Written by Matt Gillman
Nitrous oxide (N2O) is one of the leading greenhouse gases contributing to the destruction of stratospheric ozone. N2O has a destructive power three hundred times that of carbon dioxide (CO2), making it an important target in the field of environmental study (Beaulieu et al. 2010). In large rivers and pools nitrous oxide is formed mostly by microbial and biological processes. The N2O formed by these processes in both aquatic and terrestrial areas account for the majority of the emissions of the gas into our atmosphere. For N2O to form, nitrogen needs to be supplied to the environment where there processes occur. The vast majority of nitrogen is supplied by human activity from things such as agricultural soil and sewage runoffs. There are also circumstances where N2O is directly fed into rivers via waste water treatment plants. These emissions of the destructive, global warming gas are rising 0.26% per year and are clearly directly linked to human activity and therefore deserve and require study and observation (beauliueu et al. 2010).
Up until the very recent past little was known about the N2O emissions from large rivers, due to lacking model accuracy and important data. That however is not the case nowadays. These new understandings are thanks to a team of United States Environmental Protection Agency scientists lead by J. J. Beaulieu. Beaulieu and his team spent thirteen months directly measuring the production of N2O biweekly, in Markland Pool, Ohio, as well as the emissions of this N2O into the atmosphere. Markland Pool is part of the Ohio River and was chosen due to its ranking of annual discharge, which is third in America. Twenty nine sampling sites were constructed from August 2008 to August 2009 based on their distance from each other. Three kinds of samples were taken from the river. The first being samples of the gas emissions from the surface of the river. This was done using floating acrylic chambers, which then transferred that gas to storage vials. The second sets of samples were from the water over top of sediments. In these samples the concentration of dissolved N2O was measured. In the final set of samples the water column N2O production was measured by incubating the samples taken for forty eight hours in a lab setting.
The team found that the rates of N2O production from microbial activity in the water columns of the river were near double those of sediments in the river. Seasonal patterns were also very clear. Summer and surrounding months exhibited much higher production rates N2O concentration than that of the colder winter months. In fact during winter months the levels reached near equilibrium. These findings were a sign that the production of N2O was influenced greatly by seasonal cycles and form models that heavily imply that concentration of N2O is affected by water temperature. Due to the fact that microbial processes thrive in warm environments this theory is almost certain. The results also show that the processes generally take place in the pelagic zones (meaning: off shore) of the river (Beaulieu et al. 2010).
The high rate of N2O formation in water columns is partly due to the high concentration of oxygen, which is needed to covert the NH4+ ( ammonium) and other forms of nitrogen into N2O molecules (Beaulieu et al. 2010). There is nothing humans can do about the concentrations of oxygen in rivers, and trying to do something might inflict harm upon the ecosystems of these rivers but there is a different solution. The ammonium and similar molecules come from one primary source, that source being us, the human race. It can be from waste water treatment plants, which regularly run their pipelines through large rivers. The nitrogen can also be from agricultural fertilizer, which runs into streams. These streams then produce N2O themselves and also run excess nitrogen into large bodies of waters and rivers. No matter the means of introduction or the purpose thereof, these emissions can be reduced by the same sources feeding them, the problem being the lack of initiative to do so.
Original Paper Reference:
Beaulieu, J. J., Shuster, W. D., Rebholz, J. A. 2010. Nitrous oxide emissions from a large, impounded river: the Ohio River. Environmental Science and Technology, 44 (19): 7527-7533.
link to paper: http://pubs.acs.org/doi/full/10.1021/es1016735
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