R. E. Wheatley

The consequences of volatile organic compound mediated bacterial and fungal interactions.

Microbial interactions via infochemicals are fundamental to the development of spatial distribution and activity variations in ecosystems. Microorganisms produce a wide range of infochemicals, frequently secondary metabolites, most of which are soluble and many volatile. Volatile organic compounds (VOCs) have been identified in soil atmospheres and related to community structure and function. VOC profiles produced by microorganisms are consistent, relating to cultural conditions, environment and inputs, and so to population and function dynamics. VOC-mediated interactions can result in functional responses by the organisms involved that result in selective advantage to some community members. Positive, negative or neutral interactions can occur between a very wide range of soil bacteria and fungi. These effects include both stimulation and inhibition of growth, by 40 and 60%, respectively, and enzyme production. These effects are usually transient, e.g. removal of an antagonist is followed by complete recovery. Up- and down-regulation of gene expression, by mRNA and protein profiling has been demonstrated. VOCs have played an important role during the evolution of microorganisms in the context of their communities.

Spatial structure in soil chemical and microbiological properties in an upland grassland

We characterised the spatial structure of soil microbial communities in an unimproved grazed upland grassland in the Scottish Borders. A range of soil chemical parameters, cultivable microbes, protozoa, nematodes, phospholipid fatty acid (PLFA) profiles, community-level physiological profiles (CLPP), intra-radical arbuscular mycorrhizal community structure, and eubacterial, actinomycete, pseudomonad and ammonia-oxidiser 16S rRNA gene profiles, assessed by denaturing gradient gel electrophoresis (DGGE) were quantified. The botanical composition of the vegetation associated with each soil sample was also determined. Geostatistical analysis of the data revealed a gamut of spatial dependency with diverse semivariograms being apparent, ranging from pure nugget, linear and non-linear forms. Spatial autocorrelation generally accounted for 40-60% of the total variance of those properties where such autocorrelation was apparent, but accounted for 97% in the case of nitrate-N. Geostatistical ranges extending from approximately 0.6-6 m were detected, dispersed throughout both chemical and biological properties. CLPP data tended to be associated with ranges greater than 4.5 m. There was no relationship between physical distance in the field and genetic similarity based on DGGE profiles. However, analysis of samples taken as close as 1 cm apart within a subset of cores suggested some spatial dependency in community DNA-DGGE parameters below an 8 cm scale. Spatial correlation between the properties was generally weak, with some exceptions such as between microbial biomass C and total N and C. There was evidence for scale-dependence in the relationships between properties. PLFA and CLPP profiling showed some association with vegetation composition, but DGGE profiling did not. There was considerably stronger association between notional sheep urine patches, denoted by soil nutrient status, and many of the properties. These data demonstrate extreme spatial variation in community-level microbiological properties in upland grasslands, and that despite considerable numeric ranges in the majority of properties, overarching controlling factors were not apparent.

High diversity of arbuscular mycorrhizal fungi in a boreal herb-rich coniferous forest

* Here, the diversity of arbuscular mycorrhizal (AM) fungi was determined in a boreal herb-rich coniferous forest in relation to environmental variables. * Root samples of five plant species (Fragaria vesca, Galeobdolon luteum, Hepatica nobilis, Oxalis acetosella and Trifolium pratense) were analysed from stands differing in age and forest management intensity. * Thirty-four Glomeromycota taxa (small-subunit ribosomal RNA gene (SSU rDNA) sequence groups) were detected from 90 root samples (911 clones), including eight new taxa. Sequence groups related to Glomus intraradices were most common (MO-G3 and MO-G13). Samples of H. nobilis were colonized by more AM fungal taxa (3.68 +/- 0.31) than those of O. acetosella (2.69 +/- 0.34), but did not differ significantly in this respect from those of F. vesca (3.15 +/- 0.38). Effects of forest management, host plant species (except above) or season on the number or composition of fungal taxa in root samples were not detected, and neither were they explained by environmental variables (vegetation, soil and light conditions). * This is the most taxon-rich habitat described to date in terms of root-colonizing Glomeromycota. The data demonstrate the importance of temperate coniferous forests as habitats for AM fungi and plants. Lack of obvious fungal community patterns suggests more complex effects of biotic and abiotic factors, and possibly no adverse effect of common forest management practices on AM fungal diversity.

