A solvent-based extraction fails to remove mobile nitrogen from western redcedar (Thuja plicata)

The ability to accurately measure interannual variability of the stable isotope of nitrogen, ¹⁵N, in tree rings could greatly improve understanding of marine-derived nitrogen delivery to terrestrial ecosystems, soil nitrogen dynamics, and the influences of climate on nitrogen cycles. It could also provide a tool for reconstructing past salmon abundances. However, the mobility of nitrogen throughout the entire tree including between rings has confounded attempts to make full use of ¹⁵N in dendrochemistry. We report that despite solvent-based extraction pretreatment to remove nonstructural, mobile nitrogen from increment core samples, an elevated ¹⁵N signal is detected in rings formed up to about ten years prior to application of a ¹⁵N-enriched tracer in four western redcedar trees. We conclude that the interpretation of the heavy nitrogen signal in tree rings via this pretreatment extraction method is confounded. These findings corroborate work already done in the field.