While much is known about control of production of NO and N2O by nitrifying and denitrifying bacteria at the cellular level, application of this knowledge to field studies has not yielded unifying concepts that are widely applicable and that foster understanding of global sources of these atmospheric trace gases. We applied a simple conceptual model to the investigation of sources of NO and N2O and the environmental factors affecting fluxes in a drought—deciduous forest of Mexico. Fluxes of NO and N2O were higher in the wet season than the dry season, but addition of water to dry soil caused large pulses of CO2, NO, and N2O emissions. Immediate increases of extractable soil NH4+ and high rates of gross N mineralization and gross nitrification also were observed following wetting of dry soil NO2— had accumulated during the dry season, and that NO2— plus the pulse of increased soil NH4+ were mostly consumed within 24 hours of wetting. This dynamic microbial processing of soil inorganic N coincided with the pulses of NO and N2O production following wetting of dry soil. Acetylene inhibition experiments indicated that NO production was dependent on nitrification, that nitrification was the dominant source of N2O when the soil was wetted at the end of the dry season, and that dentrification might be an important source of N2O during the wet season. Post—wetting soil moisture was correlated negatively with NO fluxes and positively with N2O fluxes. These results support a conceptual model in which N trace gas production is generally constrained by the rates of N mineralization and nitrification, while the specific ratios of NO and N2O fluxes and the contributions from nitrifying and denitrifying bacteria are controlled largely by soil moisture.

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We report a field study of soil gas transport of volatile organic compounds (VOCs) into a slab-on-grade building found at a site contaminated with gasoline. Although the high VOC concentrations (30-60 g nr 3 ) measured in the soil gas at depths of 0.7 m below the building suggest a potential for high levels of indoor VOC, the measured indoor air concentrations were lower than those in the soil gas by approximately six orders of magnitude (~ 0.03 mg nr 3 ). This large ratio is explained by 1) the expected dilution of soil gas entering the building via ambient building ventilation (a factor of ~ 1000), and 2) an unexpectedly sharp gradient in soil gas VOC concentration between the depths of 0.1 and 0.7 m (a factor of ~ 1000). Measurements of the soil. physical and biological characteristics indicate that a partial physical barrier to vertical transport in combination with microbial degradation provides a likely explanation for this gradient. These factors are likely to be important to varying degrees at other sites.

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