Effect of substrate composition on production of volatile organic compounds from Trichoderma spp. Inhibitory to wood decay fungi

Abstract The production of volatile organic compounds (VOCs) by two Trichoderma isolates grown on sand infused with either malt extract broth or a minimal medium with the carbon:nitrogen levels similar to that found in Scots pine wood was examined. The VOCs were trapped in tubes packed with chromatographic adsorbant, then analysed by Integrated Automated Thermal Desorption-GC-MS. A total of 45 individual VOCs were recorded. Principle Component Analysis was used to establish that the production of individual VOCs was dependent on both the Trichoderma isolated and the growth media type. The inhibitory effect of the VOCs from the two Trichoderma isolates was also tested on four common wood decay fungi. Although VOCs from both Trichoderma species grown on a malt agar medium consistently inhibited the growth of all four target fungi, effects were negligible when the isolates were grown on a minimal agar medium. The importance of the production of individual VOCs on the inhibition of the wood decay fungi and the implications for the biological control of such fungi are discussed.

Effects and incidence of volatile organic compound interactions between soil bacterial and fungal isolates

Abstract The effects of volatile organic compounds (VOCs) produced by a random selection of soil bacteria on the growth rates and activity of several fungi were assessed using experimental systems that had only atmospheric connections. All of the bacterial isolates either stimulated or inhibited the growth rate of at least one of the fungal species. Growth rates of some of the fungi were inhibited by up to 60% in some cases and stimulated by up to 35% in others ( P P P P Phanaerochaete magnoliae ceased completely on exposure to all of the bacterial isolates and was significantly reduced in Trichoderma viride . Tyrosinase activity in T. viride was not affected by any of the bacterial isolates, but activity in P. magnoliae was increased, inhibited or not affected, depending on the bacterium to which it was exposed. These VOC-mediated effects appeared to be species-specific, with each fungus responding uniquely to the products of each of the bacterial cultures. The interactions varied according to the substrate on which the organisms were cultured, and on the size and age of the bacterial population. Interactions mediated by microbially produced VOCs could be widespread in soils, and the outcome of these interactions will be determined by the microorganisms involved.

Microbial biomass and mineral N transformations in soil planted with barley, ryegrass, pea or turnip.

Plants of barley (Hordeum vulgare), ryegrass (Lolium perenne), pea (Pisum sativum) or turnip (Brassica campestris rapifera) were grown in pots of unfertilised soil for 10 weeks together with unplanted control pots. A wide range of soil microbiological parameters was measured on bulk soil samples 2, 4, 7 and 10 weeks after seedlings were transplanted. There was no effect of planting or differential effect of plant species upon respiration rate, microbial biomass N, or biomass of microbial predators, but these parameters all varied significantly over time. Respiration, biomass N and nematode biomass all increased, whilst protozoan biomass decreased. Microbial biomass C showed no significant temporal changes or effect of planting. There was evidence for differential plant effects on potential nitrification and denitrification. Nitrification rates were depressed, compared with the fallow, in all treatments except the pea, at some time in the experiment. Conversely denitrification rates were enhanced in all treatments, except the grass, at specific times. Denitrification rates were greater in the pea treatment than the fallow on all occasions. These results demonstrate that plants do not necessarily influence the gross microbiology of the soil, but may affect physiologically distinct sub-components of the microbial biomass.

