|Soil del15N of surface soils vs. C:N. n=1000. Most of the points are from Africa and US at this point.|
Nitrogen isotopes have the potential to integrate important aspects of the N cycle. In the short-term, vegetation del15N (adjusted for mycorrhizal type) is probably the best index of N availability to plants. When N availability is high, gaseous N loss is more common, which enriches plants in 15N.
Soils are a better long-term integrator of the N cycle as plant material gets incorporated into soil organic matter.
Global patterns for soil 15N have been worked through once or twice. Amundson et al. did the first synthesis of global patterns and showed that soils in drier, hotter climates were enriched in 15N. These ecosystems should have the greatest fraction of N lost via fractionating pathways like denitrification.
Houlton and Bai (2009) looked at soil 15N patterns and calculated that terrestrial world has an average signature of +5‰. One of their advances was to show that the signatures of NO3- being leaches were similar to that of soils, which implicates gaseous N loss as the primary pathway of enrichment.
Given all that, it is still an open question as to what are the proximal controls on whether soils lose a lot of N to denitrification. Are there other characteristics of soils that are associated with isotopic enrichment (i.e. high N availability) beyond being hot and dry?
I think I'll try to tackle this in a synthesis. Above are data from 1000 soils that I've pulled together so far. One thing that's clear is that soils with high C:N are rarely enriched much. Likely these soils would have low N availability and little denitrification. Low C:N soils often have high del15N, which implies high rates of denitrification.
But there are also low C:N soils that aren't enriched in 15N. Why not? Are they cold? Wet? Do they have high pH?
That alone is a pretty interesting question.