Ecosystem consequences of introducing plant growth promoting rhizobacteria to managed systems and potential legacy effects

Abstract

The rapidly growing industry of crop biostimulants leverages the application of plant growth promoting rhizobacteria (PGPR) to promote plant growth and health. However, introducing nonnative rhizobacteria may impact other aspects of ecosystem functioning and have legacy effects; these potential consequences are largely unexplored. Nontarget consequences of PGPR may include changes in resident microbiomes, nutrient cycling, pollinator services, functioning of other herbivores, disease suppression, and organic matter persistence. Importantly, we lack knowledge of whether these ecosystem effects may manifest in adjacent ecosystems. The introduced PGPR can leave a functional legacy whether they persist in the community or not. Legacy effects include shifts in resident microbiomes and their temporal dynamics, horizontal transfer of genes from the PGPR to resident taxa, and changes in resident functional groups and interaction networks. Ecosystem functions may be affected by legacies PGPR leave following niche construction, such as when PGPR alter soil pH that in turn alters biogeochemical cycling rates. Here, we highlight new research directions to elucidate how introduced PGPR impact resident microbiomes and ecosystem functions and their capacity for legacy effects.

Keywords: agroecology; biostimulant; invasion ecology; microbiome; plant growth promoting bacteria; plant-microbe interactions.

Fig. 1

The long‐term consequences of inoculating soils with plant growth promoting rhizobacteria (PGPR) could lead to unintentional changes in soil properties, biotic communities and interactions, and ecosystem functions. Inoculating soils has the immediate intended effect of increasing plant productivity, shifting resource allocation to promote fruiting or crop yield, or reducing pathogen infections. Introducing PGPR could have further effects such as changing the resident microbiome structure and function and altering plant root exudate quantity and chemistry, which would cascade to influence soil aggregation, soil organic matter (SOM) persistence, pH, nutrient status, erodibility, and water holding capacity. Research is needed on consequences for herbivore and pollinator communities that depend on a crop’s time of fruiting or seed set or the duration of the growing season. An outstanding knowledge gap, due to lack of field‐based research, is whether PGPR inoculants disperse to adjacent ecosystems and their potential legacy effects. Artist credit: Andrew Sproles, Creative Services, Communications Division, Oak Ridge National Laboratory (TN, USA).