Methane and nitrous oxide fluxes in an acid Oxisol in western Puerto Rico: effects of tillage, liming and fertilization

Changes 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 effect of plowing, or liming and fertilizing an acid Oxisol on the soil–atmosphere exchanges of N2O and CH4. Weekly field flux measurements and field manipulation and laboratory studies were conducted over 22 months during 1993–1995. We found that N2O emissions from an Oxisol acidified 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 acidified Oxisol to pH 6 tended to increase N2O emissions to the rates from the undisturbed grassland. Fertilizing the acidified grassland increased N2O emissions but much less than when these soils were both limed and fertilized. Short-term field studies employing nitrification inhibitors in which we measured nitric oxide (NO) and N2O emissions, demonstrated that nitrification 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 nitrification. Soil consumption of atmospheric CH4 in the acidified Oxisol was about one-fourth 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 acidified 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 affect the soil–atmosphere exchange of such trace gases as N2O and CH4 which can affect global atmospheric properties.