To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (d 15 N) data, patterns of 15 N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum 15 N enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 ± 0.4% in ectomycorrhizal systems and 4.6 ± 0.5% in arbuscular mycorrhizal systems. The 15 N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling 15 N in soil profiles, fractionation against 15 N during N transfer by mycorrhizal fungi to host plants, leads to 15 N-depleted plant litter at the soil surface and 15 N-enriched nitrogen of fungal origin at depth. The preferential preservation of 15 N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large 15 N enrichments with depth in less N-limited forests and may account for soil profiles where maximum d 15 N is at intermediate depths. Mixing among soil horizons should also decrease differences among soil horizons. We suggest that dynamic models of isotope distributions within soil profiles that can incorporate multiple processes could provide additional information about the history of nitrogen movements and transformations at a site.

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