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

Abstract

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 (N 2O) and methane (CH 4). 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 N 2O and CH 4. Weekly field flux measurements and field manipulation and laboratory studies were conducted over 22 months during 1993–1995. We found that N 2O 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 N 2O emission rates from either pH soil. Liming the acidified Oxisol to pH 6 tended to increase N 2O emissions to the rates from the undisturbed grassland. Fertilizing the acidified grassland increased N 2O 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 N 2O emissions, demonstrated that nitrification rates generally control N 2O 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 CH 4 in the acidified Oxisol was about one-fourth of that in the pH 6 reference soil. Liming did not restore CH 4 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 CH 4 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 N 2O and CH 4 which can affect global atmospheric properties.