王可逸^,1, 刘晓宏^,1,2,3, 曾小敏¹, 徐国保², 张凌楠¹, 李春越¹1.陕西师范大学地理科学与旅游学院,西安 710119
2.中国科学院西北生态环境资源研究院 冰冻圈科学国家重点实验室,兰州 730000
3.祁连山国家公园青海研究中心,西宁 810008

Stable nitrogen isotope in tree rings: Progresses, problems and prospects

WANG Keyi^,1, LIU Xiaohong^,1,2,3, ZENG Xiaomin¹, XU Guobao², ZHANG Lingnan¹, LI Chunyue¹1. School of Geography and Tourism, Shaanxi Normal University, Xi'an 710119, China
2. State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, CAS, Lanzhou 730000, China
3. Qinghai Research Center of Qilian Mountain National Park, Xining, Qinghai 810008, China

通讯作者: 刘晓宏(1972-), 男, 陕西韩城人, 教授, 主要从事树轮稳定同位素记录与生物地球化学循环研究。E-mail: xhliu@snnu.edu.cn;liuxh@lzb.ac.cn

收稿日期:2020-05-18修回日期:2021-03-12网络出版日期:2021-05-25

基金资助:

国家自然科学基金项目.41971104 中央高校基础研究经费.GK202107009 中国科学院地球环境研究所黄土与第四纪地质国家重点实验室开放基金.SKLLQG1817 祁连山国家公园青海研究中心开放课题.GKQ2019-01

Received:2020-05-18Revised:2021-03-12Online:2021-05-25

Fund supported:

National Natural Science Foundation of China.41971104 The Fundamental Research Funds for the Central Universities.GK202107009 The Open Foundation of the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment.SKLLQG1817 The Qilian Mountain National Park Research Center (Qinghai).GKQ2019-01

作者简介 About authors
王可逸(1995-), 女, 内蒙古呼和浩特人, 硕士生, 主要从事树轮稳定氮同位素方面的研究。E-mail: wky@snnu.edu.cn








摘要
树轮稳定同位素比率是研究气候变化与环境演变的有效代用资料,耦合分析树轮稳定同位素记录的信息可揭示森林生态系统碳—水—氮时空变化特征及相互作用,反映环境变化对植物特定化合物的生理影响。树轮稳定氮同位素比率（δ¹⁵N）与树木生长的环境条件密切相关,其变化可以反映长时间尺度森林生态系统氮循环的动态特征,弥补监测资料的不足。本文综述了树轮稳定氮同位素的分馏机理与测定方法,解析了树轮δ¹⁵N在样品预处理和测试中需要关注的问题,系统梳理与归纳了基于树轮δ¹⁵N进行气候变化、环境演变和人类活动氮排放研究的进展。指明中国树轮稳定氮同位素研究潜力及其在未来亟需着重发展的方向,旨在推动森林生态系统氮循环时空特征深入研究。
关键词： 树木年轮;稳定氮同位素比率;气候变化;氮循环;森林生态系统

Abstract
Stable isotope in tree rings are effective proxies for the study of climate change and environmental evolution. Coupling analysis of stable isotope records of tree rings can reveal the spatiotemporal variation characteristics and interactions of carbon, water and nitrogen in forest ecosystems, and reflect the physiological effects of environmental changes on plant-specific compounds. The stable nitrogen isotope ratio (δ¹⁵N) in tree rings is closely related to the environmental conditions of tree growth, and the change of ratio can indicate the dynamic characteristics of nitrogen cycle in forest ecosystem on a long-term scale, and make up for the lack of monitoring data. Herein, we reviewed the theory of the stable nitrogen isotope fractionation and measuring method, and found out the issues that need to be paid attention to in sample pretreatment and measurement. Furthermore, we synthetically stated and evaluated the reports of climate and environment evolution based on δ¹⁵N of tree-rings, and the effects of human activities on nitrogen cycle in forest ecosystem. We suggested the great potential of tree-ring δ¹⁵N in combination of the multiple isotope proxies and its development directions in the future, which should promote further studies on the spatiotemporal characteristics of nitrogen cycle in forest ecosystems.
Keywords：tree ring;stable nitrogen isotope ratio;climate change;nitrogen cycle;forest ecosystem


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本文引用格式
王可逸, 刘晓宏, 曾小敏, 徐国保, 张凌楠, 李春越. 树轮稳定氮同位素记录的进展与展望. 地理学报[J], 2021, 76(5): 1193-1205 doi:10.11821/dlxb202105011
WANG Keyi, LIU Xiaohong, ZENG Xiaomin, XU Guobao, ZHANG Lingnan, LI Chunyue. Stable nitrogen isotope in tree rings: Progresses, problems and prospects. Acta Geographica Sinice[J], 2021, 76(5): 1193-1205 doi:10.11821/dlxb202105011

1 引言

全球气候和环境变化给人类的生活环境和生存方式带来深远影响,导致人类赖以生存的森林生态系统结构、功能和健康发生了显著变化。同时,人类活动造成全球活性氮总量翻倍,增加了大气到森林生态系统的氮沉降。从生态系统尺度来看,氮对植物的供应水平控制着生态系统的生产力^[1],氮循环特征的变化也显著影响和干扰了森林生态系统功能的各个方面^[2,3,4]。在氮为限制性因子的森林生态系统中,氮沉降增加的开始阶段会促进叶面积、光合速率和植物生长量的提高;随着生态系统的发展演化,氮沉降和磷素等会共同影响森林树木生长^[5]。然而,氮沉降对植物氮同位素分馏的影响机理以及在长时间尺度生态系统对氮沉降、气候变化和大气CO₂浓度升高的响应特征也需进一步明确^[5]。尽管氮沉降变化对氮循环很多过程的影响难以准确测定,但植物和土壤稳定氮同位素比率（δ¹⁵N）变化在一定程度上可指示氮循环关键过程的变化特征^[4]。因此,土壤和植物δ¹⁵N长期变化特征可用来研究难以直接测定的氮循环各个过程及其综合影响^[6,7]。

树木年轮以其定年准确连续性强、分辨率高、对气候环境变化敏感及复本量好等特点成为研究过去气候环境变化的首选生物代用资料,在揭示长时间尺度环境演变规律和机理方面发挥了重要作用^[8,9]。保存在树轮中的氮化合物能提供十年到千年时间尺度上的植物氮吸收利用信号,成为陆地生态系统氮循环特征的可靠综合度量指标。树轮中¹⁵N与¹⁴N的相对比值（δ¹⁵N）可以作为生态系统氮循环历史变化的生物代用记录。基于树轮 δ¹⁵N重建森林生态系统氮循环的优势在于：① 以高分辨率覆盖较长的时间尺度,② 揭示良好的地理空间相关信息,③ 反映植物直接吸收氮源的同位素信号变化^[10]。到目前为止,有关树轮稳定氮同位素的研究大多数集中在国外,研究区域广泛分布于北美洲、西欧、东亚、大洋洲、以及赤道附近的热带地区。关注点集中在以下4个方面：基于自然丰度变化研究、肥料施用与示踪研究、污染物排放研究以及海洋氮源追踪研究。其中,对树轮氮同位素自然丰度的研究最为集中。然而,国内基于树轮δ¹⁵N记录的相关研究成果相对较少,处于起步阶段,有必要对树轮δ¹⁵N研究方法和需考虑的问题给予关注,拓展树轮稳定同位素指标的应用范围。本文综述了基于树轮δ¹⁵N进行气候和环境演变方面的相关研究成果及其发展趋势,系统阐述了树轮δ¹⁵N的分馏机理、测试方法及需要注意的问题,并指出未来树轮δ¹⁵N研究需要着重发展的方向。

2 树轮稳定氮同位素研究方法

2.1 氮循环过程中树轮稳定氮同位素分馏

在自然界中,氮原子的稳定同位素有2种能够存在的形式：¹⁴N和¹⁵N,空气中¹⁴N和¹⁵N的相对丰度为99.6337%和0.3663%,通常以大气氮作为同位素测定的标准物质。一般稳定氮同位素的比率用千分偏差δ值来描述,其定义公式如下：

