CONTROLS ON NITRIC-OXIDE EMISSIONS FROM TROPICAL PASTURE AND RAIN-FOREST SOILS

In field studies, forest soils in the Atlantic Lowlands of Costa Pica emitted greater amounts of nitric oxide (NO) than soils from pastures that had been actively grazed for over 20 years following their conversion from forest. We measured NO production from intact soil cores from these land uses. Laboratory tests using ammonium (NH4+), nitrate (NO3-), nitrite (NO2-), water, and acetylene (C2H2) additions demonstrate a response consistent with field studies. Forest soil cores produced more NO than pasture cores regardless of treatment. In forest soil the response to NH4+ solution was significantly greater than response to water or an ambient moisture control. Addition of 10 kPa C2H2 caused a marked decrease in NO production in forest soil cores. These responses suggest a nitrification-linked control over NO production. Large and rapid responses to NO2- additions suggest that chemical decomposition of this ion may contribute to NO production. Pasture soil cores did not show a significant response to any of the treatments including NO2-. Low porosity in the pasture soils may restrict emission of NO produced therein.