The ecological engineering impact of a single tree species on the soil microbial community
1. Ecosystem engineering by a single species can have a cascading effect on many ecosystem processes. While the impact of above-ground ecological engineers on soil chemical properties has been studied, few studies have assessed their impact on the soil microbial community, which is largely responsible for many ecosystem functions.

2. Utilizing a long-term experiment where birch was planted on heather moorland 20 years ago, the engineering impact of a single tree species (Betula pubescens) on the soil microbial community was assessed using phospholipid fatty acid (PLFA) analysis and fungal polymerase chain reaction–denaturing gradient gel electrophoresis (PCR-DGGE). Changes in the soil microbial community were then related to soil chemical and physical variables and tree performance variables.

3. Under birch, total microbial biomass (total PLFA) declined, species richness increased and the ratio of fungal : bacterial PLFA declined. The fungal PLFA marker increased with increasing organic matter and depth of the LFH and O soil horizons, characteristics associated with moorland soils. Bacterial PLFAs increased with increasing birch canopy cover. The fungal community of the birch plots was different from that in the heather plots and changes in the fungal community composition were related to the size of the birch trees in the plots.

4. Changes in the soil microbial community were also related to changes in mineralizable N. Mineralizable N was correlated with both decreasing total soil microbial biomass and decreasing fungal : bacterial ratio.

5. The durability of the engineering effects of birch was studied in a second experiment. Plots were established in first-generation birch woodland that had developed on Calluna-dominated moorland. The plots were cleared of birch and planted with heather. After 20 years, there was no difference in the community composition of the PLFAs. In contrast, there were significant differences in the fungal community composition as judged by DGGE analysis, with the fungal community in the felled birch plots being similar to the heather moorland.

6. Synthesis. This work demonstrates that addition of a single tree species to heather moorland results in changes in below-ground soil microbial communities and in nutrient cycling.