Is wood pre-treatment essential for tree-ring nitrogen concentration and isotope analysis?

Tree-ring nitrogen concentrations and isotope ratios (delta N-15) are gaining in popularity for environmental research although their use is still debated because of nitrogen mobility in tree stems. Modern studies generally present results on wood that is pre-treated to remove soluble nitrogen compounds and to minimize the impact of radial translocation on tree-ring nitrogen environmental records. However, the necessity to use such pre-treatment has never been fully assessed. Here we compare the nitrogen concentrations and delta N-15 values of two wood preparation protocols applied to beech and red spruce tree rings for the removal of soluble compounds from ring pairs with non pre-treated tree rings. For both tree species, pre-treatment did not minimize the radial patterns of tree-ring nitrogen concentrations and the increasing concentration trends that are coincident with the heartwood-sapwood boundary. Therefore, even if the tree-ring nitrogen concentrations are slightly modified by pre-treatment, these concentrations are considered to reflect internal stem processes rather than environmental conditions in both species. The delta N-15 values were similar for untreated and pre-treated ring pairs, suggesting that wood pre-treatment did not substantially change the delta N-15 values and temporal trends in ring series. In addition, tree-ring delta N-15 series of untreated and pre-treated wood did not show any sign of influence of the heartwood-sapwood boundary in either tree species, indicating that nitrogen translocation did not generate significant isotopic fractionation. We therefore suggest that untreated ring delta N-15 values of beech and red spruce trees can be used for environmental research. Copyright (C) 2011 John Wiley & Sons, Ltd.