Natural abundance 15N is the Afghanistan of ecosystem ecology—it is all but impossible to conquer. In short, the N cycle is so complex that plant 15N becomes a single response with two many drivers. There is no consistent way to interpret any one difference in signatures between contrasts.
Although seemingly intractable, few other biogeochemical cycles rival N in their importance in determining how ecosystems function. Understanding the patterns of N cycling is so important that we have to continue to improve our ability to interpret natural abundance 15N patterns.
A number of coauthors and I just got a global review of 15N patterns published in New Phytologist. Details aside, it took a long time for this to happen. Although unfortunate, it gave us the opportunity to work with a number of editors and reviewers to understand the intellectual landscape better. Below I’ll post some summary thoughts on what needs to happen next in the discipline. In a later post, I’ll summarize what we learned in the paper.
1) We need CENTURY for 15N--a general model of the N cycle coupled with a stable isotope simulator to explore scenarios. 15N will not be raised up into the pantheon of ecological isotopes until the theoretical basis for patterns is worked out.
2) We need a survey to measure 15N in roots. Within-plant fractionation is the third major hypothesis for determining patterns of foliar 15N. Only 3 studies have compared 15N for leaves and roots. We'll need a lot more data to evaluate this hypothesis.
3) We need global maps of the N cycle. For all the N cycle has been measured, we do not have global, state-factor relationships with organic N uptake, N mineralization, nitrification, or gaseous N loss. We cannot make a global map of these fundamental processes. For example, time and time again reviewers choked on the idea that denitrification could be higher in sites with lower precipitation. A global synthesis would clear this up.
4) We need better measurements of the signature of available N. Measurements of soil inorganic 15N need to be as commonplace as available NH4+ and NO3-. Knowing these values are an incredible constraint on the key processes.
5) Mycorrhizal ecologists have been slow to assimilate 15N patterns into their understanding of the role of mycorrhizal fungi in plant N nutrition. For example, in Smith and Read's 3rd edition of Mycorrhizal Symbiosis, it was clear that the authors had gotten the story wrong. The attributed the higher 15N of ectomycorrhizal fungi to their reliance on enriched sources, and failed entirely to mention that ectomycorrhizal plants are depleted in 15N as much as the fungi are enriched. There is a lot of work that needs to be done to fully integrate mycorrhizal fungi into plant N nutrition. My guess is that one pressure point will be measuring 15N signatures of fungal mass and plant material in the field.
6) We need a central depository for 15N data that is better than my Macbook. Researchers being able to compare their values to global datasets quickly aids them in interpreting their data while facilitating new syntheses. Nothing new here.