模拟不同氮沉降量和不同氮形态组成比例对荒漠草原植物氮吸收偏好的影响

With concerns over the effects of global increasing deposition of reactive nitrogen (N), numerous N manipulation experiments have been conducted in various ecosystems. However, there is lack of experiments that simulate composition of different forms of N. The preferences of plants for different N forms among ammonium, nitrate and dissolved organic N play an important role in determining the fates of N input into natural ecosystems and in determining the ecosystem N dynamics. Nevertheless, it remains unclear whether and how N uptake preferences affect ecosystem processes and functioning... In the context of the projected increases in N deposition and decreases in the ratio of NH4+-N to NO3--N, we conduct a N addition experiment with split-plot design based on a 13-year platform in a desert steppe. Nitrogen deposition amount is the primary factor with three levels (no addition, 2 g N m-2 yr-1 and 10 g N m-2 yr-1), and the ratio of NH4+-N to NO3--N is the secondary factor with four levels (4:1, 2:1, 1:1 and ambient). We quantify plant N preferences using both 15N natural abundance method and 15N tracer technique to examine the effects of N deposition with different NH4+/NO3- ratios and different deposition amounts on plant N uptake preferences in a desert steppe. We also measure the gross rates of NH4+ production and nitrification using the N isotope dilution method, and determine the abundance of the bacterial 16S rRNA gene and bacterial and archaeal ammonia monooxygenase gene (amoA) using qPCR, in order to reveal the mechanism by which plant N preferences respond to N deposition. In addition, together with ecosystem productivity and community structure, we aim to elucidate whether and how N preferences mediate the effects of N deposition on plant composition and productivity. These findings will provide a novel explanation for the responses of grassland ecosystems to N deposition, and provide scientific data for biodiversity protection and grassland management.

随着氮沉降全球化，国内外已在各类生态系统中开展了模拟氮沉降的实验，然而模拟不同氮形态组成比例的实验鲜见报道。植物对不同氮形态的吸收偏好直接影响输入到自然生态系统中的氮的去向和氮动态，进而影响生态系统过程和功能，但其中的调控机制仍不清楚。.在未来氮沉降增加和NH4+/NO3-降低的背景下，本项目拟将在开展了13年的不同放牧压力实验平台的对照区内，建立低氮和高氮沉降量下不同NH4+/NO3-模拟实验，以短花针茅荒漠草原为研究对象，将15N自然丰度法和15N示踪结合量化植物氮吸收偏好，测定根性状、土壤总氨化和硝化速率、氨化和硝化细菌多度，阐明植物氮吸收偏好对氮沉降量和质的变化的响应规律及其机制。同时监测生态系统生产力和群落结构，明确氮偏好在氮沉降影响植物组成和生产力过程中的作用。本研究将为揭示氮沉降影响草地生态系统过程和功能的调控因子提供全新的思路和方法，为生物多样性保护和草地管理提供科学依据。

氮是决定陆地生态系统生产力的关键因子。植物对不同形态氮的吸收偏好直接影响输入到自然生态系统中的氮的去向和氮动态的变化，对于理解生态系统氮循环和预测植物对未来环境变化的响应非常重要。然而全球变化（氮沉降、增温和降水变化）如何影响植物氮吸收及其与群落稳定性和生产力的关系仍未清楚解析。因此，本研究基于内蒙古草原全球变化野外控制实验，利用氮稳定同位素技术、IsoSource模型和常规方法，对植物氮素获取能力、功能性状、生产力和群落稳定性等内容进行了研究，结果表明：(1)草原优势植物克氏针茅和冰草主要利用较丰富的硝态氮，对铵态氮利用较少，并且随着氮水供给变化而改变；揭示未来氮沉降加剧和硝态氮比例增加并非总是产生负效应，很可能促进温带草原优势植物的生长，减缓全球环境变化的后果。(2)植物获取氮素的另一个主要途径是养分回收，不同年份的研究表明回收效率随着土壤养分和水分有效性的提高而降低，受到了自然降水变异的调控。(3)温度升高对草地生产力的多样影响受到了植物激素乙烯和叶寿命的调控，暗示未来构建全球变化模型时应考虑将植物激素作为重要指标，因其驱动了植物功能性状的改变。(4)氮添加和增温均降低了荒漠草原群落稳定性，但是增温+氮处理降低的程度更大，即二者的负效应具有叠加作用。氮添加通过降低C4植物的稳定性而影响群落稳定性，增温通过降低C3植物的稳定性而影响群落稳定性。本研究揭示了草原生态系统对全球变化的响应与适应，为生物多样性的保护和草地管理提供科学依据。