Summary 1.The aim was to assess the extent to which the microbial biomass and activity, and community structure of fertilized upland grasslands are directly related to changes in soil N availability or indirectly related to individual plant species effects caused by changes in plant species composition and dominance. We investigated the short-term interactive effects of dominant plant species (Lolium perenne, Agrostis capillaris, Holcus lanatus and Festuca rubra) and nitrogen (N) amendment using an N-limited upland grassland soil. 2. In soils planted with different grass species, soil microbial biomass, and to some extent microbial activity, were determined by temporal changes in plant productivity. Variations in the way that individual plants influenced soil microbial biomass and activity were highly inconsistent over time, and largely independent of N-additions and differences in plant productivity. At the final sample date, those grass species which co-dominate the total plant biomass of intermediate fertility (H. lanatus) and semi-improved grasslands (A. capillaris and F. rubra) had a beneficial effect on the soil microbial biomass. In contrast, the dominant plant species of improved grasslands, L. perenne, had zero or a negative effect on soil microbial biomass. Two plant species (A. capillaris and H. lanatus) increased the proportion of fungi relative to bacteria in the soil microbial community, relative to the unplanted control soil and the other plant species. Lolium perenne and A. capillaris reduced the evenness of microbial PLFAs, suggesting negative effects of these plant species on the diversity of the soil microbial community. 3. The addition of N had no consistent effect on measures of soil microbial biomass or activity, but significantly altered the structure of the microbial community in favour of fungi. The lack of effects of N-addition on microbial biomass and activity were despite the finding that nitrogen addition reduced root biomass in all plant species and increased rhizosphere acidity. 4. The results suggest that in the short term, the abundance and activity of soil microorganisms in upland grasslands are regulated more by plant species traits than by a direct effect of nitrogen. These effects are likely to be related to variations amongst plant species in root exudation patterns and/or efficiency of nutrient aquisition. 5. Our study provides evidence that the functional characteristics of dominant plant species are important determinants of soil biological properties, and hence ecosystem functioning in temperate upland grasslands.

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Alterations in soil bacterial communities across a transect between a semi natural upland grassland and an agriculturally improved enclosure were assessed using culture-based methods and a nucleic-acid-based method, terminal restriction fragment length polymorphism (TRFLP). While plant diversity decreased across the transect towards the improved area, numbers of 16S rDNA terminal restriction fragments increased, indicating an increase in numbers of bacterial ribotypes. Bacterial numbers, microbial activity, and potential functional diversity also followed a similar trend, increasing with decreasing plant diversity. Alterations in bacterial community structure were coincident with changes in soil physicochemical properties which also changed across the transect. Increases in soil pH, nitrate, phosphorus, potassium, and calcium occurred toward the improved grassland, while organic matter and ammonium declined. The inverse relationship between floristic diversity and bacterial ribotype numbers suggests that soil physicochemical factors may be as influential in determining bacterial diversity in soils of upland grassland communities as floristic diversity.

Alterations in soil microfungal community structure across a transect between a semi-natural upland grassland and an agriculturally improved enclosure were assessed using an indirect measurement of active fungal biomass (ergosterol), together with a nucleic acid approach, terminal restriction fragment length polymorphism (TRFLP), which was compared to a commonly used but less sensitive community fingerprinting technique, denaturing gradient gel electrophoresis (DGGE). These techniques indicated that there was no reduction in numbers of fungal ribotypes across the floristic transect, despite decreased floristic diversity and a reduction of more than two-fold in ergosterol concentration. Although there were no differences in ribotype number, there was a decrease in diversity and an increase in dominance in only one of the transitional areas. The highest degree of variability within fungal communities was also found in this transitional area, with 84% of ribotypes only being detected in one of three replicates. Comparison of the two fungal community fingerprinting approaches indicated that TRFLP (26-33 ribotypes) was more sensitive for monitoring alterations in fungal community structure than DGGE (13-18 ribotypes). Using a measurement of the relative percentage of each ribotype within communities, a decrease in abundance of prominent ribotypes of the natural grassland soil fungal community was indicated together with an emergence of previously undetected ribotypes towards the improved area. This may have important implications for ecosystem stability or productivity, particularly if agricultural inputs to managed grasslands are suspended.