Date of Award

Spring 1999

Project Type

Dissertation

Program or Major

Natural Resources

Degree Name

Doctor of Philosophy

First Advisor

William H McDowell

Abstract

Nitrous oxide (N2O) is a trace gas that contributes to the greenhouse effect and participates in the reactions that destroy stratospheric ozone. Soil microbial processes are significant producers of this trace gas, particularly in tropical areas, which are considered major sources in the global N2O budget. Nitrous oxide fluxes to the atmosphere are variable in space and time. In this study, spatial and temporal variability in surface N2O fluxes were assessed as well as the major environmental controls on N2O production for a tropical rainforest watershed in northeastern Puerto Rico. A static chamber technique was used to assess surface fluxes and soil air probes were installed at different depths to determine soil concentrations of N2O, methane (CH4), and oxygen (O2). Suction lysimeters were installed to sample soil water for the concentrations of the major regulators of the production of N2O, specifically, nitrate (NO3-), dissolved organic carbon (DOC), and ammonium (NH4+). Water table heights were monitored manually over the course of the study. Patterns in surface N2O flux across three topographic sequences were stable through time. The three sequences had similar flux rates in aerobic, slope environments and the streambank, however, they differed in anaerobic, riparian environments. The greatest fluxes in two of the sites and lowest fluxes in the third site occurred at the junction between the slope and the riparian zone. In one of the sites, the slope-riparian break was where soil water NO3-- and DOC concentrations decreased precipitously. Soil N2O concentrations were greatest in probes that had intermediate O2 concentrations. Over the course of storm events, there were no drastic changes in N2 O fluxes or the concentrations of the controllers of its production in the break between the slope and the riparian zone. Redox status is helpful in predicting where N2O fluxes will occur on the landscape, with sites that are intermediate in O2 status having the highest N 2O fluxes. Soil surface fluxes are not predictable based on soil concentrations of N2O, so physical effects on gas transport must be important in this rainforest ecosystem. Short-term temporal variability is not important for the slope-riparian break, but may be in other topographic positions.

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