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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 y) or low (<2 kg N ha y) inputs of atmospheric N deposition. The nitrate (NO) concentration was significantly greater in high-deposition lakes (11.3 μmol l) compared to low-deposition lakes (3.3 μmol l). Background denitrification was positively related to NO 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 NO concentration did not differ between deposition regions and were 765 μmol N m h and 293 μmol l NO, 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. [ABSTRACT FROM AUTHOR]

Anthropogenic nitrogen (N) inputs have been found to influence emissions of greenhouse gases from a variety of ecosystems; however, the effects of N loading on greenhouse gas dynamics in lakes are not well documented. We measured concentrations of carbon dioxide (CO2), methane (CH4, and nitrous oxide (N2O) in 26 lakes in the Colorado Rocky Mountains (USA) receiving elevated (5 - 8 kg N ha-1 yr-1) or low (<2 kg N ha-1 y-1) levels of atmospheric N deposition. The mean CO2 concentration in surface waters was 27 μmol L-1 and did not differ between deposition regions. The CH4 concentration was greater in low-deposition lakes (167 nmol L-1) compared to high-deposition lakes (48 nmol L-1), while the opposite was true for N2O. The concentration of N2O in surface water averaged 29 nmol L-1 in high-deposition lakes compared to 22 nmol L-1 in low-deposition lakes. Nitrous oxide is of particular interest because it is more potent than CO2 as a greenhouse gas and because of its role in the destruction of stratospheric ozone. To understand the potential magnitude of lake N2O production related to atmospheric N deposition, we applied two published methodologies for determining emissions from aquatic ecosystems to available data sets. We estimated contemporary global N2O emissions from lakes to be 0.04 - 2 Tg N y-1, increasing to 0.1 - 3.4 Tg N y-1 in 2050. The contemporary estimates represent 13-95% of emissions from rivers and estuaries, suggesting that further research is required to better quantify emission rates from lentic ecosystems. [ABSTRACT FROM AUTHOR]

Human activities have more than doubled the amount of nitrogen (N) circulating in the biosphere. One major pathway of this anthropogenic N input into ecosystems has been increased regional deposition from the atmosphere. Here we show that atmospheric N deposition increased the stoichiometric ratio of N and phosphorus (P) in lakes in Norway, Sweden, and Colorado, United States, and, as a result, patterns of ecological nutrient limitation were shifted. Under low N deposition, phytoplankton growth is generally N-limited; however, in high-N deposition lakes, phytoplankton growth is consistently P-limited. Continued anthropogenic amplification of the global N cycle will further alter ecological processes, such as biogeochemical cycling, trophic dynamics, and biological diversity, in the world's lakes, even in lakes far from direct human disturbance.