rhizosphere community during plant growth also supported by changes in the bacteria (DGGE).     

Plant and soil microbial biomasses in Agrostis capillaris and Lathyrus pratensis monocultures exposed to elevated O3 and CO2 for three growing seasons

Elevated CO₂ usually promotes plant growth, whereas elevated O₃ often has a negative effect, especially on root biomass. Thus both these gases can indirectly affect the soil microbial community. We exposed Agrostis capillaris and Lathyrus pratensis to realistic levels of O₃ (40-50 ppb) and CO₂ (ambient air + 100 ppm) in open-top chambers during 2002-2004. The experiment shows negative effects of both O₃ and CO₂, especially on the bulk soil of L. pratensis, in terms of the decreased biomasses of total (25% and 31%), actinobacterial (29% and 31%), bacterial (26% and 33%) and mycorrhizal (AM fungal) (31% and 35%) indicator subgroups, analysed by the PLFA (phospholipid fatty acid) method. The fungal:bacterial PLFA biomass ratio decreased in the bulk soil of A. capillaris, especially with elevated CO₂ alone (38%). These longer-term changes are considered to arise mainly from differences between the plant functional types (i.e. grass cf. N₂-fixing legume) in litter quality and soil C:N ratio. The results also point to interactions and multi-trophic feedbacks between elevated O₃, plant, parasitic rust fungi and soil readily available P, accompanied by a shift in N balance in favour of plants rather than soil microorganisms.     

Phenological Stage, Plant Biomass, and Drought Stress Affect Microbial Biomass and Enzyme Activities in the Rhizosphere of Enteropogon macrostachyus

Indigenous grasses have been effectively used to rehabilitate degraded African drylands. Despite their success, studies examining their effects on soil bioindicators such as microbial biomass carbon(C) and enzyme activities are scarce. This study elucidates the effects of drought stress and phenological stages of a typical indigenous African grass, Enteropogon macrostachyus, on microbial biomass and enzyme activities(β-glucosidase, cellobiohydrolase, and chitinase) in the rhizosphere soil. Enteropogon macrostachyus was grown under controlled conditions. Drought stress(partial watering) was simulated during the last 10 d of plant growth, and data were compared with those from optimum moisture conditions. The rhizosphere soil was sampled after 40 d(seedling stage), 70 d(elongation stage), and 80 d(simulated drought stress). A high root:shoot ratio at seedling stage compared with elongation and reproduction stages demonstrated that E. macrostachyus invested more on root biomass in early development, to maximise the uptake of nutrients and water. Microbial biomass and enzyme activities increased with root biomass during plant growth. Ten-day drought at reproduction stage increased the microbial biomass and enzyme activities, accompanying a decrease in binding affinity and catalytic efficiency. In conclusion, drought stress controls soil organic matter decomposition and nutrient mobilization, as well as the competition between plant and microorganisms for nutrient uptake.     

Fermentation of sugar beet waste by Aspergillus niger facilitates growth and P uptake of external mycelium of mixed populations of arbuscular mycorrhizal fungi

Sugar beet waste has potential value as a soil amendment and this work studied whether fermentation of the waste by Aspergillus niger would influence the growth and P uptake of arbuscular mycorrhizal (AM) fungi. Plants were grown in compartmentalised growth units, each with a root compartment (RC) and two lateral root-free compartments (RFC). One RFC contained untreated soil while the other RFC contained soil, which was uniformly mixed with sugar beet waste, either untreated (SB) or degraded by A. niger (ASB) in a rock phosphate (RP)-supplied medium. The soil in each pair of RFC was labelled with ³³P and ³²P in order to measure P uptake by the AM fungal mycelium, of which length density was also measured. Whole cell fatty acid (WCFA) signatures were used as biomarkers of the AM fungal mycelium and other soil microorganisms. The amount of biomarkers of saprotrophic fungi and both Gram-positive and Gram-negative bacteria was higher in SB than in ASB treatments. Whilst ASB increased growth and activity of AM mycelium, SB had the opposite effect. Moreover, shoot P content was increased by the addition of ASB, and by inoculation with AM fungi. Modification of soil microbial structure and production of exudates by A. niger, as a consequence of fermentation process of sugar beet waste, could possibly explain the increase of AM growth in ASB treatments. On the other hand, the highest P uptake was a result of the solubilisation of rock phosphate by A. niger during the fermentation.     

