RIPARIAN NITROGEN DYNAMICS IN 2-GEOMORPHOLOGICALLY DISTINCT TROPICAL RAIN-FOREST WATERSHEDS - NITROUS-OXIDE FLUXES

Fluxes of N2O at the soil surface, dissolved N2O in near-surface groundwater, and potential N2O production rates were measured across riparian catenas in two rain forest watersheds in Puerto Rico. In the Icacos watershed, mean N2O fluxes were highest at topographic breaks in the landscape (approximately 40-300 mug N2O-N m-2 h-1). At other locations in the riparian zone and hillslope, fluxes were lower (less-than-or-equal-to 2 mug N2O-N m-2 h-1). This pattern of surface N2O fluxes was persistent. In the Bisley watershed, mean surface N2O fluxes were lower (< 40 mug N2O-N m-2 h-1) and no identifiable spatial or temporal pattern. Although the spatial patterns and intensities of N2O emissions differed between the two watersheds, surface soils from both sites had a high potential to reduce NO3 to N2O (and perhaps N2). This potential declined sharply with depth as did soil %C, %N, and potential N-mineralization. Simple controls on denitrification (i.e. aeration, nitrate, and carbon) explained characteristics of potential N2O production in surface and deep soils from riparian and upslope locations. In the field, spatial patterns in these controlling variables were defined by geomorphological differences between the two watersheds, which then explained the spatial patterns of observed N2O flux.