NITROUS-OXIDE, NITRIC-OXIDE, AND METHANE FLUXES FROM SOILS FOLLOWING CLEARING AND BURNING OF TROPICAL SECONDARY FOREST

Conversion of humid tropical forest to agriculture significantly alter s trace gas emissions from soils. We report nitrous oxide (N2O), nitri c oxide (NO), and methane (CH4) fluxes from secondary forest soils pri or to and during deforestation, and throughout the first agricultural cropping. Annual average nitrogen oxide emissions from forest soils we re 1.5 ng N cm(-2) h(-1) for N2O and 0.9 ng N cm(-2) h(-1) for NO. For est clearing increased the level of extractable nitrate in soils and a verage nitrogen oxides fluxes (2.7 ng N cm(-2) h(-1) for N2O, and 8.1 ng N cm(-2) h(-1) for NO). Immediately after biomass burning, short-te rm peaks of N2O and NO (123 ng N cm(-2) h(-1) for N2O, and 41 ng N cm( -2) h(-1) for NO) were superimposed on generally increased fluxes. Pea k emissions declined within 3 days after burning. Postburn fluxes stay ed higher than measured on adjacent forest sites for 3-4 months (avera ges for postburn fluxes were 17.5 ng N cm(-2) h(-1) for N2O, and 19.2 ng N cm(-2) h(-1) for NO). Increased N2O and NO emissions after cleari ng and until cropping were probably due to a combination of increased rates of nitrogen cycling and higher gaseous diffusion in drying soils . Compared to emissions from young pastures in the region, fluxes of n itrogen oxides from unfertilized agricultural areas were low (3.9 ng N cm(-2) h(-1) for N2O and 3.4 ng N cm(-2) h(-1) for NO), probably due to nitrogen uptake by fast growing corn plants and losses by leaching with draining soil water in the wet season. Variation in CH4 fluxes wa s high for ail land use periods. Forest soils consumed an average of 1 .0 mg CH4 m(-2) d(-1), which slightly increased in drier soils after c learing (1.2 mg CH4 m(-2) d(-1)). Postburn CH4 consumption by soils wa s slightly reduced (0.8 mg CH4 m(-2) d(-1)) compared to forest soils. Unfertilized agricultural soils consumed less CH4 than forest soils.