Decomposition and mycorrhizal suppression of saprotrophs remain stable after wildfire and clearcut disturbance in boreal forests

Boreal forests store a large portion of global terrestrial carbon, primarily in soils, yet their sensitivity to environmental changes remains uncertain. Disturbance is one major agent of change, with dominance shifting from wildfire to forest management over the past century. Mycorrhizal suppression of saprotrophic decomposition may be an important control on boreal soil carbon storage, but its occurrence and magnitude remain uncertain. Additionally, the distinct impacts of wildfire and clearcutting on decomposition and soil microbial communities remain largely unexplored. We quantified mycorrhizal suppression of saprotrophs and decomposition in needle and humus litter through a root-exclusion experiment in 31 sites across wildfire (4-375 yr since disturbance) and clearcut chronosequences (1-109 yr since disturbance) in northern Sweden. Mycorrhizas suppressed decomposition by 15.5 ± 5.9% overall, with no consistent change over time after disturbance. This pattern was linked to absolute fungal abundance but not to other microbial community shifts, soil temperature or soil moisture. Decomposition rates were stable across time since disturbance for all substrates and disturbance types. Our findings improve understanding of mycorrhizal-decomposition interactions and highlight the need to further explore key fungal traits and compositional changes, especially at the soil surface, that regulate carbon storage dynamics in boreal soils.