19,772 research outputs found

    Plant communities affect arbuscular mycorrhizal fungal diversity and community composition in grassland microcosms

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    The diversity of arbuscular mycorrhizal (AM) fungi was investigated in an unfertilized limestone grassland soil supporting different synthesized vascular plant assemblages that had developed for 3 yr. The experimental treatments comprised: bare soil; monocultures of the nonmycotrophic sedge Carex flacca; monocultures of the mycotrophic grass Festuca ovina; and a species-rich mixture of four forbs, four grasses and four sedges. The diversity of AM fungi was analysed in roots of Plantago lanceolata bioassay seedlings using terminal-restriction fragment length polymorphism (T-RFLP). The extent of AM colonization, shoot biomass and nitrogen and phosphorus concentrations were also measured. The AM diversity was affected significantly by the floristic composition of the microcosms and shoot phosphorus concentration was positively correlated with AM diversity. The diversity of AM fungi in P. lanceolata decreased in the order: bare soil > C. flacca > 12 species > F. ovina. The unexpectedly high diversity in the bare soil and sedge monoculture likely reflects differences in the modes of colonization and sources of inoculum in these treatments compared with the assemblages containing established AM-compatible plants

    Root traits predict decomposition across a landscape-scale grazing experiment

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    Acknowledgements We are grateful to the Woodland Trust for maintenance of and access to the Glen Finglas experiment. We thank Debbie Fielding, William Smith, Sarah McCormack, Allan Sim, Marcel Junker and Elaine Runge for help in the field and the laboratory. This research was part of the Glen Finglas project (formerly Grazing and Upland Birds (GRUB)) funded by the Scottish Government (RERAS). S.W.S. was funded by a BBSRC studentship.Peer reviewedPublisher PD

    An arbuscular mycorrhizal fungus accelerates decomposition and acquires nitrogen directly from organic material

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    Arbuscular mycorrhizal fungi (order Glomales), which form mycorrhizal symbioses with two out of three of all plant species, are believed to be obligate biotrophs that are wholly dependent on the plant partner for their carbon supply. It is thought that they possess no degradative capability and that they are unable to decompose complex organic molecules, the form in which most soil nutrients occur. Earlier suggestions that they could exist saprotrophically were based on observation of hyphal proliferation on organic materials. In contrast, other mycorrhizal types have been shown to acquire nitrogen directly from organic sources. Here we show that the arbuscular mycorrhizal symbiosis can both enhance decomposition of and increase nitrogen capture from complex organic material (grass leaves) in soil. Hyphal growth of the fungal partner was increased in the presence of the organic material, independently of the host plant

    Control of pore geometry in soil microcosms and its effect on the growth and spread of <i>Pseudomonas </i>and <i>Bacillus</i> sp.

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    Simplified experimental systems, often referred to as microcosms, have played a central role in the development of modern ecological thinking on issues ranging from competitive exclusion to examination of spatial resources and competition mechanisms, with important model-driven insights to the field. It is widely recognized that soil architecture is the key driver of biological and physical processes underpinning ecosystem services, and the role of soil architecture and soil physical conditions is receiving growing interest. The difficulty to capture the architectural heterogeneity in microcosms means that we typically disrupt physical architecture when collecting soils. We then use surrogate measures of soil architecture such as aggregate size distribution and bulk-density, in an attempt to recreate conditions encountered in the field. These bulk-measures are too crude and do not describe the heterogeneity at microscopic scales where microorganisms operate. In the current paper we therefore ask the following questions: (i) To what extent can we control the pore geometry at microscopic scales in microcosm studies through manipulation of common variables such as density and aggregate size?; (ii) What is the effect of pore geometry on the growth and spread dynamics of bacteria following introduction into soil? To answer these questions, we focus on Pseudomonas sp. and Bacillus sp. We study the growth of populations introduced in replicated microcosms packed at densities ranging from 1.2 – 1.6 g cm-3, as well as packed with different aggregate sizes at identical bulk-density. We use X-ray CT and show how pore geometrical properties at microbial scales such as connectivity and solid-pore interface area, are affected by the way we prepare microcosms. At a bulk-density of 1.6 g cm-3 the average number of Pseudomonas was 63% lower than at a bulk-density of 1.3 g cm-3. For Bacillus this reduction was 66 %. Depending on the physical conditions, bacteria in half the samples took between 1.62 and 9.22 days to spread 1.5 cm. Bacillus did spread faster than Pseudomonas and both did spread faster at a lower bulk-density. Our results highlight the importance that soil physical properties be considered in greater detail in soil microbiological studies than is currently the case

