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Sowing experiments were used to study seedling recruitment, growth and biomass allocation patterns in the perennial forest herbs Aconitum. septentrionale and Aclaea spicata in relation to the microbial soil community. Glucose and nutrients were added every second week over a 3-year period to manipulate soil microbial activity and nutrient availability. The glucose was added (400 g glucose m(-2) yr(-1)) to reduce the nutrient availability to the plants by increasing soil microbial demands. A full nutrient solution was used to increase the nutrient availability. The experiments were performed in a deciduous forest and in an open field in South East Norway, and our study is based on a consecutive sampling of whole plants with intact root systems to be able to estimate growth and allocation patterns. Both species recruited best in the forest while their growth in the open field was ca. 100 times larger than in the forest. Shoot:root ratios were surprisingly similar in the forest and the open field sites and were only marginally affected by the glucose and nutrient treatments. However, the shoot:root ratios were characterised by highly significant seasonal variations. This was the case for both species and indicates that the shoot:root ratios were under strong ontogenetic control. Recruitment was negatively affected by glucose additions, in particular in the open field. Growth was significantly and negatively affected by glucose additions in the forest. Nutrient additions gave, as expected, a significant increase in growth. The failure of seedling recruitment and inferior growth following glucose additions support the assumption that the soil microbial community is an important determinant of plant recruitment and growth. (c) 2005 Elsevier mnbH. All rights reserved.

We have added glucose and nutrients to manipulate soil microbial activity and nutrient availability in a boreal spruce forest to study the performance of birch and spruce seedlings in relation to the soil microbial community. The proportion of aboveground biomass in the seedlings was largest in plots amended with extra nutrients, while ectomycorrhizal (ECM) colonisation was low in these plots. ECM appeared beneficial for growth of both species, but only at low levels of colonisation (< 25% ECM colonised root-tips). The soil microbial biomass, as determined by total PLFA, was largest in plots treated with glucose and there was a significant negative relationship between birch seedling size and levels of total PLFA in soil. This could be taken to suggest that poor seedling growth was due to nutrient limitation caused by microbial assimilation. However, the treatment response of the birch seedlings was generally weak, and spruce often showed no response at all to the addition of nutrients and glucose. The most consistent parameter for the variation in plant performance, as well as for the microbial soil community, was the block-effect. This suggests a strong spatial structure in the soil microbial community, and that this structure was robust with respect to our treatments even though they continued over a 3-year period.