Interpretation of nitrogen isotope signatures using the NIFTE model

Area of Science:

• Ecology
• Biogeochemistry
• Soil Science

Background:

• Nitrogen is a key limiting nutrient for forest growth.
• Studying nitrogen dynamics using natural nitrogen isotopes (δ15N) is challenging due to complex soil-microbe-plant interactions.
• Previous studies observed puzzling isotopic patterns in forest nitrogen cycling.

Purpose of the Study:

• To understand the mechanisms behind observed nitrogen isotope patterns in forest ecosystems.
• To develop and validate a model simulating nitrogen dynamics and isotope signatures.
• To investigate the role of mycorrhizal fungi in nitrogen transfer and isotopic fractionation.

Main Methods:

• Development of the Nitrogen Isotope Fluxes in Terrestrial Ecosystems (NIFTE) model.
• Simulation of major nitrogen cycle transformations and isotopic signatures.
• Comparison of model predictions with field data from a successional sequence in Alaska.

Main Results:

• The NIFTE model successfully replicated observed δ15N patterns in soil, foliage, and mineral nitrogen.
• Model simulations indicated small fractionation during mineralization and no fractionation during plant/mycorrhizal uptake.
• Net fractionation during mycorrhizal transfer to vegetation best explained the isotopic data.

Conclusions:

• Mycorrhizal fungi play a significant role in nitrogen transfer, with fractionation occurring during this process.
• The importance of mycorrhizal fungi in nitrogen uptake may decrease with higher nitrogen availability.
• The NIFTE model provides a tool for rigorous isotopic assessment in ecosystem studies, enhancing understanding of nitrogen dynamics.