Dissertation Research: Conservative nitrogen cycling in wet tropical forests?

论文研究：潮湿热带森林中的保守氮循环？

• 批准号: 0910012
• 负责人: Alan Townsend
• 金额: $ 1.5万
• 依托单位: University of Colorado at Boulder
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-04-15 至 2011-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0910012&HistoricalAwards=false
• 关键词: Dissertation; Research; Conservative; nitrogen; cycling

Nitrogen is one of life's critical building blocks; all life depends on it. In many ecosystems, nitrogen limits plant growth, but in a large number of tropical forests, it appears to be present in relative excess of what life demands. This excess can cause large losses of nitrogen from tropical forests in forms that affect climate, atmospheric chemistry, and the health and function of aquatic ecosystems downstream. We are beginning to learn that, contrary to our initial expectations, the wettest of tropical forests may function somewhat differently; despite their high rainfall, which can promote high nitrogen losses, some wet tropical forests seem to show nitrogen losses far lower than their drier counterparts. In the Costa Rican forests that will be studied in this project, multiple measures of the nitrogen cycle suggest that nitrogen losses are low; despite abundant evidence from previous work showing that phosphorus is the most important nutrient control here (and thus that nitrogen ought to exist in relative excess). To address this paradox, this project will apply an isotopic nitrogen label (15N) to measure gross rates of nitrogen transformations; this technique provides a useful window into how much nitrogen cycles in an ecosystem, how fast, and if nitrogen losses are truly likely to be low here throughout the year. Additionally, measuring stable isotopic ratios of nitrogen will be used to measure two microbial processes that are likely important in controlling the ultimate fate of nitrogen in this forest; denitrification and dissimilatory nitrate reduction to ammonium (DNRA). The former can be a key mechanism for losing nitrogen from an ecosystem, as well as for producing the powerful greenhouse gas nitrous oxide; the latter can be a way to help keep highly mobile nitrate ions from leaving an ecosystem and ending up in downstream aquatic environments.This proposal addresses a fundamental question regarding nitrogen cycling and nutrient limitation in tropical forests. The work is important for several reasons. First, tropical forests appear to be a major source of the greenhouse gas nitrous oxide; this work will directly address links between a key climate variable (rainfall) and the potential for wet tropical forests to produce this gas. In addition, beyond the threats of climate change, tropical forests are threatened by high rates of land conversion and rapid increases in nitrogen deposition. Given that tropical forests are both a cradle for much of earth's biodiversity and a provider of multiple ecosystem services, this work has direct societal relevance. Scientists? ability to predict how ecosystems may respond to these perturbations depends on their understanding of processes controlling ecosystem function in relatively undisturbed forests. Finally, the wettest of tropical forests are both understudied and yet some of our best hopes for larger scale conservation efforts. This project will enhance our understanding of how these forests function at a basic nutritional level, a key component of our ability to protect and manage them most effectively in a rapidly changing world.

氮是生活中关键的基础之一。所有生活都取决于它。在许多生态系统中，氮限制了植物的生长，但是在许多热带森林中，它似乎相对超出了生命的需求。这种过量会以影响气候，大气化学以及下游水生生态系统的健康和功能的形式导致热带森林的氮损失。我们开始了解到，与我们最初的期望相反，最潮湿的热带森林可能会有所不同。尽管降雨量很高，这可以促进氮损失高，但一些湿的热带森林似乎显示出氮损失远低于其干燥的森林。在该项目中将研究的哥斯达黎加森林中，氮循环的多种测量表明氮损失很低。尽管先前工作的大量证据表明，磷是这里最重要的营养控制（因此，应该以相对过量存在氮）。为了解决这一悖论，该项目将应用同位素氮标签（15N）来测量氮转化的总速率。该技术为生态系统中的氮循环，多快的速度和氮损失是否真的很可能在整个一年中很可能很低。另外，测量氮的稳定同位素比，将用于测量两个微生物过程，这些过程可能在控制这座森林中氮的最终命运方面很重要。反硝化和异化硝酸盐还原为铵（DNRA）。前者可以是从生态系统中失去氮的关键机制，以及生产强大的温室气氧化二氮。后者可以是一种有助于防止高度移动的硝酸离子离开生态系统并最终进入下游水生环境的方法。该提案解决了热带森林中有关氮气循环和营养限制的基本问题。这项工作很重要，原因有几个。首先，热带森林似乎是温室气氧化物的主要来源。这项工作将直接解决关键气候变量（降雨）与湿热带森林产生这种气体的潜力之间的联系。此外，除了气候变化的威胁之外，热带森林受到高土地转化率和氮沉积迅速增加的威胁。鉴于热带森林既是地球生物多样性的摇篮，又是多个生态系统服务的提供商，这项工作具有直接的社会意义。科学家？能够预测生态系统如何应对这些扰动的能力取决于他们对控制生态系统功能的过程中相对不受干扰的森林中的理解。最后，最潮湿的热带森林既经过了研究，又是我们对更大规模保护工作的最佳希望。该项目将增强我们对这些森林如何在基本营养水平上运作的理解，这是我们在快速变化的世界中最有效地保护和管理它们的能力的关键组成部分。