Seasonal differences in tree species' influence on soil microbial communities

The linkage between tree diversity and the soil food web in temperate deciduous forest ecosystems remains uncertain. Using microbial phospholipid fatty acids (PLFAs), we analyzed the effect of tree species composition on microbial communities from topsoil collected in Hainich National Park, Germany. Previous results had shown minimal direct effects of tree species on the microbial community in autumn, most likely due to low plant activity and high nutrient and energy input from litterfall. However, microbial composition was affected indirectly through an influence of tree species on soil pH. In this study, we analyzed PLFA profiles in early summer and compared them with the results from autumn sampling. We hypothesized that plant-based traits would have stronger direct effects on the abundance and structure of the microbial community during the photosynthetically active period. The results showed that the soil microbial community differed more markedly 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, the measured differences in the microbial composition in early summer could be attributed primarily to litter quality. This direct influence of plant traits appeared to be eclipsed in autumn because of the high nutrient supply from fresh litter input. Following litter decomposition in the topsoil, however, litter-based plant traits emerged as a factor structuring the soil microbial community in early summer. Our results suggest that the PLFAs 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 strongly supported microbial growth in the more diverse forest stands. High proportions of arbuscular mycorrhizal fungi (PLFA 16:1ω5), root-associated microorganisms (PLFAs 16:1ω9, 16:1ω7, 17:1ω8 and 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 considered in the analysis of the effects of tree diversity or species on soil microbial communities.