Decay by ectomycorrhizal fungi couples soil organic matter to nitrogen availability

Interactions between soil nitrogen (N) availability, fungal community
composition, and soil organic matter (SOM) regulate soil carbon (C)
dynamics in many forest ecosystems, but context dependency in these
relationships has precluded general predictive theory. We found that
ectomycorrhizal (ECM) fungi with peroxidases decreased with increasing
inorganic N availability across a natural inorganic N gradient in northern
temperate forests, whereas ligninolytic fungal saprotrophs exhibited no
response. Lignin-derived SOM and soil C were negatively correlated with
ECM fungi with peroxidases and were positively correlated with inorganic N
availability, suggesting decay of lignin-derived SOM by these ECM fungi
reduced soil C storage. The correlations we observed link SOM decay in
temperate forests to tradeoffs in tree N nutrition and ECM composition,
and we propose SOM varies along a single continuum across temperate and
boreal ecosystems depending upon how tree allocation to functionally
distinct ECM taxa and environmental stress covary with soil N
availability.