Seasonal changes in the soil microbial community in a grassland plant diversity gradient four years after establishment

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 (Cmic/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 vegetation manipulations with only few significant changes four years after the establishment of the experiment.