Nematode assemblages in the rhizosphere of spring barley (Hordeum vulgare L.) depended on fertilisation and plant growth phase

Nematodes from rhizosphere soil of barley grown at three fertiliser treatments (control (0), NK and NPK) were studied in a field experiment. Sampling was done twice, during vegetative growth and flowering, respectively, to determine how fertiliser effects on nematode assemblages depended on plant growth phase. At the growth stage the proportion of fungal feeding nematodes (dominated by Aphelenchoides spp. and Aphelenchus sp.) was highest in NK. During flowering, the abundance and proportion of fungal feeders in the 0 and NPK plots had increased and reached a level similar to the NK plot. Overall densities of bacterial feeders (mainly Cephalobidae and Rhabditidae) were similar, but opportunistic bacterial feeders constituted a higher proportion in the fertilised plots compared to the unfertilised. Ectoparasitic plant feeders (Tylenchorhynchus sp.) were more numerous in NK and NPK than in the control at both sampling dates. Endoparasite (Pratylenchus spp.) numbers were lower in the NPK plot at the growth stage. Numbers of Tylenchidae increased between samplings. The classification of Tylenchidae as epidermal cell and root hair feeders as opposed to hyphal feeders is discussed. Results thus indicate that: (i) bacterial and especially fungal feeding nematodes are stimulated by unbalanced fertilisation; (ii) ectoparasitic plant feeders are stimulated by N-fertilisation, while migratory endoparasites are inhibited at high and balanced fertilisation; (iii) nutrient effects diminish after plants reach the flowering stage.     

Kemink subsoiling before and after planting

The Kemink exact soil management system is a non-inversion soil management system based on subsoiling, ridges and controlled traffic. Previous studies have documented benefits of the Kemink system used in its entirety, but the isolated effect of Kemink subsoiling has not been investigated before. To determine the isolated effect of Kemink subsoiling before and after planting two field experiments in sugar beet and barley were conducted in 1999 and 2000 under low nutrient input conditions in a conventional soil management system without recognized compaction problems. Kemink subsoiling after planting generally showed a negative effect on the growth and yield of both crops, whereas subsoiling before planting increased sugar beet yield from 8.4 to 9.5 t ha⁻¹ and sugar beet nitrogen uptake from 48.5 to 57.4 kg ha⁻¹. There was no effect of subsoiling before planting on the grain yield of barley. The negative effect of subsoiling after planting was more pronounced in 1999 than in 2000, and more pronounced in spring barley. The study shows that Kemink subsoiling after planting involves a significant risk of crop damage and cannot be expected to improve crop performance in conventional soil management systems in its current form, whereas Kemink subsoiling before planting may have potential as a measure to increase yield of sugar beet and possibly other row crops too, under low nutrient input conditions.     

Wirkung differenzierter K-Düngung auf bodenphysikalische Eigenschaften,Bodenwassergehalt und Ertrag von Zuckerrüben und Sommergerste auf einem Tschernosem im Mitteldeutschen Trockengebiet

The water balance in the soil profile and soil moisture dynamic (available water content [%]) under sugar beet 2009 and spring barley 2010 were investigated in selected treatments of a long-term K-fertilization trial on a chernozem developed from loess in Bernburg, Germany (21% clay, 73% silt, 1.5% C_org), which received an annual K-fertilization of 0/0 and 249/124.5 kg ha^?1, respectively, to sugar beet/spring barley within the last 14 years. Intensive K-fertilization increased the K content of topsoil and subsoil as well as available water capacity up to 60 cm soil depth. Under water stress conditions in August 2009, the treatment with high K-fertilization achieved the highest sugar beet yield, sugar content (°S) and white-sugar-yield at final harvest. Combined with the higher beet yield, the water use efficiency was also increased and the water pool of the subsoil was used more efficiently. In the following year (2010), weather conditions were relatively wet. Under these conditions, no differences in yield parameters and water consumption of sugar beet and spring barley between both K-levels were observed.     

Belowground interactions between intercropped wheat and Brassicas in acidic and alkaline soils

