Denitrification potential in sediments of headwater streams in the southern Appalachian Mountains, USA

We investigated variations in resource availability (NO₃-N and labile organic C [LOC]) as determinants of potential denitrification in stream sediments in the southern Appalachian Mountains, USA. Stream water and sediments were sampled seasonally in 2 streams of contrasting NO₃-N availability, Noland Creek (high NO₃-N) and Walker Branch (low NO₃-N). Eight additional streams with varying NO₃-N levels were sampled once during summer. Stream sediments were incubated at ambient stream temperatures, and nitrous oxide accumulation was quantified following acetylene inhibition of nitrous oxide reduction. Denitrification potential was greater in Noland Creek than Walker Branch. In autumn and spring, NO₃-N and LOC amendments indicated that denitrification potential in Walker Branch sediments was NO₃-N limited, whereas temperature had no effect on rates. Denitrification potential in Noland Creek sediments was not limited by NO₃-N or LOC, but was significantly affected by season and temperature. However, no differences in denitrification potential were detected when Noland Creek seasonal data were adjusted to a common temperature. NO₃-N in the 10 surveyed streams ranged from 10 to 549 μg/L, with the highest NO₃-N levels and denitrification rates generally occurring in the higher-elevation streams of Great Smoky Mountains National Park. Our results suggest that NO₃-N availability is the primary factor limiting potential denitrification in Southern Appalachian streams. Despite the ideal conditions of slurry studies, extrapolation of potential rates to estimate denitrification loss in the catchment channels indicates that the process is an insignificant N sink (1.7% of stream N export in Walker Branch and 1.5% of N export in the Noland Divide Watershed).