Effects of soil types and bacteria inoculum on the cultivation and reintroduction success of rare plant species

The interaction between plants, soils, and microbes has not received much attention in reintroduction efforts so far, although it is widely known that edaphic conditions play an essential role in local plant distribution. To analyze in how far adaption to natural soil conditions and the use of plant growth-promoting rhizobacteria (PGPR) influence reintroduction success, five rare plant species from three habitat types in north-west Germany were exposed to three different soil types, with or without the application of PGPR inoculum. Plant seeds were germinated in a greenhouse and seedlings were transplanted to natural field habitats. Growth and survival was monitored for 2 years. Natural soil from the field yielded the best results for four out of five species, indicating that the “home soil advantage” might have great potential value in the cultivation and reintroduction of rare plants. Commercial potting soil resulted in good to intermediate growth in forest and riverside species, but was the least successful option for the heathland species. Overall, plant grew least on an artificially mixed substrate, mimicking physicochemical soil properties from the natural soil, probably due to incomplete soil forming processes. The application of PGPR had no effect on germination rate and on plant growth in the field, and no consistent effect during cultivation. Based on these results, the use of a generalist PGPR inoculum does not provide a significant benefit to reintroduction efforts. In contrast, the use of home soil might have a great potential to boost reintroduction success.