Nitrogen rather than streamflow regulates the growth of riparian trees

Wang, Keyi; Zeng, Xiaomin; Liu, Xiaohong; Lanning, Matthew; Wu, Guojiao; Zhao, Liang; Xu, Guobao; Wang, Yabo; Zhang, Lingnan; Li, Xiaoqin; Lu, Qingtao; Wang, Lixin

doi:10.1016/j.chemgeo.2020.119666

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“…The average atmospheric dry and wet N deposition values over the region were 9.6 and 9.5 kg N/(hm 2 •a) during the time period of 2011-2015, respectively, both lower than the average fluxes across China (20.6 and 19.3 kg N/(hm 2 •a), respectively) (Xu et al, 2015;Jia et al, 2016Jia et al, , 2019. As a result of the limitations in the temporal resolution of N deposition data, we cannot detect reliable direct or indirect evidence of anthropogenic atmospheric N deposition in the annual variations of the δ 15 N in tree rings, in contrast with the δ 15 N values of tree rings from riparian trees in the lower reaches of the Heihe River, China (Wang et al, 2020). The more pronounced decreasing trend of the δ 15 N in tree rings might be caused by fractionation in the soil-forest N cycle after the forest was hold allochthonous N inputs with the lower δ 15 N, which requires further investigation.…”

Section: Consistent Downward Trend Of the Tree-ring δ 15 N In Differe...mentioning

confidence: 80%

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

et al. 2022

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A knowledge of the tree-ring stable nitrogen isotope ratio (δ 15 N) 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 δ 15 N values. We compared the effects of pre-treatment with organic solvents and hot ultrapure water on the N concentration and δ 15 N 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 δ 15 N 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 δ 15 N values of tree rings decreased consistently from 1960 to 2019 in both the extracted and unextracted wood samples. Extraction with organic solvents increased the δ 15 N values markedly by about 5.2‰ and reduced the variations in the δ 15 N series. However, extraction with hot ultrapure water had little effect, with only a slight decrease in the δ 15 N 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 δ 15 N variation trend of tree rings in the unextracted wood samples was not influenced by the heartwood-sapwood transition zone. We suggest that the δ 15 N 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.

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