CONSEQUENCE OF FOREST-TO-PASTURE CONVERSION ON CH4 FLUXES IN THE BRAZILIAN AMAZON BASIN

Methane (CH4) fluxes between soils and the atmosphere were measured in two tropical forest-to-pasture chronosequences in the state of Rondon ia, Brazil. Forest soils always consumed atmospheric CH4 with maximum uptake rates in the dry season. Pasture soils consumed atmospheric CH4 during the dry season, but at lower rates than those in the forests. When soil moisture increased in the pasture soils, they became a sourc e of CH4 to the atmosphere. Integrated over the year, forest soils wer e a net sink of approximately 470 mg CH4-C/m(2), while pastures were a net source of about 270 mg CH4-C/m(2). Thus forest-to-pasture convers ion resulted in a net source of CH4 from the soil of about 1 g CH4/m(2 )/yr. The total pasture-related CH4 release for the entire Brazilian A mazon increased from 0.8 Tg CH4 in 1970 to about 2.5 Tg CH4 in 1990, w ith a maximum of 3.1 Tg CH4/yr in 1988. Soils accounted for a small pa rt (about 5%) of the total CH4 release from the basin, while biomass b urning and cattle emissions accounted for 95%. The average rate of inc rease in CH4 emission from pastures was about 0.2 Tg CH4/yr between 19 75 and 1988. This represents between 12% and 14% of the global average rate of change in tropospheric CH4 content for this time period.