Interpretation of nitrogen isotope signatures using the NIFTE model

Nitrogen cycling in forest soils has been intensively studied for many year s because nitrogen is often the limiting nutrient for forest growth. Comple x interactions between soil, microbes, and plants and the consequent inabil ity to correlate delta(15)N changes with biologic processes have limited th e use of natural abundances of nitrogen isotopes to study nitrogen (N) dyna mics. During an investigation of N dynamics along the 250-year-old successi onal sequence in Glacier Bay, Alaska, United States, we observed several pu zzling isotopic patterns, including a consistent decline in delta(15)N of t he late successional dominant Picea at older sites, a lack of agreement bet ween mineral N delta(15)N and foliar delta(15)N, and high isotopic signatur es for mycorrhizal fungi. In order to understand the mechanisms creating th ese patterns, we developed a model of N dynamics and N isotopes (Nitrogen I sotope Fluxes in Terrestrial Ecosystems, NIFTE), which simulated the major transformations of the N cycle and predicted isotopic signatures of differe nt plant species and soil pools. Comparisons with field data from five site s along the successional sequence indicated that NIFTE can duplicate observ ed patterns in delta(15)N Of soil, foliage, and mineral N over time. Differ ent scenarios that could account for the observed isotopic patterns were te sted in model simulations. Possible mechanisms included increased isotopic fractionation on mineralization, fractionation during the transfer of nitro gen from mycorrhizal fungi to plants, variable fractionation on uptake by m ycorrhizal fungi compared to plants, no fractionation on mycorrhizal transf er, and elimination of mycorrhizal fungi as a pool in the model. The model results suggest that fractionation during mineralization must be small (sim ilar to 2 parts per thousand), and that no fractionation occurs during plan t or mycorrhizal uptake. A net fractionation during mycorrhizal transfer of nitrogen to vegetation provided the best fit to isotopic data on mineral N , plants, soils, and mycorrhizal fungi. The model and field results indicat e that the importance of mycorrhizal fungi to N uptake is probably less und er conditions of high N availability. Use of this model should encourage a more rigorous assessment of isotopic signatures in ecosystem studies and pr ovide insights into the biologic transformations which affect those signatu res. This should lead to an enhanced understanding of some of the fundament al controls on nitrogen dynamics.