1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
2 Paul Scherrer Institute (PSI), CH 5232 Villigen, Switzerland
3 Swiss Federal Institute of Technology (ETH), CH-8092 Zurich, Switzerland
RATIONALE: The use of stable nitrogen (N) isotope ratios (δ15N) in dendroecological studies is often preceded by an extraction procedure using organic solvents to remove mobile N compounds from tree-rings. Although these mobile N compounds may be capable of distorting potential environmental signals in the tree-ring δ15N, recent investigations question the necessity of such an extraction.
METHODS: We used an on-going experiment with simulated elevated N deposition previously labelled with 15N, in conjunction with control trees, to investigate the necessity of extracting mobile N compounds for tree-ring δ15N, δ13C, N and C concentration analyses. In addition, we examined the magnitude of radial redistribution of N across tree-rings of Norway spruce (Picea abies).
RESULTS: The 15N-label, applied in 1995/96, was found in tree-rings as far back as 1951, although the increased N availability did not cause any significant relative increase in tree growth. The extraction procedure had no significant effect on tree-ring δ15N or δ13C in either labelled or control trees, nor on N concentration. C concentrations, however, were significantly higher after extraction in control samples, with the opposite effect observed in labelled samples.
CONCLUSIONS: Our results indicate that the extraction of mobile N compounds is not necessary prior to the analysis of Norway spruce δ15N or δ13C in dendrochemical studies. δ15N data, however, must be interpreted with great care, particularly when used as proxy for the N status of trees, due to the very high mobility of N within the tree stem sapwood of Norway spruce over several decades.
Keywords: tree-ring, extraction, nitrogen translocation, stable isotopes