Nitrous oxide (N₂O) is an important greenhouse gas. It has an atmospheric lifetime of about 120 years, a global warming potential of 296 relative to CO₂ over a 100 year time horizon. Nitric oxide (NO) is considered as a secondary greenhouse gas.it can promote photochemical formation of tropospheric ozone (O₃), which has higher global warming potential (GWP) than CO₂.

China is one of the world's largest producers of pork, and swine wastewater treatment is becoming an urgent issue in the country. In recent years, constructed wetlands (CWs), an effective and inexpensive technology, have been widely used to treat swine wastewater.

In wetland systems, the dominant processes for N removal from wastewater include plant uptake, soil sorption, and microbial nitrification-denitrification. However, only the conversion into gaseous products such as NO, N₂O, and N₂ are considered as permanent removal of N, which also ensure the effective and sustainable operation of the wetland systems. NO and N₂O are direct or indirect greenhouse gases which contribute to global warming. Greenhouse gas emissions from CWs arouse concerns because high N₂O fluxes were observed in wastewater treatment processes.

Using a closed static chamber method, researchers from the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) estimated NO and N₂O emissions in wetland microcosms vegetated with Myriophyllum elatinoides (WM), Alternanthera philoxeroides (WA), Eichhornia crassipes (WE) or without vegetation (NW) during the process of treating swine wastewater.

The researchers found that the maximum emissions of NO and N₂O occurred in WE, which was significantly higher than those in WM, WA, and NW (p <0.05). "This result reflects strong nitrification and denitrification occurred in WE," said ZHANG Shunan, a doctoral researcher at ISA. "During the whole incubation period, TN removal rates of 96.4%, 74.2%, 97.2%, and 47.3% were observed for the WM, WA, WE, and NW microcosms respectively, but no significant difference occurred in WM and WE (p >0.05)."

In addition, the emission of N₂O in WE accounted for 2.10% of initial TN load and 2.17% of the total amount of TN removal, compared with less than 1% in the other microcosms. These findings indicate that wetland vegetated with M. elatinoides may be an optimal system for swine wastewater treatment, based on its higher removal of N and lower emissions of NO and N₂O.

The research was financially supported by the key CAS Programs (KZZD-EW-11, KZZD-EW-10-5), the National Science and Technology Supporting Project (2014BAD14B00, 2012BAD14B17).

The study entitled "Emissions of NO and N₂O in wetland microcosms for swine wastewater treatment" has been published in Volume 22 Issue 24, December 2015 of Environmental Science and Pollution Research, details could be found at http://link.springer.com/article/10.1007/s11356-015-5210-3.