The mobility of nitrogen across tree‐rings of Norway spruce ( Picea abies L.) and the effect of extraction method on tree‐ring δ 15 N and δ 13 C values

RATIONALE The use of stable nitrogen (N) isotope ratios (δ 15 N values) 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 δ 15 N values, recent investigations question the necessity of such an extraction. METHODS We used an on‐going experiment with simulated elevated N deposition previously labelled with 15 N, in conjunction with control trees, to investigate the necessity of extracting mobile N compounds (using a rapid extraction procedure) for tree‐ring δ 15 N and δ 13 C studies, as well as 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 15 N 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 rapid extraction procedure had no significant effect on tree‐ring δ 15 N or δ 13 C values in either labelled or control trees, or on N concentration. The 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 through the rapid extraction procedure is not necessary prior to the determination of Norway spruce δ 15 N or δ 13 C values in dendrochemical studies. δ 15 N values, however, must be interpreted with great care, particularly when used as a 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. Copyright © 2014 John Wiley & Sons, Ltd.