Collaborative Research:The Changing Seasonality of Tundra Nutrient Cycling: Implications for Ecosystem and Arctic System Functioning

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。北极土壤具有大量的碳(C)储量，并可能在变暖过程中成为一个重要的二氧化碳源。然而，了解冻土带土壤过程的关键是氮(N)，因为植物的生长和分解都受到严重的氮限制。然而，目前的冻土带生态系统及其对气候变化的响应模型假设，虽然氮限制植物生长，但碳限制分解。此外，N的可用性具有很强的季节性，在生长季节早期具有相对较高的可用性，随后出现明显的崩溃。有必要了解对这种季节性的控制，以预测北极系统对气候变化的反应，但有多个问题需要回答：1)是什么导致了季节性营养物质的崩溃？2)微生物活性是否随季节在C和N限制之间切换？3)由于养分崩溃前后不同时期的延长，生长季节的延长将如何改变整个生态系统的C和N动态？4)这些模式对北极系统的更大影响是什么？要解决这些问题，需要在非常紧迫的时间框架内跟踪植物和土壤动态，将对C和N相互作用时间的理解与对养分崩溃发生原因的增强机制理解相结合，然后使用样带采样和生态系统建模来探索这种季节性崩溃的大规模影响。本文提出的研究将通过以下方式解决我们的问题：1)通过加速积雪融化和使生态系统变暖来改变野外生长季节的长度和时间；2)建立土壤氮有效性、植物氮含量、叶片扩张、根系生长和根沉积、生态系统呼吸、微生物生物量和酶活性的精细尺度季节时程；3)进行实验室实验，确定坠机前后温度、C和N可用性对微生物活动的限制程度；4)确定养分崩溃和植物生长的时间如何在一个纬度样带上变化；5)完善针对北极生态系统开发的多元素限制模型（MEL），以更好地处理植物和微生物系统对N限制的反应，并将崩溃的具体驱动因素纳入MEL；6)测试养分有效性季节性的大尺度时空效应，以及在变暖的北极，养分有效性如何随着生长季节的延长而变化。这项工作将需要深入的机制研究，重点关注最多只发生在几周内，但对冻土带生态系统产生深远影响的过渡和转变。研究人员将通过沿纬度样带进行样带测量，将这种机械工作扩展到中间空间尺度，以验证局部发生的模式是稳健的。通过将这些机制，以及重要的n对分解的影响整合到MEL模型中，它们将扩展到整个北极系统，该模型旨在探索生态系统功能的多种限制资源影响。作为一个综合方案，本研究将探索驱动冻土带土壤氮有效性崩溃的季节模式变化如何改变冻土带的整体碳循环及其作为碳源或碳汇的作用，并通过它在全球气候系统中的作用。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Abstract Arctic soils have large stores of carbon (C) and may act as a significant CO2 source with warming. However, the key to understanding tundra soil processes is nitrogen (N), as both plant growth and decomposition are severely N limited. However, current models of tundra ecosystems and their responses to climate change assume that while N limits plant growth, C limits decomposition. In addition, N availability is strongly seasonal with relatively high availability early in the growing season followed by a pronounced crash. There is a need to understand the controls on this seasonality to predict Arctic System responses to climate change, but there are multiple questions that need answers: 1) What causes the seasonal nutrient crash? 2) Does microbial activity switch seasonally between C and N limitation? 3) How will a lengthening of the growing season alter overall ecosystem C and N dynamics, as a result of differential extension of the periods before and after the nutrient crash? 4) What will be the larger impacts of these patterns on the Arctic system? Addressing these questions requires following plant and soil dynamics in a very tight time frame, coupling this understanding of the timing of C and N interactions to an enhanced mechanistic understanding of why the nutrient crash occurs, and then using transect sampling and ecosystem modeling to explore the large-scale implications of this seasonal crash. This proposed research will address our questions by: 1) Varying the length and timing of the growing season in the field by advancing snow melt and warming the ecosystem; 2) Establishing the fine scale seasonal time-courses of soil N availability, plant N content, leaf expansion, root growth and rhizodeposition, ecosystem respiration, microbial biomass and enzyme activity; 3) Conducting lab experiments to determine the extent to which microbial activity is limited by temperature, and C and N availability before and after the crash; 4) Determining how the timing of the nutrient crash and plant growth vary across a latitudinal transect; 5) Refining the Multiple Element Limitation model (MEL) that was developed for arctic ecosystems to better handle how plant and microbial systems respond to N limitation, and incorporating the specific drivers of the crash into MEL; 6) Testing the large-scale spatial and temporal effects of the seasonality of nutrient availability and how it may change in a warming Arctic with a lengthening growing season. This work will require intense mechanistic research focusing on transitions and transformations that occur over only a few weeks at most, but which have profound impacts on the tundra ecosystem. Researchers will scale this mechanistic work to the intermediate spatial scale by conducting transect measurements along a latitudinal transect to validate that patterns that occur locally are robust. They will scale to the whole Arctic system by integrating these mechanisms, and importantly, the N-effects on decomposition, into the MEL model that is designed to explore multiple limiting resource effects on ecosystem function. As an integrated package, this research will explore how the changing seasonal pattern that drives the crash in N availability in tundra soils will alter overall tundra C-cycling and its role as a source or sink of C and through this its role in the global climate system.