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

氮是高寒和亚北极森林生态系统的主要限制性营养物质，不幸的是，我们对这些低营养环境中基本的N循环过程的了解仍然有限，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，我们阐明了高寒和亚北极生态系统中氮的主要输入(1，2)，特别是描述了蓝藻与普遍存在的优势羽毛藓--施雷伯利侧耳(Pleurozium Schreberi)和华丽侧耳(Hylodium Splendens)(3，4)之间的联系。这两种苔藓是亚北极和北方森林系统生物固定氮的主要来源，在北极高山冻原生态系统中也很重要(5)。虽然我们已经对这种联系所固定的氮的数量有了一个清晰的图景(6)，但我们对羽藓向生态系统释放氮的途径的了解仍然很不完整。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，并使从植物学家到生态学家再到生态系统模型师的一系列研究人员受益。在这项建议中，我们提出了几个令人信服的假设，以加深我们对北方苏格兰松生态系统N循环基本过程的理解，并促进我们对瑞典北部北方针叶林中羽毛苔藓-蓝藻联合的生态学的一般知识。到目前为止，我们对这种联系的了解大多围绕着关于羽毛藓生态的实地研究，很少有人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，这项研究的四个主要目标是：(1)沿着自然肥力梯度和氮沉积梯度，评估羽生苔藓固氮与立地生产力的关系；(2)确定苔藓碎屑中以有机形式存在的固定氮的部分，以及干扰在释放这些氮方面的作用；(3)评估菌根在羽状苔藓向植物转移N(氨基酸和肽N)中的作用；(4)评价固定氮与森林地表CO2、CH4和N2O跨氮富集梯度(即N沉积梯度和火灾时间序列)之间的关系。实验将在瑞典北部已建立的田间地点进行，包括一个火灾时间序列，以解决随着天然林演替而发生的氮素周转变化；总共五个自然肥力梯度，以评估可变的N和P限制对固氮、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N沉积梯度，沿该梯度，我们将评估增加的N沉积对氮素固定、周转和损失的影响。这些发现将对研究高纬度生态系统过程，包括N固定、N周转、C固定和温室气体排放具有重要意义。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。T.H.德卢卡，O.Zackrisson，M.-C.Nilsson，A.Sellstedt，自然界，419,917(2002)3.O.Zackrisson，T.H.DeLuca，M.-C.Nilsson，A.Sellstedt，L.Berglund，生态学85,3327(2004)。4.O.Zackrisson，T.H.DeLuca，F.Gentili，A.Sellstedt，A.Jäderlund，Oecologia in Press，(2009)。詹蒂利，尼尔森，扎克里森，德卢卡，塞尔斯泰特，实验植物学杂志，56,3121(2005)。T.H.德卢卡，O.Zackrisson，植物与土壤294,147(2007).6 T.H.DeLuca，O.Zackrisson，M.-C.Nilsson，M.J.Gundale，Science 320,1181(2008).

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).