Methane and Nitrous Oxide Fluxes in an Acid Oxisol in Western Puerto Rico: Effects of Tillage, Liming and Fertilization

ÐChanges in land use and management of tropical systems are considered to be major factors in the recent upsurge in increases in atmospheric nitrous oxide (N2O) and methane (CH4). Studies were initiated in western Puerto Rico grasslands to determine the e€ect of plowing, or liming and fertilizing an acid Oxisol on the soil±atmosphere exchanges of N2O and CH4. Weekly ®eld ̄ux measurements and ®eld manipulation and laboratory studies were conducted over 22 months during 1993±1995. We found that N2O emissions from an Oxisol acidi®ed to pH 4 were generally lower than from pH 6 Oxisol soils that were used as reference controls. Plowing the grasslands did not change mean N2O emission rates from either pH soil. Liming the acidi®ed Oxisol to pH 6 tended to increase N2O emissions to the rates from the undisturbed grassland. Fertilizing the acidi®ed grassland increased N2O emissions but much less than when these soils were both limed and fertilized. Short-term ®eld studies employing nitri®cation inhibitors in which we measured nitric oxide (NO) and N2O emissions, demonstrated that nitri®cation rates generally control N2O emissions; thus these were lower in unlimed soil. It is likely that NO was produced through the chemical decomposition of nitrite, which in turn, was a product of biological nitri®cation. Soil consumption of atmospheric CH4 in the acidi®ed Oxisol was about onefourth of that in the pH 6 reference soil. Liming did not restore CH4 consumption in the acid soil to rates comparable to those in the reference Oxisol. We conducted a laboratory induction study to determine if incubation of these limed or unlimed acidi®ed soils with high concentrations of CH4 could induce methanotrophic activity. Comparable uptake rates to the control soils were not induced by these incubations. These studies illustrate that management of soil can considerably a€ect the soil±atmosphere exchange of such trace gases as N2O and CH4 which can a€ect global atmospheric properties. Published by Elsevier Science Ltd