Nitrogen is a key resource for plants and animals. Thus there has been much research on what controls nitrogen retention and loss in terrestrial ecosystems. But much uncertainty remains, especially with regard to gaseous nitrogen losses. This is particularly troubling in the context of human modification of the nitrogen cycle, which is dramatically increasing nitrogen pollution, runoff, and the emissions of nitrous oxide, a potent greenhouse gas.
NSF-sponsored, long-term research at the Luquillo LTER site led to the discovery of two novel nitrogen cycling processes in soils (Fig. 1). The first, called dissimilatory nitrate reduction to ammonium (DNRA), is a microbial process that competes with nitrogen loss pathways including the production of nitrous oxide, a potent greenhouse gas (Fig. 2). The forests of the Luquillo LTER program were the first place where DNRA was quantified in soils. Subsequent research at Luquillo has shown that DNRA helps retain nitrogen in the environment where it can be used by plants and animals.
The second process, also first discovered at Luquillo, is called Feammox. Feammox is the conversion of ammonium (a key plant nutrient) to inert dinitrogen gas (the main constituent of the atmosphere) or other nitrogen forms under anaerobic conditions. The Feammox pathway uses iron oxides, which are abundant in many soils. Feammox is a newly discovered pathway that can be described as the oxidation of ammonium in the absence of molecular oxygen. This process, likely conducted by soil microbes, short circuits the nitrogen cycle, leading to gaseous nitrogen losses from soils. In tropical forest soils in Puerto Rico Feammox dominantly produced dinitrogen gas, an inert form of nitrogen that is the primary constituent of our atmosphere. There is only one other process, called denitrification, known to produce dinitrogen gas. Scientists showed that Feammox has the potential to produce nitrite and nitrate, which can subsequently lead to nitrous oxide gas production and emissions.
The discovery of Feammox will help researchers determine the sources and magnitude of greenhouse gas emissions from the biosphere. This new pathway for nitrogen loss could also ultimately help us understand the effects of management activities (irrigation, fertilizer application) on nitrogen retention and loss.
Tropical forests are the largest natural source of nitrogen-based greenhouse emissions globally. These ecosystems are undergoing rapid change, including increased rates of nitrogen pollution from human activities. Understanding how nitrogen is lost and retained in tropical forests soils will help us to better manage these ecosystems in the future. This research, as well as other studies being conducted at this LTER site demonstrates the value of established centers of research activity and collaboration. A diverse team of scientists including ecologists, biogeochemists, hydrologists, and microbiologists collborated over the long-term to make these discoveries.