PARENT SESSION
Oral Session #35: Carbon and Nitrogen Cycling.
Presiding: D. Rothstein
Tuesday, August 6. 8:00 AM to 11:30 AM. Palo Verde Room, Radisson.
Dissimilatory nitrate reduction to ammonium and nitrous oxide losses in humid Costa Rican forests.
Silver, Whendee*,1, Thompson, Andrew1, Firestone, Mary1, Ewel, John2, Reich, Alex3, 1 University of California-Berkeley, Berkeley, CA2 USDA Forest Service, Honolulu, HI3 Organization for Tropical Studies, Durham, NC
ABSTRACT- We report rates of nitrogen retention in diverse and simple humid tropical forests by a novel pathway for upland soils called dissimilatory NO3 reduction to NH4 (DNRA). We measured N retention via DNRA and N loss as N2O in old growth tropical forests on two soil types, in young single species plantations with and without N fertilization, and in older plantations in the Atlantic lowlands of Costa Rica. Using 15N tracers we measured rates of DNRA that ranged from 71 to 400 ng/g/d. Unfertilized young plantations had the lowest rates of DNRA, while rates of DNRA were similar in old growth forest on alluvial and volcanically derived soils, 11-year-old plantations, and fertilized one-year-old monocultures. Rates of DNRA exceeded N2O production via denitrification and nitrification in young and old plantations, both during the initial 3 hr sampling period and after 24 hr. Nitrous oxide emissions were similar to or greater than DNRA in the old growth forests at 3 hr, but at 24 hr only volcanically derived soils had N2O fluxes that exceeded DNRA rates. Gross mineralization rates explained approximately 50 % of the variability in DNRA over the course of the experiment. Our results suggest that N retention via DNRA represents an important N conservation strategy in these humid tropical ecosystems.
KEY WORDS: Nitrogen Retention and Loss, Dissimilatory Nitrate Reduction to Ammonium, Nitrous Oxide Fluxes, 15N