WHAT CONTROLS NITROGEN ISOTOPE PATTERNS IN TERRESTRIAL SOIL PROFILES?

To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotopes, we compiled nitrogen isotope patterns of soils from studies on tropical, temperate, and boreal systems. Because many reactions fractionate against ¹⁵N, dominant processes can be tracked by measuring patterns of ¹⁵N enrichment between surface litter and deeper soil horizons. We compared the ¹⁵N enrichment with depth to 1) mean annual temperature (MAT) and precipitation (MAP), 2) nitrification rates, and 3) mycorrhizal type of vegetation. The maximum ¹⁵N enrichment between litter and deeper soil layers was uncorrelated with MAT, MAP, or nitrification rates but varied strongly with mycorrhizal fungal association. Sites with ectomycorrhizal (ECM) vegetation increased by 10‰ in ¹⁵N with depth, whereas sites with arbuscular mycorrhizal (AM) vegetation increased by 4‰ in 15N with depth. An intrinsic fractionation factor of 8-10‰ between ECM fungi and plant hosts may also apply to AM plants. Thus, fractionation against ¹⁵N during transfer of N by ECM and AM probably accounted for much of the ¹⁵N enrichment in soil profiles, with ¹⁵N-depleted plant litter at the surface and ¹⁵N-enriched fungal material at depth. A second important factor appears to be the preferential preservation of ¹⁵N-enriched compounds during decomposition and stabilization processes. If ¹⁵N enrichment by the second process is relatively constant in deep soil horizons, then the relative contribution of plant-derived versus mycorrhizal-derived N to deep soil horizons can be estimated from ¹⁵N patterns according to the equation ä¹⁵N_soil - ä¹⁵N_litter = f_f · Ä_f + å_d, with f_f the fraction of soil N derived from mycorrhizal fungal litter, Ä_f the fractionation during transfer of N from mycorrhizal fungi to host plants, and å_d the cumulative enrichment in ¹⁵N of stable soil organic N due to decomposition and loss processes.