（1）δ=Rsample-RstandardRstandard×1000
式中：R为¹⁵N和¹⁴N的比值。

氮稳定同位素的分馏发生在生态系统氮循环各个过程中（图1）,生态系统不同含氮化合物的氮同位素组成（¹⁵N/¹⁴N）呈现动态变化,可被用来指示生态系统中氮的来源和周转过程。利用氮稳定同位素研究氮循环过程,必须考虑氮循环各个过程中同位素分馏效应（即2种物质间同位素分馏的程度）：通常以2种物质中同位素比率之商,即同位素分馏系数（α_p/s）;以及富集系数（ε_P/S）来表示,以判断分馏过程中产物与底物的同位素比值变化,定义式如下^[11]：

（2）αps=RpRS
（3）εPS=αps-1×1000
式中：R_p为产物的氮同位素比率,R_s为底物的氮同位素比率。ε > 0表示分馏过程使产物同位素比值变高;ε < 0表示分馏过程使产物同位素比值变低,ε 一般写成‰的形式。

图1

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图1氮循环过程中树轮稳定氮同位素相关分馏机理

注：蓝色字体表示分馏过程中,产物氮同位素比值降低;红色字体表示分馏过程中产物氮同位素比值升高;黑色字体表示在分馏过程中,产物氮同位素比值变化不显著。
Fig. 1The fractionation mechanism of δ¹⁵N in tree rings



在氮循环过程中,生物固氮是通过专门的固氮酶来实现的,由于全球整体大气中N₂的δ¹⁵N几乎没有变化^[12],普遍认为自然界中生物固氮过程中氮同位素分馏作用很小^[13]。通过生物固氮进入到土壤中的有机氮,在土壤中可以通过矿化和硝化等作用发生转化。许多研究揭示了根际土壤无机氮δ¹⁵N值（NO₃^-和NH₄⁺）在大多数情况下比非根际土壤的δ¹⁵N值低^[14,15,16],这是由于土壤微生物吸收和排泄氮的过程中,发生的同位素分馏作用造成的。生态系统氮输出主要通过气态形式（如反硝化作用、氨挥发）和液态形式（如淋溶作用、侵蚀）。相比于水文形式的氮输出,气态形式的氮输出会更大程度伴随着氮同位素分馏^[17]。当NH₃挥发,扩散到大气中的过程伴随着很高的分馏系数（17.9‰）,相对于残留在土壤或植物中的NH₄⁺,¹⁵N的消耗量高达40‰^[18]。尽管经过反硝化作用排放到大气中的δ¹⁵N值尚未有精准的测量,但是通过生物化学分馏因子来评估N₂O的同位素效应,结果显示N₂O的δ¹⁵N值通常在-40‰~0之间^[19,20,21],这表明反硝化过程中¹⁵N损耗很大。

植物在吸收利用土壤中的无机氮时,不同组织发生不同程度的分馏,其分馏程度取决于地面根部表层的氮浓度;当土壤氮浓度较高时,分馏作用明显（图1）。其中,矿化作用对氮同位素的分馏作用比较小,仅约2‰左右^{[18, 22-23]},而硝化作用对氮同位素的分馏作用介于-35‰~-5‰之间^[22]。此外,NO₃^–通过硝酸盐还原酶（NR）还原成NO₂^–的过程、亚硝酸盐还原酶（NIR）将NO₂^–还原为NH₄⁺的过程和谷氨酰胺（Gln）合成过程也存在氮同位素的分馏,分馏作用对¹⁵N值的歧视效应（即优先吸收¹⁴N的分馏机制）在 15‰~17‰之间变化^[24],这些分馏作用使未被吸收的NO₃^-相比植物有机氮更为富集¹⁵N ^[25]。

叶片可以通过气孔吸收大气中的可溶性无机氮。气态氮化合物（NO_x、NH_x）通过气孔进入到植物中,被转运到细胞内部,以蛋白质、氨基酸、酶等有机氮形式在植物内部迁移（图1）。Lindberg等认为干沉降是大气沉降氮输入树冠的主要形式,大气沉降中50%~70%的氮被树木冠层吸收;同时,叶片也会吸收被雨水捕获或沉降到叶表面的湿沉降氮源^[26]。叶片在吸收干湿氮沉降物的过程中会发生分馏效应,通过气孔或叶表皮直接从大气中吸收气态NH₃或湿沉降中的NH₄⁺会降低叶片氮同位素组成,而分馏的程度与氮源类型直接关联,比如针叶δ¹⁵N值偏高的直接原因是叶片气孔吸收较高δ¹⁵N值的NO₂^{[10, 27]}。

此外,植物的菌根真菌对植物δ¹⁵N值也有显著影响。与非菌根植物相比,杜鹃类菌根真菌植物和外生菌根（EM）植物叶片δ¹⁵N的消耗量更大（分别为3.2‰和5.9‰）。丛枝菌根（AM）植物的同位素值处于中等水平,与非菌根植物相比平均耗竭2‰^[10]。

虽然分馏作用引起δ¹⁵N值的变化会因发生器官（如根、茎、叶等）而不同,也会受物种、环境和氮源的影响^[28],但树木年轮对自植物根部、冠层进入到树木内部进行循环的总有机氮均能有效记录,可作为呈现植物内部与外部氮循环总体长期变化的一个综合指标（图1）,对区域尺度的森林生态系统氮循环开放程度亦有一定指示作用。

2.2 树轮稳定氮同位素比率测定方法

随着稳定同位素分析技术的进步,基于连续流分析的稳定同位素质谱仪实现了无机化合物中多种元素（C、H、O、N）的自动分析,获取可靠δ¹⁵N值所需的样品氮总量降低到约20~25 μg。树轮δ¹⁵N测定一般采用固体全木样品,借助元素分析仪—气体稳定同位素比质谱联用（EA-IRMS）系统来进行测量,不仅可以将样品自动分离与纯化,缩短样品分析时间,而且精度也接近传统的双路进样同位素质谱仪。在木材样品中氮含量较低,一般为0.05%~0.3%之间。根据氮含量的变化范围,测定自然丰度的树轮δ¹⁵N,需要更多的样品来达到气体稳定同位素比质谱仪对氮同位素比率检测信号的要求,一般所需木材干重的样品量约为10~30 mg。根据不同树种的木质特征,可以对树轮样品采取化学前处理或者不进行化学处理的方式,测定其原木样品的氮同位素比值（图2）。

图2

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图2树轮稳定氮同位素比例测定样品处理流程图

Fig. 2Flow-chart of pre-treatment of tree-ring material for stable nitrogen isotope analysis



在样品制备阶段,将剥离混合好的样品进行干燥、粉碎和研磨,使其均质化,有助于降低大样品量导致的不完全燃烧风险。其次,通过固体自动进样器将样品送入元素分析仪,样品中含氮物质依次经过元素分析仪的氧化炉和还原炉反应生成N₂,生成的N₂ 通过万用接口进入到气体同位素比质谱仪进行测定,得到¹⁵N/¹⁴N比值。此外,大量样品燃烧产生的其他含碳产物（例如CO、CO₂等）需要在进入质谱仪前被捕获并完全去除。一般采用大容量的CO₂捕捉收集管进行,以避免频繁更换捕捉剂对质谱本底值产生影响。

在测得树轮δ¹⁵N之后,需要用一个普遍接受的有机氮同位素分析的木材标准进行校准。美国地质调查局（USGS）制备了3种粉末状的全木参考标准：来源于美国黑松（Pinus contorta）的USGS 54（(-2.42±0.32)‰）、墨西哥破布木（Cordia cf.dodecandra）的USGS 55（(-0.27±0.36)‰）和南非红象牙木（Berchemia cf.zeyheri）的USGS 56 （(1.85±0.37)‰）^[29,30]。由于3种标准材料制备量有限,且价格较高,无法满足长期大量的校准工作,有必要在实验室制备一定量的木材实验室工作标准,并通过通用标准的校准使测得的树轮δ¹⁵N值与其他序列得到真正的比较。树轮δ¹⁵N样品准备和测试流程见图2。

