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Journal of Arid Land
Journal of Arid Land  2022, Vol. 14 Issue (6): 673-690    DOI: 10.1007/s40333-022-0065-1     CSTR: 32276.14.s40333-022-0065-1
Research article     
Tree-ring δ15N of Qinghai spruce in the central Qilian Mountains of China: Is pre-treatment of wood samples necessary?
WANG Ziyi1, LIU Xiaohong1,2,3,*(), WANG Keyi1,4, ZENG Xiaomin1, ZHANG Yu1, GE Wensen1, KANG Huhu2, LU Qiangqiang1
1School of Geography and Tourism, Shaanxi Normal University, Xi'an 710119, China
2State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
3Qilian Mountain National Park Research Center (Qinghai), Xining 810008, China
4School of Earth System Science, Tianjin University, Tianjin 300072, China
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Abstract  

A knowledge of the tree-ring stable nitrogen isotope ratio (δ15N) 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 δ15N values. We compared the effects of pre-treatment with organic solvents and hot ultrapure water on the N concentration and δ15N 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 δ15N 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 δ15N values of tree rings decreased consistently from 1960 to 2019 in both the extracted and unextracted wood samples. Extraction with organic solvents increased the δ15N values markedly by about 5.2‰ and reduced the variations in the δ15N series. However, extraction with hot ultrapure water had little effect, with only a slight decrease in the δ15N values of about 0.5‰. 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 δ15N variation trend of tree rings in the unextracted wood samples was not influenced by the heartwood-sapwood transition zone. We suggest that the δ15N 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.

Key words： tree rings      stable nitrogen isotope ratio (δ15N)      nitrogen concentration      solvent-extracted wood      water-extracted wood      wood pre-treatment      Qinghai spruce      Qilian Mountains     
Received: 12 November 2021      Published: 30 June 2022
Corresponding Authors: * LIU Xiaohong (E-mail: liuxh@lzb.ac.cn)
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WANG Ziyi
LIU Xiaohong
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ZENG Xiaomin
ZHANG Yu
GE Wensen
KANG Huhu
LU Qiangqiang
Cite this article:

WANG Ziyi, LIU Xiaohong, WANG Keyi, ZENG Xiaomin, ZHANG Yu, GE Wensen, KANG Huhu, LU Qiangqiang. Tree-ring δ15N of Qinghai spruce in the central Qilian Mountains of China: Is pre-treatment of wood samples necessary?. Journal of Arid Land, 2022, 14(6): 673-690.

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http://jal.xjegi.com/10.1007/s40333-022-0065-1     OR     http://jal.xjegi.com/Y2022/V14/I6/673

