Nitrification and Ammonia-oxidation of Ammonia-oxidizing Microorganisms in Sediments of Cascade Reservoirs of Lancang River Sediments

Ammonia oxidation is the limiting step in sediments nitrification and critical in the global nitrogen cycle. The discovery of ammonia-oxidizing archaea (AOA) has improved our knowledge of microbial mechanisms for ammonia oxidation in complex sediments environments. However, the relative contributions of AOA and ammonia-oxidizing bacteria (AOB) to ammonia oxidation remain unclear. In this study, through sediment sampling in cascade reservoirs of Lancang River (Dachaoshan reservoir, Manwan reservoir, Nuozhadu reservoir, Jinghong reservoir), totally five samples representing four reservoirs(two samples in Manwan reservoir) were selected for analyzing the ammonia oxidation activity and nitrifying rate. The AOA and AOB activities were separately determined by using the dicyandiamide and 1-octyne inhibition method. The community structure and nitrification characteristics of ammonia-oxidizing bacteria in sediments of cascade reservoirs were studied , and through the laboratory culture experiment, the leading mechanism of nitrification activity and ammonia oxidation rate change characteristics under different pH conditions were explored. The research showed that the nitrification rate of the four cascade reservoirs is between 13.34μmol/m2·h and 21.06μmol/m2·h during the drainage period. The nitrification rate of the Dachaoshan reservoir,Jinghong reservoir and Nuozhadu reservoir showed an increasing trend. Due to the high ammonia nitrogen concentration (140.45mg/kg) at the point of lower Manwan dam inhibits nitrification, the nitrification rate at the point of lower Manwan dam was lower. The nitrification rate range of pH1=4.5 was between 0.008 mg/ L·d and 0.02 mg/L·d. When pH2=6, the nitrification rate is between 0.013 mg/L·d and 0.029 mg/L·d. When pH3=7.5, the nitrification rate is between 0.019 mg/L·d and 0.051 mg/L·d.When pH4=10, the nitrification rate is between 0.016 mg/L·d and 0.046 mg/L·d. When pH=4.5 and pH=10, the ammonia oxidation rate at the five points was dominated by AOA. Under neutral condition and pH=6.5, AOB dominated the ammonia-oxidation rate at the lower Manwan dam, Jinghong dam and Nuozhadu dam points, and AOA dominated the ammonia-oxidation rate at the other points. Depending on the ammonia oxidation rate of AOA and AOB respectively, the relative contribution of AOA and AOB to ammonia nitrogen removal in cascade reservoirs can be accurately measured, which is of great significance for understanding the ammonia nitrogen removal process and effect in cascade reservoirs of Lancang river.