Investigating the effects of anaerobic and aerobic post-treatment on quality and stability of organic fraction of municipal solid waste as soil amendment

The use of OFMSW for biogas and compost production is considered as a sustainable strategy in saving valuable landfill space while producing valuable product for soil application. This study examines the effects of anaerobic and aerobic post-treatment of OFMSW on the stability of anaerobic digestate and compost and soil quality using seed germination tests. Anaerobic digestion of OFMSW was carried out for fifteen days after which the residual anaerobic digestate was subjected to aerobic post-treatment for seventy days. Seed germination tests showed that fresh feedstock and digestates collected during anaerobic digestion and during the early stages of aerobic post-treatment were phytotoxic. However, phytotoxic effects were not observed in soils amended with the fully stabilised anaerobic digestate compost, ADC. It was also found that seed germination increases with dilution and incubation time, suggesting that lower soil application rates and longer lag periods between soil application of ADC and planting can reduce the amount of biodegradable organics in the ADC, thus enhancing the benefits of ADC as soil amendment.

Root-induced nitrogen mineralisation: A theoretical analysis

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of inorganic nitrogen (N) to plants by stimulating microbial activity. The release of soluble C compounds from roots is assumed to occur by one of two general processes: cortical cell death or exudation from intact cells. On the basis of several assumptions chosen to allow maximal amounts of N mineralisation to be calculated, greater amounts of net N mineralisation are theoretically possible at realistic soil C:N ratios of bacteria are grazed by predators such as protozoa, than if bacteria alone are active. More N is mineralised when the substrate released from the root has a high C:N ratio (as in cell death) than when it is relatively N-rich. The amounts of N that a root might realistically cause to be mineralised are unlikely to account entirely for high nitrate inflow rates that have been measured experimentally. However there are circumstances in which the loss of C from roots is essential if any N is to be mineralised and obtained by plants.

Effects of animal manure application and crop plants upon size and activity of soil microbial biomass under organically grown spring barley

Abstract Temporal behaviour of microbial biomass C, N and respiration was measured under barley crops in two experiments on successive years in a recently converted organic production system in Scotland. Soils were fertilised with farmyard manure or poultry manure. Control soils received no manure at the start of the growing season. The effects of plants was also investigated by maintaining fallow subplots. C-flush values approximately doubled over the growing season in both years of the trial, showing a decline to pre-sowing values between the two seasons. This occurred in all soils, whether manured or not, or planted or fallow. Manure tended to increase the C-flush in the 2nd year only. N-flush in the 2nd year showed no increase in planted control plots but did increase in fallow soils. Manures significantly increased the N-flush. Respiration rates were stimulated by the presence of plants. Respiration rates were measured from soils taken from the field at post-sowing, mid-season and post-harvest occasions and incubated under constant conditions for up to 1 year. Here there was evidence that the effects of sampling and adjusting the moisture status could be as great upon microbial activity as the addition of the manures. C-flush also showed a consistent and persistent increase in these incubated soils. This suggests that the fundamental C-supplying characteristics of these soils was such that the biomass was moving towards a new equilibrium value fuelled by the relatively recent introduction of the organic farming regime.

Production of Volatile Organic Compounds by Trichoderma in Media Containing Different Amino Acids and Their Effect on Selected Wood Decay Fungi

Summary The paper describes an experiment to evaluate the effect of subtle changes to the amino acid composition of the growth medium on the production of volatile organic compounds (VOCs) by a Trichoderma aureoviride isolate and the consequent inhibition of wood decay fungi. A synthetic medium was produced to represent the C:N ratio and major amino acid content of Scots pine sapwood. Changes in the amino acid content of the medium produced very significant differences in the VOC output produced by the Trichoderma isolate and in the levels of inhibition of the target wood decay fungi. Principal component analysis of the VOC profiles indicated that aldehyde and ketone components were associated with the greatest inhibition of the target fungi. The implications of the results for the use of Trichoderma isolates as biological control agents of decay fungi in wood are discussed.