The Microbial Contribution to Litter Decomposition and Plant Growth

Soil and plant roots are colonized by highly complex and diverse communities of microbes. In contrast to knowledge on the composition of microbial communities colonizing a wide range of habitats, the ecological function of many microbes is still poorly understood, and it is unclear to which extent specific microbial kingdoms (bacteria and fungi) interact and influence important ecosystem functions such as litter decomposition or plant growth. It has been proposed that bacteria and fungi have synergistic effects on litter decomposition, but experimental evidence supporting this is weak. We manipulated the composition of two microbial kingdoms (bacteria and fungi) in experimental microcosms. The microcosms were inoculated with either bacteria (41 strains), or fungi (35 strains), bacteria and fungi together, with control microcosms remaining uninoculated. The effects of these treatments on plant growth and litter decomposition were investigated. Litter loss was 47% higher in fungi inoculated treatments compared to treatments without inoculated fungi (e.g. treatments with only bacteria or control treatments without any microbes added). Inoculation with bacteria only slightly (9.5%) enhanced decomposition compared to the control treatment. Inoculation with both bacteria and fungi did not further enhance decomposition (+2%), and as such, we found no evidence for complementary effects. Inoculation with fungi had a positive impact on plant growth after 4 and 8 weeks (4.8 and 7.13 times growth stimulation respectively), while inoculation with the isolated bacteria did not influence plant growth. After 16 weeks, plant biomass was highest in microcosms where both bacteria and fungi were present. Overall, this study suggests that fungi are the main decomposers of plant litter and that the inoculated fungi contribute to plant growth when microbial abundance is low. Further experimental studies with a wider range of microbes from different microbial kingdoms are necessary to better understand how microbial communities interact and influence plant growth, litter decomposition and ecosystem functioning.