Constructed wetlands (CWs) are increasingly used to treat animal wastewater in rural areas because of their low cost, energy-saving, and simple operation.

However, considerable nitrous oxide (N₂O) are emitted during the treatment of livestock wastewater. N₂O is a significant greenhouse gas that contributes to global warming and the destruction of the ozone layer. Therefore, effective measures are needed to control the emission of N₂O from CWs.

Through floating chamber method, researchers from the Institute of Subtropical Agriculture (ISA) of the Chinese Academy of Sciences investigated the use of rice straw as a possible measure to control N₂O emission during the treatment of swine wastewater in the surface flow CWs.

Researchers found that rice straw (RS) treatment increased 131.5% of N₂O emission factor from low strength CWs, but decreased 37.2–43.7% of N₂O emission factors for medium and high strengths compared with no rice straw (NRS) treatment.

"The RS application effectively reduced the emissions of N₂O from the surface flow CWs only when treating highly polluted swine wastewater," said lead researcher Dr. ZHANG Shunan from ISA. "The rice straw can provide a large surface structure for microorganism attachment. High nosZ/16S rRNA may be a genetic indicative to decrease N₂O emission for medium and high strengths."

Researchers also found that the RS application led to an average 10.7% increase in the potential denitrification rate, and simultaneously enhanced the gene abundances of nitrifier and denitrifier for all strengths CWs.

The proportion of nosZ gene abundance in 16SrRNA was higher in RS (0.7–1.3%) than NRS (0.4–0.9%) for medium and high strengths, while an opposite trend was observed for low strength. The discrepancy was responsible for increasing or decreasing N₂O emission by RS application among different strengths.

These results suggested that the effectiveness of RS application to control N₂O emissions from the surface flow CWs was related to the pollution level of wastewater.

The study, published in Chemosphere, was supported by the National Natural Science Foundation of China, the Key Research Project of Frontier Science of Chinese Academy of Sciences, the National Key Research and Development Program, and the Youth Innovation Team Project of Institute of Subtropical Agriculture, Chinese Academy of Sciences.

Figure 1 A picture of the surface flow CWs and N₂O emission factors from the CWs treating three strengths of swine wastewater (Image by ISA)