2.3 树轮稳定氮同位素比率测定问题考虑

2.3.1 不同区域树种木质影响 树轮稳定氮同位素相关文章涉及到硬质木与软质木。有研究表明,单一木质特征树种的生长生理特征会对森林氮循环有个体影响^[31]。因此,在树轮样芯采集时,应尽可能对林分内混合分布的多个木质特征的树种进行采样,避免在进行氮循环空间尺度分析时,单一树种个体生理趋势的影响。对不同木质特征的多个树种在洲际尺度上进行研究,其树轮稳定氮同位素比值所呈现的长期变化趋势整体一致,但区域样点间存在差异。北美洲地区研究主要集中于美国和加拿大的温带森林中,树木质地多为软木,多个树种树轮δ¹⁵N在该区域内呈现下降趋势,指示该区域较为封闭的氮循环特征^{[31,32,33,34,35,36]}（图3a）;东亚地区的相关研究也皆为软木,但是不同树种在不同站点的树轮 δ¹⁵N变化趋势差异较大^{[7, 37-40]},在一定程度上可能反映样点局地环境与人为活动的影响（图3b）;欧洲地区的相关研究集中分布在地中海沿岸,包含软木与硬木,两种木质类型的树种树轮δ¹⁵N均表现出下降趋势^[41,42,43]（图3c）;热带地区广泛研究了赤道附近分布的硬木树种,涉及北半球与南半球的8个树种,绝大多数树轮δ¹⁵N呈现为上升趋势^[44,45,46]（图3d）,表明热带地区相对其他研究区域,氮循环系统更为开放。此外,在热带森林中,由于异养土壤微生物和豆科植物根瘤菌的高固氮率,天然氮的有效性比大多数温带森林要高,将豆科植物作为树轮δ¹⁵N的研究对象,有助于理解热带地区氮循环特征的变化^[45]。不同区域的树种和木质特征存在差异,含氮化合物尤其是抽提物含量上存在差异,在验证树轮δ¹⁵N对空间氮循环的指示作用时,应针对不同树种的木材特征,采取不同的样品前处理方式或进行对比分析。

图3

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图3全球多树种树轮稳定氮同位素比率变化趋势比较

注：a图数据来源[32, 50]、b图数据来源[7, 37-40]、c图数据来源[42-43]、d图数据来源[44-46];图中虚线表示硬质木,实线表示软质木。
Fig. 3Comparison in the regional trends of global tree-ring δ¹⁵N investigations



2.3.2 氮在年轮中的径向迁移 研究表明,年轮氮含量相对较低,从心材到边材树轮氮含量都较为恒定,在最新形成的年轮中达到高值^[34]。也有研究表明,树轮氮含量在边材和心材的交界处有明显的上升趋势^[46],这种变化在很多类型的森林生态系统研究中均有报道。针对多样化的研究结果,****们从不同角度做出解释,但较为普遍的认知是：树轮中细胞壁的碳基结构上所包含的结构性蛋白质,是树轮氮的主要研究对象^[47]。氮含量较为恒定的趋势是树木生理迁移过程的结果,不会受到样品处理方法和生态系统氮有效性的影响。因此,认为树轮氮含量记录的是在树木成熟过程中有机氮的再分配利用,作为环境中氮可利用性的代用指标时具有局限性^[48]。

然而,研究发现树轮心材和边材边界处的δ¹⁵N并没有明显的变化^{[27, 40]},也不存在明显分馏作用^[48]。主要由于树轮稳定氮同位素记录了生长环境中氮源的变化,不会直接记录生理迁移作用,是环境氮有效性变化的可信指标^[49,50,51]。为更好的反映树轮结构性氮同位素的记录作用,排除非结构性氮的影响,建议利用树轮稳定氮同位素探索环境影响的长期作用时,需判定高氮含量的非结构性潜在影响,若存在一定影响则需移除非结构性氮^[39]。此外,为验证不同氮含量树种的迁移作用,建议将样芯分为心材、边材、心边材交界3个部分,并将每部分的年轮细分为更高分辨率的季节年轮来验证其科学性。

2.3.3 样品混合与化学前处理 对生长在高海拔或高纬地区的年轮相对较窄的树木来说,在树轮δ¹⁵N测定时需要采取多样芯混合的方式来获取足够的样本量进行δ¹⁵N分析。常见的混合方式有3种：① 将同一树木不同方向相同生长年份的样品进行混合^[50]。由于受到树种个体生理特征以及树木微生境等的影响,这种混合方式对于研究区域环境氮循环代表性较弱。② 根据具体实验目的,将同一树轮样芯的多个相邻年轮进行样品混合^{[34, 48, 51]}。此方式混合既不利于对氮径向迁移的研究,也掩盖了树轮作为氮循环综合指标的高分辨率（年）优势。③ 将同一树种多棵树木相同生长年份的样品进行混合^[52,53,54]。不同混合方式对获取的树轮δ¹⁵N序列的环境代表性和时间分辨率的影响不同,在对年轮δ¹⁵N时空变化进行分析时需要考虑潜在偏差。研究森林生态系统氮循环长期动态变化,高分辨率的δ¹⁵N不是必需的,但是年分辨率的δ¹⁵N可以在一定程度解译高频气候变化或者其他干扰事件的影响^[52]。因此,建议在条件允许时,尽量将相同采样点同一树种的年轮样品按高分辨率进行混合。

树轮中不存在易于识别和纯化的含氮化合物,也尚未有专门的研究测试木材中哪些氮化合物可以从树轮中去除或保留。研究表明,储存在结构化合物（细胞壁中的纤维素和木质素）上的蛋白质中的氮将被保留,而非结构化合物（单宁、多酚、脂肪、蜡、树脂,生物碱）通过萃取去除^[53,54]。对于树轮δ¹⁵N研究来说,仅结构性氮代表生态系统的氮有效性水平,但会受到不稳定的迁移氮的潜在影响。因此,移除年轮样品中不稳定性氮后得到的年轮δ¹⁵N序列可以真实反映森林生态系统氮有效性的变化历史^[54]。Sheppard等最早提出的树轮δ¹⁵N分析前处理方法可有效去除可迁移的不稳定氮的影响,保留结构性氮,但该处理流程大大降低了样品中氮含量水平,需要更多的样品来满足仪器检测需要的氮同位素信号强度^[53]。进一步的研究对Sheppard等提出方法进行修正和简化来提高样品处理效率^[39]。改进的前处理方法表明在氮输入较高的生态系统,对不稳定的迁移性氮的去除是必须的。也有研究表明,对树脂含量高的树种也需要进行前处理移除树脂的影响^[48]。因此,对于不同氮水平的生态系统来说,受树种影响,边材和心材之间的氮迁移结果存在差异,需要采取不同的前处理方法^[31]。在未来研究中,需要根据研究树种、样点环境和研究目的来确定是否和如何对样品进行前处理。建议在不明确化学处理是否可以去除生理驱动模式时,对树木年轮样品不进行化学处理,或者是用极性较大的去离子水做简单去除。

3 树轮稳定氮同位素记录研究进展

3.1 树轮δ¹⁵N与气候变化

随着植物和土壤δ¹⁵N数据的积累,对δ¹⁵N变化模式的解释随着时间推移而发生了变化,使得人们对氮循环参数如何沿气候梯度变化产生了一些困惑。Handley等首次尝试广泛地探究了气候与植物δ¹⁵N之间的综合关系,发现97个站点的叶片δ¹⁵N随降水的增加呈线性下降;虽然他们发现纬度对叶面δ¹⁵N没有影响,但是热带森林叶片平均δ¹⁵N比温带叶片平均高6.5‰^[55]。有****认为这是因为相对于其他生态系统的氮循环,热带森林的氮循环更开放,氮的输入和输出都很大^[56];也有****认为树轮δ¹⁵N在热带森林系统有更高的值是因为湿热的环境下,氮沉降和氮损失量更大^[44]。这些研究将植物δ¹⁵N的升高与湿热的气候条件建立起良好关系。

