The Boreal Nitrogen Gap: Size, fate and impacts of nitrogen fixation in Fennoscandia forest ecosystems

氮是高寒带和亚北极森林生态系统的主要限制养分，不幸的是，我们对这些贫营养环境中基本氮循环过程的理解仍然有限，这反过来又阻碍了我们有效预测生态系统碳(C)周转和温室气体排放的能力。在过去的10年里，我们已经阐明了高寒带和亚北极生态系统的主要N输入（1,2），特别是我们描述了蓝藻与普遍存在的优势羽藓Pleurozium schreberi和Hylocomium splendens之间的关联（3,4）。这两种苔藓是亚北极和北方森林系统中生物固定氮的主要来源，在北极高寒苔原生态系统中也很重要(5)。尽管我们已经对这种关联所固定的氮量有了一个清晰的认识(6)，但我们对从羽毛藓向生态系统释放氮的途径的理解仍然非常不完整。对这些途径的透彻理解将有助于更好地理解高纬度地区的碳循环过程，并使植物学家、生态学家和生态系统建模者等一系列研究人员受益。在本提案中，我们提出了几个令人信服的假设，以进一步了解北方苏格兰松生态系统N循环的基本过程，并推进我们对瑞典北部北方森林中羽毛-蓝藻关联生态学的一般知识。迄今为止，我们对这种联系的大部分理解都围绕着关于羽藓生态学的实地研究，很少努力将这种联系与地下营养循环和微生物群落联系起来。因此，本研究的四个主要目标是：(1)沿着自然肥力梯度和N沉积梯度评估芬诺斯坎迪亚北部羽苔固氮与场地生产力之间的关系；(2)确定苔藓碎屑中以有机形式保留的固定氮的比例以及干扰对释放这些氮的作用；(3)评价菌根在毛苔向植物转运N（氨基酸和肽N）中的作用；(4)评估固定N与森林地表CO2、CH4和N2O通量在N富集梯度（即N沉降梯度和火-时间序列）之间的关系。将在瑞典北部已建立的实地地点进行实验，包括一个火灾时间序列，以解决自然森林演替中氮周转的变化；利用5个自然肥力梯度，评估变氮磷限制对土壤氮素固定、周转和温室气体排放的影响；以及从瑞典南部到北部的人为氮沉降梯度，我们将沿着该梯度评估氮沉降增加对氮固定、周转和损失的影响。这些发现对研究高纬度地区的生态系统过程具有重要意义，包括氮固定、氮周转量、碳固定和温室气体排放。我们的目的是将这些结果应用到现有的生物地球化学模型中，这些模型旨在预测与生产力和温室气体排放相关的碳和氮动态。文学Cited1。t.h.德卢卡，o.s ackrisson, m.c。2. Nilsson， A. Sellstedt，《自然》419,917（2002）。O.扎克里森，T. H.德卢卡，m . c。[2]张建军，张建军，张建军，等。生态学报，2004,33(4):591 - 591。3 . O. Zackrisson， T. H. DeLuca， F. Gentili， A. Sellstedt， A. Jäderlund，生态学报，（2009）。李建军，李建军，李建军，李建军。植物学报，2005,32(4):481 - 481。6 .陈志强，《中国土壤科学》，2007年第4期t.h.德卢卡，o.s ackrisson, m.c。王志强，《科学》，2008年第1期。

Nitrogen is a primary limiting nutrient in high boreal and subarctic forest ecosystems, unfortunately, our understanding of fundamental N cycling processes in these oligotrophic environs remains limited which in turn hinders our ability to effectively predict ecosystem carbon (C) turnover and greenhouse gas emissions. Over the past 10 years we have elucidated a major N input in high boreal and subarctic ecosystems (1, 2), specifically we described the occurrence of an association between cyanobacteria and the ubiquitous and dominant feathermosses Pleurozium schreberi and Hylocomium splendens (3, 4). These two mosses act as the primary sources of biologically fixed N in subarctic and boreal forest systems and are also important in Arctic alpine tundra ecosystems (5). Although we have developed a clear picture of the quantity of N fixed by this association (6), our understanding of the pathway for N release into the ecosystems from the feathermosses remains greatly incomplete. A thorough understanding of these pathways will lead to a better understanding of C cycling processes at high latitudes and benefit a range of researchers from botanists, to ecologists, to ecosystem modellers.In this proposal we put forth several compelling hypotheses to further our understanding of fundamental processes in the N cycle of boreal Scots pine ecosystems and advance our general knowledge of the ecology of the feathermoss-cyanobacterial association in boreal forests of northern Sweden. To date most of our understanding of this association has surrounded field studies regarding the ecology of the feathermosses, little effort has been made to link the association to the belowground nutrient cycles and microbial community. Studies have been designed to address the following objectives: The four primary objectives of this proposed research are therefore to: (1) Assess the relationship between feathermoss N fixation and site productivity along natural fertility gradients and N deposition gradients in northern Fennoscandia; (2) Determine the portion of fixed N that remains in an organic form in moss detritus and the role of disturbance in liberating this N; (3) Evaluate the role of mycorrhizas in the transfer of N (amino acid and peptide N) from feather mosses to plants; (4) Evaluate the relationship between fixed N and forest floor CO2, CH4, and N2O fluxes across N enrichment gradients (i.e. N deposition gradient and fire-chronosequence). Experiments will be conducted on established field sites in northern Sweden including a fire chronosequence to address change in N turnover with natural forest succession; a total of five natural fertility gradients to assess the influence of variable N and P limitation on N fixation, turnover and greenhouse gas emissions; and an anthropogenic N deposition gradient from southern to northern Sweden along which we will assess the influence of increased N deposition on N fixation, turnover and loss. These findings will have significant implications for those studying ecosystem processes at high latitudes including N fixation, N turnover, C fixation, and greenhouse gas emissions. It is our intent to work these results into existing biogeochemistry models that are designed to predict linked C and N dynamics as related to productivity and greenhouse gas emissions.Literature Cited1. T. H. DeLuca, O. Zackrisson, M.-C. Nilsson, A. Sellstedt, Nature 419, 917 (2002).2. O. Zackrisson, T. H. DeLuca, M.-C. Nilsson, A. Sellstedt, L. Berglund, Ecology 85, 3327 (2004).3. O. Zackrisson, T. H. DeLuca, F. Gentili, A. Sellstedt, A. Jäderlund, Oecologia In Press, (2009).4. F. Gentili, M. C. Nilsson, O. Zackrisson, T. H. DeLuca, A. Sellstedt, Journal of Experimental Botany 56, 3121 (2005).5. T. H. DeLuca, O. Zackrisson, Plant and Soil 294, 147 (2007).6 T. H. DeLuca, O. Zackrisson, M.-C. Nilsson, M. J. Gundale, Science 320, 1181 (2008).