Exploring the ecological and evolutionary impacts of introducing exotic agricultural inoculants to native soil microbiomes

Microbial inoculants offer a more sustainable approach to providing crops with essential nutrients than conventional inorganic fertilisers, however the ecological and evolutionary impacts of introducing inoculants to soil microbiomes are not well understood. One group of bacteria, collectively known as rhizobia, enter into symbiotic interactions with legumes and fix atmospheric N2 into plant-available nitrogen in exchange for carbon compounds. Inoculating legumes with compatible rhizobia can result in biological nitrogen fixation, however, in an environment where compatible rhizobia are absent from the soil microbiome, introduction may have various impacts on the receiving microbial community. The relatively recent expansion of soybean (Glycine max L. Merr) growth in UK agriculture, where seed inoculation with exotic rhizobia is common, offers an opportunity to study the impact of introducing non-native rhizobia to soil microbiomes. Understanding interactions between inoculant and resident communities could improve the efficacy of soybean inoculant products, leading to increased crop yields and soybean production in the UK. Utilising a combination of greenhouse experiments, fieldwork and bioinformatics approaches, this thesis explores the ecological and evolutionary impacts of introducing soya-nodulating rhizobia (SNR) to UK soil communities. Further, the impacts of inoculating a diverse consortium, with multiple compatible rhizobia species or in combination with plant growth promoting rhizobacteria (PGPR) on temperate-adapted soybean plant productivity was explored. Results highlighted that inoculation with non-native rhizobia altered soil bacterial community dynamics transiently, however the inoculant strains persist within the community. Investigating multi-species rhizobia inoculants for temperate soybean showed that Bradyrhizobium symbionts performed better than Sinorhizobium symbionts and co-inoculation with PGPR uncovered a beneficial association between Bradyrhizobium inoculants and Rhizobium languerre PEPV16. Finally, evidence of inoculant evolution was discovered during the first season of soybean and SNR introduction into UK agricultural field sites. This research contributes to our understanding of inoculation impact for their safer utilisation in cropping systems.