Factors Regulating Denitrification in a Riparian Wetland

Soil O-2 is an important regulator of microbial processes but is rarely measured. Consequently, our understanding of temporal and spatial variation in soil O-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-2 varied seasonally in wet and dry riparian areas, (ii) how this variation in O-2 exposure translated into spatially heterogeneous areas of denitrification and denitrification potential, and (iii) how O-2, NO3-, and moisture interacted to affect denitrification rates. We collected continuous measurements of soil O-2 in wet and dry riparian soils and measured denitrification by removing the background N-2 headspace from intact soil cores, replacing it with a He-O-2 mixture, and measuring N-2 production with time. We found that soil O-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-2) during summer when the water table dropped. In contrast, the dry site remained at 20% nearly year round. Bulk soil O-2 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 O-2 exposure. Experimental manipulation of O-2, NO3-, and C may help to identify real hot and cold spots for denitrification in landscapes.