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Soil and plant roots are colonized by highly complex and diverse communities of microbes. It has been proposed that bacteria and fungi have synergistic effects on litter decomposition, but experimental evidence supporting this claim is weak. In this study, we manipulated the composition of two microbial kingdoms (Bacteria and Fungi) in experimental microcosms. In microcosms that were inoculated with fungi, litter loss was 47% higher than in microcosms that were not inoculated or only inoculated with bacteria. Combined inoculation with both bacteria and fungi did not significantly enhance decomposition compared with the fungi-only treatments, and, as such, we found no evidence for complementary effects using our experimental setup. Inoculation with fungi also had a positive impact on plant growth after 4 and 8 weeks (480% and 710% growth stimulation, respectively). After 16 weeks, plant biomass was highest in microcosms where both bacteria and fungi were present pointing to fungal-bacterial complementarity in stimulating plant growth. Overall, this study suggests that fungi are the main decomposers of plant litter and that the inoculated fungi contribute to plant growth in our experimental system.

Following publication of the original article [1], it was found that there were typographical errors in Figures 5-8. The term “AMFungal” should be “AM Fungal” and “colornization” should be “colonization” in those figures. The original article has been updated.

Soil and plant roots are colonized by highly complex and diverse communities of microbes. It has been proposed that bacteria and fungi have synergistic effects on litter decomposition, but experimental evidence supporting this claim is weak. In this study, we manipulated the composition of two microbial kingdoms (Bacteria and Fungi) in experimental microcosms. In microcosms that were inoculated with fungi, litter loss was 47% higher than in microcosms that were not inoculated or only inoculated with bacteria. Combined inoculation with both bacteria and fungi did not significantly enhance decomposition compared with the fungi-only treatments, and, as such, we found no evidence for complementary effects using our experimental setup. Inoculation with fungi also had a positive impact on plant growth after 4 and 8 weeks (480% and 710% growth stimulation, respectively). After 16 weeks, plant biomass was highest in microcosms where both bacteria and fungi were present pointing to fungal-bacterial complementarity in stimulating plant growth. Overall, this study suggests that fungi are the main decomposers of plant litter and that the inoculated fungi contribute to plant growth in our experimental system.

Plant microbiomes play crucial roles in nutrient cycling and plant growth, and are shaped by a complex interplay between plants, microbes, and the environment. The role of bacteria as mediators of the 400-million-year-old partnership between the majority of land plants and, arbuscular mycorrhizal (AM) fungi is still poorly understood. Here we test whether AM hyphae-associated bacteria influence the success of the AM symbiosis. Using partitioned microcosms containing field soil, we discovered that AM hyphae and roots selectively assemble their own microbiome from the surrounding soil. In two independent experiments, we identified several bacterial genera, including Devosia, that are consistently enriched on AM hyphae. Subsequently, we isolated 144 pure bacterial isolates from a mycorrhiza-rich sample of extraradical hyphae and isolated Devosia sp. ZB163 as root and hyphal colonizer. We show that this AM-associated bacterium synergistically acts with mycorrhiza on the plant root to strongly promote plant growth, nitrogen uptake, and mycorrhization. Our results highlight that AM fungi do not function in isolation and that the plant-mycorrhiza symbiont can recruit beneficial bacteria that support the symbiosis.

Millions of microorganisms inhabit the soil, and some of them contribute to the growth of plants. To gain a better understanding of how microbes promote plant growth, we compared the impact of fungal and bacterial communities on litter decomposition and plant growth. Our study revealed that fungi are the primary decomposers of plant litter and have a more significant impact on nutrient cycling than bacteria. Furthermore, when the fungal and bacterial communities are working together, they have a complementary effect on plant growth that is greater than when either community is working alone. Arbuscular mycorrhizal fungi (AMF) are recognized as one of the most important types of plant symbiotic fungi. AMF are widely distributed in the soil and colonize plant roots. They can functionally extend the plant root system and reach areas in the soil beyond the plant roots, enabling plants to access more water and nutrients. We investigated the microbial community that surrounds the AMF hyphae and discovered that fungal hyphae are colonized by specific bacteria and protists. We demonstrate that the microbes that colonize the fungal hyphae play a crucial role in facilitating mycorrhizal development and assisting plant nutrient uptake. We showed that a particular bacterium of the genus Devosia is associated with AMF hyphae, interacts synergistically with the mycorrhiza and promotes plant growth by facilitating nitrogen uptake. This thesis deepens our understanding of the interactions between different microbial groups and their roles in litter decomposition and mycorrhization processes. A better understanding of plant-microbe interactions is essential for the development of effective application strategies for plant-growth-promoting microbes in agriculture. Ultimately microbes can help create sustainable agricultural practices that rely less on chemical inputs for food production.