尽管如此,气候变量对树轮δ¹⁵N值的具体影响尚不一致或不明确^[57,58]。研究表明,生长季降水和春夏季温度对年轮δ¹⁵N的影响作用是相反的^[45]。也有研究表明,树轮δ¹⁵N与生长季降水量呈负或正相关,与生长季温度也呈负相关,但这种相关关系并未存在于所有研究样点^{[52, 59]},这意味着气温、降水等气候因素对树轮δ¹⁵N的影响并未形成明晰且一致的认识,可能主要是通过影响土壤氮矿化过程和植物可利用氮水平来改变树轮中的δ¹⁵N 信号^[55,56]。此外,在一些区域,树轮δ¹⁵N与研究区域的土壤生产力显著相关,表明研究地点生境特性对树轮δ¹⁵N的驱动力大于气候作用。

普遍认为,气候变化或升高的大气CO₂浓度会通过影响菌根活动和改变土壤氮循环等改变植物δ¹⁵N值^[59],气候变化导致的土壤温度和pH值、菌根活动、土壤氮转化（氨化、硝化）相对速率的变化共同影响了树轮δ¹⁵N值。但是,气候变量仅是通过影响氮循环分馏过程来影响平均值,还是会影响年轮δ¹⁵N的时间变化趋势,有待进一步验证^[59]。要阐明哪些气候变量显著影响年轮δ¹⁵N,其他因素如何交互影响这些关系,尚需进一步研究。对以上问题的回答对理解长时间尺度上气候变化如何影响森林生态系统氮循环特征及制定科学森林管理政策至关重要。

3.2 树轮δ¹⁵N与人类活动氮排放

20世纪树轮δ¹⁵N长时间变化趋势与人类活动有关。人为活动显著影响了森林生态系统的氮循环过程。在全球范围内,人为氮沉降量等于自然氮固定率^[60],但是在区域范围内,氮沉降总量差异很大,导致区域氮污染物的δ¹⁵N值变化也很大,变化范围约达20‰。不同的氮源产生的氮氧化物δ¹⁵N值变化范围不同,车辆废气排放物的δ¹⁵N在-13‰~-2‰之间,燃煤锅炉排放物的在6‰~13‰之间^[61],农业生产活动产生的硝态氮化合物排放物在-5‰~10‰之间^[62]。

车辆尾气中氮排放量增加会增加δ¹⁵N排放量,从而导致树轮δ¹⁵N的升高^[63,64],而树轮δ¹⁵N的降低也是由于排气中氮氧化物具有较低的δ¹⁵N导致^[31,32]。有研究提供了分析时间段内研究区域直接排放氮氧化物的δ¹⁵N^[63],以说明区域氮氧化物排放水平与树轮δ¹⁵N的直接关系。Savard等^[32]和Doucet等^[31]的研究给出了区域氮氧化物排放量,但没有氮沉积物的δ¹⁵N值,缺乏直接的氮沉降同位素特征以区别于生物地球化学系统内部循环特征^[64],也缺少区域NO_x排放或氮沉积数据来验证树轮δ¹⁵N的直接来源。此外,将靠近交通要道的树轮δ¹⁵N值与增加的汽车数量建立相关性,可作为树轮δ¹⁵N指示车辆排放污染物的证据,但仍然需要通过获取汽车尾气NO_x排放的同位素信号值加以验证^[31]。尽管这些研究都可能准确地报告树轮δ¹⁵N记录的污染效应,但仅Saurer等^[64]提供了污染物的δ¹⁵N数据,明确支持他们的结论。为了将树轮δ¹⁵N变化趋势归因于特定外部污染源的影响,有必要获取关于当地污染源δ¹⁵N值的变化数据。仅仅对排放源或当地大气浓度的一般分析不足以得出污染沉积对树轮δ¹⁵N趋势的直接影响^[10]。值得注意的是,虽然大气氮源和树轮δ¹⁵N的详细测量可能无法准确反映整个树轮生长期间的大气排放趋势^[34],但是在近期的短时间尺度上,树轮δ¹⁵N的时间序列和人为排放的氮沉降之间存在潜在关系。

农业生产活动是另一种重要的人为干扰生态系统氮循环的模式。在农业活动中,长期施用氮肥,使土壤中氮素含量较高,植物从土壤中吸收和利用铵态氮和硝态氮,使得植物体内的含氮化合物增加,从而影响植物（树轮）δ¹⁵N。在中国森林地区通过示踪的方式进行同位素研究,发现δ¹⁵N有记录农业肥料施用的能力^[65]。也有研究通过施用大量氮肥,量化生态系统中不同氮库之间的氮转移,并增强我们对可利用氮较高的生态系统中氮循环过程的理解。通过肥料示踪的方式模拟高氮肥输入的农业活动,有助于解释树木吸收的无机氮是如何沉积在木材组织中的,特别是含氮化合物跨年轮边界的径向迁移的去向与分配^[66]。氮在年轮之间的显著迁移可能掩盖或消除环境有效性氮的年际信号,因此氮在年轮中的迁移是所有树轮δ¹⁵N研究的一个关键问题。研究表明,在进行高δ¹⁵N标记的氮施加时,尽管该峰值在施肥处理后可能会延迟1~2年^[67],但与氮添加年限相对应的树轮中通常会显示出一个清晰而显著的峰值^[54]。这表明树轮δ¹⁵N值的变化并不直接来源于高δ¹⁵N标记的氮施加,而是通过吸收利用土壤有效氮,并在氮进入植物的过程中发生分馏。

通过示踪的方式测定树轮δ¹⁵N值有利于了解氮循环中分馏过程对树轮δ¹⁵N的影响机制,但是会掩盖树轮在百年尺度甚至更长尺度上对于农业活动影响氮循环的记录。中国是历史悠久、人口众多的农业大国,国内树轮氮同位素研究可以结合自然丰度法与示踪方式,多维度地了解生态系统氮循环过程的变化。

4 问题和展望

树轮稳定氮同位素比率在反映多时间尺度生态系统氮循环信息方面具有巨大潜力。有助于评估全球或区域环境变化,特别是不同人类活动强度影响下森林生态系统的氮循环变化。现有研究主要集中于报道树轮δ¹⁵N对气候变化、污染物排放的记录作用,但在跨区域研究中如何对比树轮δ¹⁵N的指示效应、深化耦合多个树轮同位素指标等问题还有待进一步挖掘。此外,优化和综合利用多树种δ¹⁵N指标,将树轮δ¹⁵N应用于火灾、间伐等长时间尺度森林动态研究中,有利于理解干扰事件对森林生态系统氮循环的影响。在未来研究中,应从以下几个方面着手解决与加强。

4.1 加强不同树种氮同位素样品的前处理方法研究

目前,不同木质特征树种的氮同位素特征研究广泛存在,对于不同树芯样品的前处理方式也进行了多种尝试,但是仍未得出一致或统一的样品处理方式与结论。问题在于① 受实验测试仪器限制,样品处理的混合模式易掩盖树轮年际与个体生理特征;② 对树轮中非结构性含氮化合物认知的缺乏;③ 对相邻树轮之间迁移和不同组织之间氮同位素分馏的具体机制了解较少。因此,针对不同树种和不同研究目的与要求,应加强对树轮非结构氮化合物的了解,探究氮同位素记录在植物组织之间的分馏与内部径向迁移机制。基于树种木材组成成分特征,采取相对应的样品混合与化学前处理方法。

4.2 深化多稳定同位素指标耦合的探究

树轮δ¹³C和δ¹⁸O已用来评估树木生长对CO₂浓度变化、气候因子变化的响应。不同类型树轮的δ¹³C、δ¹⁸O和δD反映的生理与生态环境信息也不同^[68,69,70]。考虑到不同指标记录气候环境信息的互补性,在树轮稳定同位素气候和环境研究中,综合多种同位素代用指标已成为一种趋势^{[68, 71-73]}。因此,需结合多个树轮稳定同位素代用指标,相互补充,获得更丰富的、详尽的生物地球化学循环变化资料,提高评估全球变化对森林树木生长、碳氮耦合循环以及污染物来源的准确性（图4）。

