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)提供资金的。摘要北极土壤储存了大量的碳(C)，随着气候变暖，它可能成为重要的二氧化碳来源。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模型及其对气候变化的反应假定，当N限制植物生长时，C限制分解。此外，N的有效性具有很强的季节性，在生长季节早期的可用性相对较高，随后出现了明显的崩溃。为了预测北极系统对气候变化的反应，有必要了解对这种季节性的控制，但有许多问题需要回答：1)是什么导致了季节性的营养崩溃？2)微生物的活动是否在季节性的C和N限制之间转换？3)由于营养崩溃前后周期的不同延长，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)这些模式将对北极系统产生什么更大的影响？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力学，将这种对C和N相互作用时间的理解与对养分崩溃发生的机制的理解结合起来，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。这项拟议的研究将通过以下方式解决我们的问题：1)通过推进积雪融化和使生态系统变暖来改变田间生长季的长度和时间；2)建立土壤N有效性、植物N含量、叶片展开、根的生长和根部沉积、生态系统呼吸、微生物生物量和酶活性的精细季节时程；3)进行实验室实验，以确定微生物活动受到温度以及C和N有效性在崩溃前后的程度的限制；4)确定养分崩溃的时间和植物生长如何在纬度横断面上变化；5)改进为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如何响应N限制，并将导致崩溃的特定驱动因素纳入MEL；6)测试养分可获得性的季节性的大范围时空影响，以及在北极变暖和生长季延长的情况下它可能发生的变化。这项工作将需要密集的机械学研究，重点是最多几周内发生的过渡和变化，但这些变化对冻土带生态系统有深远的影响。研究人员将把这一机械性工作扩展到中间空间尺度，方法是沿着纬度样带进行样带测量，以验证当地出现的模式是稳健的。它们将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，扩展到整个北极系统，该模型旨在探索对生态系统功能的多种限制资源影响。作为一个综合方案，这项研究将探索推动冻土带土壤N有效性崩溃的季节变化模式如何改变整个冻土带碳循环及其作为碳源或碳汇的角色，并通过此改变其在全球气候系统中的角色。

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.