Denitrification kinetics and denitrifier abundances in sediments of lakes receiving atmospheric nitrogen deposition (Colorado, USA)

The transport and deposition of anthropogenic nitrogen (N) to downwind ecosystems is significant and can be a dominant source of new N to many watersheds. Bacterially mediated denitrification in lake sediments may ameliorate the effects of N loading by permanently removing such inputs. We measured denitrification in sediments collected from lakes in the Colorado Rocky Mountains (USA) receiving elevated (5-8 kg N ha(-1) y(-1)) or low (< 2 kg N ha(-1) y(-1)) inputs of atmospheric N deposition. The nitrate (NO3 (-)) concentration was significantly greater in high-deposition lakes (11.3 mu mol l(-1)) compared to low-deposition lakes (3.3 mu mol l(-1)). Background denitrification was positively related to NO3 (-) concentrations and we estimate that the sampled lakes are capable of removing a significant portion of N inputs via sediment denitrification. We also conducted a dose-response experiment to determine whether chronic N loading has altered sediment denitrification capacity. Under Michaelis-Menten kinetics, the maximum denitrification rate and half-saturation NO3 (-) concentration did not differ between deposition regions and were 765 mu mol N m(-2) h(-1) and 293 mu mol l(-1) NO3 (-), respectively, for all lakes. We enumerated the abundances of nitrate- and nitrite-reducing bacteria and found no difference between high- and low-deposition lakes. The abundance of these bacteria was related to available light and bulk sediment resources. Our findings support a growing body of evidence that lakes play an important role in N removal and, furthermore, suggest that current levels of N deposition have not altered the abundance of denitrifying bacteria or saturated the capacity for sediment denitrification.