    Unravelling collembolan life belowground: Stoichiometry, metabolism and release of carbon and nitrogen

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    This thesis investigated carbon and nitrogen dynamics of soil dwelling Collembola by using direct measurements and stable isotope additions. In an isotope change experiment, collembolans exchanged between 6 and 10% of carbon and nitrogen in their body tissue per day to metabolism and between 0.5 and 2% to reproduction. When collembolans on low and high protein diets were compared, animals on the low protein quality depleted their tissue 15N values relative to those on high quality diet indicating that the nitrogen turnover decreased on the low protein quality diet. In a wheat microcosm investigating source contributions from soil, roots and isotope labelled green manure the mixing model analysis indicated that photosynthate (root derived C) was the main carbon source for collembolans (54–79% of total C) indicating that the rhizosphere channel is very important for collembolans in addition to the detritus based channel

    The effect of immigration on the adaptation of microbial communities to warming

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    Theory predicts that immigration can either enhance or impair the rate at which species and whole communities adapt to environmental change, depending on the traits of genotypes and species in the source pool relative to local conditions. These responses in turn will determine how well whole communities function in changing environments. We tested the effects of immigration and experimental warming on microbial communities during an 81 day field experiment. The effects of immigration depended on the warming treatment. In warmed communities immigration was detrimental to community growth whereas in ambient communities it was beneficial. This result is explained if colonists came from a local species pool pre-adapted to ambient conditions. Loss of metabolic diversity, however, was buffered by immigration in both environments. Communities showed increasing local adaptation to temperature conditions during the experiment and this was independent of whether or not they received immigration. Genotypes that comprised the communities were not locally adapted, however, indicating that community local adaptation can be independent of adaptation of component genotypes. Our results are consistent with a greater role for species interactions rather than adaptation of constituent species in determining local adaptation of whole communities, and confirm that immigration can either enhance or impair community responses to environmental change depending on the environmental context

    Can the irrigation of soils with Amoxicillin-enriched water cause the proliferation of Bacteria resistant to antibiotics among culturable heterotrophic aerobic soil bacteria?

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    This study investigated the short-term impact of irrigation with Amoxicillin solutions on the presence of the amoxicillin-resistance trait among culturable soil heterotrophic aerobic bacteria. The microcosm experimental design consisted of 15 days of incubation of 10 g soil samples irrigated daily with distilled water containing increasing doses of amoxicillin (0, 0.1, 1, 10, 100, 1000 µg g-1 of soil day-1). The hypothesis was that continuous daily addition of antibiotics would increase the proportion of antibiotic-resistant bacteria in soils. After the incubation period, the total and antibiotic resistance heterotrophic aerobic bacteria communities were assessed through serial dilution of soil suspensions, followed by agar plate culture enumeration, isolation, identification and microscopy observation. The presence of antibiotic-resistant bacteria was also evaluated directly on treated wastewater used for field irrigation before this microcosm study to assess the amoxicillin-resistant bacteria bioaugmentation hypothesis. Results indicated that the Amoxicillin resistance was widespread among bacteria present in both treated wastewater used for irrigation and in the receiving soil. A microcosm experiment was attempted as a ‘proof of concept’ to demonstrate that irrigation with treated wastewater containing antibiotics would exert selective pressure and promote the proliferation of antibiotic resistance bacteria. Unexpectedly, the results from the microcosm incubations indicated the daily addition of amoxicillin did not increase bacterial antibiotic resistance trait abundance in soils, which even significantly decreased for all tested doses. The antibiotic-resistant species identified among the isolates were Pseudomonas mosselii, P. otitidis, P. mendocina, P. flavescens, Stenotrophomnas maltophilia, Bacillus thuringiensis, Aeromonas veronii, Candida parapsilosis, Streptomyces violaceoruber and Microbacterium barkeri
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