Ecosystem ecologists are challenged by both the scale and complexity of their discipline. Consistent methodologies and integrative metrics can ameliorate such challenges. Stable isotope ratios of nitrogen (δ ¹⁵N) offer themselves as unique time-integrated proxies of numerous biophysical processes. The key N cycling information contained in ecosystem δ ¹⁵N values, however, is obscured by multiple competing hypotheses. Here, I examine patterns in δ¹⁵N values in fungi from around the world and in multiple ecosystem components along the soil-fungi-plant continuum in boreal black spruce forest of central Alaska. My objective was to determine both the causes of underlying δ¹⁵N variability and the utility of these measurements.

By combining previously published isotopic data with a novel dataset collected in poorly studied tropical rainforest, I was able to demonstrate the universal ability of dual isotope (δ¹⁵N and δ ¹³C) datasets to discriminate ectomycorrhizal from saprotrophic fungi in >90% of 813 samples of fungi. Furthermore, I demonstrated that the isotope values could be partially predicted by climate in a manner similar to that previously demonstrated in plants and soils.

In Alaska I focused on severely N-limited forests dominated by a single species of ectomycorrhizal tree. By examining 31 plots varying widely in foliar δ ¹⁵N values, topography, and stand density, I aimed to understand the fundamental controls over δ¹⁵N variability in plants. Using 16 experimentally fertilized plots I examined how changes to soil nutrient fertility can alter δ¹⁵N values, N sources, and pathways of cycling. Using a highly sensitive bacterial denitrifier technique I overcame a methodological impasse of previous work and demonstrated that black spruce δ ¹⁵N values were a product of the interaction of soil fertility and ectomycorrhizal activity, not merely a reflection of source N δ ¹⁵N values. In the fertilized plots I demonstrated that only the combination of N and phosphorus fertilization leads to a significant reduction of black spruce reliance upon ectomycorrhizal-derived N from 82 to 46% of total N nutrition. Phosphorus fertilization, in particular, also led to an unusual and previously undocumented response in soil N cycling.