图4

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图4树轮多指标同位素耦合分析框架图

Fig. 4The concept diagram of coupling of multiple stable isotope proxies in tree rings

4.3 拓展多来源氮同位素的对比研究

全球树轮稳定氮同位素的研究集中分布在具有独特特征的森林生态系统,代表着特定区域氮循环的变化,借助大气排放的氮同位素值、土壤沉积的氮同位素比率、河流海洋传输的氮同位素比率等其他指标,也可以直接验证树轮稳定氮同位素记录信息的直接性与准确性。

中国****对于氮同位素的研究已经深入到各个圈层的多个载体中,比如大气、降水、植物叶片、土壤、海洋沉积物、冰芯以及地下水等^{[63, 74-78]},这类研究主要集中于中国东部以及华北等人类活动较频繁的地区。这些研究丰富了氮同位素记录的氮循环相关变化信息,反映了中国的氮循环系统相对较为开放;然而利用树轮稳定氮同位素来探究区域氮循环过程的研究较少。加强区域不同载体氮同位素信息的对比,可以从不同角度剖析全球变化和人类活动对氮可利用性的影响机制和过程。

参考文献 View Option 原文顺序
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[1] Thomas R Q, Zaehle S, Templer P H, et al. Global patterns of nitrogen limitation: Confronting two global biogeochemical models with observations
Global Change Biology, 2013,19(10):2986-2998.
DOI:10.1111/gcb.12281PMID:23744637 [本文引用: 1]
Projections of future changes in land carbon (C) storage using biogeochemical models depend on accurately modeling the interactions between the C and nitrogen (N) cycles. Here, we present a framework for analyzing N limitation in global biogeochemical models to explore how C-N interactions of current models compare to field observations, identify the processes causing model divergence, and identify future observation and experiment needs. We used a set of N-fertilization simulations from two global biogeochemical models (CLM-CN and O-CN) that use different approaches to modeling C-N interactions. On the global scale, net primary productivity (NPP) in the CLM-CN model was substantially more responsive to N fertilization than in the O-CN model. The most striking difference between the two models occurred for humid tropical forests, where the CLM-CN simulated a 62% increase in NPP at high N addition levels (30 g N m(-2) yr(-1)), while the O-CN predicted a 2% decrease in NPP due to N fertilization increasing plant respiration more than photosynthesis. Across 35 temperate and boreal forest sites with field N-fertilization experiments, we show that the CLM-CN simulated a 46% increase in aboveground NPP in response to N, which exceeded the observed increase of 25%. In contrast, the O-CN only simulated a 6% increase in aboveground NPP at the N-fertilization sites. Despite the small response of NPP to N fertilization, the O-CN model accurately simulated ecosystem retention of N and the fate of added N to vegetation when compared to empirical (15) N tracer application studies. In contrast, the CLM-CN predicted lower total ecosystem N retention and partitioned more losses to volatilization than estimated from observed N budgets of small catchments. These results point to the need for model improvements in both models in order to enhance the accuracy with which global C-N cycle feedbacks are simulated.? 2013 John Wiley & Sons Ltd.

[2] Gao Yang, Yu Guirui. Biogeochemical cycle and its hydrological coupling processes and associative controlling mechanism in a watershed
Acta Geographica Sinica, 2018,73(7):1381-1393.
[本文引用: 1]

[ 高扬, 于贵瑞. 流域生物地球化学循环与水文耦合过程及其调控机制
地理学报, 2018,73(7):1381-1393.]
[本文引用: 1]

[3] Vitousek P M, Menge D N L, Reed S C, et al. Biological nitrogen fixation: Rates, patterns and ecological controls in terrestrial ecosystems
Philosophical Transactions of the Royal Society B: Biological Sciences, 2013,368(1621):20130119. DOI: http://www.geog.com.cn/article/2021/0375-5444/10.1098/rstb.2013.0119.
DOI:http://www.geog.com.cn/article/2021/0375-5444/10.1098/rstb.2013.0119URL [本文引用: 1]

[4] Craine J M, Elmore A J, Wang L, et al. Isotopic evidence for oligotrophication of terrestrial ecosystems
Nature Ecology & Evolution, 2018,2(11):1735-1744.
[本文引用: 2]

[5] Craine J M, Elmore A J, Wang L, et al. Convergence of soil nitrogen isotopes across global climate gradients
Scientific Reports, 2015,5(1):1-8. DOI: http://www.geog.com.cn/article/2021/0375-5444/10.1038/srep08280.
URL [本文引用: 2]

[6] Craine J M, Elmore A J, Aidar M P M , et al. Global patterns of foliar nitrogen isotopes and their relationships with climate, mycorrhizal fungi, foliar nutrient concentrations, and nitrogen availability
New Phytologist, 2009,183(4):980-992.
DOI:10.1111/nph.2009.183.issue-4URL [本文引用: 1]

[7] Choi W J, Lee S M, et al. Variations of δ¹³C and δ¹⁵N in Pinus densiflora tree-rings and their relationship to environmental changes in eastern Korea
Water Air and Soil Pollution, 2005,164(1):173-187.
DOI:10.1007/s11270-005-2253-yURL [本文引用: 3]

[8] Treydte K S, Schleser G H, Helle G, et al. The twentieth century was the wettest period in northern Pakistan over the past millennium
Nature, 2006,440(7088):1179-1182.
DOI:10.1038/nature04743URL [本文引用: 1]

[9] Shao X, Xu Y, Yin Z Y, et al. Climatic implications of a 3585-year tree-ring width chronology from the northeastern Qinghai-Tibetan Plateau
Quaternary Science Reviews, 2010,29(17/18):2111-2122.
DOI:10.1016/j.quascirev.2010.05.005URL [本文引用: 1]

[10] Gerhart L M, McLauchlan K K . Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood
Biogeochemistry, 2014,120(1):1-21. DOI: http://www.geog.com.cn/article/2021/0375-5444/10.1007/s10533-014-9988-8.
DOI:http://www.geog.com.cn/article/2021/0375-5444/10.1007/s10533-014-9988-8URL [本文引用: 4]

[11] Carol K. Chapter 16: Tracing nitrogen sources and cycling in catchments//Kendall C, Mcdonnell J J (eds.). Isotope Tracers in Catchment Hydrology
Amsterdam: Elsevier, 1998: 519-576.
[本文引用: 1]

[12] Mariotti A, Mariotti F, Champigny M. L, et al. Nitrogen isotope fractionation associated with nitrate reductase activity and uptake of NO₃- by pearl millet
Plant Physiology, 1982,69(4):880-884.
DOI:10.1104/pp.69.4.880URL [本文引用: 1]

[13] Hobbie E A, H?gberg P. Nitrogen isotopes link mycorrhizal fungi and plants to nitrogen dynamics
New phytologist, 2012,196(2):367-382.
DOI:10.1111/nph.2012.196.issue-2URL [本文引用: 1]

[14] Garten C T. Nitrogen isotope composition of ammonium and nitrate in bulk precipitation and forest throughfall
International Journal of Environmental Analytical Chemistry, 1992,47(1):33-45.
DOI:10.1080/03067319208027017URL [本文引用: 1]

[15] Binkley D, Sollins P, Mcgill W B, et al. Natural abundance of nitrogen-15 as a tool for tracing alder-fixed nitrogen
Soil Science Society of America Journal, 1985,49(2):444-447.
DOI:10.2136/sssaj1985.03615995004900020034xURL [本文引用: 1]

[16] Koba K, Isobe K, Takebayashi Y, et al. Delta¹⁵N of soil N and plants in a N-saturated, subtropical forest of southern China
Rapid Communications in Mass Spectrometry, 2010,24(17):2499-2506.
DOI:10.1002/rcm.4648PMID:20740523 [本文引用: 1]
We investigated the delta(15)N profile of N (extractable NH(4)(+), NO(3)(-), and organic N (EON)) in the soil of a N-saturated subtropical forest. The order of delta(15)N in the soil was EON > NH(4)(+) > NO(3)(-). Although the delta(15)N of EON had been expected to be similar to that of bulk soil N, it was higher than that of bulk soil N by 5 per thousand. The difference in delta(15)N between bulk soil N and EON (Delta(15)N(bulk-EON)) was correlated significantly with the soil C/N ratio. This correlation implies that carbon availability, which determines the balance between N assimilation and dissimilation of soil microbes, is responsible for the high delta(15)N of EON, as in the case of soil microbial biomass delta(15)N. A thorough delta(15)N survey of available N (NH(4)(+), NO(3)(-), and EON) in the soil profiles from the organic layer to 100 cm depth revealed that the delta(15)N of the available N forms did not fully overlap with the delta(15)N of plants. This mismatch in delta(15)N between that of available N and that of plants reflects apparent isotopic fractionation during N uptake by plants, emphasizing the high N availability in this N-saturated forest.Copyright 2010 John Wiley & Sons, Ltd.

