Estimating Rainy Season Nitrous Oxide and Methane Fluxes Across Forest and Pasture Landscapes in Costa Rica

The objectives of this research were to estimate exchanges of CH4 and N2O, both radiatively active gases, between soil and atmosphere on hilltop, slope and swale hillslope positions of northeastern Costa Rica; and to assess the importance of accounting for topography in making areal estimates across hilly terrain. Emission rates from soils were measured during the rainy season on three hillslope positions of both actively grazed pastures and primary forests. Emission rates from pasture and forest sites were significantly different for both gases. Differences between slope positions, though notable, were not significantly different for CH4, but were significantly different for N2O. The forest landscape was partitioned with GIS methods into hilltop, slope and swale topographic positions. The calculated areas for each of these were multiplied by their respective emission rates to calculate overall flux from the entire forested area of 618 ha. Nitrous oxide flux ranged from 16,181 to 17,100 g N d-1. Similarly, CH4 flux ranged from -6,201 to -6,658 g CH4d-1. Errors associated with both estimating mean emission rates for each hillslope position and judgmental errors in partitioning the landscape into hillslope positional classes are important to making landscape-scale estimates of flux.