Seasonal changes in the soil microbial community in a grassland plant diversity gradient four years after establishment Maike Habekost a , Nico Eisenhauer b , Stefan Scheu b , Sibylle Steinbeiss a , Alexandra Weigelt c , Gerd Gleixner a, * a Max Planck Institute for Biogeochemistry, P.O. Box 100164, 07701 Jena, Germany b Institute of Zoology, Darmstadt University of Technology, Schnittspahnstrasse 3, 64287 Darmstadt, Germany c Institute of Ecology, Friedrich-Schiller University Jena, Dornburger Strasse 159, 07749 Jena, Germany article info Article history: Received 1 February 2008 Received in revised form 15 June 2008 Accepted 26 June 2008 Available online 25 July 2008 Keywords: Biodiversity Basal respiration (BR) Chloroform fumigation extraction method (CFE) Phospholipid fatty acids (PLFA) Substrate induced respiration (SIR) The Jena Experiment abstract Aboveground plant diversity is known to influence belowground diversity and ecosystem processes. However, the knowledge on soil microbial succession from an agricultural field to grassland varying in plant diversity is scarce. Therefore, we investigated the effects of vegetation cover, varying plant biodiversity and season on soil microbial parameters in a temperate grassland ecosystem. In May and October 2006 mixed soil samples were taken from the field site from The Jena Experiment: a large biodiversity experiment in Germany which was established in 2002 on a former agricultural field. Sampled plots differed in plant species richness (0, 4, 8, 16), number of plant functional groups (0, 1, 2, 3, 4), and plant functional group composition. We measured basal respiration (BR) and microbial biomass (C mic/CFE ; chloroform fumigation extraction method), phospholipid fatty acids (PLFA), and substrate induced respiration (SIR). We found distinct seasonal variations in the microbial community structure; BR and amount of PLFAs were higher at the end of the vegetation period than in spring, which was primarily due to increased biomass of fungi and Gram negative bacteria. Furthermore, BR and the amount of PLFAs were higher on vegetated plots than on bare ground plots. Although the number of plant functional groups had no effect on microbial parameters, plant species richness affected the amount of PLFAs at the end of the vegetation period with higher biomass in 4 than in 8 and 16 species mixtures. Moreover, the proportion of Gram negative bacteria was increased whereas the proportion of fungi was decreased in presence of legumes. The present study showed distinct seasonal changes in the soil microbial community composition, which is probably driven by the availability and quality of organic resources. Further, our results highlight the time-lag of belowground responses to aboveground vege- tation manipulations with only few significant changes four years after the establishment of the experiment. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Biodiversity is known to impact ecosystem processes such as net primary productivity due to complementarity and sampling effects (Hector et al., 1999; Tilman et al., 2001). It is increasingly recognized that changes in plant diversity also affect the belowground system (Wardle et al., 2004). Above and belowground components of terrestrial ecosystems essentially depend on each other since plants provide carbon sources for the soil fauna and microflora. On the other hand, microorganisms and detritivore animals decom- pose organic matter, thereby increasing the availability of nutrients for plants and enhancing plant growth (Porazinska et al., 2003). The impact of aboveground biodiversity on soil biota may alter the functioning of microorganisms and therefore result in changes in the decomposition of organic matter (Orwin et al., 2006). Increasing plant species richness may beneficially affect the diver- sity of soil biota by including plants differing in root morphology, root chemical composition, and temporal variability of resource inputs. The increased morphological, chemical and temporal vari- ability of belowground structures and resources is likely to result in increased diversity of niches supporting more diverse assemblages of soil biota (Lavelle et al., 1995; Hooper et al., 2000). We hypoth- esized that changes in plant diversity modify resource availability for heterotrophic microbial communities in soil, and thus modify their activity, biomass and composition. Studies investigating the effects of plant diversity on soil microbial communities are scarce and mostly restricted to gross * Corresponding author. Max Planck Institute for Biogeochemistry, Hans-Kno ¨ ll- Strasse 10, 07745 Jena, Germany. Tel.: þ49 (0) 3641 576172; fax: þ49 (0) 3641 577863. E-mail address: ggleix@bgc-jena.mpg.de (G. Gleixner). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2008.06.019 Soil Biology & Biochemistry 40 (2008) 2588–2595