[17] Houlton B Z, Bai E. Imprint of denitrifying bacteria on the global terrestrial biosphere
PNAS, 2009,106(51):21713-21716.
DOI:10.1073/pnas.0912111106URL [本文引用: 1]

[18] H?gberg P. Tansley Review No.95:¹⁵N natural abundance in soil-plant systems
New Phytologist, 1997,137(2):179-203.
DOI:10.1046/j.1469-8137.1997.00808.xURL [本文引用: 2]

[19] Xiong Z Q, Khalil M A K, Xing G, et al. Isotopic signatures and concentration profiles of nitrous oxide in a rice-based ecosystem during the drained crop-growing season
Journal of Geophysical Research Biogeosciences, 2009,114(G2):G02012. DOI: http://www.geog.com.cn/article/2021/0375-5444/10.1029/2008jg000827.
URL [本文引用: 1]

[20] P?rtl K, Zechmeister-Boltenstern S, Wanek W, et al. Natural¹⁵N abundance of soil N pools and N₂O reflect the nitrogen dynamics of forest soils
Plant and Soil, 2007,295(1):79-94.
DOI:10.1007/s11104-007-9264-yURL [本文引用: 1]

[21] van Groenigen J W, Zwart K B, Harris D, et al. Vertical gradients of δ¹⁵N and δ¹⁸O in soil atmospheric N₂O: Temporal dynamics in a sandy soil
Rapid Communications in Mass Spectrometry, 2005,19(10):1289-1295.
DOI:10.1002/(ISSN)1097-0231URL [本文引用: 1]

[22] Nikolenko O, Jurado A, Borges A V, et al. Isotopic composition of nitrogen species in groundwater under agricultural areas: A review
Science of the Total Environment, 2018,621:1415-1432.
DOI:10.1016/j.scitotenv.2017.10.086URL [本文引用: 2]

[23] M?bius J. Isotope fractionation during nitrogen remineralization (ammonification): Implications for nitrogen isotope biogeochemistry
Geochimica et Cosmochimica Acta, 2013,105:422-432.
DOI:10.1016/j.gca.2012.11.048URL [本文引用: 1]

[24] Needoba J A, Sigman D M, Harrison P J. The mechanism of isotope fractionation during algal nitrate assimilation as illuminated by the ¹⁵N/¹⁴N of intracellular nitrate
Journal of Phycology, 2004,40(3):517-522.
DOI:10.1111/jpy.2004.40.issue-3URL [本文引用: 1]

[25] Yoneyama T, Fujihara S, Yagi K. Natural abundance of ¹⁵N in amino acids and polyamines from leguminous nodules: Unique ¹⁵N enrichment in homospermidine
Journal of Experimental Botany, 1998,49(320):521-526.
DOI:10.1093/jxb/49.320.521URL [本文引用: 1]

[26] Lindberg S E, Lovett G M, Richter D D, et al. Atmospheric deposition and canopy interactions of major ions in a forest
Science, 1986,231(4734):141-145.
DOI:10.1126/science.231.4734.141URL [本文引用: 1]

[27] Elmore A J, Craine J M, Nelson D M, et al. Continental scale variability of foliar nitrogen and carbon isotopes in Populus balsamifera and their relationships with climate
Scientific Reports, 2017,7(1):7759. DOI: http://www.geog.com.cn/article/2021/0375-5444/10.1038/s41598-017-08156-x.
DOI:http://www.geog.com.cn/article/2021/0375-5444/10.1038/s41598-017-08156-xURL [本文引用: 2]

[28] Robinson D, Handley L, Scrimgeour C. A theory for ¹⁵N/¹⁴N fractionation in nitrate-grown vascular plants
Planta, 1998,205(3):397-406.
DOI:10.1007/s004250050336URL [本文引用: 1]

[29] Qi H P, Coplen T B, Jordan J A, et al. Three whole-wood isotopic reference materials, USGS54, USGS55, and USGS56, for δ²H, δ¹⁸O, δ¹³C, and δ¹⁵N measurements
Chemical Geology, 2016,442:47-53.
DOI:10.1016/j.chemgeo.2016.07.017URL [本文引用: 1]

[30] Doucet A, Savard M M, Bégin C, et al. Tree-ring δ¹⁵N values to infer air quality changes at regional scale
Chemical Geology, 2012,320/321:9-16.
DOI:10.1016/j.chemgeo.2012.05.011URL [本文引用: 1]

[31] Lovett G M, Weathers K C, Arthur M A, et al. Nitrogen cycling in a northern hardwood forest: Do species matter?
Biogeochemistry, 2004,67(3):289-308.
DOI:10.1023/B:BIOG.0000015786.65466.f5URL [本文引用: 6]

[32] Savard M M, Begin C, Smirnoff A, et al. Tree-ring nitrogen isotopes reflect anthropogenic NO_X emissions and climatic effects
Environmental Science & Technology, 2009,43(3):604-609.
DOI:10.1021/es802437kURL [本文引用: 4]

[33] Mclauchlan K K, Craine J M. Species-specific trajectories of nitrogen isotopes in Indiana hardwood forests, USA
Biogeosciences, 2011,9(2):867-874.
DOI:10.5194/bg-9-867-2012URL [本文引用: 1]

[34] Bukata A R, Kyser T K. Response of the nitrogen isotopic composition of tree-rings following tree-clearing and land-use change
Environmental Science & Technology, 2005,39(20):7777-7783.
DOI:10.1021/es050733pURL [本文引用: 4]

[35] H?rdtle W, Niemeyer T, Assmann T, et al. Long-term trends in tree-ring width and isotope signatures (δ¹³C, δ¹⁵N) of Fagus sylvatics L. on soils with contrasting water supply
Ecosystems, 2013,16:1413-1428.
DOI:10.1007/s10021-013-9692-xURL [本文引用: 1]

[36] Stock W D, Bourke L, Froend R H. Dendroecological indicators of historical responses of pines to water and nutrient availability on a superficial aquifer in south-western Australia
Forest Ecology and Management, 2012,264:108-114.
DOI:10.1016/j.foreco.2011.09.033URL [本文引用: 1]

[37] Larry L C M, Chitoshi M, Toshiro Y, et al. Temporal changes in tree-ring nitrogen of Pinus thunbergii trees exposed to Black-tailed Gull (Larus crassirostris) breeding colonies
Applied Geochemistry, 2010,25(11):1699-1702.
DOI:10.1016/j.apgeochem.2010.08.017URL [本文引用: 2]

[38] Sun F, Kuang Y, Wen D, et al. Long-term tree growth rate, water use efficiency, and tree ring nitrogen isotope composition of Pinus massoniana L. in response to global climate change and local nitrogen deposition in Southern China
Journal of Soils & Sediments, 2010,10(8):1453-1465.


