Dissertation Research: Conservative nitrogen cycling in wet tropical forests?

氮是生命的关键组成部分之一；所有的生命都依赖于它。在许多生态系统中，氮限制了植物的生长，但在大量的热带森林中，氮的含量似乎相对超过了生命的需要。这种过剩会造成热带森林氮的大量损失，其形式会影响气候、大气化学以及下游水生生态系统的健康和功能。我们开始了解到，与我们最初的预期相反，最潮湿的热带森林的功能可能有些不同；尽管降雨量大，这可能会导致氮的大量流失，但一些潮湿的热带森林的氮流失似乎远低于干燥的热带森林。在本项目将研究的哥斯达黎加森林中，对氮循环的多种测量表明，氮损失很低；尽管先前的大量证据表明，磷在这里是最重要的营养控制（因此氮应该相对过量存在）。为了解决这一矛盾，该项目将采用同位素氮标签（15N）来测量氮转化的总速率；这项技术提供了一个有用的窗口，可以了解生态系统中有多少氮循环，有多快，以及这里全年的氮损失是否真的可能很低。此外，测量氮的稳定同位素比率将用于测量两种微生物过程，这两种微生物过程可能对控制该森林中氮的最终命运很重要；反硝化和异化硝态氮还原成铵（DNRA）。前者可能是从生态系统中失去氮的关键机制，以及产生强大的温室气体一氧化二氮；后者可以是一种帮助保持高流动性硝酸盐离子离开生态系统并最终进入下游水生环境的方法。这一建议解决了一个关于热带森林氮循环和养分限制的基本问题。这项工作之所以重要，有几个原因。首先，热带森林似乎是温室气体一氧化二氮的主要来源；这项工作将直接解决关键气候变量（降雨）与潮湿的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外，热带森林还受到土地转换率高和氮沉降迅速增加的威胁。鉴于热带森林既是地球大部分生物多样性的摇篮，又是多种生态系统服务的提供者，这项工作具有直接的社会意义。科学家吗?预测生态系统如何应对这些扰动的能力取决于他们对相对未受干扰的森林中控制生态系统功能的过程的理解。最后，最潮湿的热带森林既没有得到充分的研究，也是我们最大的希望进行大规模的保护工作。该项目将加强我们对这些森林在基本营养水平上如何发挥作用的理解，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。

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.