Knowledge Resource Center for Ecological Environment in Arid Area
| 中国北方草地土壤可溶性氮15N自然丰度特征 | |
| 其他题名 | The 15N natural abundance (δ15N) of soil available nitrogen in northern China grasslands |
| 刘冬伟 | |
| 出版年 | 2016 |
| 学位类型 | 博士 |
| 导师 | 方运霆 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 干旱、半干旱生态系统约占全球陆地面积的40%，在提供生态系统服务功能，调控全球碳、氮（N）循环和气候变化方面发挥重要作用。多数模型预测表明干旱生态系统的面积可能会不断扩大，而极端气候事件频发可能造成干旱生态系统更加脆弱。氮稳定性同位素技术具有示踪、整合和指示N循环的功能，有助于我们理解生态系统的N状态和发展方向。目前大尺度上的N循环与气候变化关系的研究主要基于总氮的含量和同位素信息。然而对于复杂的N循环，总氮不能充分揭示每个土壤N转化过程对气候变化的反应，这可能阻碍我们预测干旱生态系统对全球变化的响应。本研究以中国北方干旱、半干旱草地生态系统为研究对象，在甘肃和内蒙古境内设置长约3200 km的草地样带，按照约100 km的间隔设置采样点，共计36个。降水是这条样带的主要控制因素，年均降水量（MAP）为36 mm 至 436 mm。为检验干旱生态系统土壤N循环对降水变化的响应机制，本研究1）测定土壤有效氮的浓度和15N自然丰度（δ15N），分析它们随降水量变化的格局；2）进行室内15N标记试验，从氮气损失的途径和速率的角度探讨干旱生态系统N损失机制；3）通过对比植物和土壤有效氮的δ15N，探讨植物的N吸收偏好。主要研究结果如下：（1）为了避免从溶液中分离铵离子（NH4+）再进行其同位素测定，减少样品准备时间和有毒试剂的使用，本研究建立了测定铵态氮δ15N的新方法。首先，NH4+在碱性次溴酸盐（BrO–）作用下氧化成NO2–，然后在强酸性环境下，NO2–在羟胺（NH2OH）作用下还原成N2O，接下来利用同位素比例质谱仪（PT-IRMS）分析N2O的同位素组成。该方法的标准误差小于0.3‰（n = 3~5）。较以往的方法相比，该方法具有以下有点：1）无需将NH4+从溶液中分离，整个反应只需在一个反应瓶中进行，从根本上简化了制备步骤，降低了准备时间；2）更加适用于低N浓度的样品（10 ~ 20 μmol L-1），样品用量小（小于4 mL），试验空白小（每个分析0.6 ~ 2 nmol）；3）无需使用有毒、挥发性试剂HN3，以及实验条件要求较高的反硝化细菌。（2）半干旱区（142 mm < MAP < 436 mm）的大气气溶胶中无机氮离子浓度高于干旱区（36 mm < MAP < 102 mm），均以铵态氮作为主要组分。大气气溶胶中无机氮离子浓度的季节动态基本表现为冬季最高，夏季最低。大气气溶胶中铵态氮和硝态氮的δ15N值表现为干旱区普遍高于半干旱区，其中铵态氮δ15N值高于硝态氮δ15N值。大气气溶胶中铵态氮和硝态氮的δ15N值的季节动态基本表现为夏季最高，冬季最低。（3）尽管干旱区的土壤总氮库较小，但干旱区较半干旱区有更高的土壤N有效性。土壤铵态氮δ15N在干旱区（-1.2~20.2‰）显著高于半干旱区（-13.9~12.6‰），并随降水的增加而减小。硝态氮δ15N半干旱区（0.5~19.2‰）显著高于干旱区（–1.2~23.4‰），且在干旱区随降水的增加而增加，在半干旱区随降水的增加而减小。干旱区较高的N有效性很大程度上与N沉降的累积有关。同时，较高的pH造成的氨挥发是N损失的重要途径。半干旱区的水分有效性增加土壤N矿化过程，植物N吸收和微生物的反硝化作用是主要的N消耗形式。（4）厌氧氨氧化（anammox）作用在干旱生态系统矿质土壤中普遍存在。Anammox产生N2的速率在干旱区（0 ~ 14.9 nmol N g-1干土h-1）和半干旱区（0 ~ 11.7 nmol N g-1干土h-1）范围基本一致，但随降水变化没有明显规律。尽管如此，本研究中检测到的anammox速率和农田土壤，甚至海底沉积物中发现的anammox速率大小相当。反硝化作用是N2产生的主要途径，在半干旱区（4.4 ~ 65.0 nmol N g-1干土h-1）显著高于干旱区（0.1 ~ 54 nmol N g-1干土h-1）；并且在半干旱区反硝化速率随降水量的增加而增加，表明水分有效性可能是反硝化作用的主要影响因素。Anammox和反硝化作用在干旱区较高的活性潜力、anammox对N2的贡献（14%）暗示旱地土壤N循环对水分变化是十分敏感的，增加的降水可能造成干旱生态系统较大的瞬时性N损失。（5）生长在干旱区的植物，多具有较高的叶片N含量和δ15N，而生长在半干旱区的植物，多具有较低的叶片N含量和δ15N。针茅属（Stipa spp.）、隐子草属（Cleistogenes spp.）和红砂属（Reaumuria spp.）植物叶片的δ15N与土壤铵态氮的δ15N的正相关关系，以及同位素混合模型的结果表明，土壤的铵态氮可能是草地样带优势植物的主要N源。随降水量的增加，植物的主要氮源未发生变化，因此，植物δ15N随降水变化的分布格局可能与主要N源δ15N的格局有关。 |
| 英文摘要 | Drylands cover approximately 40% of the Earth’s land surface and play an essential role in providing ecosystem service and regulating carbon (C) and nitrogen (N) cycling and global climate. Drylands are projected to expand, and more extreme climatic regimes will make arid and semiarid ecosystems more vulnerable to enhanced drought. Nitrogen isotopes are tracer, integrator, and indicator of N cycling, which will provide us the N status and direction of ecosystems. Till now, our current understanding of the relationship between N cycling and climate mainly rely on the total nitrogen. However, N cycling included multiple N processes, and the total N is not sufficient to reveal the responses of each process to climate change. Incomplete understanding on