Emissions of trace gases from capoeiras enriched with leguminous trees in the Northeastern Amazon-Brazil / by Valdirene Costa de Oliveira.

Tese (Mestre)- Georg-August-University Göttingen, 2001

In the period from April´99 until March´00, the flux of the trace gases N2O, NO, CO2 and CH4 from the soil into the atmosphere in fallow vegetation (capoeiras) enriched with nitrogen fixing trees was measured. This gases play an important role in the climatic change phenomenon. Since their fluxes in the tropics is mainly influenced by the change of land use, the objectives of this study were to verify: (1) if the fallow enrichment provides a residual effect on the availability of N for soil microorganisms, and (2) if the presence of N-fixing species affects the emissions of the gases N2O, NO, CO2 and CH4 from the soil. The work was conducted on a smallholder farm in the municipality of Igarapé Açu, where the species Acacia mangium Willd. and Inga edulis Mart. were planted in a 2m x 2m spacing to enrich the fallow fields. Three plots were selected for each leguminous trees planted and 3 plots for controls without enrichement. Fluxes of NO and CO2 were measured with Scintrex and Li-Cor analyzers, respectively. Syringe samples of N2O and CH4 were collected for later analyses by gas chromotography in the laboratory. We were also measuring indices of N availability in the soils. The results show, that the fluxes of the gases N2O, NO, CO2 and CH4 vary from 0.21 to 0.55 g C m-² h-¹; 0.22 to 2.65 ng N cm-² h-¹, 0.53 to 3.51 ng N cm-² h-¹ and -1.28 to 0.54 mg m-² d-¹, respectively. A statistically significant difference between the treatments could not be found. CO2 emissions increased at the beginning of the rainy season in January 2000. This increase ("pulse") in CO2 flux indicates an increase in root and microbial activity during the rainy season. The flux of N2O and NO showed opposite trends in the rainy season in February, when N2O fluxes increased while NO fluxes decreased. Fluxes of CH4 showed positive values mostly in the rainy season, which indicate emission from the soil into the atmosphere, but negative values in the other months, which indicate consumption of atmospheric CH4 by the soils. Soils of capoeiras enriched by leguminous trees did not cause a lasting effect on N availability to the soil microorganisms, nor did N fixing trees influence the fluxes of N2O, NO, CO2 and CH4. We may conclude, that soils of capoeiras, enriched or not, generally represent sources of N2O, NO, CO2, but sinks of CH4.