[39] Caceres M L L, Mizota C, Yamanaka T, et al. Effects of pre-treatment on the nitrogen isotope composition of Japanese black pine (Pinus thunbergii) tree-rings as affected by high N input
Rapid Communications in Mass Spectrometry, 2011,25(21):3298-3302.
DOI:10.1002/rcm.5227PMID:22006393 [本文引用: 2]
Temporal changes in the acquisition of nitrogen (N) are recorded in tree-rings together with unique N isotopic values. Some debate continues regarding the importance of wood pre-treatment in isotope analysis and, thus, this study focuses on the removal of labile components to determine the intrinsic nature of N in tree-rings. The total concentration and stable isotopic value of N in annual tree-rings were determined for two cores from Japanese black pine (Pinus thunbergii) from areas colonized by black cormorant (Phalacrocorax carbo). One core sample was also collected from a control site, without cormorants. Sharp increases in tree-ring δ(15)N values associated with migration of the cormorant population indicate positive incorporation of N from soils, whereas a less pronounced trend was observed for ring samples for periods without or substantially less migration, and for those obtained from the control site. All labile N components were removed by repeated extraction with toluene/ethanol (1:1) solution. Radial translocation of labile N is limited in tree-rings from Japanese black pine, providing intrinsic records on N acquisition. The difference in N isotopic values (up to 7.0‰) following pre-treatment was statistically significant for trees affected by the avian colony, whereas the pre-treatment of the control samples did not influence N values. The implication is that in agreement with previous studies pre-treatment is not necessary when trees are exposed to natural N concentrations in the soil but the removal of enriched δ(15)N labile components is necessary when woody plants are exposed to unusually high inputs of N into soils. However, the temporal trend in tree-ring δ(15)N series of the avian N affected trees did not change. Thus, if the priority is not the value but the trend then pre-treatment is not necessary.Copyright ? 2011 John Wiley & Sons, Ltd.

[40] Wang K Y, Zeng X M, Liu X H, et al. Nitrogen rather than streamflow regulates the growth of riparian trees
Chemical Geology, 2020,547:119666. DOI: http://www.geog.com.cn/article/2021/0375-5444/10.1016/j.chemgeo.2020.119666.
DOI:http://www.geog.com.cn/article/2021/0375-5444/10.1016/j.chemgeo.2020.119666URL [本文引用: 3]

[41] Beghin R, Cherubini P, Battipaglia G, et al. Tree-ring growth and stable isotopes (δ¹³C and δ¹⁵N) detect effects of wildfires on tree physiological processes in Pinus sylvestris L
Trees, 2011,25(4):627-636.
DOI:10.1007/s00468-011-0539-9URL [本文引用: 1]

[42] Elhani S, Guehl J M, Nys C, et al. Impact of fertilization on tree-ring δ¹⁵N and δ¹³C in beech stands: A retrospective analysis
Tree Physiology, 2005,25(11):1437-1446.
DOI:10.1093/treephys/25.11.1437URL [本文引用: 2]

[43] Couto-Vázquez A, González-Prieto S J. Effects of climate, tree age, dominance and growth on δ¹⁵N in young pinewoods
Trees, 2010,24(3):507-514.
DOI:10.1007/s00468-010-0420-2URL [本文引用: 2]

[44] Hietz P, Dünisch O, Wanek W. Long-term trends in nitrogen isotope composition and nitrogen concentration in Brazilian rainforest trees suggest changes in nitrogen cycle
Environmental Science & Technology, 2010,44(4):1191-1196.
DOI:10.1021/es901383gURL [本文引用: 3]

[45] Hietz P, Turner B L, Wanek W, et al. Long-term change in the nitrogen cycle of tropical forests
Science, 2011,334(6056):664-666.
DOI:10.1126/science.1211979URL [本文引用: 3]

[46] Choi W-J, Chang S X, Bhatti J S. Drainage affects tree growth and C and N dynamics in a minerotrophic peatland
Ecology, 2007,88(2):443-453.
DOI:10.1890/0012-9658(2007)88[443:DATGAC]2.0.CO;2URL [本文引用: 3]

[47] Bao W, O'Malley D M, Sederoff R R . Wood contains a cell-wall structural protein
PNAS, 1992,89(14):6604-6608.
DOI:10.1073/pnas.89.14.6604URL [本文引用: 1]

[48] Doucet A, Savard M M, Bégin C, et al. Is wood pre-treatment essential for tree-ring nitrogen concentration and isotope analysis?
Rapid Communications in Mass Spectrometry, 2011,25(4):469-475.
DOI:10.1002/rcm.4876URL [本文引用: 4]

[49] Der Sleen P V, Vlam M, Groenendijk P, et al. ¹⁵N in tree rings as a bio-indicator of changing nitrogen cycling in tropical forests: An evaluation at three sites using two sampling methods
Frontiers in Plant Science, 2015,6:229. DOI: http://www.geog.com.cn/article/2021/0375-5444/10.3389/fpls.2015.00229.
URL [本文引用: 1]

[50] Bukata A R, Kyser T K. Carbon and nitrogen isotope variations in tree-rings as records of perturbations in regional carbon and nitrogen cycles
Environmental Science & Technology, 2007,41(4):1331-1338.
DOI:10.1021/es061414gURL [本文引用: 3]

[51] Silva L C, Gómez-Guerrero A, Doane T A, et al. Isotopic and nutritional evidence for species- and site-specific responses to N deposition and elevated CO₂ in temperate forests
Journal of Geophysical Research: Biogeosciences, 2015,120(6):1110-1123.
DOI:10.1002/2014JG002865URL [本文引用: 2]

[52] Giantomasi M A, Roig-Ju?ent F A, Villagra P E. Use of differential water sources by Prosopis flexuosa DC: A dendroecological study
Plant Ecology, 2013,214(1):11-27.
DOI:10.1007/s11258-012-0141-2URL [本文引用: 3]

[53] Sheppard P R, Thompson T L. Effect of extraction pretreatment on radial variation of nitrogen concentration in tree rings
Journal of Environmental Quality, 2000,29(6):2037-2042.
[本文引用: 3]

[54] Elhani S, Lema B F, Zeller B, et al. Inter-annual mobility of nitrogen between beech rings: A labelling experiment
Annals of Forest Science, 2003,60(6):503-508.
DOI:10.1051/forest:2003043URL [本文引用: 4]

[55] Handley L L, Azcón R, Ruiz Lozano J M , et al. Plant δ¹⁵N associated with arbuscular mycorrhization, drought and nitrogen deficiency
Rapid Communications in Mass Spectrometry, 1999,13(13):1320-1324.
PMID:10407318 [本文引用: 2]
It has long been evident that plant (15)N chiefly reflects the processes which fractionate (15)N/(14)N rather than the (15)N of plant N source(s). It has emerged recently that one of the most important fractionating processes contributing to the whole plant (15)N is the presence/absence, type or species of mycorrhiza, especially when interacting with nutrient deficiency. Ecto- and ericoid mycorrhizas are frequently associated with (15)N-depleted foliar (15)N, commonly as low as -12 per thousand. As shown by the present study, plants having no mycorrhiza, or those infected with various species of arbuscular mycorrhiza (AM)-forming fungi, interact with varying concentrations of soil nitrogen [N] and moisture to enrich plant (15)N by as much as 3.5 per thousand. Hence the lack of a mycorrhiza, or variation in the species of AM-forming fungal associations, can account for about 25% of the usually reported variations of foliar (15)N found in field situations and do so by (15)N enrichment rather than depletion. Copyright 1999 John Wiley & Sons, Ltd.

