Efficiency and impact of recurrent microbial inoculation in soil, a lab to field assessment

Overview & Synopsis

With the urgent need to adopt more sustainable practices that sustain agricultural production while protecting the environment, microbial inoculants are gaining increasing attention for their potential to reduce reliance on agrochemicals. However, despite decades of research, the benefits of soil microbial inoculants for crop yields remain highly variable across different soils, climates, plant genotypes, and inoculant strains. This variability makes outcomes difficult to predict and may ultimately reduce farmers’ confidence. This work explores the potential of recurrent inoculations as a strategy to overcome the biotic resistance of the resident soil microbial community and promote inoculant establishment. It also examines the effects of these inoculations on the resident microbial community. In a first microcosm experiment, we showed that recurrent inoculation could transiently improve the abundance of the inoculant (Pseudomonas fluorescens) with minimal impact on the resident bacterial community. A second experiment in greenhouse evidenced the unexpected potential of recurrent inoculation carried out until sowing to enhance maize growth while exerting a weaker impact on the bacterial resident community compared to recurrent inoculation starting at sowing. The third experiment conducted under field conditions reflected the challenges of translating the growth benefits observed in controlled environments to uncontrolled field conditions. Overall, this work suggests that both the timing and frequency of inoculation should be adjusted in a complementary way. Specifically, recurrent inoculation may transiently enhance the abundance of the inoculant during the critical early stages of plant growth. This may either promote successful host colonization when an adequate dose is applied, or indirectly influence the soil microbial community at sowing.