Greenhouse gas dynamics in lakes receiving atmospheric nitrogen deposition

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 (CO ₂), methane (CH ₄), and nitrous oxide (N ₂O) 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 CO ₂ concentration in surface waters was 27 mol L ^-1 and did not differ between deposition regions. The CH ₄ 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 N ₂O. The concentration of N ₂O 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 CO ₂ as a greenhouse gas and because of its role in the destruction of stratospheric ozone. To understand the potential magnitude of lake N ₂O 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 N ₂O 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.