Processes Regulating Soil Emissions of NO and N^2O in a Seasonally Dry Tropical Forest

Abstract

While much is known about control of production of NO and N 2 O 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 N 2 O and the environmental factors affecting fluxes in a drought—deciduous forest of Mexico. Fluxes of NO and N 2 O were higher in the wet season than the dry season, but addition of water to dry soil caused large pulses of CO 2 , NO, and N 2 O emissions. Immediate increases of extractable soil NH 4 + and high rates of gross N mineralization and gross nitrification also were observed following wetting of dry soil NO 2 — had accumulated during the dry season, and that NO 2 — plus the pulse of increased soil NH 4 + were mostly consumed within 24 hours of wetting. This dynamic microbial processing of soil inorganic N coincided with the pulses of NO and N 2 O production following wetting of dry soil. Acetylene inhibition experiments indicated that NO production was dependent on nitrification, that nitrification was the dominant source of N 2 O when the soil was wetted at the end of the dry season, and that dentrification might be an important source of N 2 O during the wet season. Post—wetting soil moisture was correlated negatively with NO fluxes and positively with N 2 O 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 N 2 O fluxes and the contributions from nitrifying and denitrifying bacteria are controlled largely by soil moisture.