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Abstract

There is still uncertainty in the link between tree diversity and the soil food web in temperate 19 deciduous forest ecosystems. We analysed the effect of tree species composition on microbial 20 communities from the topsoil at the Hainich National Park, Germany, using microbial 21 phospholipid fatty acids (PLFA). Previous results showed minimal direct effects of tree 22 species on the microbial community in autumn, likely due to low plant activity and high 23 nutrient and energy input from litterfall. Microbial composition was, however, affected 24 indirectly through a tree species influence on soil pH. In this study, we analyzed PLFA 25 profiles in early summer, and compared them with the autumn sampling. We hypothesized 26 that plant based traits would have stronger direct effects on the abundance and structure of the *Manuscript Click here to view linked References 2 microbial community in the photosynthetically active 27 period. The results showed that the soil 28 microbial community differed more strongly between the tree diversity levels in early summer 29 than in autumn. The acidifying character of the decaying beech litter strongly influenced the 30 soil pH values and structured the soil microbial community indirectly in early summer as it 31 had in autumn. However, in early summer the measured differences in the microbial 32 composition could be attributed mostly to litter quality. This direct influence of plant traits 33 seemed to be eclipsed in autumn because of high nutrient supply from fresh litter input, but 34 following litter decomposition in the topsoil, litter-based plant traits emerged as a factor 35 structuring the soil microbial community in early summer. Our results suggest that the PLFA 36 i14:0 and i15:0, indicative of Gram-positive bacteria, are strongly involved in decomposition 37 processes, and may be promoted by readily available nutrients. Furthermore, our results 38 indicat that a dense root network in association with arbuscular mycorrhizal fungi supported 39 strongly microbial growth in the more diverse forest stands. High proportions of arbuscular 40 mycorrhizal fungi (PLFA 16:1ω5), root associated microorganisms (PLFA 16:1ω9, 16:1ω7, 41 17:1ω8, 18:1ω7) and bacterial grazers (PLFA 20:5) characterized the microbial community in 42 early summer on these study plots. In consequence are microbial communities stongly 43 influenced beside abiotic factors by seasonal differences in litter decomposition rates and root 44 activity. These seasonal differences need to be considered when determining tree diversity or 45 species effects on soil microbial communities.