Our previous studies showed that, under P-limiting conditions, growth and P uptake were lower in the wheat genotype Janz than in three Brassica genotypes when grown in monoculture. The present study was conducted to answer the question if P mobilised by the Brassicas is available to wheat; leading to improved growth of wheat when intercropped with Brassicas compared to monocropped wheat. To assess if the interactions between the crops depend on soil type, the wheat genotype Janz and three Brassica genotypes (two canolas and one mustard) were grown for 6 weeks in monoculture or wheat intercropped with each Brassica genotype in an acidic and an alkaline soil with low P availability (with two plants per pot). Wheat grew equally well in the two soils, but the Brassicas grew better in the acidic than in the alkaline soil. In the acidic soil, monocropped Brassicas had a 3 to 4 fold greater plant dry weight (dw) and P uptake than wheat; plant dw and P uptake in wheat were decreased or not affected by intercropping and increased in the Brassicas. In the alkaline soil, dw and P uptake of the Brassicas was twice as high as in wheat, with intercropping having no effect on these parameters. The contribution of wheat to the total shoot dw and P uptake per pot was 4-21% and 32-40% in acidic and alkaline soil, respectively. Mycorrhizal colonisation was low in all genotypes in the acidic soil (1-6%). In the alkaline soil, mycorrhizal colonisation of monocropped wheat was 62%, but only 43-47% in intercropped wheat. Intercropping decreased P availability in the rhizosphere of wheat in the acidic soil but had no effect on rhizosphere P availability in the alkaline soil. Intercropping had a variable effect on rhizosphere microbial community composition (assessed by fatty acid methylester analysis (FAME) and ribosomal intergenic spacer amplification (RISA)), ranging from intercropping having no effect on the rhizosphere communities to intercropping resulting in a new and similar rhizosphere community composition in both genotypes. The results of this study show that intercropping with Brassicas does not improve growth and P uptake of wheat; thus there is no indication that P mobilised by the Brassicas is available to wheat.     

Movement of metals from soil to plant roots

The rhizobox system offers quantitative information concerning within the pH changes the range of the rhizosphere. The movement of ⁶⁵Zn, ⁵⁴Mn and ⁵⁹Fe across the rhizosphere of soybean was examined. The contributions of the rhizosphere processes to heavy metals solubilization was evaluated. Changes in pH values around the roots of barley and soybean were investigated using the rhizobox. Results indicate that the extent to which the rhizosphere pH can differ from that of the bulk soil depends mainly on the plant species and initial bulk soil pH. Apparently, soybean has greater ability than barley to solubiiize heavy metals in the rhizosphere. The range of pH change was different from that of the heavy metals, indicating that the solubility of heavy metals is not only influenced by soil pH.     

Synergists and antagonists in the rhizosphere modulate microbial communities and growth of Quercus robur L.

The synergistic and antagonistic interactions among biotic components in the rhizosphere play a crucial role in plant defence against soil-borne pathogens. We investigated if the rhizosphere helper bacterium Streptomyces sp. AcH 505 (HB) indirectly protects the plant from the parasitic nematode Pratylenchus penetrans by modifying the rhizosphere microbial community structure and whether these interactions are dependent on the growth stage of oaks. Changes in the abundance of Streptomyces sp. AcH 505 and the phospholipid fatty acid (PLFA) composition of the rhizosphere soil as well as oak shoot and root biomass were assessed. Investigated were the bud resting stage A and the bud swelling stage B with maximal root elongation of oak microcuttings at two successive harvest times. The deleterious effect of P. penetrans on oak biomass was dependent on plant development, being limited to oak microcuttings growing at the stage B. In comparison to control and HB inoculated soils, shoot biomass decreased by about 33% and 41%, and root biomass by about 33 and 48%, respectively. The antagonistic effect of Streptomyces against the nematode was linked to shifts in the rhizosphere microbial community. The Streptomyces AcH505 strain promoted growth of oak microcuttings at bud swelling stage B during maximal root elongation and enhanced the abundance of saprophytic and ectomycorrhizal fungi in the rhizosphere by 158% with respect to controls. Our results highlight the importance of Streptomyces for counteracting the damage of nematodes and promoting plant growth in natural ecosystems such as forests.     

Profiling of PLFA: Implications for nonlinear spatial gradient of PCP degradation in the vicinity of Lolium perenne L. roots

An investigation was conducted using phospholipid fatty acids (PLFAs) profiles to follow the spatial response of the microbial community at the millimeter scale with the purpose of illustrating the mechanism of nonlinear spatial dependence of PCP degradation on the distance from the root surface in the rhizosphere of Lolium perenne L. A laminar rhizobox was designed to allow the harvest of intact layers of root compartment, near-(1, 2, 3, 4, 5 mm) and far-(>5 mm) rhizosphere soil from root surfaces without the removal of the root material itself. Lolium perenne L. was grown in environmental chambers for 53 days with soil spiked with 8.7 and 18 mg kg⁻¹ PCP. PLFA profiles were found to be affected by the distance from the rhizosphere, indicating a distance-dependent selective enrichment of competent species that may be responsible for efficient PCP degradation. In particular, the five fatty acids 16:1ω5, 16:0, i17:0, a17:0 and 10Me18:0 emerged as microbiological biomarkers that may be used for assessing phytoremediation processes of PCP in soil. Their synergistic effects were shown to be most responsive to the nonlinear spatial patterns of PCP degradation in the vicinity of Lolium perenne L. roots. The results suggest that root exudates induced modifications of microbial communities in the PCP contaminated rhizosphere and spatially modified the dominant species within these communities, resulting in the nonlinear PCP degradation pattern.