Controls of nitrogen isotope patterns in soil profiles

To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (delta N-15) data, patterns of N-15 enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum N-15 enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 +/- A 0.4aEuro degrees in ectomycorrhizal systems and 4.6 +/- A 0.5aEuro degrees in arbuscular mycorrhizal systems. The N-15 enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling N-15 in soil profiles, fractionation against N-15 during N transfer by mycorrhizal fungi to host plants, leads to N-15-depleted plant litter at the soil surface and N-15-enriched nitrogen of fungal origin at depth. The preferential preservation of N-15-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large N-15 enrichments with depth in less N-limited forests and may account for soil profiles where maximum delta N-15 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.