Fig. 1 Flow chart of the wood pre-treatment methods for the determination of the stable nitrogen isotope ratio (δ15N) in tree rings from the endemic Qinghai spruce. Three pre-treatment methods were used to prepare the wood samples for the measurements of the nitrogen (N) concentration and δ15N in tree rings: (1) wood extracted with organic solvents (solvent-extracted wood); (2) wood extracted with hot ultrapure water (water-extracted wood); and (3) unextracted wood (bulk wood).
Fig. 2 N concentration (a) and δ15N values (b) in tree rings from Qinghai spruce as a function of time determined by the three different pre-treatment methods: wood extracted with organic solvents (solvent-extracted wood), wood extracted with hot ultrapure water (water-extracted wood) and unextracted wood (bulk wood). The vertical gray-shaded area indicates the heartwood-sapwood transition zone around the time period of 1995-1998.
Fig. 3 Comparisons of the time series of the tree-ring N concentration and δ15N values for the extracted wood (solvent-extracted wood and water-extracted wood) and unextracted wood (bulk wood) during the time period of 1960-2019, as well as relationships of the Z-scores of the N concentration and δ15N values between the extracted wood (solvent-extracted and water-extracted wood) and unextracted wood (bulk wood). (a and b), the N concentration; (c and d), the δ15N values. The curves in the left panel were standardized by the Z-scores. The solid lines in the right panel represent the linear regression of each dataset. The 1:1 line is shown by the dashed line. The coefficient of determination (R²) and P-value are also given.
N concentration (%)
Bulk wood Solvent-extracted wood Water-extracted wood Df. I Df. II
Whole wood 0.063±0.017 0.052±0.012 0.066±0.017 -0.011±0.006 0.002±0.003
Sapwood 0.081±0.017 0.065±0.012 0.084±0.014 -0.016±0.005 0.003±0.004
Heartwood 0.053±0.008 0.045±0.004 0.055±0.007 -0.008±0.005 0.002±0.002
P 0.000** 0.000** 0.000** 0.000** 0.127
δ15N (‰)
Bulk wood Solvent-extracted wood Water-extracted wood Df. I Df. II
Whole wood 8.4±4.1 13.6±3.7 7.8±3.9 5.2±2.1 -0.5±0.8
Sapwood 4.1±1.8 9.5±2.4 3.7±1.5 5.5±0.8 -0.3±0.6
Heartwood 11.1±2.8 15.9±1.7 10.5±2.6 4.9±2.6 -0.6±0.9
P 0.000** 0.000** 0.000** 0.322 0.228
Table 1 Summary statistical analyses among the pre-treatment methods of wood samples
Fig. 4 Box-whisker plots of the tree-ring N concentration (a) and δ15N values (b), as well as relationships between the N concentration and δ15N values (c) of the extracted wood (solvent-extracted wood and water-extracted wood) and unextracted wood (bulk wood). The boundaries of the boxes indicate the 25th and 75th percentiles. The solid lines are median values, the black dots are mean values and the black squares are outliers. Bars represent the change range of the N concentration or δ15N values. Different lowercase letters indicate significant differences (P<0.05) between the pre-treatment methods by one-way ANOVA. The solid curves represent the correlations simulated by exponential fitting.
Fig. 5 Box-whisker plots of the changes in the tree-ring N concentration (a) and δ15N values (b) for the heartwood and sapwood sections in the extracted wood (solvent-extracted wood and water-extracted wood) compared with the unextracted wood (bulk wood), as well as relationships between differences in the N concentration and δ15N values between the extracted wood and unextracted wood (c). The boundaries of the boxes indicate the 25th and 75th percentiles. The solid lines are median values, the black dots are mean values and the black squares are outliers. The bars represent the change range of differences in the N concentration or δ15N values. Offsets were calculated between the extracted wood and unextracted wood. Different lowercase letters indicate significant differences (P<0.05) between the sapwood and heartwood by the independent sample t-test.
Fig. 6 Offset in the tree-ring δ15N values (a) and percentage of offset in the N concentration (b) for different tree species after the extraction of bulk wood. Offset in the δ15N was calculated as the difference between the extracted wood and unextracted wood, so offset in the N concentration did. The percentage of offset in the N concentration was calculated as the quotients of the offset in the N concentration divided by the actual value of the unextracted wood. Tree rings in white oak were extracted with deionized water. Tree rings of other species were extracted with organic solvents. Data were obtained from previous studies as follows (see Table S3 for details): [1], Sheppard and Thompson (2000); [2], Elhani et al. (2003); [3], Elhani et al. (2005); [4], Kwak et al. (2009); [5] Hietz et al. (2010); [6], Larry et al. (2011); [7], Bukata and Kyser (2005); [8], Hart and Classen (2003). S-e, solvent-extracted wood; W-e, water-extracted wood.
Wood section N concentration
Mean sensitivity 1st autocorrelation Coefficient of variance
Bulk
wood		 Solvent-
extracted wood		 Water-
extracted wood		 Bulk
wood		 Solvent-
extracted wood		 Water-
extracted wood		 Bulk
wood		 Solvent-
extracted wood		 Water-
extracted wood
Whole wood 0.083 0.073 0.084 0.702 0.668 0.728 0.272 0.236 0.254
Sapwood 0.101 0.091 0.111 0.376 0.325 0.262 0.208 0.188 0.169
Heartwood 0.059 0.052 0.051 0.807 0.732 0.829 0.143 0.092 0.121
Wood section δ15N
Mean sensitivity 1st autocorrelation Coefficient of variance
Bulk
wood		 Solvent-
extracted wood		 Water-
extracted wood		 Bulk
wood		 Solvent-
extracted wood		 Water-
extracted wood		 Bulk
wood		 Solvent-
extracted wood		 Water-
extracted wood
Whole wood 0.139 0.115 0.135 0.876 0.765 0.889 0.494 0.271 0.492
