Journal of Geophysical Research: Biogeosciences
© 2015 American Geophysical Union
Impact Factor: 3.44
ISI Journal Citation Reports © Ranking: 2013: 24/173 (Geosciences Multidisciplinary)
Online ISSN: 2169-8961
Associated Title(s): Journal of Geophysical Research
Methane exceeds nitrous oxide in rivers' contribution to warming
Nitrous oxide (N2O) emissions have been the leading area of concern for scientists investigating the role of streams and rivers in global climate change for the past decade. A potent greenhouse gas, nitrous oxide is produced in riverbed sediments through nitrification and denitrification. Efforts to understand the rate at which nitrous oxide diffuses through the water to the atmosphere have dominated the field, yet diffusion is not the only relevant mechanism nor is nitrous oxide the only relevant gas. Observations by Baulch et al. (2011) suggest that the global warming potential of methane gas, which they measured bubbling up from several riverbeds, exceeded that of nitrous oxide. Gases produced in river sediments can travel to the atmosphere by diffusing through the water column, escaping as bubbles, or through plant-facilitated transport. The authors measured methane and nitrous oxide concentrations in the water and in riverbed bubbles and measured bubble accumulation in surface bubble traps for four Ontario streams to sort out whether diffusion or ebullition is dominant for each gas. They found that 10%-80% of methane emissions were in the form of bubbles, while nitrous oxide emissions were almost completely through diffusion. Additionally, the authors used streambed sediment samples to identify a relationship between gas emissions and sediment properties. They found that high levels of fine materials such as silt or clay were associated with increased emissions of both nitrous oxide and methane. They suspect the fine sediments could limit the availability of oxygen in the sediment. Depleted oxygen levels increase rates of denitrification and methanogenesis, thus increasing gas production rates. The authors also found that methane bubbles surpass diffused nitrous oxide in terms of global warming potential, which they suggest could warrant a rethinking of the importance of streams and rivers to global warming.