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Soil [O.sub.2] is an important regulator of microbial processes but is rarely measured. Consequently, our understanding of temporal and spatial variation in soil [O.sub.2] 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 [O.sub.2] varied seasonally in wet and dry riparian areas, (ii) how this variation in [O.sub.2] exposure translated into spatially heterogeneous areas of denitrification and denitrification potential, and (iii) how [O.sub.2], N[O.sub.3.sup.-], and moisture interacted to affect denitrification rates. We collected continuous measurements of soil [O.sub.2] in 'wet' and ~dry' riparian soils and measured denitrification by removing the background [N.sub.2] headspace from intact soil cores, replacing it with a He-[O.sub.2] mixture, and measuring [N.sub.2] production with time. We found that soil [O.sub.2] varied considerably in the wet site, ranging from anoxic conditions when the water table was high in late spring to completely oxic conditions (20% [O.sub.2)] during summer when the water table dropped. In contrast, the dry site remained at 20% nearly year round Bulk soil [O.sub.2] strongly controlled denitrification rates in the wet site but not in the dry site, which only denitrified when N[O.sub.3.sup.-] 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 02 exposure. Experimental manipulation of [O.sub.2], N[O.sub.3.sup.-], and C may help to identify real hot and cold spots for denitrification in landscapes. doi: 10.2136/sssaj2009.0463