Sapwood 0.262 0.168 0.307 0.472 0.385 0.259 0.450 0.254 0.394
Heartwood 0.101 0.088 0.086 0.828 0.452 0.874 0.256 0.108 0.244
Table S1 Summary statistical analyses, including mean sensitivity, 1st autocorrelation and coefficient of variance of the N concentration and stable nitrogen isotope ratio (δ15N) in tree rings of the extracted wood (solvent-extracted wood and water-extracted wood) and unextracted wood (bulk wood)
Fig. S1 Trends simulated by the linear regression of the N concentration (a) and δ15N (c) in tree rings of the extracted wood (solvent-extracted wood and water-extracted wood) and unextracted wood (bulk wood), and trends of the N concentration (b) and δ15N (d) of the heartwood and sapwood sections in tree rings of the extracted wood (solvent-extracted wood and water-extracted wood) and unextracted wood (bulk wood) from 1960 to 2019. The vertical grey-shaded area indicates the heartwood-sapwood transition zone around the period of 1995-1998.
Wood N concentration (%)
Heartwood Sapwood Whole wood
Slope R2 Slope R2 Slope R2
Solvent-extracted wood 0.00030 0.532 0.00096 0.230 0.00056 0.629
Water-extracted wood 0.00058 0.808 0.00077 0.110 0.00082 0.730
Bulk wood 0.00068 0.831 0.00113 0.165 0.00082 0.689
Wood δ15N (‰)
Heartwood Sapwood Whole wood
Slope R2 Slope R2 Slope R2
Solvent-extracted wood -0.08 0.216 -0.15 0.129 -0.17 0.659
Water-extracted wood -0.23 0.853 -0.07 0.092 -0.21 0.888
Bulk wood -0.25 0.802 -0.12 0.167 -0.22 0.869
Table S2 Statistical analyses of the variations in the N concentration and δ15N in tree rings of the extracted wood (solvent-extracted wood and water-extracted wood) and unextracted wood (bulk wood)
Method Extraction protocol Tree species N concentration δ15N Determination Reference
Sheppard/
Thompson		 S1: 4 h in a 1:1 mixture of toluene/ethanol;
S2: 4 h in ethanol;
S3: 1 or 4 h in
distilled
water		 Ponderosa pine
(Pinus ponderosa) and Douglas-fir		 Decrease (about 45.000%) NA The variation in N concentration of tree
rings was reduced substantially.		 Sheppard and Thompson (2000)
Beech (Fagus sylvatica) Decrease (about 36.000%, from 0.220% to 0.150%) Decrease (about 0.4‰) The interannual resolution of the N and δ15N
was improved, but not
all mobile N was
removed.		 Elhani et al. (2003)
Beech (F.
sylvatica)		 NA Increase (-6.0‰- -5.0‰ to
-4.5‰- -3.5‰
in the control trees)		 Variation in the δ15N signal was decreased, but differences between trees before and after treatment were enhanced. Elhani et al. (2005)
Red pine (Pinus densiflora) Decrease (19.400%- 31.600%) NA Higher extractable N in the sapwood was removed, while not affecting the overall trend of N. Kwak et al. (2009)
Spanish cedar
(Cedrela odorata) and big-leaf mahogany (Swietenia macrophylla)		 Decrease
(not significantly lower)		 Increase (about 1.0‰ higher in
the treated wood)		 The proportion of N extractable in the
sapwood was not higher than that in the heartwood.		 Hietz et al. (2010)
Norway spruce
(Picea abies)		 Decrease Increase (slight) No significant effect
on the tree-ring N and δ15N.		 Tomlinson et al. (2014)
Western redcedar (Thuja plicata)
and Douglas-fir		 NA Elevated 15N signal found
10 a before the 15N-labeled		 The mobile N in tree rings was eliminated ineffectively. Bunn et al. (2017)
Simplified
Sheppard/
Thompson		 Keep overnight in a 1:1 mixture of toluene/ethanol Japanese black
pine (Pinus thunbergii)		 NA Increase of 3.0‰
in the control trees, and
decrease of
-7.0‰ in the sites with the high N		 Significant effect on the tree-ring δ15N under high-N conditions. Larry et al. (2011)
Altered
Sheppard/
Thompson		 S1: in 1:1 mixture of benzene/methanol;
S2: 12 or 48 h in acetone; S3: 1 h in
deionized (DI) water		 Beech (Fagus grandifolia)
and red spruce
(Picea rubens)		 Decrease
(8.000% and
7.000%, respectively)		 Decrease (0.4‰ and 0.3‰, respectively) The difference is always within the analytical error, with no significant change in the N or δ15N values or temporal trends. Doucet et al. (2011)
Methods Extraction protocol Tree species N concentration δ15N Determination Reference
Water-bath Fresh DI water (shaking 4 times each for 3 d) Red oak (Quercus rubra) and white oak (Quercus alba) Decrease
(10.500% in the sapwood)		 Increase
(0.6‰ in the sapwood)		 No significant change
in the δ15N.		 Bukata and Kyser (2005)
Holocellulose extraction S1: 16-18 h in a 2:1 mixture of toluene/
ethanol; S2: 16-18 h
in ethanol; S3: 6 h in DI water		 Ponderosa pine
(P. ponderosa)		 Decrease
(about 10.000%)		 Decrease
(2.0‰-3.0‰ in the control trees;
4.0‰-34.0‰ in the labeled trees)		 Relatively small and consistent effect on the control trees, while relatively large and variable effect on the highly 15N. Hart and Classen (2003)
Table S3 Retrospect of extraction protocols used in the tree-ring δ15N analysis from previous studies
Fig. S2 Growth pattern of Qinghai Spruce during the period of 1840-2019. Variations in the tree growth were estimated as the basal area increment (BAI) during the period of 1840-2019. The orange line is smoothed using a 20 a low-pass Fast Fourier Transform (FFT) filter to emphasize the low-frequency variations.
Fig. S3 Temporal trends of annual mean temperature (a) and total precipitation (b) from 1958 to 2019 based on averaging data from two nearby meteorological stations (Qilian and Minle) of the study area. The dashed lines represent the trends simulated by the linear regressions. Annual variation is represented by the slope of the trend line. The coefficient of determination (R2) and significance P-value are also provided.
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