Tree-ring δ15N of Qinghai spruce in the central Qilian Mountains of China: is pre-treatment of wood samples necessary?

A knowledge of the tree-ring stable nitrogen isotope ratio (delta N-15) can deepen our understanding of forest ecosystem dynamics by indicating the long-term availability, cycling and sources of nitrogen (N). However, the radial mobility of N blurs the interannual variations in the long-term N records. Previous studies of the chemical extraction of tree rings before analysis had produced inconsistent results and it is still unclear whether it is necessary to pre-treat wood samples from specific tree species to remove soluble N compounds before determining the delta N-15 values. We compared the effects of pre-treatment with organic solvents and hot ultrapure water on the N concentration and delta N-15 of tree rings from endemic Qinghai spruce (Picea crassifolia) growing in the interior of the central Qilian Mountains, China, during the last 60 a. We assessed the effects of different preparation protocols on the removal of the labile N compounds and investigated the need to pre-treat wood samples before determining the delta N-15 values of tree rings. Increasing trends of the tree-ring N concentration were consistently observed in both the extracted and unextracted wood samples. The total N removed by extraction with organic solvents was about 17.60%, with a significantly higher amount in the sapwood section (P<0.01). The delta N-15 values of tree rings decreased consistently from 1960 to 2019 in both the extracted and unextracted wood samples. Extraction with organic solvents increased the delta N-15 values markedly by about 5.2 parts per thousand and reduced the variations in the delta N-15 series. However, extraction with hot ultrapure water had little effect, with only a slight decrease in the delta N-15 values of about 0.5 parts per thousand. Our results showed that the radial pattern in the inter-ring movement of N in Qinghai spruce was not minimized by extraction with either organic solvents or hot ultrapure water. It is unnecessary to conduct hot ultrapure water extraction for the wood samples from Qinghai spruce because of its negligible effect on the removal of the labile N. The delta N-15 variation trend of tree rings in the unextracted wood samples was not influenced by the heartwood-sapwood transition zone. We suggest that the delta N-15 values of the unextracted wood samples of the climate-sensitive Qinghai spruce could be used to explore the ecophysiological dynamics while focusing on the long-term variations.

Automated summary

This study compared the effects of pre-treatment with organic solvents and hot ultrapure water on the nitrogen concentration and delta N-15 values of tree rings from Qinghai spruce. The results showed that extraction with organic solvents significantly affected the delta N-15 values, while extraction with hot ultrapure water had little effect. Therefore, unextracted wood samples can be used to study the delta N-15 values of Qinghai spruce.