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Soil Biology and Biochemistry
(Article in Press): 2013

Seasonal differences in tree species' influence on soil microbial communities

Carolin Thoms, Gerd Gleixner

Max Planck Institute for Biogeochemistry, Postbox 100164, 07701 Jena, Germany.


There is still uncertainty in the link between tree diversity and the soil food web in temperate deciduous forest ecosystems. We analyzed the effect of tree species composition on microbial communities from the topsoil at the Hainich National Park, Germany, using microbial phospholipid fatty acids (PLFA). Previous results showed minimal direct effects of tree species on the microbial community in autumn, likely due to low plant activity and high nutrient and energy input from litterfall. Microbial composition was, however, affected indirectly through a tree species influence on soil pH. In this study, we analyzed PLFA profiles in early summer, and compared them with the autumn sampling. We hypothesized that plant based traits would have stronger direct effects on the abundance and structure of the microbial community in the photosynthetically active period. The results showed that the soil microbial community differed more strongly between the tree diversity levels in early summer than in autumn. The acidifying character of the decaying beech litter strongly influenced the soil pH values and structured the soil microbial community indirectly in early summer as it had in autumn. However, in early summer the measured differences in the microbial composition could be attributed mostly to litter quality. This direct influence of plant traits seemed to be eclipsed in autumn because of high nutrient supply from fresh litter input, but following litter decomposition in the topsoil, litter-based plant traits emerged as a factor structuring the soil microbial community in early summer. Our results suggest that the PLFA i14:0 and i15:0, indicative of Gram-positive bacteria, are strongly involved in decomposition processes, and may be promoted by readily available nutrients. Furthermore, our results indicate that a dense root network in association with arbuscular mycorrhizal fungi supported strongly microbial growth in the more diverse forest stands. High proportions of arbuscular mycorrhizal fungi (PLFA 16:1ω5), root associated microorganisms (PLFA 16:1ω9, 16:1ω7, 17:1ω8, 18:1ω7) and bacterial grazers (PLFA 20:5) characterized the microbial community in early summer on these study plots. We conclude that microbial communities are strongly influenced by abiotic controls, however, seasonal differences in litter decomposition rates and root activity should be taken into account when determining tree diversity or species effects on soil microbial communities.

Keywords: Seasons; Tree species; Site conditions; Soil microorganisms; Beech; Fagus sylvatica; Forest; Phospholipid fatty acids (PLFA); Decomposition



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