N cycling thus impedes our ability of predicting the responses of dryland ecosystems to global changes.This study was conducted in the arid and semiarid grassland ecosystems in northern China. We set a 3200 km grassland transect in Gansu and Inner Mongolia. A total of 36 sites were selected at the interval of about 100 km. The mean annual precipitation was from 36 mm to 436 mm. To investigate the mechanisms of N cycling of dryland ecosystems to precipitation change, we measured the concentration and isotopes of soil available N. We also quantified N2 loss and the potential of anammox in soil N2 production using the isotope pairing technique, and measured the concentrations and isotope signatures of atmospherically deposited NH4+ and NO3– at two ends of the transect. The main results are following:(1) We report a new chemical method to determine the δ15N of NH4+. This method is based on the isotopic analysis of nitrous oxide (N2O). Ammonium is initially oxidized to nitrite (NO2–) by hypobromite (BrO–) using previously established procedures. NO2– is then quantitatively converted into N2O by hydroxylamine (NH2OH) under strongly acid conditions. The produced N2O is analyzed by a commercially available purge and cryogenic trap system coupled to isotope ratio mass spectrometer (PT-IRMS). Based on a typical analysis size of 4 mL, the standard deviation of δ15N measurements is less than 0.3‰ and often better than 0.1‰ (3 to 5 replicates). Compared to previous methods, the technique here has several advantages and the potential to be used as a routine method for 15N/14N analysis of NH4+, 1) substantially simplified preparation procedures and reduced preparation time particularly compared to the methods in which diffusion or distillation is involved since all reactions occur in the same vial and separation of NH4+ from solution is not required; 2) more suitability for low volume samples including those with low N concentration, having a blank size of 0.6 to 2 nmol; 3) elimination of the use of extremely toxic reagents (e.g., HN3) and/or the use of specialized denitrifying bacterial cultures which may be impractical for many laboratories.(2) The concentrations of inorganic N (ammonium and nitrate) of aerosol particles were higher in semiarid zone (142 mm < MAP < 436 mm) than those in arid zone (36 mm < MAP < 102 mm), with the larger fraction of ammonium. Nitrogen concentration of aerosol ammonium and nitrate presented an apparent seasonality, with the highest in winter, and lowest in summer. The δ15N value of inorganic N basically showed the opposite pattern of the N concentration. The δ15N of aerosol ammonium and nitrate were higher in arid zone, and lower in semiarid zone. The δ15N of ammonium was higher than that of nitrate in both arid and semiarid zone. In addition, the δ15N of ammonium and nitrate were higher in summer compared to those in winter.