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Mobile forms of nitrogen leach from upland environments into aquatic systems, often discharging to coastal zones. Addition of nitrogen to once N-limited systems results in a host of changes ranging from eutrophication to loss of biodiversity. Floodplains can ameliorate these changes by removing and sequestering nitrogen. In many coastal floodplains, sedimentation causes lakes to transition to baldcypress swamps, and ultimately to bottomland hardwood (BLHW) forests. These habitats differ in their contact with floodwater, which directly and indirectly affects their ability to process nutrients, but the effects of habitat change on denitrification at the floodplain scale cannot be predicted because of lack of suitable data. This study compared denitrification characteristics among the aforementioned habitats within the Atchafalaya River Basin (ARB). Microcosms were established in the laboratory, and the acetylene block technique was used to estimate four denitrification characteristics: background denitrification rates, maximum rates, time to reach maximum rates, and the linear response of denitrification to nitrate concentration. There were significant differences in denitrification characteristics among the three habitat types; specifically, all habitats differed in the time required for denitrification to respond to nitrate in the overlying water, and denitrification in lake habitats differed from both BLHWs and baldcypress swamps. Landscape-scale models should account for different linear relationships between denitrification and nitrate concentrations, and different response times to nitrate concentrations for different habitats. Because denitrification characteristics differ across habitats within the ARB, continued habitat change within the floodplain will alter nutrient discharge to coastal waters.

Ecosystem-wide denitrification estimates generally depend on the degree of spatial variability in the system, but spatial variability is rarely assessed. To model nitrogen removal rates in the Atchafalaya River Basin we first identified trends in background and potential denitrification across this large floodplain. We conducted a laboratory study to quantify background and potential denitrification rates. Background and potential denitrification rates were significantly different. Background rates ranged from ranged from 0-1.35 μgN g −1 d −1 and potential rates ranged from ranged from 26.72-710.47 μgN g −1 d −1 , illustrating the existence of denitrification hotspots across the landscape. Background rates were related to soil characteristics (carbon, nitrogen, nitrate), but potential rates appeared to be related to landscape position (spatial coordinates). Background denitrification showed a strong positive correlation with soil nitrate, and a negative correlation with soil nitrogen and soil carbon. Potential denitrification rates showed no significant correlations with any parameters tested. We observed a significant relationship between location and potential denitrification rates, with greater potential downstream than upstream, but not between location and background rates. This suggests that landscape scale studies should include additional qualifiers, such as habitat type and organic matter quality, for more reliable estimates of denitrification rates.