『Abstract
　To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (δ¹⁵N) data, patterns of ¹⁵N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum ¹⁵N 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 ¹⁵N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling ¹⁵N in soil profiles, fractionation against ¹⁵N during N transfer by mycorrhizal fungi to host plants, leads to ¹⁵N-depleted plant litter at the soil surface and ¹⁵N-enriched nitrogen of fungal origin at depth. The preferential preservation of ¹⁵N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for late ¹⁵N enrichments with depth in less N-limited forests and may account for soil profiles where maximum δ¹⁵N 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.

Keywords: Nitrogen isotopes; Soil horizons; Isotopic fractionation; Modeling; Mycorrhizal fungi; Soil mixing; Denitrification』

Introduction
Methods
　Analytical and computational solutions of ¹⁵N enrichment patterns in soil profiles
Results
Discussion
　Climate
　Nitrification and denitrification
　Mycorrhizal fractionation
　Fractionation during decomposition
　Soil fauna, soil foodwebs, bioturbation, and disturbance
　Synthesizing the processes influencing soil δ¹⁵N into a conceptual model
Conclusion
Acknowledgments
References