Dynamics of Soil Bacterial Communities Over a Vegetation Season Relate to Both Soil Nutrient Status and Plant Growth Phenology.

Soil microorganisms regulate element cycling and plant nutrition, mediate co-existence of neighbors, and stabilize plant communities. Many of these effects are dependent upon environmental conditions and, in particular, on nutrient quality and availability in soils. In this context, we set up a pot experiment in order to examine the combined effects of soil nutrient availability and microbial communities on plant-soil interactions and to investigate assemblage rules for soil bacterial communities under changed nutrient conditions. Four gamma-sterilized soils, strongly differing in their nutrient contents, were obtained from different fertilization treatments of a centenary field experiment and used to grow communities of grassland plants. The sterilized soils were either self- or cross-inoculated with microbial consortia from the same four soils. Molecular fingerprinting analyses were carried out at several time points in order to identify drivers and underlying processes of microbial community assemblage. We observed that the bacterial communities that developed in the inoculated sterilized soils differed from those in the original soils, displaying dynamic shifts over time. These shifts were illustrated by the appearance of numerous OTUs that had not been detected in the original soils. The community patterns observed in the inoculated treatments suggested that bacterial community assembly was determined by both niche-mediated and stochastic-neutral processes, whereby the relative impacts of these processes changed over the course of the vegetation season. Moreover, our experimental approach allowed us not only to evaluate the effects of soil nutrients on plant performance but also to recognize a negative effect of the microbial community present in the soil that had not been fertilized for more than 100 years on plant biomass. Our findings demonstrate that soil inoculation-based approaches are valid for investigating plant-soil-microbe interactions and for examining rules that shape soil microbial community assemblages under variable ecological conditions.