SUMMARYThe semi-arid region of Chiapas is dominated by N 2 -fixing shrubs, e.g., Acacia angustissima. Urea-fertilized soil samples under maize were collected from areas covered and uncovered by A. angustissima in different seasons and N 2 O and CO 2 emissions were monitored. The objective of this study was to determine the effects of urea and of the rainy and dry season on gas emissions from semi-arid soil under laboratory conditions. Urea and soil use had no effect on CO 2 production. Nitrons oxide emission from soil was three times higher in the dry than in the rainy season, while urea fertilization doubled emissions. Emissions were twice as high from soil sampled under A. angustissima canopy than from arable land, but 1.2 lower than from soil sampled outside the canopy, and five times higher from soil incubated at 40 % of the water-holding capacity (WHC) than at soil moisture content, but 15 times lower than from soil incubated at 100 WHC. It was found that the soil sampling time and water content had a significant effect on N 2 O emissions, while N fertilizer and sampling location were less influent. Index-terms: Acacia angustissima, denitrification, nitrification, soil water content, urea. RESUMO: EMISSÕES DE ÓXIDO NITROSO E DIÓXIDO DE CARBONO EM SOLOS TROPICAIS SEMIÁRIDOS DE CHIAPAS -MÉXICO