Reduction of forest soil biota impacts tree performance but not greenhouse gas fluxes

Soil communities are essential to ecosystem functioning, yet the impact of reducing soil biota on root-associated communities, tree performance, and greenhouse gas (GHG) fluxes remains unclear. This study examines how different size fractions of soil biota from young and mature forests influence Alnus glutinosa performance, root-associated community composition, and GHG fluxes. We conducted a mesocosm experiment using soil community fractions (wet sieving through 250, 20, 11, and 3 μm) from young and mature forest developmental stages as inocula. The results indicate that the root-associated community composition was shaped by forest developmental stage but not by the size of the community fractions. Inoculation with the largest size fraction from mature forests negatively affected tree growth, likely due to increased competition between the plants and soil biota. In addition, GHG fluxes were not significantly impacted by either size fraction or forest developmental stage despite the different community composition supplied. Overall, our research indicates that A. glutinosa strongly selects the composition of the root-associated community, despite differences in the initial inoculum, and this composition varies depending on the stage of ecosystem development, impacting the performance of the trees but not GHG fluxes. • Simplified biotic community by wet sieving soils from young and mature forests. • Root associated communities differed based on development stage but not size fractions. • Removing soil biota from mature forests increases biomass production. • Removing soil biota does not affect GHG fluxes likely due to functional redundancy. • Soil biota in mature forests potentially inhibit Frankiales symbionts.