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）。摘要北极土壤有大量的碳（C）储存，并可能作为一个重要的二氧化碳源与变暖。然而，理解冻原土壤过程的关键是氮（N），因为植物生长和分解都受到严重的N限制。然而，目前的冻原生态系统及其对气候变化的响应模型假设，虽然N限制植物生长，C限制分解。此外，N的可用性是强烈的季节性与相对较高的可用性在生长季节早期，随后明显崩溃。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）是什么原因导致季节性营养崩溃？2)微生物活动是否会在C和N限制之间季节性转换？3)生长季节的延长将如何改变整个生态系统的C和N动态，作为营养崩溃之前和之后的时期差异延长的结果？4)这些模式对北极系统的更大影响是什么？解决这些问题需要以下植物和土壤动态在一个非常紧迫的时间框架，耦合这种理解的C和N相互作用的时间，以增强机械的理解，为什么营养崩溃发生，然后使用样带采样和生态系统建模，探索大规模的影响，这个季节性崩溃。本研究将通过以下几个方面来解决我们的问题：1）通过提前融雪和加热生态系统来改变田间生长季节的长度和时间：2）建立土壤氮有效性、植物氮含量、叶片扩展、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性时间进程; 3）进行实验室实验，以确定温度以及崩溃前后的C和N可用性对微生物活性的限制程度; 4）确定养分崩溃和植物生长的时间如何在纬度样带上变化; 5）完善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对氮限制做出反应，并将崩溃的特定驱动因素纳入MEL; 6）测试大-养分可利用性的季节性的尺度空间和时间效应，以及在气候变暖、生长季节延长的北极地区养分可利用性如何变化。最多只在几周内发生的过渡和转变，但对冻土带生态系统产生了深远的影响。研究人员将通过沿着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，以验证当地发生的模式是否可靠。他们将通过将这些机制（重要的是，分解的N效应）整合到MEL模型中，以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性资源影响。作为一个综合的包，这项研究将探讨如何不断变化的季节性模式，驱动在冻原土壤中的氮可用性的崩溃将改变整体冻原碳循环及其作为源或汇的C，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.