Plant species and nitrogen effects on soil biological properties of temperate upland grasslands

1. The aim was to assess the extent to which the microbial biomass and acti vity, and community structure of fertilized upland grasslands are directly related to changes in soil N availability or indirectly related to individu al 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 Fest uca rubra) and nitrogen (N) amendment using an N-limited upland grassland s oil. 2. In soils planted with different grass species, soil microbial biomass, a nd to some extent microbial activity, were determined by temporal changes i n plant productivity. Variations in the way that individual plants influenc ed soil microbial biomass and activity were highly inconsistent over time, and largely independent of N-additions and differences in plant productivit y. At the final sample date, those grass species which co-dominate the tota l plant biomass of intermediate fertility (H. lanatus) and semi-improved gr asslands (A. capillaris and f. rubra) had a beneficial effect on the soil m icrobial biomass. In contrast, the dominant plant species of improved grass lands, 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 commu nity. 3, The addition of N had no consistent effect on measures of soil microbial biomass or activity, but significantly altered the structure of the microb ial community in favour of fungi. The lack of effects of N-addition on micr obial 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 o f soil microorganisms in upland grasslands are regulated more by plant spec ies 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 domin ant plant species are important determinants of soil biological properties, and hence ecosystem functioning in temperate upland grasslands.