Methane and nitrous oxide fluxes in grasslands in western Puerto Rico

Changes in land use and agricultural practices in tropical soils are thought to make major contributions to the global soil source of nitrous oxide (N2O) and sink for atmospheric methane (CH4). The variety of measurements of these gas fluxes in tropical systems are, however, limited. From November 1992 until February 1995 we conducted weekly CH4 and N2O flux measurements within three grassland locations in western Puerto Rico that had not been cultivated for at least 25 years. Sites were located in either Vertisol, Ultisol or Oxisol soils that are typical of soils in the tropics. Plots within each site included non-fertilized controls and plots where fertilizer was applied at recommended rates (300 kg N ha−1 y−1 quarterly) for forage production in the region. During the measurement period, CH4 uptake rates averaged 5.8 μg CH4-C m−2 hr−1 with no significant differences across sites. These uptake rates were generally 10-fold lower than those reported for tropical forests. Fertilizer addition had a small negative affect on CH4 uptake in the Vertisol, tended to enhance CH4 uptake in the Ultisol and significantly decreased CH4 uptake in the Oxisol. Nitrous oxide fluxes averaged 16.9 μg N m−2 hr−1 from unfertilized sites, with no significant differences across sites. These background emission rates were typically higher than those in temperate grasslands. In fertilized soils, N2O emissions averaged about 46 μg N m−2 hr−1 in the Ultisol and Oxisol and 130 μg N m−2 hr−1 from the Vertisol, about 0.8 and 3.3% of the N applied. Fluxes from fertilized sites were within the range of emission rates observed for fertilized temperate grasslands.