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.
