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.

Characterization of root colonization profiles by a microcosm community of arbuscular mycorrhizal fungi using 25S rDNA-targeted nested PCR.

The aim of the present work was to study colonization patterns in roots by different arbuscular mycorrhizal fungi developing from a mixed community in soil. As different fungi cannot be distinguished with certainty in planta on the basis of fungal structures, taxon-discriminating molecular probes were developed. The 5' end of the large ribosomal subunit containing the variable domains D1 and D2 was amplified by PCR from Glomus mosseae (BEG12), G. intraradices (LPA8), Gigaspora rosea (BEG9) and Scutellospora castanea (BEG1) using newly designed eukaryote-specific primers. Sequences of the amplification products showed high interspecies variability and PCR taxon-discriminating primers were designed to distinguish between each of these four fungi. A nested PCR, using universal eukaryotic primers for the first amplification and taxon-discriminating primers for the second, was performed on individual trypan blue-stained mycorrhizal root fragments of onion and leek, and root colonization by four fungi inoculated together in a microcosm experiment was estimated. More than one fungus was detected in the majority of root fragments and all four fungi frequently co-existed within the same root fragment. Root colonization by G. mosseae and G. intraradices was similar from individual mixed inoculum, whilst the frequency of S. castanea and Gig. rosea increased in the presence of the two Glomus species, suggesting that synergistic interactions may exist between some arbuscular mycorrhizal fungi.

Localization of ß (1-3) glucan in walls of the endomycorrhizal fungi Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe and Acaulospora laevis Gerd. & Trappe during colonization of host roots.

Previous studies showed that cell walls of endomycorrhizal fungi belonging to the Acaulosporaceae and Glomaceae contain ß (1-3) glucan polymers as well as chitin. Indirect immunolabelling with monoclonal and polyclonal antibodies has been used to investigate the distribution of these structural polysaccharides in cell walls of Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe and Acaulospora laevis Gerd. & Trappe as they interact with pea and tobacco roots, respectively. The (l-3) glucans were detected in the walls of external hyphae, and of hyphal coils and intercellular hyphae developing in outer root tissues. The glucan component was alkali-insoluble but treatment with chitinase resulted in solubilization of most of the ß(1-3) glucans from the fungal wall. A decrease in immunolabelling was associated with thinning out of the hyphal wall as the fungi colonized deeper in the host root, and ß(1-3) glucans could not be detected in walls of intercellular hyphae or arbuscules in the parenchyma cortical tissue. The molecular configuration of cell walls of G. mosseae and A. laevis is discussed in relation to fungal morphogenesis and the symbiotic state in endomycorrhiza.