Below-ground organic matter accumulation along a boreal forest fertility gradient relates to guild interaction within fungal communities.

Plant-soil interactions link ecosystem fertility and organic matter accumulation below ground. Soil microorganisms play a central role as mediators of these interactions, but mechanistic understanding is still largely lacking. Correlative data from a coniferous forest ecosystem support the hypothesis that interactions between fungal guilds play a central role in regulating organic matter accumulation in relation to fertility. With increasing ecosystem fertility, the proportion of saprotrophic basidiomycetes increased in deeper organic layers, at the expense of ectomycorrhizal fungal species. Saprotrophs correlated positively with the activity of oxidative enzymes, which in turn favoured organic matter turnover and nitrogen recycling to plants. Combined, our findings are consistent with a fungus-mediated feedback loop, which results in a negative correlation between ecosystem fertility and below-ground carbon storage. These findings call for a shift in focus from plant litter traits to fungal traits in explaining organic matter dynamics and ecosystem fertility in boreal forests.