Broad-scale analysis of soil microbial community DNA from Upland grasslands

We have applied a broad-scale approach to the analysis of DNA extracted from soils which support characteristic grasslands at an upland site in the UK. To test for the degree of coherence between microbial and vascular communities, grasslands were characterised as improved, semi-improved, or unimproved, according to the degree of management they had received and consequent botanical composition. Microbial DNA was extracted directly from the grassland soils and analysed by three techniques: (i) thermal denaturation, which profiles the guanine and cytosine (G÷C) base distribution within the community; (ii) cross hybridisation of the DNA which measures the degree of similarity between the samples; (iii) measurement of reassociation kinetics of denatured DNA, which provides a measure of the complexity of the DNA. Thermal denaturation revealed significant differences in the %G+C composition of the communities. DNA from the improved soil had the highest median %G+C value, whilst that from the unimproved soil had the lowest. The relative distribution of G+C bases also differed significantly between the samples from the three grasslands. Cross hybridisation of DNA from the different soils also indicated significant differences in the degree of similarity between the DNA from the grasslands, with unimproved showing 59% similarity to improved. Indices from the cross hybridisation assay suggested that, in terms of complexity, the samples ranked unimproved > semi-improved > improved. Reassociation kinetics supported this conclusion, but the rates of reassociation were such that less than 40% reassociation occurred over a 31-day period, thus preventing calculation of C _o t_1/2.