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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Soil emissions of NO were measured at the Chamela Biological Station, México, using soil covers and a field apparatus for NO detection based on CrO3 conversion of NO to NO2 and detection of NO2 by chemiluminescence with Luminol. Mean NO fluxes from forest soils ranged from 0.14 to 0.52 ng NO‐N cm−2 hr−1 during the dry season and from 0.73 to 1.27 ng NO‐N cm−2 hr−1 during the wet season. A fertilized floodplain pasture exhibited higher fluxes, but an unfertilized upland pasture, which represents the fastest growing land use in the region, had flux rates similar to the forest sites. Wetting experiments at the end of the dry season caused large pulses of NO flux, equaling 10% to 20% of the estimated annual NO emissions of 0.5–1.0 kg N ha−1 from the forest sites. Absence of a forest canopy during the dry season and the first wet season rain probably results in substantial NOx export from the forest system that may be important to regional atmospheric chemical processes. Wetting experiments during the wet season and a natural rain event had little or no stimulatory effect on NO flux rates.

Intact cores from the upper soil profile and surface litter were collected at the peak of the dry season and during the rainy period in the tropical deciduous forest of the Chamela region, Jalisco, México, to (1) analyze upper soil phosphorus (P) movement and retention, (2) compare soil P dynamic pools (soluble, bicarbonate, and microbial) in dry and rainy seasons, and (3) determine the response of these P pools to wetting. Unperturbed litter-soil cores were treated in the laboratory with either 10 mm or 30 mm of simulated rain with carrier-free P and compared to a control (no water addition) to determine the fate and retention of added P.P concentrations and pools in most litter and soil fractions were higher in the dry than in the rainy season. Soluble P was 0.306 g/m and microbial P was 0.923 g/m in the dry season (litter plus soil) versus 0.041 (soluble) and 0.526 (microbial) g P/m in the rainy season. After water addition, rainy-season cores retained 99.9 and 94% of P in the 10- and 30-mm treatments, respectively. Dry-season samples retained 98.9 and 80% of inputs in the same treatments. Retention after wetting occurred mostly in soil (bicarbonate and microbial fractions). Simulated rainfall on rainy-season soils increased P immobilization. On the other hand, simulated rainfall on dry-season soils released P through mineralization. The P release represents between 46 and 99% of the annual litterfall return. Our results suggest that both soluble and microbial P constitute important sources for initiation of plant growth at the onset of the rainy season in tropical dry forest.

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