Soil O2 is an important regulator of microbial processes but is rarely measured. Consequently, our understanding of temporal and spatial variation in soil O2 is limited. This, in turn, limits our understanding of a key regulator of N loss through microbial denitrification. In this study, we explored: (i) how soil O2 varied seasonally in wet and dry riparian areas, (ii) how this variation in O2 exposure translated into spatially heterogeneous areas of denitrification and denitrification potential, and (iii) how O2, NO3−, and moisture interacted to affect denitrification rates. We collected continuous measurements of soil O2 in “wet” and “dry” riparian soils and measured denitrification by removing the background N2 headspace from intact soil cores, replacing it with a He–O2 mixture, and measuring N2 production with time. We found that soil O2 varied considerably in the wet site, ranging from anoxic conditions when the water table was high in late spring to completely oxic conditions (20% O2) during summer when the water table dropped. In contrast, the dry site remained at 20% nearly year round. Bulk soil O2 strongly controlled denitrification rates in the wet site but not in the dry site, which only denitrified when NO3− was added. Denitrification enzyme activity was approximately twice as high in the wet site compared with the dry site, both of which responded predictably to O2 exposure. Experimental manipulation of O2, NO3−, and C may help to identify real hot and cold spots for denitrification in landscapes.

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There is great uncertainty about the fate of nitrogen (N) added to urban and suburban lawns. We used direct flux and in situ chamber methods to measure N and NO fluxes from lawns instrumented with soil O sensors. We hypothesized that soil O, moisture, and available NO were the most important controls on denitrification and that N and NO fluxes would be high following fertilizer addition and precipitation events. While our results support these hypotheses, the thresholds of soil O, moisture, and NO availability required to see significant N fluxes were greater than expected. Denitrification rates were high in saturated, fertilized soils, but low under all other conditions. Annual denitrification was calculated to be 14.0 ± 3.6 kg N ha yr, with 5% of the growing season accounting for >80% of the annual activity. Denitrification is thus an important means of removing reactive N in residential landscapes, but varies markedly in space, time, and with factors that affect soil saturation (texture, structure, compaction) and NO availability (fertilization). Rates of in situ NO flux were low; however, when recently fertilized soils saturated with water were incubated in the laboratory, we saw extraordinarily high rates of NO production for the first few hours of incubation, followed by rapid NO consumption later in the experiment. These findings indicate a lag time between accelerated NO production and counterbalancing increases in NO consumption; thus, we cannot yet conclude that lawns are an insignificant source of NO in our study area.

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NOTES 1401 ody at higkar tempefatures. The monofluorides are inferior anesthetic tigents whlle the difluorides have no such effect. These behaviors are in good agreement with the facts recently demonstrated that the atomic distances between the C and F atoms is 1.41 A. in the monofluorides, while ft is only 1 36 A. in the difluorides.2 P g cent F Percept CI R P , ' C Calcd Found Cdcd Pound CHQF 8 9 to 9 0 18 4 18 1 68 D ti8 2 CHClFd -40 8 to -40 6 43 9 43 5 40 1 40 4 CHZClF -9 0 to -9 1 27 7 27 4 $1 8 51 8 CHIFZ -51 6 74 1 74 I