[56] Martinelli L A, Piccolo M C, Townsend A R, et al. Nitrogen stable isotopic composition of leaves and soil: Tropical versus temperate forests
Biogeochemistry, 1999,46(1):45-65.
[本文引用: 2]

[57] Zeng X, Liu X, Xu G, et al. Tree-ring growth recovers, but δ¹³C and δ¹⁵N do not change, after the removal of point-source air pollution: A case study for poplar (Populus cathayana) in northwestern China
Environmental Earth Sciences, 2014,72(6):2173-2182.
DOI:10.1007/s12665-014-3127-7URL [本文引用: 1]

[58] Chang C T, Wang L J, Huang J C, et al. Precipitation controls on nutrient budgets in subtropical and tropical forests and the implications under changing climate
Advances in Water Resources, 2017,103:44-50.
DOI:10.1016/j.advwatres.2017.02.013URL [本文引用: 1]

[59] Bassirirad H, Constable J V, Lussenhop J, et al. Widespread foliage δ¹⁵N depletion under elevated CO₂: Inferences for the nitrogen cycle
Global Change Biology, 2003,9(11):1582-1590.
DOI:10.1046/j.1365-2486.2003.00679.xURL [本文引用: 3]

[60] Galloway J N, Townsend A R, Erisman J W, et al. Transformation of the nitrogen cycle: Recent trends, questions, and potential solutions
Science, 2008,320(5878):889-892.
DOI:10.1126/science.1136674URL [本文引用: 1]

[61] Heaton T, Spiro B, Robertson S. Potential canopy influences on the isotopic composition of nitrogen and sulphur in atmospheric deposition
Oecologia, 1997,109(4):600-607.
DOI:10.1007/s004420050122PMID:28307345 [本文引用: 1]
Isotopic studies of nitrogen and sulphur inputs to plant/soil systems commonly rely on limited published data for the N/N and S/S ratios of nitrate, ammonium and sulphate in rainfall. For systems with well-developed plant canopies, however, inputs of these ions from dry deposition or particulates may be more important than rainfall. The manner in which isotopic fractionation between ions and gases may lead to dry deposition and particulates having N/N or S/S ratios different from those of rainfall is considered. Data for rainfall and throughfall in coniferous plantations are then discussed, and suggest that: (1) in line with expectations, nitrate washed from the canopy has N/N ratios higher than those in rainfall; (2) the N/N ratios of ammonium washed from the canopy are variable, with high ratios being found for canopies of higher pH?in conditions of elevated ambient ammonia gas concentrations; and (3) in accord with expectations and previous work, S/S ratios of sulphate washed from the canopy are not substantially different from those in rainfall. The study suggests that if atmospheric inputs are relevant to isotopic studies of the sources of nitrogen for canopied systems, then confident interpretation will require analysis of these inputs.

[62] Yu L, Zheng T Y, Zheng X L, et al. Nitrate source apportionment in groundwater using Bayesian isotope mixing model based on nitrogen isotope fractionation
Science of the Total Environment, 2020,718:137242. DOI: http://www.geog.com.cn/article/2021/0375-5444/10.1016/j.scitotenv.2020.137242.
DOI:http://www.geog.com.cn/article/2021/0375-5444/10.1016/j.scitotenv.2020.137242URL [本文引用: 1]

[63] Guerrieri M R, Siegwolf R T W, Saurer M, et al. Impact of different nitrogen emission sources on tree physiology as assessed by a triple stable isotope approach
Atmospheric Environment, 2009,43(2):410-418.
DOI:10.1016/j.atmosenv.2008.08.042URL [本文引用: 3]

[64] Saurer M, Cherubini P, Ammann M, et al. First detection of nitrogen from NOx in tree rings: A ¹⁵N/¹⁴N study near a motorway
Atmospheric Environment, 2004,38(18):2779-2787.
DOI:10.1016/j.atmosenv.2004.02.037URL [本文引用: 3]

[65] Lu Lu, Dai Erfu, Cheng Qianding, et al. The sources and fate of nitrogen in groundwater under different land use types: Stable isotope combined with a hydrochemical approach
Acta Geographica Sinica, 2019,74(9):1878-1889.
[本文引用: 1]

[ 路路, 戴尔阜, 程千钉, 等. 基于水环境化学及稳定同位素联合示踪的土地利用类型对地下水体氮素归趋影响
地理学报, 2019,74(9):1878-1889.]
[本文引用: 1]

[66] Hart S C, Classen A T. Potential for assessing long-term dynamics in soil nitrogen availability from variations in δ¹⁵N of tree rings
Isotopes in Environmental and Health Studies, 2003,39(1):15-28.
DOI:10.1080/1025601031000102206URL [本文引用: 1]

[67] Cullen L E, Adams M A, Anderson M J, et al. Analyses of δ¹³C and δ¹⁸O in tree rings of Callitris columellaris provide evidence of a change in stomatal control of photosynthesis in response to regional changes in climate
Tree Physiology, 2008,28(10):1525-1533.
DOI:10.1093/treephys/28.10.1525URL [本文引用: 1]

[68] An Wenling, Liu Xiaohong, Chen Tuo, et al. Atmospheric circulation information recorded in tree-ring δ¹⁸O at Lijiang, Yunnan Province
Acta Geographica Sinica, 2009,64(9):1103-1112.
[本文引用: 2]

[ 安文玲, 刘晓宏, 陈拓, 等. 云南丽江树轮δ¹⁸O记录的大气环流变化信息
地理学报, 2009,64(9):1103-1112.]
[本文引用: 2]

[69] Cullen L E, Grierson P F. A stable oxygen, but not carbon, isotope chronology of Callitris columellaris reflects recent climate change in north-western Australia
Climatic Change, 2007,85(1/2):213-229. DOI: http://www.geog.com.cn/article/2021/0375-5444/10.1007/s10584-006-9206-3.
DOI:http://www.geog.com.cn/article/2021/0375-5444/10.1007/s10584-006-9206-3URL [本文引用: 1]

[70] Liu X H, Zeng X M, Leavitt S W, et al. A 400-year tree-ring δ¹⁸O chronology for the southeastern Tibetan Plateau: Implications for inferring variations of the regional hydroclimate
Global and Planetary Change, 2013,104:23-33.
DOI:10.1016/j.gloplacha.2013.02.005URL [本文引用: 1]

[71] Liu X H, Wang W Z, Xu G B, et al. Tree growth and intrinsic water-use efficiency of inland riparian forests in northwestern China: Evaluation via δ¹³C and δ¹⁸O analysis of tree rings
Tree Physiology, 2014,34(9):966-980.
DOI:10.1093/treephys/tpu067URL [本文引用: 1]

[72] Loader N J, Santillo P M, Woodman-Ralph J P , et al. Multiple stable isotopes from oak trees in southwestern Scotland and the potential for stable isotope dendroclimatology in maritime climatic regions
Chemical Geology, 2008,252(1/2):62-71.
DOI:10.1016/j.chemgeo.2008.01.006URL

[73] Liu Xiaohong, Xu Guobao, Wang Wenzhi, et al. Tree-ring stable isotopes proxies: Progress, problems and prospects
Quaternary Sciences, 2015,35(5):1245-1260.
[本文引用: 1]

[ 刘晓宏, 徐国保, 王文志, 等. 树轮稳定同位素记录: 进展、问题及展望
第四纪研究, 2015,35(5):1245-1260.]
[本文引用: 1]

[74] Zhou Tao, Jiang Zhuang, Geng Lei. Atmospheric reactive nitrogen cycle and stable nitrogen isotope processes: Progresses and perspectives
Advances in Earth Science, 2019,34(9):922-935.
[本文引用: 1]

[ 周涛, 蒋壮, 耿雷. 大气氧化态活性氮循环与稳定同位素过程: 问题与展望
地球科学进展, 2019,34(9):922-935.]
[本文引用: 1]

[75] Wang A, Fang Y T, Chen D X, et al. High nitrogen isotope fractionation of nitrate during denitrification in four forest soils and its implications for denitrification rate estimates
Science of the Total Environment, 2018,633:1078-1088.
DOI:10.1016/j.scitotenv.2018.03.261URL

[76] Yan Maojun, Dong Shuhang, Zhong Xiaosong, et al. A study on particulate nitrogen isotope distribution, isotope characteristics and controlling factors in the southern Yellow Sea in summer
Acta Oceanologica Sinica, 2019,41(12):14-25.


[ 晏茂军, 董书航, 钟晓松, 等. 夏季南黄海颗粒氮同位素分布特征及影响因素研究
海洋学报, 2019,41(12):14-25.]


[77] Jiang S, Shi G T, Cole-Dal J, et al. Nitrate preservation in snow at Dome A, East Antarctica from ice core concentration and isotope records
Atmospheric Environment, 2019,213(9):405-412.
DOI:10.1016/j.atmosenv.2019.06.031URL

[78] Chen C J, Li J Z, Wang G A, et al. Accounting for the effect of temperature in clarifying the response of foliar nitrogen isotope ratios to atmospheric nitrogen deposition
Science of the Total Environment, 2017,609:1295-1302.
DOI:10.1016/j.scitotenv.2017.06.088URL [本文引用: 1]