Does rice straw application reduce NO emissions from surface flow constructed wetlands for swine wastewater treatment?

Rice straw was applied often as a carbon source to improve nitrogen removal; however, few studies have considered the effect of rice straw on nitrous oxide (NO) emission during nitrogen removal in constructed wetlands (CWs). We constructed eighteen combined systems, consisting of rice straw ponds and surface flow CWs to investigate the effect of rice straw application on NO emission in three strengths of swine wastewater treatments. The results showed rice straw (RS) treatment increased 131.5% of NO emission factor from low strength CWs, but decreased 37.2-43.7% of NO emission factors for medium and high strengths compared with no rice straw (NRS) treatment. The RS application led to an average 10.7% increase in the potential denitrification rate, and simultaneously enhanced gene abundances of the total bacteria (16S rRNA), ammonia-oxidising archaea, ammonia-oxidising bacteria, nitrate reductase, and NO reductase (nosZ) for all strengths CWs. The multiple regression model revealed NO emissions were strongly related to water temperature, nitrate, chemical oxygen demand, and denitrification genes. The proportion of nosZ gene abundance in 16S rRNA 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 NO emission by RS application among different strengths. These findings indicated the effectiveness of RS application to control NO emissions from the surface flow CWs was related to the pollution level of wastewater.