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Soil organic material (SOM) is usually enriched in (15)N in deeper soil layers. This has been explained by discrimination against the heavier isotope during decomposition or by the accumulation of (15)N-enriched microbial biomass versus plant biomass in older SOM. In particular, ectomycorrhizal (EM) fungi have been suggested to accumulate in old SOM since this group is among the most (15)N-enriche