(3) Our results suggested a higher inorganic N availability in the arid zone than in semiarid zone despite a smaller total N pool therein. The δ15N of soil ammonium (NH4+) was higher in arid zone (-1.2~20.2‰) than those in semiarid zone (-13.9~12.6‰), and decreased with increasing precipitation. In contrast, the δ15N of NO3– was significantly higher in the semiarid zone (0.5 to 19.2‰) than those in arid zone (-1.2 to 23.4‰). With increasing MAP, the δ15N of NO3– increased in arid zone but decreased in semiarid zone. The relatively higher N availability in arid zone was largely from the accumulation of N deposition. Meanwhile, ammonia volatilization caused by high pH was an important pathway of N loss. In contrast, in the semiarid zone, enhanced water availability stimulated N mineralization, and plant N uptake and microbial denitrification became the major pathway of N output. (4) For the first time, we recognized that anaerobic ammonium oxidation (anammox) existed in the mineral soils of dryland ecosystems. The potential N2 rates caused by anammox were 0 ~ 14.9 nmol N g-1 dry soil h-1 and 0 ~ 11.7 nmol N g-1 dry soil h-1 in the arid zone and semiarid zone, respectively, but did not change across the precipitation gradient. Nevertheless, anammox rates in our study were comparable to those determined in agriculture soils and sediments from previous studies. Denitrification was the main pathway to N2 loss. The potential denitrification rates were higher in semiarid zone (4.4 ~ 65.0 nmol N g-1 dry soil h-1) than in arid zone (0.1 ~ 54 nmol N g-1 dry soil h-1), and increased with the precipitation change in semiarid zone, indicating the response of denitrification to water availability. The potential of anammox and denitrification, the contribution of anammox to total N2 losses (14%) in arid zone implied that the N cycling in dryland ecosystems can be sensitive to the precipitation change, and the increasing precipitation might cause large transient N loss in dryland ecosystems.(5) Across the whole grassland transect, the dominant plant species from the relatively hot and dry areas have much higher N concentration and δ15N, while those living in the relatively cold and wet areas have much lower N concentration and δ15N. The δ15N of Stipa spp., Cleistogenes spp., and Reaumuria spp. were positively correlated with the δ15N of soil NH4+, suggesting that soil NH4+ could be the major N source for the dominant species. This was further confirmed by the isotope mixing model. Our results suggested that the pattern of plant δ15N changing with precipitation was related to the δ15N of their major N source. |
| 中文关键词 | 15N自然丰度 ; 降水梯度 ; 干旱生态系统 ; 土壤无机氮 ; 厌氧氨氧化 |
| 英文关键词 | 15N natural abundance precipitation gradient dryland ecosystem soil inorganic N anammox |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 生态学 |
| 来源机构 | 中国科学院沈阳应用生态研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/287745 |
| 推荐引用方式 GB/T 7714 | 刘冬伟. 中国北方草地土壤可溶性氮15N自然丰度特征[D]. 中国科学院大学,2016. |
| 条目包含的文件 | 条目无相关文件。 | |||||
| 个性服务 |
| 推荐该条目 |
| 保存到收藏夹 |
| 导出为Endnote文件 |
| 谷歌学术 |
| 谷歌学术中相似的文章 |
| [刘冬伟]的文章 |
| 百度学术 |
| 百度学术中相似的文章 |
| [刘冬伟]的文章 |
| 必应学术 |
| 必应学术中相似的文章 |
| [刘冬伟]的文章 |
| 相关权益政策 |
| 暂无数据 |
| 收藏/分享 |
除非特别说明，本系统中所有内容都受版权保护，并保留所有权利。
