DISSERTATION RESEARCH: Nitrous Oxide and Dinitrogen Production from Tropical Forest Soils Under Ambient and Fertilized Conditions

Brown University, Providence RI

Investigators

Abstract

Nitrous oxide is a greenhouse gas that is emitted from soils where nitrogen is abundant. In human dominated landscapes, fertilized farm fields are the most important source, and in natural landscapes, intact lowland tropical forests, which are often very nitrogen rich, are the greatest source. This research aims to measure emissions of nitrous oxide in tropical forests, where it is not clear if nitrogen pollution is emitted as nitrous oxide or as nitrogen gas (which is inert and already makes up 78% of the atmosphere). While nitrous oxide emissions are relatively easy to measure, nitrogen gas emissions are notoriously difficult to measure. To overcome this problem, the researchers will use a newly developed technique to directly measure nitrogen gas and nitrous oxide production from soils. By comparing fertilized and unfertilized soils in an intact tropical forest in Luquillo, Puerto Rico, they will provide information on nitrous oxide emissions, and the impact of nitrogen pollution on tropical forests. The researchers will share their findings with the general audience through an education website that they have established, and with other academics. The researchers will use the Nitrogen Free Air Recirculation Method (N-FARM) to quantify dinitrogen (N2) and nitrous oxide (N2O) production from intact soil cores taken from soils with different native oxygen status (ridges, slopes and valleys) under ambient and N enriched conditions. Because N-FARM allows gas fluxes to be measured under different atmospheric oxygen conditions, coupling these measurements with field-based continuous measurements of soil oxygen will allow results to be scaled to generate annual landscape scale estimates of N2 and N2O production. Specifically, the researchers will test two hypotheses: H1: a) lower soil oxygen in valleys will result in higher total N gas loss than on more upland portions of the landscape and b) N gas losses will have a lower N2O:N2 ratio in valleys, since lower soil oxygen favors reduction of nitrate to N2, rather than N2O. H2: Nitrogen fertilization will a) induce greater nitrogen gas loss, via both nitrification and denitrification, and b) gas losses will have a higher N2O:N2 ratio since nitrification does not produce N2. To test these hypotheses, the researchers will examine replicated fertilized plots on ridge/slope/valley transects in the Luquillo Mountains. They will collect soil cores from beneath control and fertilized plots and run them on the NFARM apparatus within a week of collection. These data will provide the first direct N2 emissions measurements from tropical soils, and will add much needed understanding to the way N losses occur from tropical forest today, and potentially in the future.