All molecular analyses of soil bacterial diversity are based on the extraction of a representative fraction of cellular DNA. Methods of DNA extraction for this purpose are divided into two categories: those in which cells are lysed within the soil (direct extraction) and those in which cells are first removed from soil (cell extraction) and then lysed. The purpose of this study was to compare a method of direct extraction with a method in which cells were first separated from the soil matrix by Nycodenz gradient centrifugation in order to evaluate the effect of these different approaches on the analysis of the spectrum of diversity in a microbial community. We used a method based on polymerase chain reaction (PCR) amplification of a 16S rRNA gene fragment, followed by hybridization of the amplified fragments to a set of specific probes to assess the phylogenetic diversity of our samples. Control parameters, such as the relationship between amount of DNA template and amount of PCR product and the influence of competing DNA on PCR amplification, were first examined. Comparison between extraction methods showed that less DNA was extracted when cells were first separated from the soil matrix (0.4 microg g(-1) dry weight soil versus 38-93 microg g(-1) obtained by in situ lysis methods). However, with the exception of the gamma-subclass of Proteobacteria, there was no significant difference in the spectrum of diversity resulting from the two extraction strategies.

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Anthropogenic acidification has reduced soil pH and released potentially toxic aluminium (Al) ions in many regions. This investigation examines whether increased acidity has caused genetic adaptation to acidic conditions within the grass species Elymus caninus, Poa nemoralis, Deschampsia cespitosa and D. flexuosa. We sampled tussocks (genets) of each species in two regions of southern Sweden, differing in their exposure to acidifying deposition. The tolerance of the genets was tested in a solution experiment with different pH and Al concentrations. Our data suggest that species found at lower pH field locations have a greater tolerance to low pH and high Al levels than species found on less acidic soils. Analysis of variance showed a significant average effect of population and (or) genet in most species; however, we found little evidence of genetic adaptation to acidic conditions at the regional, population and micro-site level. In fact, there was no consistent change in the ranking of populations or genets with varying pH or Al concentration. Based on these results, we hypothesize that phenotypic plasticity rather than genetic adaptation has been favoured as the predominant mechanism to cope with soil acidity in the four grass species.

Genetically based adaptation and phenotypic plasticity represent important means of coping with natural or human‐induced increases in soil acidity. In the present study, we examined the role of phenotypic plasticity in the grass Deschampsia cespitosa by testing for general and trait‐specific responses to acid and aluminium (Al) stress. We sampled tussocks (genets) from sites in southern Sweden differing in their exposure to acid deposition, and quantified the performance of each genet under low pH and high Al levels in a solution experiment using the length and biomass of both shoots and roots as response variables. In agreement with results from a previous solution experiment, the overall performance (expressed as total biomass) declined under acid and Al stress, and there was no evidence for local genetic adaptation with respect to acidity. Three Öland populations showed signs of being stimulated by high Al levels, despite originating from relatively basic soils. We observed a significant increase in root length under high Al levels and hypothesize that this response may be adaptive in the natural soil environment, allowing growing roots to “escape” patches of soil with toxic concentrations of this element. Our results for D. cespitosa indicate that phenotypic plasticity has the potential to mitigate the negative effects of soil acidity in this species.