Northeastern Section - 42nd Annual Meeting (12–14 March 2007)

Paper No. 6
Presentation Time: 8:15 AM-12:00 PM


HOBBIE, Erik A., Department of Natural Resources, Complex Systems Research Center, Morse Hall, University of New Hampshire, Durham, NH 03824 and OUIMETTE, Andrew P., Complex Systems Research Center, Morse Hall, University of New Hampshire, Durham, NH 03824,

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 ä15Nsoil - ä15Nlitter = ff · Äf + åd, with ff the fraction of soil N derived from mycorrhizal fungal litter, Äf the fractionation during transfer of N from mycorrhizal fungi to host plants, and åd the cumulative enrichment in 15N of stable soil organic N due to decomposition and loss processes.